Andy Resnick said:
Edit- I forgot to mention your comment "And there's almost no sane reason why would ever reduce your aperture.", because for me, there are at least 2 good reasons. First, (slightly) stopping down the aperture makes the images less susceptible to poor seeing conditions. Second, (slightly) stopping down the lens improves the images by decreasing aberrations.
Drakkith said:
My understanding was that this would only improve visual quality since you can take advantage of those few seconds where the turbulence along the incoming cone of light is minimal. Thoughts?
Andy Resnick said:
Not exactly- the amount of image degradation caused by clear air turbulence is related to the aperture diameter. I have a copy of an excellent dissertation discussing/measuring this at my office, but I am currently 'snowed in' so I can't get you the reference right now... IIRC, the aperture diameter sets a cutoff to the relevant length scales of the turbulence- smaller diameter, smaller cutoff.
In deep sky astrophotography, where you're taking sub-frame images of at at least tens of seconds, but more typically several minutes, stopping down the aperture won't help more than they hurt (other physical abberations aside, i.e., chromatic, spherical, coma and astigmatism -- stopping down the aperture might help for those). The seeing changes are just way too fast.
Ignoring the other aberrations, you'll get more bang for the buck by increasing the aperture to raise the signal further above the noise floor.
A smaller aperture might have a slightly better effect on your guiding though, maybe, where exposures are only a couple of seconds. But I'm talking about the primary imaging here.
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Planetary. Let's talk planetary now.
For visual use, when viewing Jupiter, Saturn, Mars, Venus, the Moon or the Sun, they're pretty bright through a telescope. You can afford to stop down the aperture for
visual use. As
@Andy Resnick points out, the planet might seem a bit more stable in the eyepiece during bad seeing conditions if you stop down the aperture. And since they're bright enough to begin with, a slightly dimmer view can be acceptable.
But for goodness sake, don't stop down the aperture when doing planetary astrophotography. Yes, in planetary astrophotography, seeing is king for any given setup. Meaning the seeing conditions trump scattered clouds, haze, bad transparency, and light pollution. If you find yourself in bad seeing conditions, wait until the seeing is better. Try to do you imaging when the target is near the meridian where there's less atmosphere between your telescope and the target. But stopping down the aperture is conceding to failure.
Now assuming the seeing conditions are average or better, the benefits of a larger aperture are twofold.
- Using lucky imaging techniques, the goal is to take as many short of exposures as possible within the limited timeframe (you can't let the target rotate for too long), and as many exposures as possible, with exposure times on the order of 10 ms or so, to produce individual frames just enough above the noise floor where your lucky imaging processing software (such as AutoStakkert!) can just make out planetary features. The individual images will be noisy -- very noisy, but just barely above the noise floor where planetary features are visible. Because of seeing, not all of these individual images will be good. But that's OK, because you throw out the bad frames (roughly half of them). The high noise (compared to deep sky sub-frames) is acceptable, because you're taking so many images that you can let the Central Limit Theorem come to the rescue. Having a larger aperture means you can reduce the exposure time, all else being the same, and increase the number of frames. Thousands of frames. Maybe tens of thousands of frames, if your camera is fast enough. Or another way of looking at it, having a larger aperture means surface details will be above the noise floor (for the same, short exposure time) in the individual frames when they would otherwise fall below the read noise.
- Properly using lucky imaging techniques, and if the seeing conditions are pretty good to begin with, you can get images that are pretty close to your telescope's diffraction limit. Striving for anything less is akin to resigning to failure, in my opinion. And your telescope's diffraction limit is a function of the telescope's aperture.
I have a 10" scope that I love. But love alone can't make this scope compete with a quality 14" scope, particularly for planetary imaging, no matter how dear it is to me.