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  #401  
Old 07-05-2021, 11:58 AM
legswilly (Werner)
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Thank you Ray and Astro.

We have an observatory at Port Macquarie and I will contact the person there and they might know someone in the area, were I could have a look at a setup for testing mirrors. No hurry because I ordered a 200mm mirror from China. It most likely will be only spherical and not suitable for a telescope, but I thought I could use it to find out how testing works. I may have squandered some money, it won't be the first time. I am waiting on some books and they might tell me a bit more.
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  #402  
Old 09-05-2021, 11:28 AM
legswilly (Werner)
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How can I make a flat mirror? Ronchi screen, where to get?

I am looking at how to test a mirror and I would like to build an autocollimation test rig.

The one I have read up on had a round flat mirror. Would it make a difference if the mirror is square? Surely a square one would reflect the round image of the Primary as a round. How is a mirror made flat?
I have tried to print a ronchi screen on the laser printer. It did not come out as well and everyone that makes a mirror would use a ronchi. Where can I get one at a reasonable cost.
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  #403  
Old 09-05-2021, 05:35 PM
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mura_gadi (Steve)
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Hello,

I am currently looking at ronchi eyepieces and an artificial star setup for my testing. From looking around the ronchi screen on glass slides are fairly expensive.

Some smaller printers/startups do offer printing to glass, I haven't got any feed back on pricing for those.

Try and print the whole transparency as A4, then scan over the sheet and cut your best two square inches out.


Steve
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  #404  
Old 09-05-2021, 06:15 PM
Saturnine (Jeff)
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Hi Werner

Try looking up the Stellafane site https://stellafane.org/stellafane-ma...atm/index.html

Plenty of other mirror making and testing sites on the web as well



Quote:
Originally Posted by legswilly View Post
I am looking at how to test a mirror and I would like to build an autocollimation test rig.

The one I have read up on had a round flat mirror. Would it make a difference if the mirror is square? Surely a square one would reflect the round image of the Primary as a round. How is a mirror made flat?
I have tried to print a ronchi screen on the laser printer. It did not come out as well and everyone that makes a mirror would use a ronchi. Where can I get one at a reasonable cost.
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  #405  
Old 15-05-2021, 06:27 PM
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Sunfish (Ray)
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Look on the Stellafane website in the testing section for a video lecture regarding auto collimation testing. The mirror is required to be optically flat first surface . The video describes the specification of flatness required and the history and design of these kinds of testing rigs.
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  #406  
Old 15-05-2021, 08:49 PM
legswilly (Werner)
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Quote:
Originally Posted by Sunfish View Post
Look on the Stellafane website in the testing section for a video lecture regarding auto collimation testing. The mirror is required to be optically flat first surface . The video describes the specification of flatness required and the history and design of these kinds of testing rigs.

Thank you for the link. I did have a look at the auto collimation test setup and will build one as soon as I can get hold of a reasonably priced 300mm flat mirror, or 300mm blanks.

Sydney glass quoted me $319.00 for one 25mm blank, and $154.00 for polishing.
Telescopes and Astronomy sells 305mm x 25mm for $162.00. Need three blanks.

My concern is the thickness. It should be OK for a flat mirror, but I would eventually want to use the remaining flats to make a mirror 300mm and it might be too thin.
One guy tried to laminate, but said he was not successful with epoxy. Have to look him up and see what went wrong.
I am looking around on AliBaba and will get some answers maybe on monday.
Where else could I look?
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  #407  
Old 15-05-2021, 09:18 PM
legswilly (Werner)
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Quote:
Originally Posted by sopticals View Post
Hi Leslie,

Not sure you should attempt this route. My experience with gluing glass goes back a year or two when I attempted a 25"f4.25 sandwich mirror project. I used a not too fast curing super strong epoxy glue. At first during hogging through to fine grinding, and even when part way through polishing all looked good, (Ronchi test was ok), but there was a particular point nearer the end of polishing when the Ronchi test showed that some of the glue joints at the junction glass pillars(7mm thick) to front and back plates(each 19mm thick float glass) were failing. At that point I laid aside this project. The only way to get two glass surfaces to successfully adhere long term is too fuse those surfaces together by heating to the required temperature in a glass or ceramic kiln. In the future I may take my sandwich project to a kiln and do the "fuse". I dont know what diameter mirror your thinking on working. My advice would be to use a single good quality 19mm float glass blank (for anything 14" in diameter and less). Larger will need a thicker substrate.
Why did you have to use glass pillars? Why not poor the resin in the centre of the bottom glass and place the the top on make sure the glue squeezes out everywhere and let it sit for a few hours with the slow curing epoxy.
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  #408  
Old 18-05-2021, 03:00 PM
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Quote:
Originally Posted by legswilly View Post
Why did you have to use glass pillars? Why not poor the resin in the centre of the bottom glass and place the the top on make sure the glue squeezes out everywhere and let it sit for a few hours with the slow curing epoxy.

Hi Werner, I tried the "pillars between" experiment because I had past experience with the Hubble Optics "sandwich" mirrors (3 x 14") and they performed well. Anyway, was to subsequently obtain a large quantity of thicker (35-37mm) good quality plate glass so never had to try any alternative ways of bonding together thinner glass sheets.

