Re: [ATM] Banishing the diagonal

[Gary Fuchs <gary@rcn.com>]

 >> Is the original poster still reading this thread? 

Yes, I'm still reading it. The tangential discussions are quite interesting.

 >>What was your goal? 

To understand more about the effects of changes to the secondary size 
and placement. I've been going on the assumption that secondary size 
could be varied somewhat, provided the overall design, f-ratio, etc., 
allowed it, without compromising the apparent quality of the view - what 
I'm realistically going to be able to discern. I'd like to know how much 
variation I could get away with, and if adding a lens might be a 
workable way to manage a significantly smaller secondary obstruction 
while maintaining the quality of view.

Gary Fuchs



Ken Lowther wrote:

> Well, if we are worried about the quality of images, we can just stay 
> home and
> browse the web for Hubble pix.....  I am not saying ccd's don't have 
> their
> place.  We are just getting into a different realm here.  Is the original
> poster still reading this thread?  What was your goal?
>
> Ken
>
> Quoting David Whysong <dwhysong@gmail.com>:
>
>>> So, in order to reduce diffraction you put the ccd in the light path 
>>> and then
>>> run cooling tubes to it?   Sounds like we have come back around to the
>>> original problem. ;-)
>>
>>
>> Secondary diffraction is rarely an issue for CCD imaging because the
>> exposures are long enough that seeing dominates the PSF. (Spikes
>> from a spider do show up, of course.) As for thermal issues, you simply
>> insulate the camera.
>>
>> If I had the money, I'd build a 20" f/4-ish Newtonian with a large CCD
>> and paracorr at the focus. The only reason to have a diagonal is that
>> the SBIG research series cameras have a large and asymmetric shape,
>> which might cause a "lumpy" PSF in good seeing conditions. That could
>> affect astrometric measurements.
>>
>> -- 
>> David
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>
>
>
>
> Hermit, holed up in Youngstown, Ohio
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