View Full Version here: : Any amateur mirror makers still alive in Oz/Nz?
10-10-2011, 07:59 PM
Any amateur mirror makers still alive in Oz/Nz? I dont see any evidence in recent times of posting in the down unders of backyard mirror makers.Anybody out there? If so what are you up to at the present? Mirrors being made? Successes /failures? Biggest/smallest? Newts/Dobs? Future projects planed etc. As one who has returned to telescope making (in this last 18 months, after about a 40 year absence), I would love to hear from you. Currently I am working on a nearly finished 25" f4.7 Dob the mirror of which I am figuring at the moment. Also at the figuring stage on a 22" f5.2 mirror. I must admit Im maving lots of fun. I had almost forgotten how exciting and challenging making a good mirror can be. Thanks for your imput (in advance).:)
10-10-2011, 08:51 PM
Yes, still at it in Oz.
I finished a 20inch a few months ago.
I do not see anything amaterish about your endeavours.
I have mirrors to be made better before getting any more glass.
Where do you get yours from?
Hi Stephen and Colin,
I make my own mirrors too but only in the 4 to 12 inch range (so far).
I have a long term project to complete - 14 inch F4. Originally intended it to be a Dall Kirkham Cassegrain. Currently I am trying to decide whether just to make it into a Fast newtonian. Mirror is polished spherical but needs figuring.
I also recently picked up an 8 inch classical cassegrain mirror set very cheaply. I may make my own secondary for it as the one that came with it may not match. The seller was not sure if they were part of the same set. I recently acquired an 8 inch F2 sphere so testing it should not be a problem.
Good to hear from other mirror makers. I think we are a rare breed!
11-10-2011, 08:29 AM
Rod-Great stuff. My first project was a 7" EQ Newt back around 1962 as a 17 year old,(no Dobsonian options in those days).Even with the f7.5 mirror the scope was a monster with its 1/8" alluminium tube and massive EQ mount (my own design) made of cast iron having 1" bright steel shafts, a 3" pier (boiler pipe) with "hens foot" base also 3" pipe. The thing must have weighed 100kg. However it performed for me, being able to use out to 40-50x per inch of aperture.Compared to the 3" reflector (Japanese origin) bought from a local camera shop, its performance was the difference between night and day. In those days in NZ with the import restictions and such it was almost impossible to buy a reflecting telescope of greater than 3" or 4.25". The only 6" and 8" (biggies in those days) were home made. Now days biggies start at around 14", and again the only affordable way to go (if your budget is limited), is to fabricate ones own from the ground up. I figure that in real money terms that my current 25" Dob project will cost less (thanks to the brilliance of John Dobson and his simple concept), and actually weigh less than my original 7" Newtonian. With all the information available online its as easy to grind a 20"+ scope today as it would have been an 8" 50 years ago.
11-10-2011, 06:45 PM
Hi Colin, thanks for the post. I agree with your philosophy, regarding improving the glass you have. When I put my Dob together I will first of all test the primary without coating on moon, planets and stellar objects.I know only around 5% of incoming light will be reflected (about the light one would expect of a 6" reflector), but even with the diminished light one can evaluate to some degree resolution etc. If that test is passed satisfactorily then the next step would be to silver coat, which I can have done for around NZ$50. If everything was ok I would Aluminize. Otherwise a further figuring iteration would be the order of the day.
I have a son connected to the domestic glass/mirror industry so my glass is not too expensive. However the thickess I can get here in NZ seems to be 19mm float so a little limited on how big a mirror one can go. I have tried suppliers in asia for large blanks but invariably (so far) have been quoted US$5000 plus for blanks (borosilicate) of 30" diameter with thicknesses of 30 to 40mm. So well outside my budget. My current Dob (completed) should come in at around NZ$1000-1200. Where do you source your glass in OZ? I hear 1" plate can be got in Sydney.
11-10-2011, 10:04 PM
Hi guys Ive made a few mirrors/telescopes. My first mirror was an F7 Newtonian. For a first mirror it wasn't too bad it had a small turned down edge but it I used it successfully for a few years. I made a couple of others that never made it into telescopes. I've also tried to make a classical cassegrain that I could use as a fast Newtonian as well, just exchange the secondaries. The primary is an F4.55 and it is a good mirror. The secondary is still a work in progress. I'll get back to it sooner or later. I have a 16 inch plate glass blank which I'll make as practice run and then I'll make a 20 inch. I have a 20 inch Pyrex blank, but my next project is going to be a Lurie Haughton Newtonian. I will reuse the first mirror I made and turn it into an F4 mirror. I should use BK7 glass but as a trial I'll use plate glass for the 2 correctors. About 2 years ago Rod recommended I use plate glass for the correctors instead. I managed to get my hand on two 19 mm 10 inch blanks so I'll follow Rod's recommendation. If it works then I'll do it again with the BK7 glass.
12-10-2011, 07:30 PM
Hi guys , I've never tried mirror making but plan to soon , I have a 12" scope that I'd like to put a homemade mirror in one day but realise
thats a fair way off .
So for now I've bought a 6 " blank and materials to have a go at it .
As I built a test stand and started building a tester a couple of years back :rolleyes: I thought I'd build and finish the scope first so as to maybe move things along a little quicker .
I'll start a thread here sometime on how it goes and your sage advice is most welcome :thumbsup:
12-10-2011, 08:09 PM
is it started yet? :)
12-10-2011, 08:51 PM
fair comment!! :lol: soon ,, real soon
13-10-2011, 01:18 PM
Hi Graham, the 6" size is great to start with. My first mirror was a 5.75" and was a great exercise, but was I admit (in hindsight) a bit rough. However gave not bad views. In those days you started with a rough cut (about a dozen sided sawn) piece of 1" plate. I asked for a diameter of 6.25" hoping that by the time I had ground it into a circle it would have been 6", but lost a full half inch in the translation.:(
With your 6" blank I would suggest you make it to f10 as at this optical speed a spherical figure would be ok,and assuming your 12" dob is f5 then the focal lengths of both mirrors being the same, the 12" dob structure could double as a test bed for your 6" mirror.:)
13-10-2011, 07:00 PM
Thanks Stephan, I never thought of the 12 " as a test bed .
The 6" scope itself componant wise is pretty close to finished so I'll probably run with it,, its a single pole assembly at the moment
and will be adjustable out to around f 7.5 .. longer than that and I have to buy another bit of tube;)
13-10-2011, 07:08 PM
I like this thread.
I think it would be a great idea to have a mirror making thread section on IIS which would allow people to start threads on their mirror making progress and swap ideas rather than a thread being buried in the ATM section which tends to be mainly about builds and rebuilds using commercial mirrors.. What do you guys think ? . I would be happy to be moderator on it in the sense that I would try to keep things 'focused' and keep my eyes on the section and lend advice where I can.
Mirror making is something that is almost approaching a lost art in comparison to 40 years ago where if you wanted a `huge' 6" reflector you would probably have to grind one yourself as a 6" mirror would have been a weeks wages. I don't think any amount of the advanced work I do these days can replace the romance of the art I felt as a 14 year old , walking around an old washing machine with a 6" blank on top - the smell and sound of fresh Carborundum cutting glass and the smell of resin on a lap and the excitement of seeing my frosty glass take on a shine which I could reflect light from.
I read somewhere that it was the introduction of the C5 and C8 that spelled the dell knell for the ubiquitous 6" or 8" reflector on a pipe equatorial that we aspired to . The Asian Dobs revived the Newtonian in the 90's but I don't think mirror making as a hobby ever recovered.
I think that would be great! Personally I think it would help keep me motivated if I could share a project via a thread like you suggest. Having advice from people like youself would be a fantastic opportunity.
14-10-2011, 02:56 PM
Just a comment for those starting a 6" scope. An 8" is almost exactly the same cost and effort/time to make but you get 2x the amount of light with it.
My first telescope, a 15" f5.6 Dob was made in '86 and is still in use at ASSA's Stockport facility. Subsequently, I made dozens of smaller ones for other people but not for many years now. I've got that medical problem of Chronic Repetetive Birthdays! I've still got some 10" Plate blanks somewhere if anyone is interested.
14-10-2011, 07:34 PM
Hi Mark,(and all), a mirror makers thread would be a great idea. But are there enough of us (glass pushers) "down under" to keep such a thread alive? In these "instant this and instant that" days with commercial options for everything, its hard going for the "do it yourselfer" to get motivated, unless (as in my own case), one is restricted by budget limitations which make it necessary to make ones own as the commercial equivalent is out of the question. Plus there is immense satisfaction in successfully grinding, polishing and figuring your own mirror, which when built into a scope, can put you way out there in the midst of that great celestial wonderland of planets, DSOs, and "fly over" views of the moon.
