[Aavso-photometry] new flat fielder's question

[Yenal Ogmen]

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