[QUOTE=Shiraz;1032317]
thanks Greg.
Under what conditions do you see the grid structure? - my flats and bias images are very smooth at f4. I have seen fixed noise structure in narrow band images, but that is due to thermal noise and cannot be handled with flats (flats can deal with variability of pixel sensitivity, but most cameras seem to have negligible pixel gain variation anyway).
This is a crop of about 1/8th of my SX694 bias image. You can see the grid pattern of the pixels. F ratio would be irrelevant here as this is a bias not a light.
It showed up clearly in 1x1 downloads in CCDsoft for some reason. I also saw some very short exposures that showed banding. I don't see any of that in darks and flats or lights though.
http://upload.pbase.com/image/153330567
Here is a 10 minute dark at -20C.
http://www.pbase.com/gregbradley/image/153330583
If you zoom in (click on original at the bottom of the page) you can see the hot pixels. Also what appears to be amp glow. So not subtracting darks with this camera could produce minor gradients in dim images in those areas.
Also it would leave a considerable number of hot pixels. The number of hot pixels is less than my Proline 16803 but not greatly so.
If I try to use flats on my CDK 17 using the 16803 without accurate dark subtraction also it winds up a mess.
Dithering no doubt reduces a lot of thos hot pixels. I note that The Sky X now provides dithering in the camera control. Its probably time for me to ditch CCDsoft and start using The Sky X for camera control. The limit of only 5 filters in the colour series part of CCDsoft plus you can't change the naming of the saved filter so if you want to do a different sequence than LRGB (because otherwise blue is too often taken low in the sky where it performs the worst out of the colour filters) you can't.
Greg.