14-10-2011, 08:19 PM
I think a thread souly for mirror making would build to be a good resource in time for those wanting to try there hand at it.
14-10-2011, 08:58 PM
has it started yet? (I have a 6" pyrex blank here somewhere)
15-10-2011, 11:43 AM
Great idea Mark!
15-10-2011, 01:05 PM
I had in mind not a thread as such but a separate section like ATM but perhaps called Mirror Making
So far there seems to be good support for the idea. I'll point Mike to it if he hasn't already seen it .
19-10-2011, 12:29 AM
I'm at the figuring stage of my first mirror blank, an 178mm F4.43, after working on and off for about a year. This is my first attempt and I'm happy with the results so far. It has been a very good learning curve which in experience alone will be great in making my next mirror.
It would be nice to a mirror making section in ATM.
09-11-2011, 04:49 PM
I cobbled together a tester out of some bits and pieces but am a little unsure how to finish it on a couple of points
Regarding the head , were do I mount the lightsource in relation to a ronchi screen and knife edge , some pics around show it in front of the screen but others behind the knife edge ?
Secondly does the light source get centred directly at the centre of the test stand ?
09-11-2011, 07:03 PM
Firstly, can you guys recommend some good books to get started on ?
11-11-2011, 06:42 PM
good luck with that Mark :thumbsup:
Steve (8" f/7 )
18-11-2011, 08:59 PM
Something I definitely want to do also at some stage, nothing like the satisfaction of using something you made yourself. Prob is I'd probably bite off more than I can chew and screw it up, I don't think I could be satisfied doing a spherical mirror as I have a weakness for fast scopes.
21-11-2011, 06:42 PM
I would love to someday make a mirror..... and telescope (when I have time :D). But wouldnt knopw where to start, so having a threaded group with info would be great.
21-11-2011, 11:27 PM
My only claim to fame was belonging to a mirror making club run by the guy who was incharge of figuring the optics of the AAT! Brings back memories; that was thirty years ago :eyepop:
A great chap and enthusiastic amateur astronomer. We used to make 6" mirrors in exchange for materials to make bigger scopes - I still have a 8.75" f6 in a box somewhere.
The biggest take home lesson I learned from David, was build a machine to do the hard work, and still have enough energy and enthusiasm to enjoy the figuring!!!
22-11-2011, 08:41 AM
Hi, i made my own 8" f6. Mirror, tube, mount, stepper driver circuits, computer control, gear box -the lot. I bought the secondary and the focuser. It was very therapeutic and i learnt so much about the science and theory whilst doing it. My aim was to do astrophotography and i did some average sort of stuff, but i could not get the quality of control in my gearbox - limited budget, and have since converted it to a Dob. I now have a Celestron C8+ for photography. But my hand made mirror Dob outclasses it for general viewing. I was really proud of it, still am. I love the history of astronomy and I felt I was following a line of endeavour that went back to Galileo. Hope i donít sound too pompous but its like being part of the continuum of history. I have three books, collectors items i believe, that i used as background research; Telescopes and Accessories, George Z Dimitroff and James G Baker 1st Ed 1948; Amateur Telescope Making, Edited by Albert G Ingalls 4th Ed. 1935 - with a forward by Harlow Shapley; Amateur Telescope Making Advanced, Edited by Albert G Ingalls 1st Ed. 1937. I also have How to Make a Telescope by Jean Texereau which I used extensively. Modern materials make it easier to obtain the quality finish but the science of optics has not changed.
25-11-2011, 06:38 PM
Good stuff Peter. It gives great satisfaction when you turn a telescope one has made to the heavens and see the cosmos revealed in all its glory.There is that special feeling when what you are seeing is due to the work of your own hands,thats the reward for the effort and time put in.
I think it'd be fascinating to have the skill and time to make your own mirror. No doubt very educational too. Interesting to follow the exploits of you guys taking on the challenge.
26-11-2011, 12:25 PM
I can recommend "How to Make a Telescope by Jean Texereau" if itís still available. Its old but it is very straightforward. Lots of diagrams and a simple explanation of the how to figure and test using a knife edge tester. You can not imagine the pleasure I felt when I first used my knife edge tester, the figuring was way out of course, it was still spherical, but to see my blank light up with the pattern as predicted was a real buzz. I used to knife edge test just to see it.
For you other guys that talk about not having the skill, without wishing to insult the great mirror makers or belittling their skill, it requires patience and time. A small mirror of say 6 inches to start should be within anyoneís capabilities. I wonít lie and will admit that when figuring I probably produced all possible figures, and maybe some that arenít in any book, I never got frustrated I just stopped for a day or two and then kept going till I got it right, and as I say knife edging was real fun.
Remember the shorter the focal length the more demanding the figuring, but if your using it just for casual observing then extend the focal length.
When I made my mirror the cost of a figured mirror was, for me at least, prohibitive - over US$600, so making them was more common. But I did it for the challenge in the end. I calculate that I worked for about US$3 per hour, and I am considering doing it all over again to make an 18" f4 Dob.
26-11-2011, 01:29 PM
Usefull site and reasonable prices.
11-12-2011, 02:12 PM
Hi all, just been googling up on Graham Loftus, (a well known New Zealand mirror/telescope maker),who was a great friend of John Dobson. Both of these men made mirrors up to 24" in diameter out of 19mm thick float glass.In fact the largest of the Loftus mirrors (please correct me if Im wrong), is 36" in diameter and only about 1" thick plate glass. Graham said that 19mm thick float is fine for up to 20" diameter mirrors. He said it was more difficult to finish larger diameter mirrors of this thickness.
Loftus made a comment: a large aperture with even a fairly mediocre figure will out perform a high quality telescope with small to medium aperture,(my paraphrase).
I would encourage anyone who has been teetering on the brink of "whether to,or whether not to" have a go at making their own "light bucket" mirror/telescope to "have a go".If your game then your in for an adventure.
11-12-2011, 02:30 PM
I would be frightened of fracturing a 20" 19mm mirror during grinding. I guess it would be mounted on something but then you would have flexure problems.:)
12-12-2011, 06:21 AM
There was a 24" Loftus Dob appeared on TradeMe over here about a year ago. Had an angled EP/Focusser arrangment. Was up in a barn about 30 km from me but the wife said 'NO' :shrug:
I think the asking was about $2500 or thereabouts. Not sure if anyone bought it or where it went if it did sell. Was a huge wooden box of a thing with steel frame base on wheels. Can't rmember if any detailed specs were given on the mirror, I don't think so.
12-12-2011, 06:16 PM
Hi Peter, I have personally ground out the following sized mirrors using 19mm thick float glass: 18.5", 20.25", 22", 22.15", 25". No problems with fracturing, easy to grind and polish.The key with working with mirrors this thin is to have a solid flat surface for support to work on, plus to float the mirror on several sheets of wet newspaper, or artificial grass, or such like. Because float is softer than pyrex or quartz it polishes out very quickly, so in the figuring stage its a good idea to use rouge instead of cerium oxide, as the cutting action is much slower, this gives one much more control over developing the figure required. Also by using rouge one is more likely to avoid a rough (dog biscuit) finish, which can be image destroying.:)
12-12-2011, 09:47 PM
I'd certainly disagree with this idea. A smaller telescope with excellent figure will show fainter stars than a larger one with a mediochre figure - so less weight to carry around and less thermal mass. I've seen this myself with an excellent 20" against a 36". When it comes to non - stellar sources things probably get more complicated.
13-12-2011, 08:02 PM
My copy of "How to Make a Telescope by Jean Texereau" arrived today. Second hand off Amazon.
Not sure if I've missed it in the various threads but is anyone able to comment on options for getting a mirror aluminised. I'm Brisbane based.
Ballpark prices for different sizes, who does it etc.
Still not quite decided if I'm going to try pushing some glass and that's one part of the process I've not seen any info on.
13-12-2011, 09:11 PM
The mathematical reductions in Texeraux probably put off a lot of people - nowadays this can all be handled by software.
Another great book in the French tradition and more recent is 'A Manual for Advanced Telescope Makers' also published by Willmann Bell.
The Stellafane website is a great source of links for mirror making stuff ( probably the best there is ).
Aluminising - worth checking with Oplass in Sydney ( see older ads for CQin Co, in Australia Sky and Telescope for a # ) . They are only sending stuff for coating to and from China now. I have another contact for quality coatings just coming on line shortly . Costs depend on size- a 12" mirror with Quartz overcoat is about $350.
