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calibration frames summary and how many
i'm just starting in astrophotography and have been using APP which is a great program and easy to get started with. I've only been doing darks but will start doing flats and bias for the next session. I've read through the 7 page thread about calibration frames. Here's my take away from that, please correct me if i've got anything backwards.
I'm using a canon 6d, full frame sensor, unmodified, based on that
darks and lights: same iso, same temp, and exposure; i shoot the darks after the lights are done. How many darks should i take?
flats: my main takeaway from the APP discussion is make sure the exposure time is at least 0.5-1.0 secs, iso doesn't need to match darks/lights iso, same focus as darks/lights. How many darks should i take?
bias: iso: match flat frames iso value; fastest shutter speed and view finder and lens cap on; using this to correct the flats. if i match the bias frames to the flats then that should take care of any vignettes and i don't need to shoot flat darks.
do i have this right?
Regarding the number of darks: that is a point of a lot of discussion. Some people claim you need over one 100 though I have read reports where only 20 were deemed sufficient. A general rule of thumb seems to be between 50 and 100 though it depends on the camera used. For a DSLR I'd say between 30 and 60 should be enough, mostly because a DSLR sensor is not cooled and even putting it in the fridge (as some people do) doesn't prevent the sensor from warming up while taking darks.
Regarding your second question: I suppose you meant to ask how many lights you need to take? For DSLRs I'd recommend taking at least 1.0 sec flats to avoid the mirror flip to cast a shadow on the sensor. In general about the same number of flats are recommended as the number of lights though more than some 50 or 60 should not really make a difference.
Regarding bias frames: for both dark and flat calibration you'll need them so it's best to match the iso of the darks and that means that the flats need to have the same iso as well or that you need to shoot two sets of bias. Here the more really is better so you could go as high as several hundreds of bias frames. However, at least some 50 to 60 should suffice in most cases.
Regarding dark flats: depending on the exposure time of the flats you may want to shoot those as well though if you manage to keep the flat exposure time in the 1 second range then you can do without them. If is possible to use the darks for the lights with the flats as well and in that case you do need bias frames for the darks. However, if you take darks that match the lights and dark flats that match the flats then you don't need bias at all. In short: if you want to use the darks for the lights and flats then you need bias and then the iso of all frames needs to be the same. If you want to shoot dark flats then the iso of the flats doesn't need to be the same as the lights but the iso of the dark flats and the flats needs to be the same and the iso of the darks and the lights as well.
All this stacking business is of course for the purpose of reducing noise, increasing SNR. This improvement is increased by root(2) for each doubling of your stack, so each doubling of SNR is achieved with each quadrupling of your stack. You can judge for yourself whether its worth it:
its easy enough to take 64 biases or 64 flats but you probably don't want to take the time to take 64 darks if you have 10 minute exposures.
Here again is a trade off, doubling the exposure doubles your SNR whereas you would have to take 4x the exposures for the same SNR improvement.
So I try to lengthen my light exposure and reduce the (light and dark) count - by the way, there is a club out there that does all AP using 1 exposure. 🙂
When I started out, I bought Nebulosity by Craig Stark, the inventor of PHD. The help file provided by them is an excellent learning tool. I've attached a copy here if you want to give it a read.
@astrogee Thanks for the great addition! Note that an SNR improvement in the darks may not result in a similar SNR improvement when calibrating the lights with those darks. I mean: taking 16 or 64 darks may not result in a 4x better SNR in the calibrated lights. So the trade off not only depends on the amount of time it takes to take so many darks and other calibration frames but also on the effect on the final stack. It's a complicated topic that doesn't really have a well established answer that works for everyone.