Mirror testing

[Guest]

I've just finished making a Foucault tester before I start polishing the mirror and I had an idea to use my laser collimater as an aid to lining up the mirror and tester. Then I wondered if anyone has developed a way of actually testing a mirror using a laser.  It seemed to me that if you lined up the laser parallel to the axis of the mirror and then had an arrangement to traverse the laser across the width of the mirror you could detect errors by looking at the position of the reflect laser spot. Also If you put the screen as far away from the mirror as possible it would have the effect of magnifying any error in the figure. Then you could use a bit of geometry to work out the details.

[MrBill]

Sounds like you just invented the laser interferometer...

[Astrobits]

When the return beam from your light source is as close to the source as possible your tester and mirror are in alignment. No laser needed and will probably be too bright to use anyway.

Nigel

[Guest]

Thanks. The way I was thinking of doesn't work out but I found some information about building a bath interferometer and using it to test a mirror which looks very interesting.

Has anyone here actually done it?

 

[mapstar]

The foucault test is cheap, tried and tested.  

I honestly couldn't get on with it but there is no need to over complicate things although it is useful to have more than one test method.

[Guest]

Hi Damian,

Sorry, I wasn't quite sure what you meant. Did you mean that you used the laser interferometer instead of the Foucault test?

I think testing the mirror is one of the most interesting parts of the mirror making process. I'd be interested to try two different methods and see if they agree.

Cheers

Steve

 

[mapstar]

10 hours ago, woodblock said:

Hi Damian,

Sorry, I wasn't quite sure what you meant. Did you mean that you used the laser interferometer instead of the Foucault test?

I think testing the mirror is one of the most interesting parts of the mirror making process. I'd be interested to try two different methods and see if they agree.

Cheers

Steve

 

Hi Steve,

I used the Ronchi test which is a form of the foucault knife edge test.

A lot of mirror makers use it although like everything in mirror making views on it are mixed. 

Some use the foucault knife edge test with a couder/zonal mask and data reduction software such as figure XP to help the figuring process.

Overall though it is about a smoothly corrected mirror. 

The star test is the ultimate arbiter of a good mirror and many things can be gleaned from it.

I am not even a good amateur at it but I have learned a lot from my time testing and looking at the images.

I've even used the Ronchi test for testing my mirror when it was mounted in the cell.

Hope that helps

[Guest]

Thanks Damian,

Just been looking through your Mapstar Mirror Thread. 22 inches is  some mirror. In the book I have it says that the difficulty of making a mirror goes up according to the square of the diameter so I think I'll complete my 8.5 inch mirror before I go on to anything like that.

One quick question. Did you make or buy your Ronchi screen? If you bought it where did you get it from and if you made it how did you make it?

Cheers

Steve

 

[mapstar]

Thanks for reading the thread Steve 

Yes it was a very ambitious task one which I couldn't quite complete through the lack of my knowledge and experience. I got there in the end with help from a certain kind gent.

I got my Ronchi screens from here but to be fair you only require the 133lpi screen

http://www.ronchiscreens.com/screens.php

Funny thing is its a US site and when you get the screen it says made in holland! 

There is also a program you can use to generate what the Ronchi should look like for your mirror written by John Upton. It can be found here

http://www.atm-workshop.com/ronchi-test.html

Good to see you taking up the challenge of mirror making.I'm sure with the help on here it will be a great experience.