13-12-2011, 10:06 PM
Mark thanks for that. I've found the Stellafane ATM page and have the 12" pricing gives a ballpark idea. Some reading ahead of me yet.
20-12-2011, 03:44 PM
I have a question for someone who has made their own mirror. I have figured a 17" blank of 19mm plateglass, and put it into the future telescope assembly. I can view the moon sharply at 840x magnification, Jupiter to about 350x (due to bad Sydney seeing at present), and defocussed stars have fairly equal rings both sides of focus, with no sign of astigmatism. Does anyone know if this is a good enough indication that the mirror could be coated? Mel Bartels says it should magnify to between 30 and 50 times per inch of aperture.
20-12-2011, 03:57 PM
there is a guy in Brisbane that does it, the company is called Optical Coating Associates with HQ based in sydney. I think he can do up to 20" mirrors.
20-12-2011, 08:58 PM
I think you have answered your own question in that if you can do the moon at 840x that equates to better than 49x per inch of aperture.I would definitely go ahead with the coating. If I can figure my current mirrors (22" and 25" float glass 19mm thick) to do even 30x to the inch I would be well pleased. I also have almost completed my dob structure and hope to test the first of my uncoated mirrors (22") sometime after the new year, when we get a little more night down under. Im doing my figuring outside and the temps are not to my liking (too warm).I like to work in cooler temps (around 10-15deg C with no sun), so may have to wait a month or so to fine tune the final figure.
20-12-2011, 11:26 PM
thank you very much for your reply Stephan as I hadn't actually realised it was at that level of mag. I have read that it if there is any slight problem it looks really bad after coating , so I am not confident. Is the moon a real test or should it always be something more subtle such as Jupiter? Also I have looked around for a cheap mirror coater and as I can't find one I guess the next thing is to build my own coater. I have already priced the fabrication of a chamber and it seems to be about the same price as getting one good coating done. So another project!! As this is my first time on these Ice in Space thread things (or any others), is there a mirror making thread? Hoping to hear from you and thank you again, cheers Frank
21-12-2011, 08:55 AM
Frank thanks for that, found this one http://www.opticalcoating.com.au/
"We have 2 chambers at 2 locations. The smaller (500mm) original system is based in SE Qld and the other (590mm dia) is based in Sydney." - the SE Qld office is in Southport.
21-12-2011, 09:31 AM
Frank - there is no mirror making section at the moment - why not ask Mike to start one on IIS ?
You could start a new thread on your 17" mirror project in the ATM section.
21-12-2011, 10:05 AM
Thank you Mark, I'll try to do that.. cheers Frank
21-12-2011, 10:37 AM
I have to confess to a mistake with my numbers in previous post. The 840x was OK for the moon but the 350 for Jupiter should have been 270, which gives about 17x per inch before the start of image breakdown. Also the moons do look like tiny balls. My 12" commercial mirror (F5) is also only useful to about 17x per inch on Jupiter as well. I suppose the real question for you is this, can you test a mirrors magnification using the moon and is this a definitive test.
21-12-2011, 03:18 PM
No, a visual observation of the moon or Jupiter is not a meaningful test. Sure it is satisfying to know you are getting an image, but it tells you nothing about how good the figure is, nor what you should do to correct it.
You could make these measurements with the mirror in a telescope pointed at a bright star, with a knife edge at focus and no eyepiece you can do something similar to a Foucault test - however its very awkward to do this in practice, and atmospheric turbulence makes it impossible most of the time on mirrors more than 6" or perhaps 8".
You should be setting up the mirror in a place where you have still air and using one of the various tests described in the ATM books - a Foucault test, Ronchi, Ross null test, Dall null test, caustic test or perhaps interferometry. Plenty of sources on the net describe these.
These tests will tell you how good the figure is,and also show you where the surface is high or low.
My mirror making days were long ago... Maybe one day I'll try again.
21-12-2011, 06:30 PM
thank you for the info. It passes using my foucault tester, just. I will try your knife edge idea, Cheers Frank
22-12-2011, 02:46 PM
I"ve made 50+ mirrors in Hamilton up to 20". Recently completed a 16" Dall-Kirkham refigure (both primary and secondary made by Loftus) and currently polishing a 12" that'll be used in a dob.
23-12-2011, 06:45 PM
Looks like you have moved a bit of glass.How do the Graham Loftus mirrors rate in your opinion? there seem to be a number of his mirrors in circulation.Where do you source your glass? Most all the Loftus mirrors are 19mm to 25mm (from what I can ascertain). I have been trying to find a source for thicker suitable glass at a reasonable price, but without success up to this point in time.
Am finding the 25" diameter by 19mm thick float glass mirror (f4.7) a little difficult to figure, but making better progress with a 22" f5.3 of the same thickness. Hope to have the latter up and working in the near future.
24-12-2011, 11:07 AM
(hopefully) I've attached a before and after of the Loftus 16". I've seen a few reasonable Loftus mirrors but have also seen a number of others similar to the 16". I've never seen one without a TDE.
The early mirrors I've done were 19mm plate or float glass (Alert or Smith & Smith). My 20" is Pyrex from Newport and have done 3 16" Pyrex from Newport. Other mirror blanks have been accumulated over the years from various people, alot that were half finished. The 12" I'm doing now is actually 320mm, 30mm thick plate from an Auckland Astro Soc. garage sale.
24-12-2011, 01:02 PM
Nice work Dave ! You certainly fix the TDE and it is so smooth.:thumbsup:
24-12-2011, 03:34 PM
When asked to do a refigure, it's actually nice to see such a bad test to start with because I know the owner will see a big improvement.
26-12-2011, 02:22 PM
Hi guys, I used to grind mirrors some 20 years ago, sizes from 5 inch to 15.5 inch. In these pics I'm polishing the 15.5 inch with 3 dobs of 6, 8 and 15.5 inch aperture. The little 6" f6 was my first.
It would be interesting to make a 20+ inch scope!
28-12-2011, 10:03 AM
Nice improvement with the 16" Loftus mirror. Is this a float glass substrate, and if so what thickness? What is the f ratio?
For the refiguring, did you use Cerium oxide, or rouge? What pitch hardness? Full sized laps, or sub diameter?
28-12-2011, 01:03 PM
The 16" primary and secondary were both Float or Plate (I don't know how to tell the difference). I don't recall actually running a tape on the primary edge but I think it was 1". Definitely not thicker. ROC was 2460mm so f/3.075 for the primary, conic was -.555. Final f/ratio for the scope was 6.4!! The owner uses it mainly for imaging with a coma corrector. I just refigured the existing optics, not design the system :lol:
The refiguring was done with thin cerium oxide with a 110mm soft lap. See pic. A small amount of polishing was initially done with a 6" lap, which just fit between the outer edge and the central hole but 95% with the smaller one.
04-01-2012, 03:26 PM
Thank you to those who replied to my testing dilemma. FYI, I have tested my 17" mirror in the scope with a ronchi grating and also performed a foucault test with a razor blade. This was stuck in an eyepiece tube unscrewed from a 2" eyepiece. Doing the foucault test in the telescope proved challenging but showed turned edge with the rest of the mirror going dark grey during that brief moment when everything lined up. When right at the null zone the whole mirror just blanked out. It is a very sensitive test to perform! This is about how it tested inside with my foucault tester setup with TDE showing much the same area. The ronchi test showed straight lines except the outer 1/4" which is the TDE. This is a very easy test to perform so I may just stick to that in future. cheers, Frank
01-02-2012, 06:42 PM
Been away awhile. Thought I would post a few pics to do with the progress of what tooling I have been using to grind and polish my three mirrors (2x22"f5.3 and 25"f4.7) currently being worked on.All three now polished, two (22" and 25") at the figuring stage. The 22" nearly there, just waiting for cooler steadier temps in my "out in the open" work area.
01-02-2012, 06:46 PM
Just a few more shots.:eyepop:
01-02-2012, 09:08 PM
Is that the mirror support for polishing : a thin square of non slip rubber mat ?
02-02-2012, 08:47 AM
the mirror is floated on several layers of wet newspaper(not easily seen in the pictures). Between support board (plywood) and pine trunk, I have used a piece of thick rubber carpet underlay to float the whole mirror/support board assembly, as interface between support board, and log top dont exactly match. In this way I am attempting to avoid astigmatism of the support board itself.
02-02-2012, 09:07 AM
So I'd imagine that you can only polish on an overcast day. The heating effects of the Sun on the mirrors would make figuring near impossible.
02-02-2012, 01:13 PM
A rough sagitta calculation indicates that the central thickness of a 25" f4.2 made from 19mm float will be around 10mm, and a bit over 12mm for the 22's...
