Re: [ATM] Dog biscuit ripple -Bad?

[Michael Peck <mpeck1@ix.netcom.com>]

At 04:10 6/26/05, dave g wrote:
>G'day Everybody,
>
>I'm making a 10inch f6 and I have just about got the paraboloid correction 
>right as the numbers repeatedly come out very good but I'm little worried 
>about the dog biscuit ripple.   Please look at the image at this link...
>http://users.tpg.com.au/users/daveg/10inchf6_050626.jpg
>

The stock answer to that question is if you can see surface roughness you 
should get rid of it.

On the other hand, I have a mirror that looks at least that bad in a 
digital shadowgram, and I have pretty good evidence that the RMS amplitude 
of the surface roughness is on the order of 4.5 - 5 nm. If that were the 
only thing wrong with the mirror that amount of dog biscuit would probably 
be acceptable, depending of course on the intended use of the telescope.

If anyone wants to know how I can make a precise sounding statement about 
the magnitude of surface roughness I'll explain. If nobody cares I won't 
bother, but this is an interesting case study of the amount of information 
that can be extracted from interferograms.

Judging from the lack of a complete diffraction ring in the image you may 
have a turned down edge. That could be a bigger problem than the surface 
roughness.

Mike Peck


_________________

Michael Peck
email mpeck1@ix.netcom.com
Wildlife photography page http://home.netcom.com/~mpeck1/index.html
Amateur telescope making http://home.earthlink.net/~mlpeck54/astro/astro.html 

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