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  #21  
Old 03-12-2016, 09:45 AM
Rod
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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.
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  #22  
Old 03-12-2016, 11:10 AM
Stefan Buda
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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.
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  #23  
Old 04-12-2016, 03:47 PM
Stefan Buda
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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.
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  #24  
Old 04-12-2016, 03:56 PM
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Tinderboxsky (Steve)
I can see clearly now ...

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Fascinating. I am enjoying following progress.

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

Cheers

Steve.
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  #25  
Old 04-12-2016, 07:20 PM
bratislav (Bratislav)
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Quote:
Originally Posted by Stefan Buda View Post
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.
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

Last edited by bratislav; 04-12-2016 at 08:31 PM. Reason: typo
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  #26  
Old 06-12-2016, 08:57 PM
Stefan Buda
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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.
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Last edited by Stefan Buda; 06-12-2016 at 09:07 PM.
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  #27  
Old 09-12-2016, 10:49 AM
Stefan Buda
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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.
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  #28  
Old 15-12-2016, 09:38 PM
Stefan Buda
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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.
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  #29  
Old 20-12-2016, 09:22 AM
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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.
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  #30  
Old 20-12-2016, 11:35 AM
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OzEclipse (Joe Cali)
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Quote:
Originally Posted by Stefan Buda View Post
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.
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
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  #31  
Old 20-12-2016, 03:40 PM
Stefan Buda
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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.
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  #32  
Old 21-12-2016, 09:31 PM
Wavytone (Nick)
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http://www.opticalcoating.com.au/services.htm
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  #33  
Old 23-12-2016, 07:53 AM
Stefan Buda
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Thanks for lead number 3.
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  #34  
Old 27-12-2016, 02:47 PM
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Well done Stefan,

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


Stewart
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  #35  
Old 27-12-2016, 09:28 PM
Stefan Buda
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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.
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  #36  
Old 12-01-2017, 02:15 PM
Stefan Buda
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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.
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  #37  
Old 12-01-2017, 05:31 PM
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Very impressive Stephan...

Incidentally, what is the resolution of that printer... it looks impossibly fine.
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  #38  
Old 13-01-2017, 09:36 AM
Stefan Buda
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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.
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  #39  
Old 13-01-2017, 03:09 PM
Wavytone (Nick)
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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.
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  #40  
Old 14-01-2017, 08:10 AM
Stefan Buda
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Originally Posted by Wavytone View Post
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.
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.
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