02-02-2012, 01:24 PM
fwiw) The club scope in Bunbury WA has (or had?) a 20" primary made from a slumped 3/4" pyrex (meniscus) blank. The lateral support was effected via a central plug. Whilst it is true that it did form an image of sorts, I cannot say that it was ever a satisfying instrument to use. My old 12.5" f6 with a Parkes mirror produces a more pleasing view on any (and every) night that they were operating from the same location. (and that is being polite)
02-02-2012, 07:54 PM
Overcast, cool, (10-15deg C), little or no wind. Mornings and evenings best. May have to wait another month for the right conditions to do a fine tune on the figuring.
03-02-2012, 02:05 PM
I've now got a 200mm blank and a kit of compounds from Telescopes and Astronomy. I've cast a plaster disk ready to set up the tile tool.
I'm currently thinking some more about what F ratio to aim for.
My impression is that a high F ratio is much more likely to give me some success as I learn how to do this.
03-02-2012, 06:55 PM
This is as I understand it , F4 being difficult F10 easy, F6 maybe ?
Would like to see some discussion of the various strokes used? is there someone who can post up a vid (or a linkie)? :D
03-02-2012, 07:59 PM
Roger that's about what I was thinking. Have you looked at the Stellafane site eg http://stellafane.org/tm/atm/mirror-refs/strokes.html I've not played any yet but there are video's linked from that page.
03-02-2012, 08:15 PM
Bob not bad that :) does anyone know , the "w" stroke as shown there seems not to "flow" looks a bit odd or is that how it should be done?
04-02-2012, 10:18 AM
Bob , for a first 8" I'd recommend F7.5 ( or even F8) as the 60" focus is a good eyepiece height for standing and the mirror will only require a small amount of parabolising.
04-02-2012, 10:35 AM
Mark thanks for that. Be nice to have success on the first go (more inspiration for future work).
04-02-2012, 10:57 AM
fwiw) One of the most inspiring telescopes I have ever looked through was Mark's 8" f8 Newtonian binoculars.
27-02-2012, 05:19 PM
For an interested party I said I would put on a brief log regarding time taken to go through the grind of a 22" float glass mirror blank.
Grit #46 Sic. Using a 7"diameter 2.5kg barbell weight disc it took 4.75 hours (TOT) to hog out a sagitta of 5.8mm.(My aim for final sag depth 6.7mm(0.264")).
Grit #80 Sic.Continued with the 7" hogging tool for 3.75 hours (TOT) sagitta to 6.9mm (looks like an overshoot-but not so as you will see).
Grit #120 Sic. Changed to a larger tool 14" ceramic tile tool (50mm ply substrate).1 hour only (TOT). Sagitta back to 6.7mm due to the larger tool diameter moving the surface figure away from the hyperbolic formed by previous smaller diameter hogging tool. So now figure more spherical as action occuring simultaneously over a larger area of the blank.
Grit #220 Sic. Moved up to a full sized (22" diameter by 2.5" thick plywood) tool. This one overlaid with glass tiles (1" squares). Weight applied 15kg to facilitate grinding action.Very short strokes (TOT). Took 4.5 Hours to remove all pits from previous grits. In retrospect I should have spent another hour with #120 to take out all artifacts from #80. Sagitta remains constant at 6.7mm (my target sag). The full size glass tile tool further improves the evolving spherical figure appearing on the fledgling mirror. The sagitta dosent change much at this stage because tool and mirror diameters match and wear is across the whole area of the mirror.
Grit #400 Sic. Same 22" glass tile tool. 3 hours (TOT). All #220 pits gone. Sagitta still 6.7mm.
Grit #800 Sic. This is where the drama started. In all 13.5 hours was spent grinding with a mix of (TOT and MOT). For the first time part way through the grind I noticed scatches appearing. I would barely work one out when another (sometimes several others), would appear. I discovered the reason (towards the 8 hour point), why the scratching was occuring was because of the vacuum generated by mirror passing across the channels of the tool was drawing earlier grit particles up into the swarth. ( My strategy when changing down in grit size was to wash down the tool and scrub out the channels, but it can be difficult to get all of the artifacts from the previous level 100%. So I would paint in all the channels with a waterbased paint to attempt to capture such particles. unfortunatly the waterbased paint was inefective in the capture of all such unwanted particles. So I switched to an oil based paint mixed with a penetrating oil and painted the channels this eliminated the problem and no more scratches. Sagitta depth 6.7mm (target depth).
So in all time taken to fully grind this 22" mirror was 31 hours
I more recently ground a second 22" mirror in 20 hours
20-03-2012, 06:41 PM
Very,very brief polishing, figuring log.(22" f5.36 mirror).
Time taken to polish to 99% : 7 hours. (14" lap #55 G and CeO).
Time figuring (first iteration) : 16.5 hours. (5", 8.5", 11", 14", 17" laps).
My method was to start the deepening towards parabola from the center out. Progress through lap sizes from smaller to larger, thus moving the correction outwards as lap size increased.
Noted: when the lap size was small, figure tended to rough and chaotic, but as lap size increased and correction moved out towards the mirror edge, figure began to smooth out and become less chaotic.
Have ended up, (for the first iteration, yet to be tried as an uncoated optic under the night sky). The figure is fairly smooth, (no high or low zones), a bit of TDE/rolled edge. Possibly a little over correction.
I have made two aperture masks, to give 1) 19" f6.21 (this takes out the edge defects, and should provide reasonable performance for solar system viewing). 2) 17.5" f6.74 ( experimental only). Am in process of making a 20.5" mask as well.(this may be sufficient to hide edge defect).
Currently figuring second 22" primary. Will take a similar time, but no appreciable edge defect at this point in time.
As for the first mirror: If it dosent perform , then another figuring iteration wil be in order.
20-03-2012, 07:37 PM
When using small laps for figuring I've had success with circular strokes to get a smooth figure. I've just finished a 14.3" f/4.4 using a 4" lap.
What strokes do you use?
21-03-2012, 03:19 PM
Thanks for your imput. For the first of the 22"ers I polished with a sub diameter (14") lap (TOT). Stroke "Rosetta", with approx 3" over hang (F-B), coming to the edge side,(no over hang). Took 7 hours to achieve a 99% polish.
The second 22"er, (currently working on), I used a full sized (22") lap. Stroke straight through the center, with a 2"-2.5" over hang (B-F). Took only 3 hours to reach a 95%-99% polish. Used both (TOT) and (MOT), but mainly the former. This was actually very hard work for a 67 year old,(a real good workout).
In the figuring strokes I used all laps from 4" to 17" (TOT). With the smaller laps 4" and 5" a variety of strokes, including, very short straight, to circular (tight and not so tight), to take down high centers, and high zones. The larger laps from 7"- 17", long straight strokes across, but with an off center bias to avoid forming zones.
Love your nice pics.Im not having a lot of joy with my camera in this respect, always seem to look rougher than the visuals. If I can clean up my act (photography), will post something.
27-03-2012, 03:45 PM
As I had heard of other ATMers, using 1" plate glass for mirror blanks, I contacted Sydney Glass Pty Ltd, in respect to the 25mm plate sheets, they currently have in stock.
I received the following reply:
We have got some 25mm clear float, but is so badly annealed:shrug: it is impossible to cut it conventionally.
We use a waterjet and often supply panels of this to telescope enthusiasts.
(I was then provided with prices for the various sized discs I had enquired about-waterjet cut-edges look flat ground-no bevels).
(I also asked about cut sheets ie. Full sheet cut in half/quarter.)
The reply continued:
There is no point in providing you with "stock" glass because of the cutting problem. (end of reply).
I wont give the name of the correspondent.(Writer).
I agonized, over whether I should post this information, or not, but felt if not posted, many ATMers may fall into the trap, of trying to make a mirror, out of the above.
If one is going to use cheaper glass, glass by the Pilkington process, (float glass), is generally ok, if it comes from a reliable source. Good float, is relatively free from strain, has acceptable anneal, reasonably flat (usually within 4 to 5 waves of flat), and wedge free. (front/back parallel).
I feel sorry for those, who may have tried mirror making, and have failed,:( due to poor anneal, of the substrate they selected, as a blank.
04-04-2012, 07:06 PM
Hi interested glass pushers,
Just come inside from first light experience of my recently finished 22" f5.36 (fl 3000mm), mirror, (as yet uncoated). Unfortunatly my sky was 90% plus cloud covered and my dob structure has yet to birth a finder scope, so views were confined to the moon (when I eventually located it) as it briefly appeared for a minute or so from time to time.
