[Aavso-photometry] Adequate FOV size?

[Radu Corlan]

On Thu, 30 Dec 2004, Tandy, David wrote:

> I'm about ready to buy a CCD camera to get started with a photometry program
> (I'll also take the occasional pretty picture, too).  I'm a photometry
> novice but I do have about a year's worth of CCD experience.
> 
> I have a Meade 14" F10 on a AP 1200GTO mount in a permanent setup.   My
> polar alignment is good and the mount points well with the mirror locked in
> the OTA. 
> 
> Obviously, a large FOV has advantages but a large format camera required to
> get a large FOV at 3556mm fl is very expensive  I can always add a focal
> reducer to widen the FOV but I've read that doing so can hurt the quality of
> the observation due to the refractive nature of the focal reducer,
> vignetting, etc.  

Tandy,

The reducer is not that bad - many people use refractors for photometry, 
and the effect of the reducer is in fact smaller. As for vignetting, yes, 
you get some, but is you try using a larger sensor directly on the scope, 
that would vignet too. Simply placing the reducer closer to the sensor 
will reduce the amount of reduction (so to speak ;-)) and help with 
illumination. You can fully illuminate a KAF1602 with the stock meade 
reducer used at 0.7 or so.

The SCT has a large field curvature - the fact that the reducer flattens
the field helps. You may get into trouble with a large sensor as well.

A nice alternative to the kodak sensor cameras is one that uses a Marconi 
back-illuminated chip (i believe Apogee makes one). I never used the 
camera, but the sensor looks perfectly suited for photometry, with 
excellent blue response and a nice, smooth QE curve. it's a 13u, 1kx1k 
camera.

Radu


> 
> My question is:  Do I need a 30' x 30' FOV or is 10' x 10' adequate for most
> type of photometric observations?   For most variable stars are the
> reference stars within a few minutes of the target star?
> 
> My seeing is average so I'm thinking that a 1" or so pixel scale is good.
> At F10, that translates to around 17 microns.  At F6.3, 10 microns.  It
> seems like a 9 micron pixel chip would work well.  I'll be a bit oversampled
> at F10, but that's better than being undersampled, right?  Plus, when I want
> to shoot some pretty pictures through my widefield scope, 9 microns is just
> about perfect.  Lastly, I've seen only one 16 micron pixel camera and it is
> nearly $8k.
> 
> So, for such a long focal length setup (3556mm) am I going to need a larger
> format camera (SBIG STL) or will a ST-8XE (9' x 13' FOV) be perfectly
> adequate?    Any camera suggestions are welcome!
> 
> Thanks,
> 
> Dave
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> 

-- 

-------------
Radu Corlan       Snail Mail: Bucuresti sect. 1, 
rcorlan at pcnet.ro  str. Argentina nr. 28, Romania

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