FLATS & DARKS, questions...

First of all, are they the same?
Second, I know they are truly necessary in order to remove much of the noise found in the images we take, but how often do you take them? must you take a new set if you make changes in your imaging setup? (ie. when changing from f/10 to f/6.3 with r/c?)... What about if ambient temperature changes, do you take a new set of them also?


Thank you for your attention, and clear skies to all!


Ivan
8)

Ivan,
Darks are different from flats. Dark frames remove "noise" from an image that is caused from a heating of the chip from the electronics. A CCD chip is photon sensitive. Since heat and light are the same, a CCD chip can not tell the difference between the two so a dark frame is basicly a picture of the heat being created by the chip. Most software will take a dark frame and subtract it from the images you are taking. It uses the same dark no matter if you take one or a hundred.Darks Must be the same exposure and temperture as your light frames. A one minute light image at -20 must have a one minute dark exposure at -20. Some people will take dozens of darks and average them together and make one single "Master dark."
Flats are different. A flat field is basicly a picture of your telescopes optical path and is used to evenly illuminate the CCD chip. It also removes any images of dust that may show up in longer exposures. Your flats will be at a different exposure and can be at a different tempreture. However since your taking an image of your optics, you MUST be at the same focal plane as your image. If you change your focus the flat will not work. Flats are a little harder to do. You want to saturate the pixels to about 30 to 50% so you need to check your manual to see what the saturation level is. You also need an evenly illuminated surface to image for your flats. Most will use a "sky flat". Taking an image of the sky about 20 minutes after the sun has gone down or comes up.
There are dozens of web sites about both of these that can explain in much more detail. Just do a search for "Dark frames and Flat fields" and you should find everything you need to know.

hi Ivan,

Email me at [email protected] with your phone # ( if you wish ) and I'll give you a call ... it's a complex subject .. so far you have gotton good advice.

clear skies,

Larry Citro

Thank You guys!

Ivan
8)