Making a Busack-Riccardi-Honders Astrograph

[Stefan Buda]

One of my current ATM projects is the making of a small RH astrograph.
I was wondering that, if there is enough interest, I could make my progress public on this forum. It would allow people to see what goes into making a precision instrument and why they cost so much.

[Kunama]

I doubt I would ever build one but it would be great if you could post a progressive build thread !!!! and Lots of Pictures !!!! My favourite part of the forum is the ATM/DIY section..... (Yes, I know I also frequent the classifieds Col !!!)

[ZeroID]

Me too, we can all drool together .. :P

[Rod]

That would be fascinating Stefan. Please keep us up to date on your project.

Rod.

[Shiraz]

yes please - we would all benefit from hearing what is involved.

sounds exciting.

[Stefan Buda]

Thanks guys,
Four followers is more than enough for me :-)
Ok, but let me first explain what the Korean War has to do with it.
Back in the early 50s the Australian government purchased a stockpile of optical glass for military purposes. Some 20 odd years later they concluded that WW3 was not going to happen for a while, or if it did, it would not last long enough to make any optics out of it and so they sent the glass to land fill. Fortunately some people managed to save some (unknown proportion) of it.
Some of this glass started circulating through various markets and by various individuals and a large porion of it was stored in ammo boxes on a farm near Warrnambool. I started collecting optical glass back in the 80s and came to poses about 40kg of various size slabs and discs, mostly purchased for around $5 per Kg.
One of my ambitions is to turn as much of my glass into optics as I can and this is where the Korean War connects with my BRH astrograph project.
As a first step I selected the largest slabs of suitable glass from my collection and that is what determined the size of the clear apperture of the instrument, which will be 150mm.
Busach's prescrption calls for all elements to be made from the same type of borosilicate crown, but my largest suitable slabs were of a Schott glass called PSK3 and after much raytracing I decided to use BK7 for the small corrector lenses - mainly because I had a couple of old laser mirrors with Ohara BK7 substrates.
There is a neat raytracer on Busack's website called PointSpread, http://www.busack-medial.de/download.htm that allows you to play with your own version, although I used Zemax to optimize mine.

[Satchmo]

HI Stefan

I'm afraid without the provenance of the 'melt data' on the glass, giving a designer the figures of the actual measured dispersion and refractive index of the individual pieces , this glass is really just `land fill ' .

If you were going to actually manufacture such an instrument you would just buy new glass with all the required melt data - BK7 is not very expensive compared to the value of such a finished instrument . I'm not trying to be picky - its just one of those cold hard realities of using optical glass. You may be able to fudge the result by sticking to the design and aspherising one surface via null test in the end result ?

I've thought about these all spherical designs , but to be honest polishing and figuring 8 optical surfaces just doesn't seem to make any sense compared to one paraboloid and an off the shelf Wynne corrector ( at F3 ) . Maybe I have missed something !

[Stefan Buda]

Thanks Mark for the comments but I don't think the accuracy of the glass dispersion data for this instrument is as critical as it would be for an apochromat, for instance, where all the power comes from refraction. Here it is important that the two main lenses come from the same melt and I don't think Schott or Ohara glasses deviate too much from their intended specs. Anyway, a few years ago I made a small Companar using "landfill glass" and it works very well. The worse that can happen is that I will be required to make a couple of precision prisms from the offcuts and measure the glass myself and then adjust the curvatures accordingly. And yes, I will null test it but I don't know at this stage how useful such a test will be as the design is not meant to be diffraction limited.
Please don't try to understand why I'm making it because probably has something to do with insanity.
Regarding the corrected Newtonian: I count 8 optical surfaces and at least one of them is not an easy one. Besides, it is such a 20th century thing.:lol:

[clive milne]

You made a companar?

Dude,
respect!
:thumbsup:

[Stefan Buda]

Thanks Clive.
I believe that it may be the only one ever made or at least the only successful one made by an amateur, as I have never seen any proof for the existence of other ones.
http://www.parkes.atnf.csiro.au/news_events/astrofest/awards/2014images/deep/HM_DeepSky_Stefan_Buda_Wiches_Head. jpg

And of course, it should make the BRH project feel like a walk in the park. :-)

[clive milne]

Stefan.. I always loved that design on an intellectual level. practicality aside, so much win!

That may be unfair,
it is a practicable design even with common glass. Coatings probably being the biggest obstacle.

Love your work.

~c

[Wavytone]

Clive if you liked that on an intellectual level, try this

https://www.google.com/patents/US3926505

Esp. Figure 3. Now if Stefan can make a 150mm BRH, maybe one of these could be the next challenge.

Or a Baker-Nunn...:eyepop:

[clive milne]

You would think at some point the designer would say... yeah... nahh.. a bit much glass there.
But yes... brilliant none the less.

[Stefan Buda]

Thanks for the link. I bookmarked it and will have a closer look when time permits.
I've been thinking about making a small solid Schmidt of about f/1 for years but other projects keep getting in the way.

[Stefan Buda]

So back to the BRH.
In fact I started working on it early last year and then had the project on hold for about a year.
I started by slicing off the corners of the PSK slabs and two other slabs of optical glass, one a barium flint and the other a dense barium crown, to be used as grinding tools. I already had two glass disks that were suitable for making up the 4 grinding discs needed for the PSK lenses. The cutting was done on my milling machine and the mess was confined with the use of an ice cream container. The diamond plated tool used was one that I made many years ago and it still cuts well.

[clive milne]

Stefan... have you ever considered moving to a cooler climate?
https://www.kiwistaroptics.com/careers

Incidentally... you might enjoy some of the designs, here:
http://www.cloudynights.com/topic/486498-new-astrograph/

I like the idea of the prescription detailed in post#14 (uses a trepanned C11 primary) 12" clear aperture @ f2.5 and 6 microns in the extreme corners of a 16803 format chip... lots of complimentary radii and only 1 full aperture convex surface. I'd be curious to know what BK7 costs in blanks that size?

