[Aavso-photometry] new flat fielder's question

[Richard Huziak]

Hi Wolfgang, Yenal,

Common wisdom is to shoot flats with minimum 1 second exposure to reduce 
shutter effects, but that depends, as you said, on your camera shutter 
construction.  If you have a photometric shutter you don't have to worry 
that much.  With SBIG cameras I generally start shooting flats when I am 
at the 0.1 sec brightness level.  I'd rather shoot a bit slower just to 
be certain, but that is more paranoia than reality, since the 0.1 sec 
flats turn out OK.  The problem with waiting for slower shooting times 
is that you run out of sky illumination FAST, especially if you are 
trying do to 16 flats in 5 colours.  As you said - compare the short exp 
flat to long exposure flats, and stay out of the non-linear short 
exposure part of the CCD which can happen with ultra-short exposure times. 

I think you also asked about focus.  You should be near your working 
focus to shoot the flats, but everyone is always out by the difference 
between last night's and tonite's thermal expansion/contraction  focus 
differences.  Tweaking the focus small amounts doesn't change the flat 
properties enough to worry about.  So if you don't know your focus 
before shooting flats, focus and shoot uncalibrated and then shoot flats 
in the morning or next eve and add them in after.

rick

Wolfgang Renz wrote:
> Hi Yenal
>
>   
>> It was 20 minutes before sunset  ...
>>     
>
> So you are not taking twilight but daylight flats ;-)
>
>   
>> I got ~31000ADU ...
>> Does it not matter how long you expose as long as you get
>> 30000-35000ADU?
>>     
>
> Usually any value will be OK as long as the ADU values are still
> in the linear range of the camera (which you should determine
> anyway).
> But as higher the values are, as better the SNR will be and as
> less flats will you have to take to reach a level that makes the
> contribution of the flats to the total error insignificant.
>
>   
>> ...  at 0.0075 sec.
>> Is this exposure suitable for flats ... ?
>>     
>
> For full-frame chips with a mechanical shutter that cann't open
> and close in a 0 time, one should take flats of at least a few
> seconds exposure time. The possible shutter (exposure time)
> error and the uneven illumination due to the shutter construction
> is of course worse with an isris shutter than with a rotating shutter.
>
> But as you have a Sony ICX429ALL interline chip in your DSI
> Pro II, its probable OK to use such very short flat exposure
> times. You will know if you compare them with some several
> sec flats.
>
>
> Maybe someone can chime in who takes short exposure time
> flats with an interline chip.
>
>
>   
>> One last question, after taking flats, if you fine tune focus,
>> does this destroy flat fielding?
>>     
>
> If the flats are taken severely defocused, it has for sure an influ-
> ence on the flats. Maybe not so much  on the pixrl-to pixel values
> but e.g. dust on the cover glas of the CCD chip, on the camera
> chaimvber window and on the filters will get illuminated a bit dif-
> ferently. So its probably best to take the flats a well focused as
> possible.
>
> Clear skies
>  Wolfgang
>
>   

-- 
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Richard Huziak
Manufacturing Engineering
SED Systems
Saskatoon, SK, Canada
tel. (306) 933-1676
<huziak at SEDSystems.ca>
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