## 72497Re: Another Q: Dark Noise/Dark Current

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• Mar 16, 2013
> it would simply be a matter of exposing long enough to make
> the small difference in brightness ...
> by overcoming the shot noise with lots of data.

It is possible to calculate the exp time necessary to achieve a particular SNR for an object by knowing the flux of the object and the ratio of object to sky brightness:

SNR = sqrt(t) * f/ sqrt(f + f*r)
Where t = time (seconds)
f= total flux of object (detected photons per second)
r = ratio of sky/object

some simple algebra yields:
t = SNR^2 * (1+r)/f

approximation for large r:
t = SNR^2 * r/f

An object S/N > 3 is required for certain detection so a good approximation is:

t = 10 * r/f

I guestimate that daytime sky (varies by alt-az, time of day, atmosphere...) is about a million times brighter than the brighter parts of a bright galaxy. A 12-14" aperture and decent camera might detect about 1000 photons/sec from that galaxy (for the extent of the brighter area). So:

t = 10 * 1,000,000 / 1,000 = 10,000 sec = 2.8 hours

The sky would quickly swamp a CCD and thus require a fast video camera. 30fps would generate about a quarter million frames. That's actually not so bad, I've handled that many doing high speed intensified DS imaging.

So with the right equipment it might not be terribly difficult to image a galaxy in the daytime. I might try it except that I do not have a goto mount and so the real challenge would be to aim the scope and guiding would be nearly impossible so it would have to be very well polar aligned and such. But it might be interesting to image a random patch of daytime sky for several hours...

Stan

(revised and replaced this msg)
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