[bratislav]

Harrie Rutten told me that he is aware of several Companars successfully built, the largest one being 12 inches. It was also commercially offered (at least at some stage) by Oldham Optical in UK. I don't know how many of those mentioned by Harrie were amateur creations though.

Bratislav

PS most of that old "landfill" glass was made by Chance Pilkington and on larger pieces it has melt data numbers scribed on the glass

[clive milne]

And as Stefan has intimated with respect to the huff and puff about melt data... it really isn't that important on a two element, zero power corrector (of the same glass). It has very little bearing on over all system performance other than to shift the wavelength at which spherical aberration is at a minima.

It's a bit like agonising over the spherochromatism of a Schmidt plate.... you don't need anything more exotic than white water plate (as long as it is homogeneous) and have a rough idea of the refractive index.

~2c

[Stefan Buda]

Bratislav,
It would be nice to actually see some proof as I don't think that Harrie has actually seen any of those instruments. Why would professional astronomers waste money on such a large and expensive instrument?
Remember that the Companar is huge relative to its focal length. And if those were amateur instruments then we should have seen some results.

Anyway, back to the topic:
In the late 90s I cobbled together a curve generating machine for my 16" Dall-Kirkham project and I've been using it ever since for making all sorts of lenses and mirrors. It is this machine that I used for shaping the glass for the BRH.
Here you can see the shaping of one of the 6" grinding tools.

[Stefan Buda]

Clive,
Even Melbourne too cold for me..
Yes I have seen that CN thread. I prefer the ultra compact formula of the BRH. Especially from a commercial/manufacturing point of view.
Let me know if you find out the cost of BK7 blanks.

[Rod]

Hi Stefan

Thanks for the pics of the curve generator. How do the glass blanks attach to each other? I'm assuming some sort of blocking pitch. Could you explain how that kind of pitch is made and how you apply it?

Thanks

Rod.

[Stefan Buda]

Rod,
The bottom piece of glass is held in place by the wooden blocks that were glued lightly around their periphery to both the rotary table and the glass, with epoxy resin. After the job was done, I used a wood chisel to carefully get rid of the wood and free the glass. I had to be careful not to let the epoxy to seep under the glass and permanently attach it to the table. After the epoxy had set, I ground the top flat and used double sided mounting tape to attach the second piece of glass on top. When the top one was done, I used a hacksaw blade to cut through the mounting tape and free the top piece. Hope that make sense.

[Stefan Buda]

Shaping the Mangin mirror.
First I ground it to circular shape and then generated the convex curve with a large diamond cup wheel. After that I removed it and generated a matching concave curve on the top of the thick glass piece. I don't have photos for two of the stages. Then I reattached it with mounting tape; generated the concave curve and partially trepanned it to free as much of any residual internal stress as possible. I used a similar procedure for making the primary for the Companar, and a bit of internal stress was released when I completed the trepanning, after polishing, and the mirror went slightly astigmatic. I hope for better luck this time.

[Tinderboxsky]

Fascinating. I am enjoying following progress.

It is a long way from my school days in the 60’s when I ground and polished a 6” f8 mirror for my home made Newt. No DOB's in those days!

Cheers

Steve.

[bratislav]

Well, for what is worth, this is what Harrie said to me in his email:

"There are made more Companars. The biggest is 12”. I got pictures of it, but they are at an older computer and not accessable anymore ;-( "
This was enough of a proof for me.

There are plenty of instruments whose results never made it to www and Google, but that doesn't mean they don't exist. Can anyone outside of ASV find any proof that you have made a Schmidt camera for example? Or a Wright? Or that Barry made a Coelostat?

When I said "not amateur made" I didn't mean it was meant for professionals. Just that it was professionally made, meaning made and sold for profit. Just as any Celestron, Meade and what have you are today.

Now, back to B-R-H ;)

[Stefan Buda]

A couple more pictures.
The first one shows all the grinding tools associated with the two large lenses. Three of these grinding tools will be used to make polishing laps after the grinding is done. The fourth one (R2) is too thick so I cast an Ultracal30 disk to be used instead.
The second pic shows the improvised wedge error tester I knocked up for the job. The lens sits on three steel balls that have been partially embedded into the steel plate.

[Stefan Buda]

Well if you think that is a lot of glass then don't forget: That is only half the story. The same number of discs are needed for making the two small lenses.

[Stefan Buda]

I finished the small lenses, except for the AR coatings.
For the coatings I can think of three possible options:
1. Find someone that can BBAR coat them for me.
2. Just do my own single layer magnesium fluoride AR coating.
3. Upgrade my vacuum system and do my own BBAR coating.

Problem with the first option is that I'm not aware of anyone doing commercial BBAR coatings in Australia, except for spectacle makers perhaps.
Problem with the second option is that single layer AR coating on low index crown glass is not as good as BBAR - 1% vs 0.25% reflectivity per surface across the spectrum.
Problem with number three: Big learning curve.

[Roger Davis]

Nice to see that things are progressing nicely for you Stefan! First time back to IIS for a few years and nice to see all those familiar names. Looks like you will be having some fun putting it all together. Hope you don't mind if I just hover around in the background to see how you go.

[OzEclipse]

Stefan,

Longman Optical in Tasmania do BBAR coatings. Have not used this service but have used their aluminizing service. Coating was 10/10. I didn't appreciate how good until a subsequent coating which changed my mirrors apparent optical quality from outstanding to mediocre. They have been in the business since the 1940's.

http://www.longmanoptical.com.au/optical-coatings/


Joe

[Stefan Buda]

Roger,
Good to hear from you and I think you are the fifth person that expressed interest in following this project, so I will keep posting.

Joe,
Thanks for the link. I have two leads now to follow up.

If anyone else knows of a place that can do BBAR coatings please advise.

[Wavytone]

http://www.opticalcoating.com.au/services.htm

[Stefan Buda]

Thanks for lead number 3.

[StewartBeveridg]

Well done Stefan,

The project is looking good.
Are you using glass from the lodge?