Stephen.

Last edited by sopticals; 18-05-2021 at 06:13 PM.
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  #409  
Old 20-05-2021, 08:29 AM
JohnF (John)
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I certainly hope there are still amateur mirror makers as am still trying to sell a pair of 6 inch blanks.
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  #410  
Old 22-05-2021, 09:42 AM
legswilly (Werner)
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Quote:
Originally Posted by sopticals View Post
Hi Werner, I tried the "pillars between" experiment because I had past experience with the Hubble Optics "sandwich" mirrors (3 x 14") and they performed well. Anyway, was to subsequently obtain a large quantity of thicker (35-37mm) good quality plate glass so never had to try any alternative ways of bonding together thinner glass sheets.

Stephen.
Thank you for replying Stephen. Have already bought some glass and will try to epoxy together. I also want to build a auto collimation test stand and needed a flat mirror for it. Could not find one to buy, so will make one. Still need to go to Sydney to pick up the glass which is waiting for me.
Found a Ronchi eyepiece maker https://www.gerdneumann.net/english/...lar-5l-mm.html. He makes a 10lines per mm, which equates to 254 lines per inch. Is 254 lines too many? It's a lot cheaper.
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  #411  
Old 22-05-2021, 01:22 PM
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Quote:
Originally Posted by legswilly View Post
Thank you for replying Stephen. Have already bought some glass and will try to epoxy together. I also want to build a auto collimation test stand and needed a flat mirror for it. Could not find one to buy, so will make one. Still need to go to Sydney to pick up the glass which is waiting for me.
Found a Ronchi eyepiece maker https://www.gerdneumann.net/english/...lar-5l-mm.html. He makes a 10lines per mm, which equates to 254 lines per inch. Is 254 lines too many? It's a lot cheaper.
254 lines per inch should be ok. I use a 133 lpi eyepiece and it also works well for testing the whole system using an actual star. At 254 lpi sensitivity to optical defects should be higher. Shouldn't be any negative diffraction issues.

Stephen.
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  #412  
Old 07-06-2021, 07:51 PM
legswilly (Werner)
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Another Pich request

I have got all the grit I need for grinding and polishing. Bought from Naxos in Sydney which I know from the time I was working.
Now I am looking where to buy pitch from. Is there someone in OZ? Last time I got a quote for things from the USA, the postage was quite expensive and I would try to get is somewhere else. Is there somewhere in the UK? I looked also in Germany, but they use a different classification for it. So I am still looking.
Bite the bullet and buy from the USA ?
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  #413  
Old 08-06-2021, 06:18 PM
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Merlin66 (Ken)
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PM sent.
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  #414  
Old 08-06-2021, 09:30 PM
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Quote:
Originally Posted by sopticals View Post
254 lines per inch should be ok. I use a 133 lpi eyepiece and it also works well for testing the whole system using an actual star. At 254 lpi sensitivity to optical defects should be higher. Shouldn't be any negative diffraction issues.

Stephen.
I find 250LPI ok in the scope on a star but too fine on the bench.
Dave
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  #415  
Old 09-06-2021, 04:59 PM
legswilly (Werner)
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Quote:
Originally Posted by dave brock View Post
I find 250LPI ok in the scope on a star but too fine on the bench.
Dave

Too late, it's already on order from Germany. I did not know where to get one with less lines.

Where did you get your 250 line one from. What make is it?
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  #416  
Old 10-06-2021, 06:02 PM
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I got my latest ones from ronchiscreens.com but they are no longer available.
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  #417  
Old 10-06-2021, 06:12 PM
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mura_gadi (Steve)
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Ronchi Eyepieces

Hello,

Try looking for "Gerd Neumann (jr)" products, there are 10L/mm for 250 per inch for around 35euro, 5L/mm as well, but can be a little more expensive, $55euro.



Steve
Ps. https://www.gerdneumann.net/
PPs. https://www.gerdneumann.net/deutsch/...-eyepiece.html (a few shops hold them as well)

Should add Ronchi slides are available in Oz from places like Edmunds optics and Proscitech, but expect to pay between $250-$500+ depending on L/mm and the size/material of the slide.

Last edited by mura_gadi; 17-06-2021 at 06:17 AM. Reason: Ppps
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  #418  
Old 13-06-2021, 10:19 AM
legswilly (Werner)
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[QUOTE=mura_gadi;1522464]Hello,

Try looking for "Gerd Neumann (jr)" products, there are 10L/mm for 250 per inch for around 35euro, 5L/mm as well, but can be a little more expensive, $55euro.

Thank you for the link. I got the 10L/mm and it's coming. Reading about testing most people use closer to 5L/mm. Being a skinflint, I bought the cheaper one. I'll see how it goes.

I am nowhere near testing. At the this time, I am building a turntable for grinding and polishing. I thought I needed a flat mirror for the auto collimation test as is written on the Stellafane site. Now I know I don't need a glass mirror. Too late. Got most of the stuff for the turntable.
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