Mirror Polishing

[Jas.Mc (Jason)]

Hi all. I’m making my first mirror. I seem to be stuck on the polishing and hoping to get some advice. The polishing preceded great for around the first four hours with the surface quality improving at a steady pace. After that point and another 5 hours polishing there has been no noticeable change at all when testing with a low powered red laser. I’m still getting a surface reflection which looks like tiny barely see able pits not touching each other but close together. Sorry that’s the best description I can think of and it’s too small for a photo. Anyway the mirror is six inch. Glass is suprax. The last grit I used for fine grinding was 1600 not sure what it was actually made from the seller never said. White cerium oxide for polishing mixed with water to a milk like consistency. I’m also unsure of what kind the pitch is but to my unexperienced eye it seems to be fairly soft. I’m certain, but could be wrong, I finished of the fine grinding correctly. Maybe I have just not polished for long enough? I have run out of the cerium oxide supplied with the blank which was meant be enough to finish the mirror. I live in an isolated area and have no one to ask for some hands on help and would appreciate any advice or thoughts. Also I'm in Goughs Bay Vic if there are any other amateur telescope makers in the area it would great to here from you.Thanks Jason

[astro744]

What resources are you using as a guide to making your mirror?

You should have considerable polish after two to three hours enough to test the surface using either the Foucault or Ronchi Test. The edge may still be translucent after this time but the remaining three to four hours will correct this and also give you a change to correct any surface defects by altering the stroke depending on what correction you need.

Note a pitch that is too soft will likely cause a severe turned down edge and central depression. The correct pitch hardness is critical and it differs summer to winter.

What grits did you use and for how long each? It could be that you have not completely removed some of the larger grit pits during a grinding stage.

[Jas.Mc (Jason)]

Thanks for your reply astro744. I’ve been using mostly the guide on the Stellafane website. The whole surface is quite shinny and creates a nice reflection in the light but just doesn’t measure up with the laser test. Grits size is as follows 80, 120, 220, 320, 500, 1000, and 1600. I used each for 2 hours or more give or take a bit and ran out of most of them before going on to the next. As for the pitch it started out fairly hard when the weather was warmer but seems to be getting softer as time goes on. I can only get about an hour polishing before the channels close up. I have built a tester modelled on the Stallafane one but have not tested the mirror yet as they recommend for a beginner to not test the mirror until it is fully polished out. I am thinking of regrinding with the 1600 but worried if that’s not the problem I will be back to square one with the polish and still not have solved the issue. Hence my post to see if anyone else has had a similar trouble or maybe knows some other causes. Plus it's nice to connect with some other people with similar interests.

[astro744]

I have only made two mirrors in the late 80's; an 8" f9 and 10.1" f6.3. I was very happy with the 8" and made a Dob and EQ mount for it but sold it to help fund the 1991 Eclipse trip to Hawaii. (I would buy this one back if I could). The 10.1" sat incomplete for quite some time and then I rushed the figuring stage only because an offer of low cost aluminising came up. Many years later I got Mark Sucthing to re-figure it for me and it ended up f6.4 and absolutely first class.

I used a book called "Building a Low Cost Telescope" by Eric Witcombe, Founder, Amateur Astronomers Supply Co. Sydney.

Later I invested in a few more books including "How to Build a Telescope", Second Edition" by Jean Texereau (an industry standard) as well as most other Willmann-Bell telescope making publications. Nowadays the internet provides all you may want or need but the Willmann-Bell publications are very valuable reference books.

See http://www.willbell.com/ under telescope making & optics.

Note Willmann-Bell also sell mirror blanks, powders & gratings, either separately or in a kit. Look under Mirror Kits & Supplies.

I'm not sure how much help I can be as it was a very long time when I made my mirror but I do know I had to pour a second pitch lap as I had an issue with the first. The first book I mentioned says to test the mirror before completing polishing in case you have a significant deformation that needs a certain stroke to remove. In essence you are figuring as you are polishing and then at the very end getting the final paraboloid is easier.

May I suggest you join a Yahoo Group on mirror making. There may also be someone else on this forum who has completed more mirrors than I and certainly more recently that may be able to help.

[DavidU (Dave)]

One of the worlds best mirror makers is on here, Mark Sutching, I hope he see's this post and posts a reply.