Stewart

[Stefan Buda]

Thanks Stewart,

The glass I'm using for the large lenses is from the same source as the lodge glass is from. The small lenses are made from old laser mirrors.

[Stefan Buda]

One of the difficulties I face each time I make a new instrument tube is the necessity of making a suitable mandrel for laminating the composite material, usually carbon fibre/epoxy. My astrographs don't have adjustments for collimating the optics and that can only be achieved with very accurate tubes and housings for the optics. The same way that camera lenses are made.
I've been scratching my head for a while about how to make the mandrels for this baby until it suddenly dawned on me that I could perhaps 3D print them. I will need two of them - one for the actual OTA and one, almost as long, for the light baffle. The OTA being extremely short, has a large acceptance angle for stray light without a decent baffle.
I started with the less important one so that I can apply the knowledge gained to the second one.
The 3D printing took 4 days, with the printer running 17/24.
I used PLA because of its brittleness as this is a sacrificial mandrel that will have to be broken up and removed piece by piece when the carbon tube is finished.
After gluing all the pieces together, I skimmed the OD and the ends on the lathe, between centres.

[clive milne]

Very impressive Stephan...

Incidentally, what is the resolution of that printer... it looks impossibly fine.

[Stefan Buda]

Thanks Clive! The printing was done at medium resolution - 200 micron. Anything finer would have taken forever. Probably could have gained time by slicing at 250.

[Wavytone]

Good thinking Stefan. I can think of another way to do it but that's probably the best way.

However as you did say you'd finished this on a lathe, I'd have included some steps in the mould and faced them on the lathe also, to mould-in accurate shoulders in the carbon fibre tube for alignment purposes. And sure they'll probably need a skim in the lathe afterwards to make sure they're true to 0.01mm.

But personally I'd probably have obtained (or cast) a lump of duralumin alloy big enough and hogged it out. An 8kg sledgehammer will punch holes in engine blocks if you swing it hard, and I used to have a mate with a furnace who melted them and cast the blanks for me for a 10" worm and clutch. A couple of engine blocks = a LOT of alloy :) and free.

[Stefan Buda]

The alignment shoulders will be on the aluminium rings that get bonded to the ends of the tube. The ends of the tube will be accurately machined before bonding.

[Stefan Buda]

Baffle tube build.
The 3D printed mandrel did its job quite well. I used Al foil to stop the carbon-epoxy from sticking to it and in the end I think I could have just pushed the tube off it but I started breaking it up with a pair of plyers first. The reason is that I had to make sure it was removable in bits and pieces, as the other tube for the OTA will have aluminium parts bonded to its ends, while still on the spindle, and the mandrel will have to come out in pieces.
Originally this baffle tube was going to have an aluminium reinforcing ring at the front end, but a couple of weeks ago I learned that a local manufacturer was about to start making a carbon fibre reinforced PETG filament for 3D printing. I obtained a sample quantity for trial and I used it to 3D print the front ring.
The tube has 4 layers of carbon cloth and I gave it a textured, slightly matte finish, rather than the smooth and shiny finish of commercial ones.
And don't worry about the pile of leftover plastic as it is biodegradable - made from corn starch.

[bojan]

I am watching this space from the beginning.. 3D printer is very helpful everywhere, optical design is no exception :thumbsup:

[pixelsaurus]

I do enjoy a good read. Keep it coming. :thumbsup:

[Wavytone]

Stefan, nice work on laying up the CF tube. My effort some years ago to make a truss looked pretty awful.

[Stefan Buda]

I had to put this project on hold for the whole of February due to piled up pay work but now I'm getting to a point where I can start moving ahead with it again.
Over the last few days I made some improvements to my Bath interferometer that I intend on using to test the Mangin mirror for astigmatism, before and after removing the central part that has been partially trepanned.
Last night I tested a 6" Bushmaster primary mirror, or rather used it to test the interferometer - as the mirror has already been measured with the previous iteration of the Bath.
You can still see the trefoil distortion caused by the three clamps, although I have slacken them already.

[Logieberra]

Wow. I wish I could could stuff like this! I've subscribed to this thread to enjoy the journey with you. Thanks for sharing!

[Stefan Buda]

I'm still overloaded with work so I made very little progress on this project.
The grinding of the front lens is finished and, before I start reinventing the wheel, perhaps someone can suggest some good ways of testing it after the polishing.
The front surface is convex with a radius of 6770 and the second one is also convex with a radius of -822. Centre thickness is 14.3
I could use the grinding tools to make test plates but R1 is way too long for that and, if I can, I would like to avoid test plates altogether.

[Wavytone]

Hi Stefan,

To test a convex surface at 6770 my first thought is a concave test plate, polished spherical and test the two by putting them almost in contact (use 3 pieces of paper as spacers) and counting interference fringes (newtons rings will be visible) under a diffuse monochromatic light source. For that to work though you'll have to partially polish the surface being tested.

Alternatively make an auto-coliimator setup with say an accurate larger parabolic mirror say f/5, and adjusting the position of the light source to create a beam converging on the centre of curvature of the convex surface to be tested. Reflect this off the convex surface back to the mirror and examine the resulting focus.

There will be some spherical aberration in this due to the converging beam and umyou could either calculate the expected amount and figure the lens surface accordingly, or if you want a null test, insert a lens near the focal plane that will give the right amount to correct it. Shouldn't be hard to calculate the required lens, similar to a Ross null test.

[Stefan Buda]

Thanks Wavytone for your suggestions.

I agree that test plates would be the preferred way but I have no idea how to test a 6" sphere with a 7m radius and prefer to avoid the extra work if possible.
However I may do that for the second surface.

I do own a precision 8" flat that will allow me to test the lens in auto collimation with a Ronchi or Ross Null test, but that would show me the combined errors of both surfaces.

So a combination of one test plate and auto collimation with the flat, should do the trick.

Another possibility I'm thinking of is to use the Bath interferometer on both sides of the lens and compare the result to what is expected from ray tracing. The reflecting (back) surface will dominate the seen error and I should be able to correct it iteratively.

