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  #81  
Old 06-04-2012, 02:50 PM
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Stephen

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.
Hi Mark,

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.

Stephen.
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  #82  
Old 14-04-2012, 06:33 PM
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Stephen

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.
Hi guys,

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.

Stephen.
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  #83  
Old 07-05-2012, 12:14 PM
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Hi Grinder/Polishers,

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.

Stephen.
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  #84  
Old 02-06-2012, 12:43 PM
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Hi all,

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.

Stephen.
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  #85  
Old 04-06-2012, 09:08 AM
starman345 (Brian)
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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?
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  #86  
Old 04-06-2012, 05:30 PM
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Smile Pine Butt Bench.

Hi Brian,

welcome to the forum. Have you checked out all of this thread?? Some where I posted a few pictures of my "out door workshop", complete with pine butt, about which I do multiple rotations almost on a daily basis.

Stephen.
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  #87  
Old 04-06-2012, 10:04 PM
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I skimmed throught but must have missed it. I'll have another look
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  #88  
Old 07-06-2012, 09:38 AM
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I saw your pictures, very nice setup, Now I see how you do things.
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  #89  
Old 07-06-2012, 10:42 AM
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I saw your pictures, very nice setup, Now I see how you do things.
Hi Brian,

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 on the computer, (next best thing).

Stephen.
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  #90  
Old 07-06-2012, 09:06 PM
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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
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
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  #91  
Old 14-06-2012, 06:28 PM
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Hi grinders,

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" 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. 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 theory) will have an increase in stiffness of 13.285. This should enable me to finish figuring without a flexture problem.
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  #92  
Old 14-06-2012, 09:41 PM
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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.

Last edited by JB80; 15-06-2012 at 03:58 AM.
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  #93  
Old 15-06-2012, 05:41 PM
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Steven

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



Quote:
Originally Posted by sopticals View Post
Hi grinders,

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" 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. 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 theory) will have an increase in stiffness of 13.285. This should enable me to finish figuring without a flexture problem.
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  #94  
Old 16-06-2012, 06:36 PM
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Quote:
Originally Posted by Satchmo View Post
Steven

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.
Hi Mark
Why didn't he reduce the correction so that on a typical night the mirror did not overcorrect so much?

Dave
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  #95  
Old 17-06-2012, 08:41 AM
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[QUOTE=Satchmo;864058]Steven

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.

[/QUOTE

Hi Mark,

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 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.

Stephen.
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  #96  
Old 17-06-2012, 02:05 PM
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Quote:
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Hi Mark
Why didn't he reduce the correction so that on a typical night the mirror did not overcorrect so much?

Dave
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 ?
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  #97  
Old 17-06-2012, 07:58 PM
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Hi Mark
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 12.
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.

Dave
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  #98  
Old 25-06-2012, 02:07 PM
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Smile Gugolz #55 too hard?

Hi Guys,

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 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?)
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  #99  
Old 25-06-2012, 07:56 PM
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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.
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  #100  
Old 25-06-2012, 08:48 PM
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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
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
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