[Aavso-photometry] Flat Stuff

[Michael Koppelman]

Arne will tell all in his new book I'm sure, but he once told me that 
he thought a good flat should have about a million electrons. I'm at 
2.5 e/ADU so I need 400,000 ADU to get a million electrons. If I get 
32,000 ADU per exposure I need about 13 of them. I like to do a min/max 
clipped mean which throws out the highest and lowest value for each 
pixel so I usually take 16 or more in each color.

Another thing about flats is that one should not take too short of 
exposures or there might be an effect from the shutter. I try to stay > 
5 seconds on my flats to avoid any chance of this.

Finally, redo your flats often! When you change focus the dust donuts 
change size and your flat is invalidated. I take new flats every time I 
focus if not more often than that.

Just some random flats thoughts that have occurred to me on the 
discussion lately.

Cheers,
Michael Koppelman
http://www.lolife.com/astronomy/