[Merlin66]

Could you use the Enrique Gaviola method?
I used it when making a Cass secondary many years ago....
(See "Advanced Telescope Making Techniques", Vol 1, p63)
Ken

[Stefan Buda]

It should work for R2 but I don't have a suitable lens.

[bratislav]

Test R2 with a test plate, then flash polish R1 and make sure your lens focuses starlight (*) where expected (can't remember what glass you used, let's assume BK7, so for given prescription it should focus at 1414.5mm). Make sure you test it reversed (with R2 facing the sky) as this way residual spherical is much lower.

If it focuses as calculated, polish the R1 fully and re-check. If it is still at 1414.5mm or thereabouts, check if wavefront seems smooth (as per Zemax simulation attached). No need to test the lens any further.
It takes more than 20 waves of asphericity to change focus and shape of the Ronchi bands significantly. If surface was this bumpy your full size lap would be dragging and skipping and jumping all over the place.

(of course you don't need to use real starlight, any decent scope will provide good collimated beam)

[Stefan Buda]

Thanks Bratislav, but I think auto collimation with my 8" flat would be easier to set up than a star test and would be more sensitive due to the double pass arrangement.

[bratislav]

Most of the power and correction comes from R2, in fact R1 has almost no influence. Here's the wavefront that lens generates when eccentricity of R1 is zero (perfect sphere), 20 (EXTREME oblate) and -20 (EXTREME hyperbola).
Spot the difference :P

Bottom line is R1 doesn't even need to be checked that much.

[Stefan Buda]

Looks like using the Bath on R2 may be the way to go then. I do not intend to do any testing until both surfaces have been fully polished as I'm confident that the shape parameters are accurate enough.

[bratislav]

You can definitely test R2 directly through R1 by Foucault and follow the expected longitudinal aberration (see pic #2). This surface when tested through the lens behaves like an oblate with eccentricity of approx 0.5. Problem is Bath interferometry - I have no idea how much Bath reference beam gets affected by the lens ... so no idea what DFTF would report as final eccentricity. I can model a narrow laser beam through R1, reflect it from R2 and back though R1 and tell you how much it will get deformed, but I have no idea how those two fronts will interfere (what pattern the interferogram will look like). Zemax can't do that. Or rather I can't do that in Zemax :shrug:

[Stefan Buda]

Very good point about the reference beam. I haven't thought about that.
It may not be a problem though, as the wave front will remain symmetrical, I think, and it should only affect the amount of spherical aberration seen. A good question is whether it will add or subtract from the 0.5, and I'm assuming that I can hit the vertex of R1 with the reference beam.
And thanks for the ray tracing too - you saved me some time.

[bratislav]

I've modelled 2mm laser beam hitting the vertex of R2. It doesn't look good. At 500mm (where Bath beamsplitter/lens would be) it looks like it gets severely affected, nearly 10 waves. See pic.
I think Bath might not be the best way to do this. Use Bath to figure the concave reference and then contact test it. You can still use DFTF or OpenFringe to analyse results ...

[Stefan Buda]

I'm surprised that you got such a massive path difference. Based on a simple calculation I expected less than two waves.

Another thing to investigate through ray racing is whether it is possible to do a Ross null setup using one of my two null lenses. If possible, such a test should be accurate enough for this f ratio. Or would it require a negative null lens?

[bratislav]

The problem is, R2 (when set up as reflective) with two passes through the glass lens becomes quite strong in power, with effective focal length of around 250mm. This is short enough to bend laser beam quite a bit before it exits back.
We'll have to sit down and discuss the options as well as results. Perhaps when you come over to pick up the silicone lap mold and some Acculap?

[Stefan Buda]

I've done some ray tracing of my own and, not surprisingly, got the same results.
I also explored the possibility of a null test, for back reflection from R2 through R1 and for autocollimation with a flat. I could not get a good enough null for either setup with a simple null lens.
Looks like I have to make the test plate for R2 and leave the figuring of R1 last, as a complete instrument in autocollimation.

[Stefan Buda]

Still extremely slow progress with this project. One reason is that I took on the job of salvaging a brand new 10" astrograph that came with unusable optics and for which the manufacturer failed to take the responsibility of replacing it with a usable one. Well, it turned out to be a much bigger job that I thought it would be and it's been using up much of my spare time. The primary mirror needed complete regrinding/figuring to get rid of major astigmatism, plus a very long list of other mechanical and optical problems that needed fixing.

Anyway, I purchased 3 slices of aluminium alloy 6061, 178 diameter, for the main structural parts, and I cast an Ultracal30 disc for the R2 test plate lap. I prefer alloy 7075 for my projects, for its hardness, but the 6061 alloy also anodizes very well and it is much easier to obtain.

[SimmoW]

Amazing work as ever Stefan.

Can you at least pm me the manufacturer of the 10 inch so I avoid them in future? Publication of their negligence would be preferred though.

Hey, I'm thinking of getting some aluminium tube rings made for my new 10 inch made by Alistair Sam, 314mm external diameter, do you know of a good cnc guy who charges reasonably? And will want them red anodized :-)

Oh, good luck with your Malin award shortlisted pics.

[Stefan Buda]

Hi Simon,
I bumped into you today before I saw your post.
On this link you can read about the scope, but the problems listed are only minor compared to what I found when I started pulling it apart and began testing and measuring all components: http://myastroshop.com.au/ssmassey/reviews/odk-10-review.htm

Getting back to the Honders, I did put into it only a few hours since my last post, but not so my 3D printer. The second set of 3D prints for the sacrificial OTA mandrel is done. Another marathon, 52 hours of printing.
This mandrel is 10mm smaller in diameter than the first one.

[Stefan Buda]

I took advantage of the heatwave today (17C) to cast the lap for the R2 test plate.

[blink138]

hi stefan i am a spectacle maker and i may be able to get the latest multi coatings done for you / glass is rarely used in opthalmics these days and a different mixture, from memory is used, but i have a lot of contacts of independant grind laboratories around australia
pat

[Stefan Buda]

Hi Pat,

That sounds very promising. Please let me know if you find someone willing to do it. The refractive index of BK7 is not all that different from the ophthalmic plastics, so the coatings could be very similar I guess.

