On 23 Dec 2008, at 20:31, Chuck Hards wrote:
Patrick makes a good point about an uncooled chip generating "noise" on long exposures.
Tonight's sucker hole didn't permit doing anything serious but Chuck's message prompted me to shoot a "visual aid". First I shot a 1 minute exposure of a random galaxy. That's called the "raw" image. Next I shot 15 one minute exposures with the scope covered (called "darks") and then using software I median combined them into a single master dark. Then, again using the software I subtracted the master dark (which is basically a picture of the noise in the chip) from the raw and got the final image. All three shots are posted here: http://users.wirelessbeehive.com/~paw/temp/RAWMINUSDARK.JPG As you might expect this is a very simplified version of what's done to get Tyler-like images but maybe it will give an idea of what "noise" is and what's involved in removing it (and how important it can be to remove it). patrick