There are many variables to consider not just the ISO settings, exposure length and ambient temperature. Something as simple as the brightness of the object you are shoot can also play a integral part in the quantity of noise or the lack off. I do like the mathematical formulas provided in the link. Though I'm still crunching the numbers.
Slightly off topic, attached is the US S&T article from the November 2006 issue (which subsequently I received yesterday). The article talks about going deep with DSLRs with renowned astrophotographer Chuck Vaughn.
http://astrophotography.aa6g.org/
As quoted from the article:
"I consider 3 hours to be my minimum duration per subject when imaging under dark skies. As with cooled CCD cameras, creating long cumulative exposures is best done with many shorter ones combined; due to the
greater sensitivity of digital sensors, they reach the skyfog limit far quicker than film. From my location, I’ve standardized my exposure length to 10 minutes per frame at ISO 800. Lower ISO values cause undesirable
posterization (visible steps between brightness levels) in the faintest areas of my images, and I don’t see much advantage to ISO 1600, with its decreased dynamic range. I save all my images in RAW format, to preserve the entire 12 bits of data produced by the camera. I also turn off the automatic noise reduction and instead record dark exposures (images of the same duration as the light frames, recorded with the camera’s lens
covered) at roughly the same ambient temperature so I can calibrate my images later. Of course, accurate guiding of the telescope is a must for any image longer than a few seconds, so I use an old SBIG ST-4 autoguider to ensure round stars every time."
Enjoy
