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Diagonal flat mirror

Guest

By Guest
in DIY Astronomer

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Guest

Guest

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I'm currently making a mirror for a home made Newtonian and just looking ahead. Do you telescope makers out there normally make the diagonal secondary flat mirror or is it possible to buy them?

Cheers

Steve

 

 

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Guest

Guest

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I haven't worked out the size yet. I have a book which gives some information about how to make one but it doesn't look easy so that's why I wondered what other people do. If you can buy them at a reasonable price then I'd just as soon buy it.

Cheers

Steve

 

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Astrobits

Astrobits

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Posted

It depends.

Do you want to make your own? If so then follow the instructions you already have.

It is tricky but you are only making a spherical surface with a radius of nearly infinity. Actually the radius can be much shorter than that and be within an acceptable accuracy, but it is possible to test your flat against the primary mirror once it is finished using the kit and techniques that you will use to test your primary as you figure it. Just test the primary by reflection in the flat and any difference between the primary alone and with the flat must be due to the flat. The standard way of testing flats is against a known flat reference surface--a catch 22 situation ( to make a flat you need a flat first ). A liquid surface is flat enough for our purposes, the National Physical Laboratory use oil as the liquid, I have used water to successfully make my 8" dia reference flat.

Nigel

 

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Guest

Guest

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Thanks Nigel,

The book I'm using is N E Howard. One of his methods of making a flat mirror without a flat reference is to make 3 mirrors and test them one against the other. Then there's a bit of maths to work out the shape of each one. It's not to clear where you go from there but I get the gist.

Your second post arrived while I was typing this. I see that Galvoptics also offer Newtonian secondaries but I don't have to decide just yet. Perhaps I should see how I get on with the primary first. I've finished grinding and I'm just making a Foucault tester before I move on to polishing.

Thanks again

Steve

 

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