Stefan

[blink138]

blank size may be the limiting factor but i will make some enquiries
pat

[Stefan Buda]

Pat,

At this stage I would prefer to have only the two small lenses coated and they are only 50.8 diameter.
I would wait with the three large surfaces until I have the complete OTA tested under the stars.

Stefan

[Stefan Buda]

R2 test plate finished.

[Stefan Buda]

Some progress on the metal front. The rear reinforcement ring for the dew shield is done, as far as the turning is concerned.
First I skimmed the three aluminium discs on the lathe to make them flat and reduced the OD closer to the required size. Then I trepanned one of the discs and glued it onto another so that a can machine it distortion free.
The deep groove (2.5x2.5) in the middle is for a string of heating resistors. The shallow, angled groove is for engaging three spring loaded pins, when the dew shield is fully extended. There is also a 5mm wide undercut for a strip of self adhesive synthetic velvet, intended to prevent the scratching of the OTA when the shield is moved back and forth.

[Stefan Buda]

Good progress on the metal front. All aluminium parts finished on the lathe, apart from the focuser components. After I drill all the required holes this lot will go to the anodizers.

[Wavytone]

All very nice Stefan !

[Stefan Buda]

Thanks Wavytone.

The weather has warmed up finally and I was able to proceed with some of the glass work.
The front lens is fully polished and R2 is figured against the test plate.
The R1 figuring will be done last, when the whole OTA is complete.

[Stefan Buda]

Anodising done after a hiccup.
I've been getting my anodising done by Electomold, in Thomastown, for many years, because they work to military specs and are quite reliable. But their minimum charge has been going up way beyond de CPI and I had a bit of a shock when they told me that now it is $200 plus GST. So I took my bits to Collins Anodic and had them done for $70 plus GST - their minimum charge.

Even bigger hiccup with the front ring engraving.
I intended to have it laser engraved but for some obscure reason I got knocked back. Then I tried another laser engraver on Toorak road. They had a more primitive, CO2 laser based, engraving machine and wanted $85 for the job. I was not convinced that they were going to do it well, so I decided to mechanically engrave it myself. For that I had to cobble together a rotary platform and use my old CNC engraver. In the end I could not get rid of the backlash without making the axis too hard to turn for the stepper motor and the ancient software that runs it does not have backlash compensation. The end result is not much better than a hand engraving but it will have to do.

[Stefan Buda]

Second CF tube done.
Back plate and front lens housing glued in.
Concentricity and parallelism held to better than 0.006mm on both the front and the back elements.

[Troy]

Just stumbled across this thread - wow

Stefan this is really taking the diy projects to the next level :eyepop:

looking forward to first light - you will have to do some astrophotography also?

[Stefan Buda]

Thanks Troy, and yes, I will use it for AP just like all my previous DIY instruments.
Crazy, but for me an astrophoto starts with designing the instrument with which to capture it.

As an example, I made a dedicated instrument for capturing the Aureole effect during the transit of Venus in 2012. http://asv.org.au/images/gallery/Stefan%20Buda/TwoMinutesOfVenusTransit.png
..and I haven't used it since, because we haven't had any more Venus transits.:(

[Troy]

Sharp images of Venus and it shows the rare phenomenon well (aureole effect).
I know from your planetary images the 16" D-K is a fine scope also.

Stefan is optics/design - is that your background or is it self taught?

⌐■_■

Cheers

[Stefan Buda]

Troy,
Thanks for the compliments.
My background is mechanical engineering. Optical design is self thought, based on an early interest in optics going back to my primary school years.

Talking of optics, I have nearly finished grinding the mangin mirror for this project. I have been scratching my head for a long time trying to come up with a procedure that is not too difficult and will result in a usable optic.
The main problem is the big hole in the middle, as it causes all sorts of difficulties.
I decided to polish and figure the front surface with the "plug" still attached by a couple of millimetres of glass. That should help with the testing, and the polishing can be done with a normal full lap. After that I will finish the trepanning and I will do a precision cylindrical grinding operation on the internal diameter. Hopefully not too much residual stress will be released and the front (finished) surface will cope better with a bit of distortion, being a refractive surface, even though it works in a double pass mode. The second surface will be done with a full diameter polisher that will have its centre dug out to match the hole in the mirror. Testing of the second surface will be difficult.

[Kunama]

I keep pressing the "LIKE" button but nothing is happening !!!

Just awesome work Stefan, my kingdom for your lathe, mill and machining skill .....

[Stefan Buda]

:lol: Thanks, but wait until you see the focuser. Originally I designed something along the lines of the Officina Stellare RH200 but I could only get about 27 millimetres max between the focuser and the focal plane. That was perfectly ok for my QHY8 OSC camera but not enough to eventually move on to a mono camera with filter wheel. So back to the drawing board I went and came up with a design similar to the Clement focuser, but without flexural hinges... Stay tuned.

[Stefan Buda]

Here comes the focuser. About 40 hours work.

[FlashDrive]

Just gobsmacked at the work and care that has gone into making this beauty ..... what craftsmanship when you have the right gear ... lathe / milling machine / etc

A labour of Love ...

Col..

[multiweb]

+1 Been following this and it's just sub zero cool. The mechanical engineering is incredible but the optics making is mind boggling. The focuse looks like a clement design. Not too sure how the little yellow wheel makes the whole thing go up and down. Is it just a simple threaded bush? Couldn't quite make it from the spare parts shot :question:

[Stefan Buda]

Thanks Col, but it also takes a bit of insanity, I think.



Thanks Mark,
The phosphor bronze part, second part from bottom right corner, screws into the back plate and the thumb wheel has a threaded shaft that engages it. The threaded shaft has been split to act as a spring and has an M3 grub screw down the middle in order to spread it apart in a controlled way. The spring action is needed to get rid of backlash. The split shaft is made of hard alloy - Al7075 - and is hard anodised.

