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Thanks for all the replies. Yes, the photo is the default stretch in CCDStack but magnified 400x. The only reason I'm looking at this is that stacks of luminance images taken with my KAF830 chip show a lot of these dark blotches. The individual subs also show the darkies but they seem to come to greater prominence after stacking and especially when stretching.
I've just gone back again and tried to see differences between uncalibrated and calibrated subs, and I've taken Rick's advice and looked at values before and after. I suspect something isn't going according to plan during calibration. The dark spots do not calibrate out.
Here are some typical numbers coming out of CCDStack (I selected the entire image);
Sub 480 seconds
No calibration:
mean 2,312.68
STD 598.94
S/N 3.86
median 2,298.00
int mode 2,297
min 915.00
max 65,535.00
Dark Only:
mean 1,835.27
STD 584.95
S/N 3.14
median 1,822.61
int mode 1,828
min -986.60
max 65,069.50
(Why is S/N getting worse? Why is min a neg number? Why median decreasing?)
Flat Only:
mean 2,181.57
STD 589.93
S/N 3.70
median 2,167.16
int mode 2,166
min 1,784.35
max 67,345.16
Dark + Flat:
mean 1,809.41
STD 577.36
S/N 3.13
median 1,796.61
int mode 1,792
min -1,009.29
max 66,947.62
It looks like calibration is only making things worse! Is that a correct interpretation?
I've taken 20 flats with a light box (and bias subtracted):
mean 33,300.01
STD 420.41
S/N 79.21
median 33,343.47
int mode 33,288
min 9,102.81
max 51,980.00
40 bias frames and 20 darks. I probably should take double the darks and perhaps triple the flats since I'm not dithering (yet,,,two scopes on one mount is a challenge!).
Much appreciated if someone here can shed light on this data.
Thanks,
Peter
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