Camera came to me already modded so not sure Glen. Which lines are unusual? Verticle or Horizontal? In the image I took of B33 it was only the horizontal lines (the initial image I posted was rotated) I noticed and bothered me. This was with the battery eliminator. Maybe the Bias frames are giving me some false confidence that using the regular battery will solve this problem?
Okay, the lines do look a bit extreme compared to Bias of my unmodded 600D but then again it is much less used and a more recent model. Can still see some vertical banding in the 600D Bias but nowhere near as obvious as the 40D.
You should expect the 40D bias frames to show significantly more read noise than the 600D. CMOS sensor technology has come a long way.
You may need to modify your acquisition techniques to manage the higher noise levels.
If the battery adapter is noisy, grounding the camera chassis (which is connected to sensor ground) through the tripod mounting hole, to 12V supply ground, may fix interference. Worth a try if you think the adapter might be a problem.
Switching adapters might cause horizontal lines in your images.
Thanks Rowland. It's good to know what might be expected from a 40D Bias frame. After googling some images it seems the regular battery Bias I gathered isn't too far off the norm for this camera, although advice seems to take more than the 50 which is what these master biases are made up from.
Any suggestions on how to modify acquisition techniques? The Horsehead image was 11x300s light frames at ISO800 with 30s pauses between frames and dithered through Backyard Eos. Dark frames were taken with the same ISO, at roughly the same temperature (+/- 2 or 3 degrees), with the same exposure length. Flats were taken at the end of the session (40 all up also at ISO800 with peaks in the histogram 1/3 of the way across) and Biases followed this (again 40 with the quickest shutter speed at ISO800). These were then processed in Nebulosity 4.
Not sure quite how to go about grounding everything. I'm no good with electric circuits, but am thinking an insulated wire attached to the mounting hole and the negative wire or terminal of...?
Hi Ben. Sounds like you have it almost mastered. Flats should be approx half well or 50 - 60% histogram. I wasn't sure, but I did think your flat was a little underexposed. The bias was beginning to show through. Flats can be more problematic than darks on occasions.
You may find the first few subs are cooler than subsequent frames as the sensor temperature increases. Main thing is to use the same acquisition technique for darks and lights. That way you are better assured of temperature consistency.
"Ideally" - the more bias the better. Some stacking programmes create a superbias, which is similar to taking hundreds of bias frames. 50 sounds OK.
Dark scaling is an option in some software. Basically, take longer darks and let the software scale the dark noise to the lights. In this case the bias is subtracted from the dark and light frames.
Otherwise, DSLR RAW data is safely processed to better handle temperature and linearity differences as follows;
Calibrated light = (light sub - master dark)/ master flat
In this case, only the flats are bias subtracted.
Fix a wire to the tripod hole by some means. You could use a tripod plate or a 1/4" bolt and washer. Then connect the other end to the -ve line/terminal of your battery. The idea is to ground any switching interference to a linear source. If you think it's a problem. I'm not sure it's that much of an issue - switching signal will manifest in the same way as other noise sources.
Thanks again Rowland. Will definitely up the exposure on the flats and may as well try grounding the battery (I'm assuming you mean the big 12v battery that powers the battery eliminator). Otherwise I'll just go back to in-camera batteries and hopefully that will solve the issue as the Bias frames seem to suggest. Many thanks everyone.
Yet to work out the battery but did work out fitswork to remove lines (previously applied this to a .tiff file rather than the .fits file) and started to remove some of the lens flare in GIMP (although far from perfect). Managed to keep the colors I prefer as well. As always any further advice welcome :-)
