Re: [ATM] Dog biscuit ripple -Bad?

[clnoble@email.arizona.edu]

Mike,

I would to hear  about how you can gauge the magnitude of surface roughness by
just looking at an interferogram.

Colton Noble
Tucson, AZ

Quoting 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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