[Placidus (Mike and Trish)]

Originally Posted by Shiraz

You must have read my mind. Attached quick test on the edge of the Helix - not in any way comparable with your effort, but nonetheless, there is clearly something beginning to show in there. This is with 7.5 hours of 10 minute subs with the ASI1600 on a 10 inch f4 scope, software binned, but no noise reduction applied. Taking everything into account, my system would have about 1/2 the geometric sensitivity cf yours and I exposed for about 0.4x as long, so this possibly looks like a confirmation of sorts - the theory may just work after all

I don't normally like to post an image in someone else's thread, so if you find it annoying, will happily remove the post.

Wow! Your image is highly relevant and much appreciated! We had to do 2x2 on-chip binning to get anything like that result.

Originally Posted by lazjen

How do I measure the sky signal for this equation?
I assume the Q figure is different for each channel or narrowband being used?
And for the ASI1600, the gain setting determines the read noise, right?

If I know these 3 things, I can get a good value for T? Then effectively after that, increasing N should increase SNR at nearly the best possible "rate" I can theoretically achieve (for the sky, camera, etc)?

Is this right?

I just had a crack at measuring the sky signal. I looked at a single 1 hour dark and flat corrected frame near the south galactic pole (in Sculptor) taken around midnight. The brightness of the starless background was 1650 ADU.

That very much surprised me. It is a high number, much higher than our dark current or the read noise, expressed in consistent units.

With a gain of 1.5 e-/ADU, 1650 ADU is 2,475 electrons. Taking Q = 0.5, that's a photon flux of 4950 photons per pixel per hour, or 1.375 photon per pixel per second.

Is that typical for rural sky? To tell, we need to divide by the unobstructed aperture, and also divide by the angular view of the sky per pixel.

1.375 photons per pixel per second / 0.14 sq meters / 0.3 square arcsec view per pixel = 32.7 photons per square meter of unobstructed aperture per square arcsec of sky.

From that, one can work out the sky brightness in mag/square arcsec, but the calculations are horrific, requiring knowledge of sky absorption, reflectance of primary and secondary mirrors, bandpass of filters, coatings on correctors and ccd chamber, etc, etc, etc. I end up with a value somewhere between 21 and 22 mag/sq arcsec, which is pretty pleasing, but there are so many assumptions that all I can say is yes, 1650 ADU per hour is actually pretty much to be expected .

The really good news is that for working out signal to noise ratio, you don't actually need to do any of that. You just need your actual raw measurement of photons per pixel per hour, and plug it into the formula for snr.