Issue with dark subtract

[tloebl]

Hi,

For testing I took a series of 120 sec subs and a 120 sec dark. I tried  subtracting the dark using Image Calibration but it does not seem to subtract. I thought the issue might be the fits header but now I'm not so sure. I wonder if someone could run this on their copy of PI to see if it's only an issue with my machine. I can supply a light and dark for this.

Tom 

[mschuster]

Yes, post up the light and dark in dropbox.

Sometimes it is helpful to check statistics and histogram of light and dark.

And then do PixelMath light-dark (create new image), and statistics/histogram on that as a sanity check.

Mike

[tloebl]

Mike,

posted at

https://www.dropbox.com/s/kx5slypwtpe22fg/Target%20Set%201_120sec_1x1__frame1.fit?dl=0
https://www.dropbox.com/s/9g8jtt9an4ef3vb/Target%20Set%201_120sec_1x1__frame4.fit?dl=0

Tom

[mschuster]

Tom,

Turn off the Optimize option in the Master Dark pane of ImageCalibration and rerun. See the pic below.

With a single frame as dark master, PI dark optimization does not do a good job on your frames. It finds a scaling factor of about 0.2 rather than a value near 1.0, as it should be for equal exposure light and dark and no bias specified.

With the Optimized turned off, ImageCalibration will simply do the calculation light - dark and give nearly the same results as the PixelMath sanity check I posted previously.

Note: even when using a master dark, you may find turning off Optimize works better when dark/light exposure times match. My NB lights and dark masters are 2400s and I find no optimize works better. I think Juan is working on improving dark optimization so things should be better eventually.

Mike

[tloebl]

Thanks Mike. Still learning. Unchecking Dark Optimize worked.  As a rule, I make a master dark and process a bunch of images  and this was the first time I tried a quick and dirty single frame of each.

Tom