Combining Multiple Imaging-Runs

[m37]

Hey all, hope you are all getting/enjoying relatively clear skies.

I've decided to have a go at some longer imaging runs and I'm currently shooting some wide field galaxy/nebula shots with lots of pretty stars around them.

I've only ever done short runs before so I have a couple of questions:

I use an unmodded, uncooled DSLR (Canon EOS 1000D) so I have no control over chip temperature. I thought it would be a good idea to spend a whole night imaging and getting Backyard EOS to record the temperature in the file name. This way I can shoot a load of different darks and build up a library catalogued by time/temperature which I can then match up to the relevant exposure time/temperature for the light frames.

My question really is how do I go about processing and combining these? From each session I will have a bunch of lights all at the same ISO/exposure time but different temperatures, a series of darks at the same exposure and ISO but different temperatures, stock bias frames and flats specific to each session.

Would I take all the lights from a session, process them with all the matching dark frames I have accumulated, plus bias, plus flats for that session? If the lights are all at different temperatures, how do I tackle this? Do I need to match each light with a dark? Confusing.

If I then end up with a load of different but calibrated  images of the same target, is there an optimum way to further combine these to reduce noise?

Hope that made sense, cheers

Chris

[IanL]

Taking the last point first, you need to stack the lights using some kind of weighting. This will usually be based on the amount of noise in the image; you can weight by other things like exposure length but since noise is a direct indicator of the quality of the image and can be evaluated directly from the image without reference to any external factors it us most often used.

In PixInsight for example, noise analysis is undertaken during image calibration (bias/darks/flats) and recorded for each image (in the FITS header).  Then there is a "weight" setting in the ImageIntegration (stacking) process that will weight each sub based on the results noise evaluation, with noisier subs receiving a lower weight and therefore contributing less to the final image.

Most stacking software will have an equivalent concept for weighting of subs (either based on noise or exposure length).

The rest of the process is relatively straightforward; you'd just calibrate the images with master bias, dark and flat prior to stacking exactly as you do now.

The only gotcha is that matching darks to lights with non-temperature controlled DSLRs is very difficult. Start with darks that are the same ISO and exposure length as the lights, then organise them in to batches that are within 3C of each other (say all the 7C, 8C and 9C darks) and stack that in to a master dark which you'd then use to calibrate all the lights that are 7C, 8C or 9C.  Then a master dark of 10C, 11C and 12C to make a master for the 10C, 11C and 12C lights, etc.

The reason I suggest batching like this is that you really want 15 to 30 darks per master to avoid adding too much noise to the lights.  Making 30 darks per 1C is going to take a long time (and a fridge), whereas making 30 in a 3C batch is more doable.  If time is short, go for say 15 darks and maybe a slightly wider temperature range per batch.  Personally I find that darks are worth the effort required, but other people will tell you that they're not worth the effort with a DSLR - the only way you're going to find out is to try for yourself and see what results you get.

[Russe]

in DSS - all the way at the bottom is a group tab - you put your subs/lights into groups according to when they were taken - add darks in every group (easy enough, if you have a darks library - stick your camera in your freezer wrapped in a towel and take 100 images at your preferred exposure length - put them into folders in accordance to their temperature) - add bias in every group (easy enough - make a library) - and add flats (more difficult - you gotta take them every time you go imaging).

Here's Doug German's youtube video: http://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&cad=rja&ved=0CDIQtwIwAQ&url=http%3A%2F%2Fwww.youtube.com%2Fwatch%3Fv%3DcmDy80d2krk&ei=vfMVU_n0B-_e7Aao1oGIDg&usg=AFQjCNFghdEZfO8E95NvMjyBtfhG_aeJcg&bvm=bv.62286460,d.ZGU

Now more and more people are on about dark flats - I don't know how to make them, so can't help you on them.