The basic technique, when you cannot rely on autoguiding, is to take short exposures.
While “classical” astrophotography, since the time when film was used instead of CCD/CMOS sensors, is based on acquiring a relatively small number of long exposures of the same subject (usually just one picture when film was used), relying on some form of active guiding to track without errors the stars for a long time, “simplified” astrophotography is based on acquiring many short exposures with just passive tracking.
What do “many” and “short” mean?
Well, in the context of this blog, I will always mainly refer to my own experience, so my answer is “usually from 50 to 150 exposures of 15s to 45s each”, but there is also a new trend, called “lucky imaging” and inspired to planetary astrophotography, that promotes the use of many hundreds, or even thousands of exposures ranging from less than one second to a few seconds.
This newer technique essentially requires a lot of aperture and a very sensitive monochromatic camera. While the latter is in my plans for the future, the former is outside of my logistical capabilities.
What is the relationship or the difference between the classical approach and the one based on short exposures?
Well this is definitely an open and debated issue, but at least some theoretical framework does exist.
Generally speaking, nobody will tell you that acquiring 120 exposures of 30 seconds each is the same thing as acquiring a single 60 minutes exposure.
The debate is on how big the difference between the two things is.
The stalwarts of the long exposure approach usually belittle, not to say besmirch, the idea that one can do serious astrophotography with exposures ranging in the tens of seconds.
For instance Allan Hall in “Getting Started: Long Exposure Astrophotography” says:
One issue I face when explaining short exposure versus short exposure is that you can get the same amount of total exposure time by taking one hundred 1 minute images as if you take ten 10 minute images, so why should I do long exposures?
A Salvation Army bell ringer stands outside a store for five hours and collects $600 in donations. If you do the math this works out to an average of just over $0.03 per second. If that same bell ringer set up for only one second, do you think he would get $0.03? No? It’s the same idea here. You have to be there long enough to collect some photons to stack in the first place, this is where long exposure AP [Astro Photography] comes in.
The analogy with the bell ringer is quite suggestive, but may be misleading: if bell ringers where there to collect dollars falling down from the sky with a constant uniform flux, then indeed the bell ringer would collect $0.03 per second, or rather a dollar every about 33 seconds, in a quantized view.
Indeed it can be shown that if a camera had zero read noise and the stacking algorithms were not weighing down or rejecting “rarely lit” pixels, there would be absolute equivalence between very long and very short exposure, provided that the total integration time be equal.
Key take aways
When you are imaging under light polluted skies, you should not worry about your exposure times. It will make very little to no difference if you are using subexposures of 30sec. or 3minutes. It’s the total integration time that matters.
If you are imaging under dark skies, you will benefit from longer exposures in terms of SNR. However, it’s not worth it to go to extremes and keep in mind the cost of throwing out a subexposure due to poor tracking for instance.
Hence, since I am imaging under definitely light polluted sky, I must only worry about total integration time, albeit for practical reasons (disk space, processing time) trying to keep the number of exposures as low as possible is sensible.
In practice, then, my acquisition technique is that of using a software like Astro Photography Tool (APT) for the DSLR, or the vintage Meade Envisage for the equally vintage Meade DSI Pro, to shoot and store on disk a few tens to a few hundreds exposures ranging in the 15-45s range when using telescopes and slightly longer (up to 120s) exposures when using camera lenses in the 50-135mm range.
Since I have begun to use ZWO cameras Nebulosity has become my main acquisition program, sometimes, in the bin 2×2 mode, even for my Canon DSLRs.
After having initially neglected calibration frames, I now regularly take dark, flat (mainly) and sometimes also bias frames: the quality of my final images has taken advantage of that in a manifest way.
I will then process these pics with a dedicated software like Deep Sky Stacker (DSS), Maxim DL or, more recently, Astro Pixel Processor (APP) and post-process (or “image process” … it is very much a matter of choice of words) the output of the stacking process with StarTools.
I will deal in dedicated articles with most of these software programs and also with hardware items (i.e. cameras, mounts and telescopes).