[multiweb]

That's a very peculiar system. Is it your own brew?
I would imagine with all the moving parts, angles and hinges the machining tolerances and fit would be extremely tight and challenging.

[Atmos]

My building ability starts and ends with a hammer and a pinch bar (tools for destruction!), it’s great seeing this slowly come into action.

[Stefan Buda]

I like coming up with unorthodox solutions..
You are absolutely right about the tolerances. I even had to shim the Z-axis of my milling machine to get the necessary orthogonality.
The axle rods for the hinges are precision guide rods, recycled from CD and DVD drives, and the holes in the hinges had to be diamond lapped after anodizing. Not agricultural machinery standard.:)

[multiweb]

I'm just amazed that given the geometry of the whole assembly that third hinge axle rod went in without a fight. :lol:

[Stefan Buda]

Yep, it did require a well thought out procedure and many tricks to insure success. The first picture shows one trick that guaranteed that all the positions of the holes in the hinges were the same. Keep in mind that the machine has digital readout to 0.005mm.

[ZeroID]

OK, I give up, I can't see how it works. But geez, it's beautifully made.

[Stefan Buda]

I intended to finish this project before the end of the year but things got in the way as usual..
At least I managed to get the Mangin out of the way. I just have to do some coatings now and will be ready for preliminary assembly over the coming weeks.
The front surface of the Mangin is the fastest surface I ever tested with a Ronchi setup. Amazingly my light source was able to illuminate it evenly, despite the f/1.29 ratio. The lines looked very straight on both sides of focus, however I don't think that is due to my wonderful skills and perfect polishing lap, but rather has to do with the lack of sensitivity of the Ronchi test as this sort of extreme f-ratio.
For the back surface I also used the Ronchi tester, through the front surface. Due to the near concentricity of the two surfaces the Ronchi lines were nearly straight. The longitudinal focal spread, according to ray tracing, was only 0.25mm and it was very difficult to measure even though I 3D printed a zonal mask for it.

[Wavytone]

Sheesh... wow. Just wow. Stefan you could make that focuser a commercial product, if you wanted to.

[Stefan Buda]

I may be forced to go in that direction as the laser industry is declining in Australia. When I became self employed, in 2000, there were three businesses building laser cutters in Melbourne - now there are none.

Seeing other people's battles in trying to source adequate focusers for their astrographs, I think there is a niche for a good motorised 3" focuser.
The main difficulty is in coming up with a design that can be produced cost effectively.

[Wavytone]

Make a 2" as well. The Bellerophon (www.clementfocuser.com) is no longer available as a 2".

Looks like I'll have to settle for a Starlight FTF2015BCR-RP and have a backplate made locally to suit.

[Paul Haese]

I always look for this thread to see how things have developed with this project. I bow down to you Stefan, your skills are exemplary and I marvel at this project. I look forward to more updates and the final product. Incredible workmanship. :bowdown:

[Stefan Buda]

I think the Clement focuser is still protected by patent, although it does not apply to my version with plain hinges.
However I'm thinking about something more along the line of the Atlas focuser, but cheaper if possible.



Thanks Paul - shame that I have to work for a living, as I wish I had more time for this sort of projects.

Meanwhile, I managed to evaporate some aluminium on the optics and assembled the OTA on the 31st of December.
I found that the focal plane is about 1.5mm further out from where it should be, indicating a potential problem. Maybe it is the unfigured back surface of the Mangin responsible. It will be a long iterative process to get this sorted.

And nearly forgot: Happy New Year everyone!

[Stefan Buda]

Last night I captured some extra focal images of Canopus and sent them over to Bratislav for Roddier analysis.
This morning he sent me the results and I'm happy to report that the instrument is free from astigmatism. However the spherical aberration needs to be cured.
So, I pulled out the Mangin and measured R4 as I suspected that it was the culprit. Sure enough the spherometer showed a sagitta increase of 5 micron, corresponding to an increase of radius of 1.5mm. The damn polishing lap was working a lot harder on the inner edge than on the outer, despite of me using rather long strokes on the polishing machine - never trust a polishing lap even if you have performed exorcism on it. Optical polishing remains a black art, but I could have noticed the problem by periodically testing the surface with the spherometer.
Anyway, I decided to go back to fine grinding rather than try to polish it back to specs.

[Wavytone]

Yup you have SA with a touch of astigmatism, the astigmatic axis runs from 10 o'clock to 4 o'clock. Could have told you that from the image, never mind the roddier analysis.

The $64 question is whether the astigmatism is induced by strain in the optic by whatever is holding it (pinching) when the image was taken, or whether you've actually figured it that way. Only you can work this out.

You know what you have to do... though to remove a high zone of 5 microns I'd be practicing the gentle art of hand figuring with pitch and rouge, pressing with petal-shaped card templates to control where the contact zones are... forget fine grinding - far too drastic, and even cerium oxide is too drastic. Rouge is far more gentle when it comes to figuring.

Only downside is rouge is very, very, messy stuff to use, though it does come off in a washing machine. Just hope your wife just doesn't look closely at what goes in.

[Stefan Buda]

Thanks Wavytone for the advice, but I did make my move yesterday on the Mangin. One single "wet" with 302 1/2 emery brought the RC back to what it should be and then I finished it off with 2 wets each of 1200 and 1600 emery.
I'm about 3 hours into polishing now and the RC is holding nicely.
One of the problems with the first polishing was that we had a heat wave and the ambient temperature was about 28 degrees - way too high for Acculap Standard hardness. Now I'm polishing at 22 degrees and another heatwave is upon us. Must finish it today or wait for the cool change.
I am not going to worry about 15nm of residual astigmatism - it is completely insignificant and quite small considering the complexity of the instrument and the lack of collimation adjustments.

[bratislav]

The residual spherical aberration can be turned off in software. ALL residuals (including astigmatism) then amount to about 22nm RMS (less than 1/25 wave) which is excellent even by visual standards. Astigmatism itself is about 15nm (1/36 wave RMS), just about undetectable.

