Re: [APML] Rosette comparison (was New Rosette Mosaic)

[Aplanatic@aol.com]

Tony writes:

> Dave,
>  
>    How the heck can you tell Rob has a TDE from looking at an image? Maybe he has convoluted chromo-spherism in his 
> version of MaxIm DL...   : - )
>  
>      Tony

Yes, some sort of strange imaging processing could create or enhance the appearance of the bright stars in Rob's image, but I don't think so.

I've built and tested quite a few telescopes.  There are many different types of optical defects that *could* give rise to the fuzzy bright stars in Rob's image.  In my experience, the only two that actually show up in practice are: TDE and surface roughness.  If the cause was surface roughness then blue stars would show much more scatter than orange stars. (Scatter goes like 1/lambda^4.)  Since this is not the case from inspection of Rob's image, my conclusion is that one of the mirrors has some TDE.  Making a fast hyperbolic primary mirror or a hyperbolic secondary mirror *good to the edge* is quite a difficult undertaking.  This also supports my conjecture.

Matt,

>Great to hear from you...

You too!  Great to see some of your very fine images.

>If the two images are taken using different focal length, how >should that be taken into account?

>If the two images have different integration times, how >should that be taken into account?

> Matt

I'm only considering the effect of the detector, not the telescope or its focal length.  For film images the detector is the emulsion scanned at 4000 dpi.

Because of the nonlinear nature of film there are some difficulties with my statements about MTF over the full range of exposures.  The calculations were done using Kodak's contrast transfer function from their published information.


Roland writes:

>You can't tell the cause but if you analyze the raw image >histogram in Maxim, you can tell quite a lot about the optic, >especially if you compare to a standard. 

>Roland Christen 

Yes, I agree.  Don't you think with a reasonable degree of certainty that Rob's RC has a bit of TDE?  By far the most common problem that would yield the bright star images.

-Dave-





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