Using a GSO 30mm 2" Superview (67deg FOV), the magnification 100x. The 91% Gibbous moon took most of the field of view. Focus came up nice and sharp, with small craters and surface wrinkles standing out with good contrast. At the boundary where the moons bright limb meets with the dark background of deep space, I was pleased with the sharpness of the boundary going from the brightness of the lunar fully lit surface to the sudden blackness of empty space.
Even though an uncoated mirror reflects around 5% of the light falling on it, a mirror of 22" (uncoated) still captures the light of a 5" coated mirror, so not going to be great on DSOs. So next step is to put on a coat of silver, and it should be ready to go.
PS. Would have liked to try more magnification, but with the limiting viewing, and difficulty in locating a target object without a finder decided to call it a night. My finder is going to be a Skywatcher 4.5" f4.44 newt.
04-04-2012, 07:12 PM
Thanks for the report . You really need to test your mirror on stars as that will tell you something about your mirror. A quality 100 lpi grating at focus will give you even more detail about overall correction and zones. It is hard to tell anything much by looking at the Moon in between sharp and blurry at 5 X magnification per inch of aperture.
06-04-2012, 03:50 PM
Todays been spent getting my dob to give birth to a finder. Now its done. 4.5" f4.44 Skywatcher newt,(with GSO 20mm Superview 70deg ocular). Will spend tomorrow getting all optics lined up, and then I shall be able to sight up on a suitable star, for testing,(I do have a 133LPI Ronchi grating already instaled in an eyepiece), so will give a report at the earliest.
14-04-2012, 07:33 PM
Been away awhile. Back to the task.Put the current 22" mirror to the 133 lpi grating at the eyepiece. Clear sky, but atmosphere somewhat unsteady (even Mars and Saturn displaying some twinkle).
Sighted scope up on Sirius (Altitude around 70deg). First tested at full aperture: figure appears well corrected across the greater part of the radius,( very smooth, straight lines (used 6 lines)), but most definitely the evidence of rolled down edge.
How large is the rolled edge? I have a number of aperture masks. 20.5", 19", 17.5". So tried the 20.5" and 19" masks. With the 20.5" still evidence of RE. Tried the 19" and still a trifle RE. When the 17.5" mask was in place no further sign of edge condition. At this aperture the Ronchi bars appeared dead straight, from edge to edge, of uniform thickness (parallel), were very smooth, this would suggest (to me) that the central 17.5" is in the "envelope". From the observation this central portion is free of humps,hollows, and zones. No observable astigmatism.
Would welcome suggestions as to how best to deal with the rolled edge, which from the above is evident on the outer 1.5"+ of the radius.
Currently well into the early figuring of the second 22"er. Also with high/ low zones near the edge.
07-05-2012, 01:14 PM
Have been doing more critical testing of my 22" mirror. On re-appraisal, what I have is a paraboloidal figure, (conic -1), across the central 17.5" of the mirror diameter, between 17.5" and 17.75", to mirrors edge, the surface curvature more approximates a hyperbole (conic -1.5). ( The curves (ronchi test 133 lpi at the focus), do not actually "hook" at all.)
I have carried out visual tests with the uncoated mirror in the scope using aperture masks of various apertures from 17.5" to 20.5", and at full aperture 22.2". Test objects Moon, and Saturn (with its moons). Interestingly the images focused up identically sharp regardless of aperture employed. In fact I preferred full aperture, as images were brighter ( I could clearly see at least four of Saturns moons at full aperture, whereas only Titan could be seen at the 17.5" setting). Had stunning views of the 2/3 gibbous moon, (full aperture), with detail sharp, with strong contrast terminator to edge. Features in the fully lit moonscape,which normaly tend to be some what washed out, stood out in sharp relief.
When viewing stellar/DSO objects the uncoated 22" gave less spectacular views than the 4.5" Skywatcher Newt finder. The finder using a 20mm 70deg eyepiece for 25x, with a 2.8deg field of view, compared to the 22" at 100x with 0.7deg field. (So actual area of the FOV being 1/16 the size of the finders(so fewer stars)). Therefore not a great jaw dropper on DSOs.
Still working the second of the 22"ers. Intending to leave one of these mirrors uncoated, for exclusive use on moon and planets. One advantage with an uncoated optic, is that there is no coat to deteriorate, and when the surface gets dirty/dusty, then a good easy washup, with a window cloth, and clean water will do the job.
02-06-2012, 01:43 PM
I am just going back to work on my 25"f4.7 mirror, after a long lay off. I now have two "pine butt" benches,(side by side), for working the 22" and 25" mirrors. Will be doing a few minutes figuring on each, day by day. Currently using a 17" lap (TOT) on the 22", usually 10 or 15 mins a session (one or two sessions a day). The present aim for this optic, is to take the correction from the inner 50% of radius out to the circumference, and deal with a TUE (last 22" mirror had TDE). Also smooth out some of the roughness left from using 5" to 11" laps which were used to deepen center/remove central hill.
As I said before I have just pulled the 25" from the shed (yesterday). Had to set about, and conform my 14" lap to the mirror, using "hot presses". To hot press, I place the lap face down in a tub of hot water, (at water cylinder temperature (65-70degC), for about 90-120sec. At this time I also use warm (45-50degC) water to warm the mirror itself, (sloshing water onto the mirror, as it rests on the pine log bench). Warmed lap then pressed onto warmed mirror, (with a micro cloth in between-no Cerox), pressure applied, 12.5kg, and some considerable body weight for about 75-90sec. This procedure I had to repeat four times, with three channel recuts, (used an electric soldering bolt for the cuts), before getting the 100% contact needed between mirror and lap. Two hours later in the cool of the evening, (7degC) was able to put in two 20min sessions of figuring. In this case I need to fix an oblate condition, so used a 'w' parabolizing stroke with a bias to the left side of center to avoid "zones" forming.
04-06-2012, 10:08 AM
Stephen, thanks for posting your progress. I'm following along with interest. I'm not familiar with "pine butt benches", are you able to walk around the mirror as you do the figuring with this setup?
04-06-2012, 06:30 PM
welcome to the forum. Have you checked out all of this thread?? Some where I posted a few pictures:eyepop: of my "out door workshop", complete with pine butt, about which I do multiple rotations almost on a daily basis.
04-06-2012, 11:04 PM
I skimmed throught but must have missed it. I'll have another look:)
07-06-2012, 10:38 AM
I saw your pictures, very nice setup, Now I see how you do things.:thanx:
07-06-2012, 11:42 AM
At the moment my workshop floor, and work benches, have a 4" of snow covering(from yesterday). This happened while the Venus transit was in progress, so needless to say, the sun didnt make an appearance. However watched it live:eyepop: on the computer, (next best thing).
07-06-2012, 10:06 PM
The Canadian Weather Network was showing video of the snow in Christ Church yesterday...I was thinking of you out there in your parka and mukluks grinding away:lol:
I completely forgot about the Venus transit but have seen some footage on tv, I think they said it will happen again in 105 years so I'll catch it then;)
14-06-2012, 07:28 PM
Really having some trouble making progress with this 25" f4.7 (19mm thick) float glass mirror. Its polished up really nicely, but just no joy with the figuring:(. What I think is happening is that because of its extreme thinness, its flexing too much as I work the laps (TOT). I may have pushed beyond the "thinness limit":shrug: on this one. Was ok with 22"x 19mm with one usable mirror so far, and will get a second 22" to an acceptable stage before too long. What I am intending with the 25" is to make it a "sandwich" by marrying a second 19mm float disc to it:thumbsup:. In between will be approximately 100 25mmx25mmx7mm glass tiles centers around 2 1/2" arranged in concentric pattern. The overall thickness of the composite will go up from 19mm(3/4") to about 45mm(1.77"). As stiffness increases with the cube of thickness the substate (in :question:theory) will have an increase in stiffness of 13.285. This should enable me to finish figuring without a flexture problem.:D
Just finished reading this very interesting thread, good work guys. :)
I am looking to grind my first mirror soonish as I decided I wanted to make a dob but the prospect of that task has left me like an Alsation after a head swap operation considering all the variation I could choose so I thought I could start with a mirror and hopefully develop my ideas as I go.
I've spent a bit of time reading various ATM sites and am in the middle of reading an old telescope making book too with hopefully another on the way but still I would like to gather a bit more info before I jump in head first.
Finding blanks or better yet blank kits with grinding tool supplied in Europe is proving a harder task than I had hoped. There are a few places to choose from so I should be fine but not that many and there doesn't seem to be a wide variety of choice.
At the moment I have two suppliers I'm looking at one has Schott borosilicate blanks and the other has Pyrex or Suprax. I don't really know what the major differences between the three are.
What would be the consensus on which would be better?