And tackling 5 microns (10 waves!) with rouge would take better part of one's life. And most likely result in scratches (too many sessions and Mr Murphy doesn't sleep). Rouge is way to slow to do anything, and best left for massaging in those last tenths of a wave. Utterly out of place for an astrograph with more than 50% obstruction.

BTW this "high zone" isn't so much on glass - most of this residual third order SA comes from having a wrong radius at a wrong (actually WORST) place. Most of it could have been sorted out by respacing the components (at a cost of some softening of off axis images); but Stefan's design is already been cast in stone (and everything, including spacings) set to within a micron or so, so having R4 back to spec is the only solution.

Seeing that Stefan has already reground and mostly repolished the Mangin (I wanted to suggest using pads for raw polish as they won't flow at all, all but guaranteeing the steady curve on R4), it is all now academic ...

[Stefan Buda]

Managed to finish the polishing of R4, yesterday, and the Ronchi figure looks much nicer than the first time round - most likely due to a more correct polishing temperature. The previous figure had slightly rolled down inner and outer edges.
Last night I put the OTA back together without aluminizing R4 to try recording some extra focal images. It was difficult to capture enough photons but during my tests I noticed that there was quite severe coma present, just like the previous time when I just ignored it. The SA looked definitely smaller but the coma should have been absent - clearly I had another problem to trace.
I got up at 6am this morning and frantically started measuring everything. It did not take long to discover that the small biconvex lens, which is almost symmetrical, was installed back to front. I took so much care to avoid this very problem and somehow still managed to screw up.
So by 8am the Mangin was in the vacuum chamber and by 9 it had a nice fresh aluminium coating. By 10am I had done the autocollimation test that shows nice, very gently curving Ronchi lines.
And now I ready to hit St Kilda beach with some good reading and try to survive the 42 degrees day.

[Stefan Buda]

Well, a lot has happened since my last post and not all good.

After reworking R4 and reversing the incorrectly installed lens, star testing showed that the coma was gone, but the Roddier test showed an increase of the astigmatism. Looks like the lens may not have been evenly supported during polishing. The instrument seems extremely sensitive to miscollimation, that shows up as astigmatism, and I don't know at this stage how much of the stig is due to what.

I also had an adventure with the AR coating of the corrector lenses.
After searching far and wide to find someone willing to do BBAR coatings, I gave up and decided to do my own single layer MgF2 coating. I had a larger than usual evaporation boat already installed, one that I had used for evaporating silicone monoxide, and decided to use it rather than swap it out for one that I was used to for MgF2. It turned out to be a big mistake. As I was adjusting the current, I managed to overheat the boat and the MgF2 started spattering and I ended up with two lens surfaces with tiny droplets of MgF2.
Doing the second sides of the lenses I was more careful and had no problem.
Since then, I reground, polished and recoated one of the spattered lenses.
The second one can wait as it is not important at this stage.
Last night I sent Bratislav another set of test images for Roddier analysis.

Here's Bratislav's comment: "Yup, close enough. If stig is taken out in software, it is diffraction limited (1/22 wave RMS). PSF with and without astigmatism included in the snapshot. With stig components "on", Strehl drops to about 0.5 (L/8 RMS)."

[Stefan Buda]

I wanted to do some test imaging before doing any more work on the optics and last night the sky cleared with the moon putting on a low 35 degree altitude appearance.
Visually, with a 5x Barlow and 20mm eyepiece, giving 112.5x magnification, the moon looked nice and crisp with very well defined focus.
Then I captured a 1000 frame AVI with my unfiltered ASA120 mono cam.
So the attached image is wide spectrum, not even IR/UV cut filter used, and it should show a lot of atmospheric dispersion if I had used a colour camera.

I think that all I need to do is a bit of figuring to reduce the astigmatism and then I can move on to doing all the final coatings.

[multiweb]

That moon shot looks good. Very close to the finished product now. :thumbsup:

[Stefan Buda]

Thanks Mark, and you are absolutely right about being close with the optics: With Bratislav's help, I did a couple more Roddier tests and it turns out that no figuring is required.
The first one was done a few days ago and the only change from the last posted Roddier test is that I rotated the Mangin about 90 degrees. Interestingly the orientation of the astigmatism remained the same, suggesting that the problem may not be the Mangin.
I was planning to do some more testing to isolate the problem and, last night, as a first thing to try, I slackened the retaining ring of the front lens, and Bingo!, the astigmatism was gone. See the second attached test result.
Bratislav reckons that it is as good as astrographs get.

[Troy]

Stefan, when you made the carbon fibre tube. Did you use a single sheet of carbon fibre to wrap around the mandrel?

Cheers

[Stefan Buda]

Hi Troy,

I did it in several stages. First I used a strip of cloth that was long enough to make 3 layers. When the epoxy had set, I painted on a bit more epoxy, and when that set too I skimmed the surface on the lathe to even out irregularities. After that I laminated one more layer of carbon fibre, followed by more epoxy build-up and skimming. The wall thickness is about 1.5mm.

Cheers,
Stefan

[Troy]

:thumbsup:

[Stefan Buda]

I had to take a bit of a detour, as I promised someone to refigure the secondary for an ODK10 and that was a big job that required the making of a precision test plate as well.
With that job completed, I got back onto this project a few days ago and I practically finished it. I did seven coating runs, three for magnesium fluoride AR coating, two for aluminium and two for silicon monoxide.
I made three spring loaded pins that live inside the periphery of the front lens cell and lock the dew shield in place when the later gets extended.
Also installed a string of resistors into the dew shield and made a connector block for a super expensive little connector socket that I bought from RS Components.
One very obvious mistake I made is that I did not pay attention to the helical pattern on the two carbon tubes and ended up with a mismatch. It won't be easy, but I will have to learn to live with that. I will probably wrap the dew shield with reflective material anyway.
All I need now is a dust cover and the heater cable, plus installing the guide scope.

[RickS]

Beautiful :thumbsup:

[LewisM]

Amazing!

