From: Stanley Moore [stanleymm@earthlink.net] Sent: Wednesday, January 14, 2004 4:16 PM To: John Smith
Subject: RE: Noise and sub-exposures revisited

John,

OK, I got those FITS and took a look.  The 6 x 10 min image is measurably superior to the 13 x 5 min image, even though the 13 x 5 min image is 5 minutes longer (and probably takes more than 10 minutes more to acquire, due to download time, etc.).

It was an error for you to calculate S/N as simply the mean adu divided by std of a given area.  Signal refers only to the photons coming from the astronomical object. It is inappropriate and completely misleading to include sky glow as "signal". (e.g. the 13x5 image will obviously have higher sky signal and thus better "sky S/N").

Here is a correct calculation of actual measured S/N in your images:

Background levels:
	5 min stack 	mean=703
	10 min stack	mean=930

nebula area at {X=401, Y=281, width=25, Height=20}:
	5 min stack 	mean=1,228	std=54.94
	10 min stack	mean=2,005	std=110.24

S/N of that nebula area:
	5 min stack 	signal = 1,228 - 703; noise = 54.94; S/N = 9.56
	10 min stack	signal = 2,005 - 930; noise = 110.24; S/N = 9.75

As you can see, the 10 min stack has better S/N.

If you like, I will not post this and allow you to make the appropriate corrections...

Stan