Glass for mirrors

[bloodhound31]

Can anyone tell me if Annealed float glass 19mm to 25mm thick and 16 to 20 inches across would be good enough for mirrors to grind at home?

I have never build one before but I would like to give it a crack.

I know it may be better to try a smaller mirror as a first go, but I already have a commercial 11" SCT and it doesnt make sense to me to make a smaller telescope.

Information overload is good, but please try to keep it simple as I am a bit slow....

Baz.

[Ian Robinson]

I believe George has used it for that range of diameters.

Nova Optical did potatoe chips up to 30" in 2" and 1.5" substrates.

[Satchmo]

Originally Posted by bloodhound31

Can anyone tell me if Annealed float glass 19mm to 25mm thick and 16 to 20 inches across would be good enough for mirrors to grind at home?

If you are one of the `journey is the goal ' types , then it would be a worthwhile project. If you hoped to have any chance of making something ( as an inexperienced amateur mirror maker ) that would outperform your C11 then you would be wasting your time. Why not grind a 6" F8 mirror, leave it spherical in shape, and at least enjoy the thrill of observing the sky with a relatively sharp set of optics you have crafted with your own hands?

[Geoff45 (Geoff)]

It would be quicker and more successful to try your hand at a 6" and a couple of 8" mirrors first. Chances of success with a 16" as a first mirror are slightly better than zero, but not much.

[Ian Robinson]

You could also try a preradiused blank from somewhere like Newport Glass Works (USA) that would make the job easier I think.

They also have plate glass , pyrex and other substrates in fine and precision annealed.

[Satchmo]

Originally Posted by Ian Robinson

Nova Optical did potatoe chips up to 30" in 2" and 1.5" substrates.

30" X 2" mirrors shouldn't show any image defects at the horizon if the sling is designed and fitted properly, and the seeing is pretty lousy down there anyway.. FEA analysis only suggests 6nm RMS error for a correctly positioned sling. I suspect most of what people see as potato chipping are actually scopes that have stretching slings that allow the scopes to go out of collimation when they are pointed low. Warm air pooling at the top of the mirror box can also give an astigmatic effect.

The tolerances for angle of the sling support and edge position are very small though: a few mm off the centre of gravity at the edge of the mirror and things will go pear shaped pretty quickly. A 2mm positional error on th edge for a cable sling on a 30" X 2" mirror results in a Strehl ratio reduction of 7%. On the test bench, I can still see a quite recognisable airy disc with a 24" x 1.75" thickness.

[bloodhound31]

Originally Posted by bloodhound31

Information overload is good, but please try to keep it simple as I am a bit slow....

Originally Posted by Satchmo

30" X 2" mirrors shouldn't show any image defects at the horizon if the sling is designed and fitted properly, and the seeing is pretty lousy down there anyway.. FEA analysis only suggests 6nm RMS error for a correctly positioned sling. I suspect most of what people see as potato chipping are actually scopes that have stretching slings that allow the scopes to go out of collimation when they are pointed low. Warm air pooling at the top of the mirror box can also give an astigmatic effect.

The tolerances for angle of the sling support and edge position are very small though: a few mm off the centre of gravity at the edge of the mirror and things will go pear shaped pretty quickly. A 2mm positional error on th edge for a cable sling on a 30" X 2" mirror results in a Strehl ratio reduction of 7%. On the test bench, I can still see a quite recognisable airy disc with a 24" x 1.75" thickness.

You see, this stuff at this stage means absolutely nothing to me..

I get things like this done by getting my hands dirty, (or in this case, wet) and having a go. That's why I like U-Tube. You can actually see how things are done, then ask the author simple questions.

I am guilty of the same thing though. Now that I know what I am doing (kind of) with astrophotography, I tend to blow the newcomer out of the water with tech stuff they have no grasp of yet. Gotta watch out for that.

Maybe thats why so few people try new things, because it comes across as too complicated, when really, it may be quite simple.

I love doing things for myself and I think something like this is just a question of how much patience you have. In time you should get it right?

Baz.

[GrahamL]

In time you should get it right?

I think you have given yourself good info right there baz,the learning curve your going to travel needs those small steps others are suggesting

[Satchmo]

Originally Posted by bloodhound31

I am guilty of the same thing though. Now that I know what I am doing (kind of) with astrophotography, I tend to blow the newcomer out of the water with tech stuff they have no grasp of yet. Gotta watch out for that.

Baz

I'm not actually guilty of anything, other than perhaps hi-jacking the thread for a moment . My reply clearly referenced a comment I quoted by Robinson about `potato chipping ' of mirrors and was not aimed at beginners.

Cheers