[ATMoB-discuss] First photo from C14/ST7

[John Boudreau]

Agreed. A good set of sky (twilight) flats is the best way to do it. However, the camera will most certainly detect stars, even in a sky too bright to take good flats--- especially with a scope the size of a C14. Even my tiny FS60C will pick up stars with sky flat.

For sky flats, it's best to take 5 or more through each filter used in the imaging run. The scope should be pointed in a slightly different section of sky (preferably in the zone from the zenith to the region opposite the rising or setting Sun), for each flat so that the star pattern doesn't repeat from frame to frame. The resulting flats should be median combined to best remove the stars in the result--- averaging doesn't do as good a job at that. The flats should be taken at an exposure that results in a 30,000 ADU average (+/- 5000 ADU or so) across the flat --- ADU values in CCDSoft are given in the lower left corner of the screen next to the x and y pixel positions of the mouse cursor. Some imagers use a piece of white cloth or a plain white T-shirt stretched over the front of the scope and held in place with rubber bands--- that way you don't have to worry about stars in the flats, and the cloth adds to the sky background diffusion.

Since the time window for good sky flats can be fleeting, a more practical way is to use a good light box. Then the flats can be taken at any time, even with the observatory roof in place <g>. I use a light box for my scopes ranging from a C11 to FS60C that's designed very much like the one described here: http://www.astrodon.com/oldsite/LearningCurve.html
Testing the light box flats vs. a set of sky flats revealed no discernable differences at all, so now that's all I use. 


---John


----- Original Message ----- 
From: "Steve Beckwith" <stevebeckwith at comcast.net>
To: "'Bruce Berger'" <bruce at scopemaker.com>; "'Buonomo, John'" <Jbuonomo at uhs.harvard.edu>; <atmob-discuss at atmob.org>
Sent: Wednesday, August 09, 2006 7:54 AM
Subject: RE: [ATMoB-discuss] First photo from C14/ST7


> If the camera has already been taken off the scope, then it's too late
> to take a flat. You can use the sky to take flat fields prior to your
> imaging run.  This needs to be done just as dusk begins. However, it's a
> bit tricky as it needs to be done before the camera can detect stars.
> Take a number of them and average them together. 
> 
> 
> 
> -----Original Message-----
> From: atmob-discuss-bounces at atmob.org
> [mailto:atmob-discuss-bounces at atmob.org] On Behalf Of Bruce Berger
> Sent: Tuesday, August 08, 2006 6:05 PM
> To: 'Buonomo, John'; atmob-discuss at atmob.org
> Subject: RE: [ATMoB-discuss] First photo from C14/ST7
> 
> Hi John,
> 
> Thanks for your suggestions.
> 
> These were guided 3-5 minute exposures using the built-in guide chip on
> the
> ST7. I have 4 darks and 4 bias, and about 9 unfiltered images to stack.
> I
> will try your advice tonite and repost the result. 
> 
> How do I take a flat? I've seen an elaborate setup that Gary Walker
> uses. Is
> there an 'easy' way?
> 
> Bruce
>