The one thing I have decided is that when I start I want to start with an 8" if possible.
Anyway I am looking forward to wherever this journey may take me.
15-06-2012, 06:41 PM
NZ amateur optician Graeme Loftus told me about 20 years ago that he never had any success with any plate glass mirrors where the ratio of the saggitta to edge thickness was less than 5/8...or 0.5625. Your 25" comes in at 0.44 which does not pass what he called ` the 5/8 rule'. What he was referring to was figure bending effect of having more thermal inertia at the edge than the center. The 36" X 1" thick mirror he made gave apparently fair images for about 5 minutes each night as it passed from under to overcorrected at sunset.
The issues you speak of of the mirror sagging under the weight of the tool sound quite plausable. I'm not sure why you want to push so big with this thin glass. The thinnest I would ever conceive of trying would be about 14 inch at 19mm thickness :)
16-06-2012, 07:36 PM
Why didn't he reduce the correction so that on a typical night the mirror did not overcorrect so much?
17-06-2012, 09:41 AM
The 36" X 1" thick mirror he made gave apparently fair images for about 5 minutes each night as it passed from under to overcorrected at sunset.
I always do my optical testing in the evening, (outside), to more accurately track the actual observing temperatures the mirrors would experience when pressed into service. In this, I expect performance to be at its best, at full cool down (around 5deg C).
Why so thin? "Budgetary restraint". This being a "raw" experiment,as I have limited finances, but time on hand, I decided to build a large aperture dob with a view to limiting amount spent to around $1500.
First I looked to obtain the mirror blank. My research came up with several options, most not cheap. Some options: 20.5" x1.25" quartz blank A$2000+ with freight extra several hundred $, so out of the question. Another option was a 30" x 2" borofloat blank out of China US$5000:eyepop: plus freight. Then there was the option of a 600mm(23.6") x25mm plate glass blank out of Oz for around NZ$1000 delivered (and these blanks have very poor anneal-cant even be cut conventionally without shattering), so out of the question.
Finally settled on a 25.2" x 3/4" float glass(origin:Belgium) very good anneal,flat (within 4 waves),no wedge,bubble free, etc. admittedly very thin, but cost to me delivered NZ$186. I already have a very usable 22" using this same substrate so figured on trying to push the "envelope" a little further.
17-06-2012, 03:05 PM
Dave : Because the glass is high expansion and the ratio between the mass in the edge and the lack of glass in the center means extreme sensitivity and effect to the figure from a temperature differential ... what part of a hypothetical 'typical' night of falling temperature do you correct for ?
17-06-2012, 08:58 PM
With the majority of mirrors I've made I have made the telescope at the same time so have been able to check the final correction by star test.
Particularly with plate glass when the scope is first taken outside the figure shows some undercorrection. The mirror and surrounding air inside the tube at this point is at ambient storage temperature so the undercorrected figure is what I have put on the glass (no worse than 1/4 wave). After a short time as everything starts to cool the correction increases. After another period the correction stabilises. I've found that the time of year (summer/winter) doesn't matter too much but sky condition does. If it's partly cloudy the correction won't increase as much as on a totally clear night. Obviously the important thing is the rate of temperature drop. In Hamilton the temp. difference between daytime high and nighttime low, on average (or on a typical day), is similar all year round. Off the top of my head around 12į.
I'm not talking huge amounts of correction change here but visible in the star test. I see the same thing in my 20" x 1.6" Pyrex mirrored dob.
I assume the Loftus mirror would behave similarly so if it passes through good correction in early cooling (sunset) I suggest a reduction in overall correction would be an improvement.
Did he say the mirror continued to gain correction the whole night or did the gain eventually stabilise?
If I'm figuring a mirror but not able to star test then I Foucault test in the early evening under falling temperature conditions. I test for smoothness under stable conditions though.
Incidently, loftus once ate his dinner off that same mirror.:confused2:
25-06-2012, 03:07 PM
Winter here in NZ. Working outside (5deg-8deg C),trying to move surface figure, of my current, (on bench) 22" mirror, in the right direction. In this temperature range #55 G is too hard. What do I need to do:help: to soften it, to be like around an equivalent to #44 G. In the old days, (50 years back) when I used a resin instead of pitch, we used pure turpentine, and beeswax to "condition/temper" the resin for the lap. Can I use turps, or do I need some other substance to do this, (and about what quantity of additive is needed per KG of Pitch?):question:
25-06-2012, 08:56 PM
Turps will do it, but I doubt I personally would be figuring outside. Apart from the temperature, one gust of wind and you could pick up a speck of grit that would spoil your year!
Can't help regading exact qty, but the last time I poured a lap, I recall it taking tablespoons, not cups to get the right viscosity.
I vaguely recall reading on the ATMlist years n years ago that pure turpentine is recommended for Gugolz.
I do have a bottle of it sitting next to my Gugolz so I guess my memory is good :shrug:
Love the intense pine smell of pure turps ;)
edit: regarding quantity... no idea. Wouldn't be too hard to knock-up a makeshift penetrometer tho, ala Texereau ;)
I have used castor oil following a suggestion on the ATM list. It kelps the lap soft longer than turps.
26-06-2012, 07:44 AM
You want to use Gum Turpentine for softening not turps !
26-06-2012, 05:21 PM
If it helps, I've only ever used rosin mixed with Stockholm Tar.
29-06-2012, 06:17 PM
Hi Dave, :)
where do you get rosin and Stockholm Tar these days? :question:
At the moment I am experimenting, by adding very small quantities of 30 grade motor oil, to a brew I have, of 50/50 Gugolz, #55/#64. Doing incremental tests, by adding a teaspoon plus, to the approximate 1Kg of G, then letting cool, and doing the small coin edge test, to get the right hardness/softness, for my current 5-7deg working area.
The motor oil suggestion:thumbsup:, came from a well known mirror maker from the US. My aim to arrive at a mix, equivalent to G#40.
30-06-2012, 07:13 PM
I have had a stock of rosin for some time so I don't know where to get it now but the Stockholm tar can be bought from most vet supply stores or saddlery/equestrian supplies. I use 7 parts rosin to 1 part S/t for 20į temp. An extra teaspoon of tar will soften 1 ltr mix a reasonable amount.
I'm sure it would work with pitch.
05-07-2012, 04:04 PM
When you say gum turpentine,:confused2: do you mean, pure turpentine, or is this something different? If so where does one source it?
Have tried motor oil (not impressed). Seems not to mix, for when the mixture was cooled, the pitch was as hard as ever, the oil located itself around the edge of the heating pot (aluminium hot pot). Problem mineral/not mineral combination?:shrug:
I'm guessing it is pure turps, made fron pine resin/gum.
Smells very strongly of pine, I love it.
I don't remember but I think I got mine from Bunnings, I remember it being about twice the price of the regular stuff.
05-07-2012, 05:56 PM
I see still no mirror making subject has become a sticky yet.
06-07-2012, 01:18 PM
As far as I know, gums turps and pure turps are the same thing. :thumbsup: Don't confuse this with mineral turps for cleaning paint brushes etc. You can get it at any decent hardware store and is used for mixing quality timber furniture finishes.
06-07-2012, 06:38 PM
I used to use pure turps, (seemingly, same as "gum turpentine"), in the sixtys, with beeswax, to temper my rosin/resin. I remember the aroma of the lap, was "outa this world:) (nice)". Not impressed with the aroma of Gugolz, but thats progress I suppose. Any way have tossed up whether, I should use the "pure turpentine", or " Stockholm Tar", to soften the Gugolz. The toss went the way of the S.Tar, for two reasons. Firstly, I know that the p.turps, is very "volatile",in which case the lap would not retain its new "softness level" for very long. Secondly, I would have to do a return journey, of at least 100km to aquire a supply. Fortunately, the local vet, just down the road, (less than 10minutes drive), had the s.tar on the shelf, so the deal was done. I assume the tar being less volatile :question:will mean greater "softness" longivity, for the new lap. Looking forward to getting lap, and mirror reunited in the 5-8deg work enviroment.
07-07-2012, 08:33 AM
Yes, I've asked mike for a mirror making thread section. He offered to make this thread a sticky a while back. Perhaps a if a few more people raise it then it will happen :)
07-07-2012, 08:34 AM
Yes I meant 'turps' to mean `mineral turps'. You should be able top get Gum Turpentine from and hardware store.
07-07-2012, 02:32 PM
Unless we read the thread, we don't know (and no we don't read every thread :P). A pm to one of us Mods would of sufficed.
