The Debate over Short Exposures
There is a raging debate over short exposures vs long. The decision to use one over the other is multi-faceted. Here are some reasons to consider short exposures over longer ones:
1. Your mount isn’t up to the task.
2. You live in a zone where aircraft frequently buzz by.
3. You have clear skies but with intermittent clouds.
4. You are a EAA practitioner.
If none of these apply to you then you should open yourself up to the benefits of longer exposures. I’d like to present two examples. The first one has both signal and noise so it easily applies to imaging:
1. Imagine it is the dead of night and the world is asleep. You wake up suddenly and whisper to your partner: “Did you lock the front door?” Their reply is “Yes, now go back to sleep.” This conversation was possible due to a low-noise environment. Now consider a high-noise environment like Niagara Falls and try to whisper. Nope, doesn’t work. You need to raise your volume. Some people might interpret this as the reason why you need longer exposures with CCDs. This is true but please understand that CMOS and CCD both benefit from longer exposures. Imagine that you are back at Niagara Falls and your partner finally finds a voice level that you can just make out, barely. Doesn’t it seem reasonable to ask for louder voice levels just to be absolutely, positively clear what they said?
This next example is an analogy. Take it for what it is worth but I think it does a good job of explaining why increasing exposure is beneficial even when it means that you are recording high levels of sky glow:
2. Imagine that you are at an automobile drag strip. One car can travel at a maximum speed of 100 feet per second and the other at 110 fps. Assume that when the light turns green that they can immediately accelerate to their maximum speed. After 1 second they have traveled 100 ft and 110 ft, respectively. Only 10 ft separate the two. After 2 seconds they traveled 200 ft and 220 ft. Now 20 ft separate the two. You can see that the distance separating the two vehicles steadily increases as time goes by. This is the same with imaging. After 1 second you have only 10 units of brightness separating galaxy from sky glow. But after 2 seconds you have 20 units of brightness separating them. And so on.
Like I said not everyone can run longer exposures for the reasons that I listed above but there is one other consideration to be mindful of. Foreground stars are the brightest objects in your field of view. The longer you keep your shutter open the more the risk of saturating the brightest stars. The saturation level is very much dependent on your camera. CCDs are very good at this because they generally have much deeper ‘wells’ than CMOS and can hold more electrons (i.e. photons). But this is all a matter of taste. Personally I don’t like saturated stars so I try to crop them out when I can.
Finally, what about stacking? Stacking helps increase signal-to-noise in both CMOS and CCD, however keep in mind this inequality: 100x 1-second exposures does not equal 1x 100-second exposure. You will always get better results in less total time when increasing exposure. You don’t have to go crazy. Just try doubling and then go from there.
The fallacy of the perfect exposure:
Your choice of exposure is akin to a multi-lane highway — both have boundaries. Highways have shoulders — veering one way sends you into a gully, and the other way into oncoming traffic. Likewise with imaging, too short of an exposure records nothing of interest, and too long saturates stars and potentially damages the frame by passing aircraft and clouds. What lane you travel in depends on your skill level and risk tolerance.