Short exposure flat frames?

[chd]

Trying to get my calibration done more correctly this time...

Am I correct in thinking that the only reason not to do very short exposure flat frames is that if the exposure is short enough then the travel time of the shutter can introduce unevenness in the exposure?

And that therefore, if I am using a camera like the Atik 428ex that does not have a shutter (I don't think - right?) then this is a non-issue and however short an exposure gives me the saturation I want should be fine, right?

Second, how important is the sensor temperature for flats? Since I shot these before it was dark it wasn't as cold, and the sensor temperature was about 2 degress warmer than for my bias frames and dark frames. Is this a significant issue?

Thanks!

[lensman57]

alway

Trying to get my calibration done more correctly this time...

Am I correct in thinking that the only reason not to do very short exposure flat frames is that if the exposure is short enough then the travel time of the shutter can introduce unevenness in the exposure?

And that therefore, if I am using a camera like the Atik 428ex that does not have a shutter (I don't think - right?) then this is a non-issue and however short an exposure gives me the saturation I want should be fine, right?

Second, how important is the sensor temperature for flats? Since I shot these before it was dark it wasn't as cold, and the sensor temperature was about 2 degress warmer than for my bias frames and dark frames. Is this a significant issue?

Thanks!

What exactly are you concerened about? You should always take your flats at the same condition, camera orientation and focus position of the  corresponding lights. The duration of the flat exposure should not matter unless the exposure  is long enough for the  noise to appear in which case you need Dark Flat frames ( same exposure your light flat frames ) to correct for the dark noise. The sensor temperature is crucial if you want to do things right ( hence the need for cooling the sensor ),, that is why the flats, darks and even the bias have to be taken at the same time as lights and at the same temp.

Regards,

A.G

[dph1nm]

I don't think there is any real reason to take flats at the same temperature as the lights. They are mostly there to take out optical vignetting or dust on the sensor, neither of which are particularly sensitive to temperature.

NigelM

[chd]

That's what I thought, but just wanted to confirm. I can see needing some dark flats at the same temperature as the flats to remove noise from the flats, though I doubt that's much of an issue for me right now since I was taking twilight flats at 0.3 second exposures; the noise should be minimal anyway.