Sticky done. :thumbsup:
07-07-2012, 10:36 PM
Hi John- yes I was referring to a PM conversation with Mike a while back. I know he is a busy man. Thanx for the sticky :)
09-07-2012, 12:28 PM
I think the S/T is going to "hit the spot". Yesterday, did a melt mix of 1kg of a 50/50 mixture of Gugolz #55/#64, and about 150ml of S/T. I let this mix cool to the ambient temperature, (7deg C), and tested, failed:sadeyes: the fingernail/edge of small coin test, as I could easily depress the cooled pitch surface, in a couple of secs, with forefinger. OK, overdone the S/T. Added another KG of #55G, to the "mess", and had a second run, (NO MORE S/T). Upon cooling, the coin test gave a 1/4" indent at 10sec,:thumbsup: (temperature at the time 4deg C). Poured two sub diameter laps, (12" and 7"), with this new mix, which I shall call #40G. Armed with these new laps should be able to now carry on "winter season figuring".:)
09-07-2012, 12:54 PM
Just a note on testing pitch. I like to leave samples for a while-fast cooling them under water doesn't seem to give a reliable test for some reason, tends to read too soft.
04-08-2012, 06:43 PM
Hi All,(and special thanks to Dave Brock:thanx:),
The stockholm tar tip has provided me with a real breakthrough in my being able to continuing figuring my 22" and 25" mirrors at my current 4-10deg C temps,(working outside in my back yard). Previously with #55 Gugolz I was unable to successfully make any headway with figuring, due to the pitch laps being too:sadeyes: hard.
Now with the right mix of pitch/stockholm tar have produced three very complient sub-diameter laps,(7",12.25" and 14"). With the use of these, I have brought my second 22"er, to a point where star testing, using the Ronchi eyepiece (133lpi) is the next step. I found with these large very (very) thin mirrors, that astigmatism during figuring presents a real problem,(if stoke is not quite right, one can produce what appears to be major:shrug: "stig" within 3-5minutes). The good news is that the defects polished in can be polished out, (though corrective therapy takes considerable longer).
During the final 1-2 hours of figuring, while I was working with the larger 14" lap to remove some residual stig, and improve overall smoothness to the figure, I was greatly aided by the climatic:) conditions, gently raining, (greatly helped to lubricate the action between mirror and lap-was just using the minimum amount of CeO), temperature at 8.5deg C. The result, a very smooth finish. As I mentioned in a previous post I estimate the pitch hardness as equivalent to about #40G.
Also making progress with the 25"er, again coming in and out of stig problems but overall I am making headway and expect this optic to be ready for star testing soon.:thumbsup:
19-08-2012, 07:47 PM
Hi grinders/polishers/ and figurers,:)
Just like to report that I have finished figuring, (first iteration),the 25" f4.72 by 3/4" thick float glass mirror, after 33hrs of fighting all the contortions:mad2: that astigmatism could throw at me.
The present spell of cool (7-8deg C) damp weather, has given me the ideal conditions to successfully use my #40G pitch laps, to gently move the mirrors figure in the right direction.:thumbsup:(Neither of the 22"ers presented such a challenge, though "stig" did present in both but not so aggressively).
When using the 85lpi Ronchi test I found the 25"er very sensitive to the position of edge :shrug:supports while on the testing rig, (if not exactly +-45deg from mirror bottom then the mirror would show physical warp as :screwy:astigmatism). The further the edge supports from the ideal, the worse the stig appeared to be, but when placement of supports was;) correct, stig disappeared. Neither 22" was as sensitive to edge support placement.
Testing is done at night with temps around 5deg C so as to approximate those temps which one would experience, when mirror would be working when night viewing in my part of the world, Autumn/Winter/Spring seasons particularly.(Dosent get really dark here in Summer anyway). Now just waiting for the weather to clear so star:eyepop: testing can be undertaken.
19-08-2012, 09:52 PM
Great to hear this Stephen, yoiu certainly have had quite a wrestling match with that mirror, now the wait for clear dark skies.
12-09-2012, 02:15 PM
here is a picture of a couple of laps I made using Ardit instead of using plaster. Ardit is a floor leveling concrete compound used before tiling of floors. I used it when I ran out of plaster one time and found it to be really strong and good to use. It sets quickly and just before it dries you can shape it with a rasp to get a good shape. A few coats of paint on the back and rim of underside and then add pitch. I used the larger one with hard pitch to polish my mirror and it turned out spherical right to the edge. The smaller one was used for figuring using soft pitch.
25-09-2012, 04:09 PM
Recent star testing of my 64cm mirror revealed two areas in the figure that need further:shrug: work. From the 88% radius, some rolling down of the edge exists, and the central 55-60% of the mirror is still undercorrected. Interestingly enough, when I fitted a 22" aperture mask, (thus hidding the turned down 12% of the radius), I was intrigued:question: by what was seen at the eyepiece, (ES82deg 24mm giving 125x). When viewing through the eye lens center, was unable to get a clear focus, but when viewing through the ocular nearer the edge of the eye lens, a sharp focus was possible, and star:rolleyes: images were decidedly smaller than those of my 14" dob, with double stars being separated by a larger margin, when similar magnifications used in both scopes (and my 14" is no slug). Aperture:eyepop: does rule, even when the larger is considerably short of being perfect. Currently I rate this mirror as a 2/10. So some more work on the figuring block. Again I noted how much more sensitive the Ronchi (133 lpi) eyepiece star test is compared to the Ronchi (65 lpi) bench test at ROC.
PS. Currently would rate my other two mirrors: 22.2" f5.36 @ 3.5/10 and 22" f5.23 @ 5.5/10.
03-10-2012, 08:48 PM
I'll add my 2c to the 'any mirror makers' thread. About 14 years ago I made an 8" f/6 mirror as my first project. It was OK but ended up with a TDE and a rough zone inside that where I'd tried to fix it.
The coating has had it, so I decided to have a bother go before I got it recoated. I tried fixing the edge itself, but in the end went back to a sphere and reworked it from there. To parabolise I used a 6" star lap - not really necessary for this mirror, but there's an unfinished 14" next and I was trying a few things out on this one.
The biggest difference since my first effort was that I built a Bath interferometer and used that for monitoring the figuring progress. I have to say, it's a wonderful thing to just take a photo of the mirror, load it on the computer and see the exact shape of the surface. I'ma bit surprised I haven't seen more mention of them on here.
The final result is that I've gone from a rough surface, -0.8 conic and 0.75 strehl to a smooth surface, -0.98 conic, 0.95 strehl, and 1/29 wave RMS. the only downside is a small scratch. You can hardly see it, but it shows up in the photos, and I'm betting it will stand out when the mirror gets coated. I have no idea how it happened, and 2hrs polishing didn't improve it. I decided in the end that I wasn't going back to grinding to get it out, so it's going to get a bit of texta after it's coated.
I've attached the before and after images and reports. I'd like opinions from the experts here, but I think it's done.
05-10-2012, 03:38 PM
Looks like a big improvement there Bruce. Well done.
Not sure what you mean by "texta" but if it involves some sort of painting or similar I would urge you to just leave it alone. The light diffracted from 1 scratch is not going to be noticeable.
05-10-2012, 04:41 PM
Thanks for the reassurance. The original plan was to ignore it, but it looked so bad in that photo that I started to think about blacking it out. I'll go back to ignoring it.
24-10-2012, 08:30 AM
Just returned my reworked #1 22.2" mirror to the dob structure, after further figuring work was required, to remove a rolled down edge 88% radius out, and further improve correction over the inner regions.
Star testing, using a 133 lpi Ronchi:thumbsup: ocular last night, revealed a vast improvement since test previously carried out, after the original figuring. As a test subject I used Rigel Kent. The test was carried out using the eyepiece focused at various numbers of lines from 1-8. The most useful numbers being in the 6-8 region. The image seen revealed straight,clean,parallel lines edge to edge except at the extreme top and bottom of lines which indicates the existence of some TDE. However as I let the star image transverse the FOV I was unable to notice any bowing of the lines as they came onto/exited FOV. I conclude from this that TDE must be quite narrow. When image was reduced to 1-3 lines seeing caused too much image movement to see the TDE condition.
Replacing the ronchi eyepiece with my regular 2" eyepieces (ES30,24,TV Nagler 20) I viewed Rigel as the spectacular double it is, nice separation. Did note at this point that collimation is off by quite a bit,(not easy to collimate an uncoated optic with no center spot), as star images flared somewhat to one side. Also looked :eyepop:at the 63% waxing moon, which gave nice sharp high relief images in both ES eyepieces, but a smidgen off being absolutely sharp in the Nagler. Placing on a 20.7" aperture mask cleaned that up somewhat. This removed the TDE, slowed the focal speed,and probably compensated to a degree for the miscollimation.
