Flat Field & Dark Frame - Rosette Nebula

ultraman

Astroandy asked me to start my image processing question by starting a new posting.  Here it is.

This is a single frame 720s Ha image of Rosette Nebula from last night.  Calibrated only with dark frame and flat field.  Taken by SXV-M25C with Bin 2x2, Sky90 with Reducer, EM200.

I was not able to get the gradient out in my image due to it generated in my dark frame (see attached dark frame).  Is this normal to see gradient in the dark frame?  I had adjusted the level in PS CS2 to show the gradient effect.  See the noise that are elongated pixels, which are many on the left hand side of the dark frame.  The dark frame was taken in 600s.  I actually had another dark frames done in the matching 720s with my light frame.

I was also not able get the vignetting out by calibrating my field field (see attached flat field).  I have my flat field saturated at about 2/3 of the pixel well.   Exposure time was 18 seconds. my flat fleld all right?  I did not adjust anything in PS CS2 on the flat field image.

I have my bias frames which I have not yet applied to the light frame.

As for the dark and bias frame as well as light frame and flat field, I have multiple of each to stack.  I am just trying out with this single frame image to see how good I can calibrate the image with the dark frame and flat field.  I did not do a good job as you can see.

How can I get rid of both problem?

I got all valuable advices from anguslau from my last posting.

[ 本帖最後由 ultraman 於 2008-1-5 04:19 編輯 ]

Astroandy

There's something wrong in your dark frame, I think you have to retake it. Have you tried substracting the flat only? You should be able to remove the vignetting with the flat. Do another dark to remove the hot pixels later on, or use SD mask stacking then you don't need to take darks.

anguslau

ultraman, as Andy said, there are something wrong with your dark frame. Can you tell us how you take your dark?

18s for a flat frame is also a bit long because dark current will become significant. I would try to limit it to a second or so. Otherwise you have to calibrate your flat frame (with an appropriate dark) before use. Your light source for flat seems very weak. Can you tell us how you take your flat?

What software/procedures did you use to do the calibration?