Quote Originally Posted by Jon Ruyle


Thanks, Paul.


Tim-when yous say the dark points are clipped to much, do you mean I subtracted too much or not enough? On my monitor, space looks totally black.

I mean you adjusted it so that the dark points of you picture are too dark.


Here is a pretty good overview of using DSS and post processing.
[url="http://astrochat.co.uk/forum/viewtopic.php?t=13241]Astro Image Tweaking.[/url]


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As for darks and flats, I can't either to work right with DSS- maybe you can help.


When I dark frames in DSS (instead of in camera NR), I get little streaks all over the picture. I think it has to do with the fact that I'm getting a little drift (separate issue), but I don't know. I haven't heard of other people having the problem. Of course for the darks I use the same ISO and exposure time and make sure the temperature is close.


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Make sure you are shooting your dark frames the same exposure as you shoot your light frames.
Dark frames don't correct for all the noise! Some of the random noise as well as hot or cold pixels, but there is also what some might refer to as read noise or fixed pattern noise (aka Bias or Offset) that is generated by the camera internal processor reading the data collected from the sensor. This is why you need to shoot bias frames. The ugly streaks you see all over are probably the bias error.


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Whenever I've tried flats the thing always overcompensates (gives me a bright edges and a dim center). Maybe I'm doing something wrong.


My refractor has almost no vignetting, so I never needed flats.
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Flats can be a bit tricky to shoot and get the right exposure. How are you setting the camera up to shoot them?


Flats should be brighter in the center and darken as you get closer to the outer edges. The transition will be more pronounced if you have serious vignetting. The trick is to get the exposure so the center where it is brightest is not blown out(over-exposed). You will probably have to take some test shots keeping a close eye on the histogram. You want to expose to the right, but not too far to the point that the histogram is jammed against the right side.


Maybe this will help give you a better idea. &gt;&gt;&gt; Flat Field Calibration


Additionally, Flats are not just used to correct for vignetting, but for other optical anomalies such as dust spots on your sensor, and artifacts caused by pixel to pixel variations.