Overall the test revealed a great improvement in the mirror figure. Previously I rated this optic as a 3.5/10. Now at this point I would rate at a 7/10. Excluding the TDE section I would think mirror would be close to 1/4 wave accuracy.:)
24-10-2012, 08:40 AM
Congrats on mirror improvement. Just use a whiteboard marker to make a temporary collimation dot in the center- it will wipe off when you need to. I use them to mark areas to polish in normal figuring operation.
24-10-2012, 08:42 AM
I'm glad to read this Stephen, you certainly have put your time in on this one, nice to see it coming together for you. It must be a bear to collimate with no center spot:)
25-10-2012, 07:57 AM
Thanks for the useful tip, Mark.
11-12-2012, 07:37 AM
We need some sort of revival of amateur telescope making, nice to see it's still alive to some extent back home. Does anyone know how Graeme Loftus is doing? He must be really old now, almost 90? Haven't heard he died.
Funny, Graeme got me into making mirrors years ago. Now I've worked on a bunch of the world's most interesting telescope projects in some capacity, designed and (optics) contract managed a few into existence, now contract manager for E-ELT optics (fab for M1 segments through M4 adaptive mirror and M5).
Thanks to Graeme.
A number of the top professionals I've met in the field of optics over the years started their carreers as teenage ATMs, and there's probably more opportunities for employment in this area than ever now, keep that in mind all you young guys thinking about making a mirror. Go for it!
13-12-2012, 05:28 PM
I haven't heard from Graeme for 2-3 years but I'm sure I'd have heard (on the NZastronomers group) if he had passed away.
Like Andrew I am one of the many whom Graeme introduced to atm.
Graeme is still going (87) however stopped making telescopes 1-2 years ago. I dropped in to see him yesterday and even though his body has slowed down his mind is as sharp as ever!
Merry Christmas all.:)
03-02-2013, 08:23 PM
Great to hear Graeme Loftus is still with us. What a inspiration:thumbsup: he has been to so many a modern day amateur mirror maker in "down under" Oz/Nz.
Seems the price for 19mm float glass in NZ has taken a big hike over the last two or so years. Just been quoted :eyepop:NZD$930.90 by Metro glass for the supply of a 640mm (25.2") diameter circle of 19mm float. The last circle I purchased this size was NZD$168 from the same source. A price jump of 554% seems a bit:shrug: over the top.
Sounds like they don't want to do it so have quoted a price that makes it worth the effort if they have to do it.
Might be worth shopping around. Maybe ask for a square cut quote.
15-02-2013, 02:35 PM
I have just had my 17" mirror coated and thought I should report in and thank those who offered advice for this newby glass pusher. The field tests so far give great views and stars are quite small dots (proportional to magnitude). Sydney has terrible seeing and so accurate tests outside are impossible these days. I did do a ronchi test and lines appeared straight with the usual TDE appearing in the outside 1/2". Like Stephan, I found 6 - 8 lines were easier to read, and showed up the TDE which seemed to get masked with just a few lines. Looking at the trapezium in Orion I can regularly see 6 stars there, and the gas cloud gives plenty of detail. The LMC looked fabulous and Jupiter shows details in the bands and the moons are round and bright. One thing I noticed after using it uncoated for many months was that the coating made no visual difference to the images only they were brighter. I was worried that after coating the images would be far worse so I was relieved when this wasn't the case. So overall it seems to be a reasonable mirror and I am happy with it.
20-02-2013, 08:57 AM
Congratulations.:hi: Great effort. You will be feeling good after all your hard work, and perseverance having borne fruit. There is something special about the view :eyepop:through a scope, when the scope, and mirror has been forged from ones own personal vision, and effort.
I am still working on getting my 25" mirror to work in my dob. Still having edge:screwy: support bothers. I can put the mirror into the cell and have "stig",(star images as streaks, which change orientation when moving through focus), take it out and put it in again and "no stig", (star images point like, and inter/extra focal images similar). Put it (mirror) in again last evening for optical testing in scope, some :rolleyes:stig is back, but still did a good job of separating the a1/a2 (4sec arc),components of Alpha Crucis with plenty of dark sky between. In fact a much clearer separation than the 14" dob gave set up beside. The 25 is yet to be coated, this I intend to do myself *silvered*, when I get the edge;) support issue sorted.
24-02-2013, 07:27 PM
I'd say hello fellow mirror makers, but I am yet to finish my first mirror. I am fine grinding at the moment.
I am making a 150mm F6 Newt (at least I hope it is an F6). While it was wet, I held a 40mm eyepiece up to it at arms length and tried to focus on the neighbour's roof. I figured that as it was around F6, and a sphere is close to the Raleigh Criteria, I would get a reasonable image off axis if my arms were long enough.
It surprised me that I could focus on a near object holding the objective at arm's length. I could not get it to focus at infiinity, which was a relief, because I don't know how easily i could figure an F4 or F3.
It is also amazing how much light is reflected off glass during the daytime. I am considering leaving it uncoated for use as a drag and drop lunar telescope.
Anyway, I am glad that there are still mirror makers out there and this thread has had some good tips.
24-02-2013, 07:31 PM
Thanks Stephen for the congrats. I get a little astig sometimes too, but according to a thread at Cloudy Nights, as you mentioned, the edge support makes a difference. I use packing tape, approx 10mm wide for lower 180 degrees of mirror. I have large air vents in the bottom of the mirror cell for cooling and can check that the tape is sitting dead centre. This seems to help for me. I notice temperature seems to move the glass around a fair bit, I guess as various stresses come in to play as I only used thick sheet glass from Sydney Glass, but it doesn't seem to make a lot of difference to the image, however as it moves from say under corrected to over corrected during the evening there is image deterioration if you are observing planets. As I want this for looking at faint fuzzies I am not worried.
I am working on a long focus 6 inch mirror for a schiefspiegler. RC is 200 inches approx.
After a lot of trouble with all kinds of zones I have a rough hyperbola, looks like an f6 parabola. I am hoping that will be relatively easy to reduce to a sphere.
I am finding the long focus sphere a bit more of a challenge than I expected but it is interesting and I like being able to use the ronchi test as a null test.
Does anyone know anything about a polishing compound called TK 68?
I was given a jar of it by a friend who used to make optics professionally and have used it for polishing this mirror. I read a vague reference to it elsewhere being an aggressive polishing compound in which case it sounds unsuitable for finishing a mirror like this. I have some red rouge so could switch to that.
24-02-2013, 10:28 PM
That's a little worrying because as you focus on closer objects, the focal point moves away from the primary. That is, if the mirror is 36" focal length for infinity then the distance you need to hold the eyepice for a closer object will be, say, 37"-39" for example. Have you done a focal length test?
25-02-2013, 10:43 AM
TK-68 is a very old cerium - I'm not sure if its available any more. I used to use it - don't remeber it being particularly aggressive. if your surface doesn't clean up to a nice sphere quickly maybe your pitch is too hard and you are not pressing enough . I always use polishers on the soft side and get nice smooth zone free surfaces cold pressing fairly regularly. What kind of pitch are you using ?
25-02-2013, 10:49 AM
This does sound fishy . Just project the Sun image onto a piece of paper and move until you get focus ( get someone to hold the paper just beside the Sun for you and hold the end of the tape measure) . A sanity check while grinding is to calculate the saggita and then buy a drill bit whose diameter is just shy of the required depth and lay a steel straight edge over the drill bit as it lays in the centre of the mirror and grind until you can sit the edge down on the mirror flush with the drill bit in contact. Your saggitta is about 1.56mm so a standard 1.5mm drill will do the job !
If you are still grinding just spray a light mist of water mixed with Glycerine onto the surface for quick reflectivity. If you have a polish the full Moon can also be used as a FL check. Always use the Sun or Moon - don't complicate things as a beginner by trying to focus anything else.
That doesn't surprise me I got it 16 years ago and I suspect my friend had it for several years before that. Good to know you found it ok. Do you recommend finishing with rouge? I have some I bought from willmann-bell. I found it needs elutriating or else you get lots of sleeks.
I am using gugolz pitch which again I have had for several years. I softened it with castor oil. The lap needs trimming every 3 or 4 hours so I assumed it was soft enough.
I think the zones were caused by too aggressive work by me to reduce a central hill - led to a deep depression. I am used to faster mirrors so have had to learn to be more gentle!
25-02-2013, 07:12 PM
OK Thanks for that. I will try the moon tonight.
26-02-2013, 10:29 PM
Yup. You were right. It is around F2.5. I ground out a scratch in it. I guess I went a little deep.
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