So, when does full scale production start :) :lol:

[multiweb]

That looks good Stefan. When do we see first light? Did I hear big Mike say "Eta Carina"? In green please. :thumbsup:

[croweater]

Talk about do it yourself projects! You did everything Stefan. Just brilliant. Cheers, Richard.

[Stefan Buda]

Thanks guys for the kind words. I'm afraid there will be no full scale production because this would not be a viable commercial product, even if I could get the price down to an acceptable level.
Some of the reasons:
It can only cover APS size sensors.
Barely enough back focus to accommodate thin filter wheels.
Only manual focusing.

Some clear sky is predicted tomorrow, so may be able to have first light soon.

[strongmanmike]

Looks quite amazing Stefan...wow.

Mike

[Paul Haese]

That is a fantastic looking piece of kit and I am sure it works very well. If I go to Vic South this year I hope you attend so I can take a closer look and drool. Incredible skills that you should consider utilising to make a business for these scopes. I am sure you would have a few line up.

[Stefan Buda]

Thanks Michael and Paul for the encouragements.

As I alluded to before, this type of instrument would need to be at least one extra inch larger in aperture to be commercially viable.
However my next project will be very much in that category, but it will not be a Honders.

Last night I plugged in my QHY8 for the first time and I can report that the stars were sharp corner to corner. The polar alignment was not good and exposures longer than a few seconds were trailing so I don't have images to prove it. Also realised that I need to print a Bahtinov mask for it because it is very difficult to judge best focus with the tiny under sampled stars.

Paul,
Yes I will take it to VicSouth if the weather cooperates. I always make a last minute decision on attending, based on the weather forecast.

[Peter Ward]

I am simply in awe of what you have accomplished.

I'm very much looking forward to your seeing examples of your first images with this amazing project.

Amateur astronomy at its very best :bowdown::bowdown:

[Stefan Buda]

Thanks Peter, I appreciate your kind words. This little instrument combined with my OSC camera should be good for recording brighter comets, which we lack at the moment. I have to think of a suitable target for the Messier star party. I do not intend to image the usual bright targets where people with mono cameras and fancy filters would leave me for dead, regardless of how good the Honders is. I think we are over due for a half decent comet.

[Lognic04]

I saw this scope in person at yesterday's ASV meeting. A really awesome idea and even cooler scope!!! Cant wait to see what pics you get! :D

...I want one now :rofl:

[Stefan Buda]

Thanks Logan,
The way you are going, it won't be long before you can make your own.

[Lognic04]

:thumbsup: :D

[Stefan Buda]

Just back from the Messier star party - the driest and warmest night that I remember at the LMDSS.

The bad news: The astrograph is almost unusable as it is, due to internal reflections.

The good news: It produces perfect stars, corner to corner, and the internal reflection is easy to rectify - so I'm completely happy with it.

I noticed that on 5min exposures, that I stretched severely, faint reflection arcs were visible, that appeared to be produced by off the field bright stars.
In the end I used Jupiter, as the brightest light in the sky, to characterise the problem. With Jupiter in the field I was getting only the expected halo circles, due to faint multipath reflections. When Jupiter was just off the field, I was getting no spurious reflections at all, but as I kept increasing the offset, at some point, perhaps 5 degrees off the field, a bright arc appeared across the chip.
It is quite clear now where I went wrong. When I designed the primary baffle, I concentrated only on cutting out stray light that can enter through the front lens and reflect off the inside of the baffle at a grazing angle.
It turns out that this optical arrangement is trying to produce a much larger image circle, that what can actually fit through the aperture of the small corrector lenses.
So, these partially focused stars that cannot fit through, end up hitting the inside of the baffle tube at an incidence angle that will allow them to reflect onto the chip.
The fix should be as simple as machining out the small ridges from the inside of the baffle tube and applying a strip of self adhesive synthetic velvet to absorb that unwanted photons.
I will post some test images in a few days, when I had time to do some processing.

[Stefan Buda]

Here's a 32 minute exposure (32x1min) stacked and flat/dark corrected in DSS. The only other processing is a stretch in StarTools to reveal the background and all the faint stars.
The first image is a highly compressed postage stamp version of the full frame, and the rest are 100% crops of the corners and centre.
Looks like there is a bit of a sensor tilt in the camera, but I think it may not be worth worrying about.
Also this set does not seem to suffer noticeable reflections from off the field stars, but not only bright stars produce reflections and so the image must have some extra photons adding false background brightness.

[Troy]

Good images,
Glad things are working out.

[Stefan Buda]

Thanks Troy, the reflection issue will be very easy to fix and even without the fix, it is not too bad.

Here's a StarTools processed version of the same image, slightly cropped to improve the framing and reduced to 33% with high jpeg compression.

[Lognic04]

Saw this in person again at the MSP! Beautiful pics!

[Atmos]

The telescope looks to perform magnificently!

[Stefan Buda]

Thanks Logan and Colin for the feedback.

I used the blue moon a couple of days ago to illuminate the inside of the OTA while visually looking trough the focuser, trying to identify the nasty reflections I noticed during my recent dark sky testing.

It turns out that nothing is wrong with my baffle tube design. The reflections are coming from the two inch bore of the focuser. My camera nose piece is very short and that leaves part of the smooth bore of the focuser exposed to off axis illumination.
I won't even need to pull the OTA apart to fix the problem.

I'll be ready for that, long overdue, half decent comet.

[Lognic04]

Good thing its a quick, easy fix! :thumbsup:

[DaveGee]

Stefan, I just found this thread. (I should look at IIS more often)...

Bloody hell! What workmanship! What an instrument! What a first-light image! Inspirational. Wow, just Wow!!!

[netwolf]

Wow. this is a true work of passion and looks stunning. Can you estimate how much time this has taken for you to make. And I see two on your mount, did you make 2?

[Stefan Buda]

Thanks Dave and Fahim,

Yes I made two. The smaller one was a practice run for the larger one.
Both have the same focal length but the smaller one is f/4.5 having 100mm aperture.

Last weekend, at the Galactic Centre SP, I wanted to test it for any remaining stray reflections, but the weather did not cooperate.