Any amateur mirror makers still alive in Oz/Nz?

[legswilly (Werner)]

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.

[legswilly (Werner)]

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.

[mura_gadi (Steve)]

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

[Saturnine (Jeff)]

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

Originally Posted by legswilly

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.

[Sunfish (Ray)]

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.

[legswilly (Werner)]

Originally Posted by Sunfish

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?

[legswilly (Werner)]

Originally Posted by sopticals

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.

[sopticals (Stephen)]

Originally Posted by legswilly

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.

[JohnF (John)]

I certainly hope there are still amateur mirror makers as am still trying to sell a pair of 6 inch blanks.

[legswilly (Werner)]

Originally Posted by sopticals

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.

[sopticals (Stephen)]

Originally Posted by legswilly

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.

[legswilly (Werner)]

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 ?

[Merlin66 (Ken)]

PM sent.

[dave brock]

Originally Posted by sopticals

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

[legswilly (Werner)]

Originally Posted by dave brock

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?

[dave brock]

I got my latest ones from ronchiscreens.com but they are no longer available.

[mura_gadi (Steve)]

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.

[legswilly (Werner)]

[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.

[legswilly (Werner)]

I got some square glass from Sydney and wanted to cut the rounds with a glass cutter as shown on youtube. Would not work with the cutter I bought and homemade tools. Got my grinding/polishing turntable built and cut the rounds on the turntable. First one was not too bad and the next 2 turned out OK. Chamfered them and I thought the polishing should be OK to do.

I read about grinding/polishing flats in Making your own telescope by Allyn J. Thompson. With the three blanks method and float glass, 600 grit is recommended, because it should be reasonably flat. I bought an optical flat and calibrated a homemade spherometer and it gives me a difference of appr. 0.0005". That is over 75mm, I only got a 80mm optical flat. So I thought that is not too much.
I have never polished glass and what happened is that after a very few rotations (4 rpm) the top blank seized on me. I used plenty of water in which I had mixed the 600 grit. It was like I was wringing precision steel flats together. Very hard to shift. Has to stop the turntable and slide it off. On looking at the blanks after cleaning, some dull areas showed where it was ground. If I would keep going, I could not hold onto it, too much grabbing.

What am I doing wrong? Is it not possible to do it this way? Would I have to make a lap and selectively grind/polish it flat?
Sorry for the long post.

[legswilly (Werner)]

Would I have to make a lap and selectively grind/polish it flat?
[/QUOTE]


I persevered and changed to 400 grit. Went through a whole cycle with the four disks and finally have some success.

What was wrong with the 600 grit, nothing, I used more water to dilute it and it worked, but slowly. Wanted all the shine off and I am nearly there. One more cycle with the 400 and I start the 600 cycle again.