[Aavso-photometry] new flat fielder's question

[Wolfgang Renz]

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

-- 
Wolfgang Renz, Karlsruhe, Germany
Rz.BAV = WRe.vsnet = RWG.AAVSO



----- Original Message ----- 
From: "Yenal Ogmen" <yenalogmen at yahoo.com>
To: <aavso-photometry at mira.aavso.org>
Sent: Wednesday, May 28, 2008 8:14 PM
Subject: [Aavso-photometry] new flat fielder's question


Hi all,

Today, for the first time, I tried flat fielding,
using double t-shirt and twilight zenith. It was 20
minutes before sunset and I got ~31000ADU at 0.0075
sec. Is this exposure suitable for flats or it does
not matter how long you expose as long as you get
30000-35000ADU?
One last question, after taking flats, if you fine
tune focus, does this destroy flat fielding?

Thank you.

Yenal Öðmen
  Green Island Observatory Homepage
www.geocities.com/yenalogmen 
Geçitkale 
North Cyprus