EOS Bodies - For Stills / Re: Full Frame and Bigger Pixels vs. APS-C and Smaller Pixels - The Reach War« on: August 14, 2014, 12:31:09 PM »
Astrophotography seems very complicated - making the results that jrista and others share all the more impressive.In case people don't understand "seeing limited", this refers to atmospheric interference when viewing celestial objects like the moon. Twinkle twinkle little star - the star is a point light source to us and should be a non-twinkling dot, but air turbulence, temperature differential, humidity in air all affect (refract) the image of that dot. That's why astrophotographers stack ("average") large numbers of identical moon or planet photos.
Stacking is for reducing noise. The " lucky imaging technique" where you select the best frames from many (usually from video) is for getting around the seeing limit. You stack just those best frames to reduce noise on already high resolution images.
It's not an insignificant difference. In some cases, you can improve from a seeing limited image at 2 arc seconds to a diffraction limited one at better than half an arc second.
It's pretty complicated, for sure. It should be noted seeing limited spot size is rarely as good as 2" (arcseconds). Because of seeing in Colorado, it's usually somewhere between 3.8-4.1", even worse at times. In most of the US, seeing-limited spot size is usually a little over 3"...if you have a 2" seeing limited spot size, your seeing is pretty good. If you have seeing limited spots that are smaller than 1.5", your seeing is excellent. Assuming you use drizzling to increase the resolution beyond that, if you used full 3x drizzling, resolution could improve to 0.5". With seeing of 3.8", 3x drizzling might be able to improve resolution to 1.3" (you really need a LOT of frames to achieve that, though).
Also, getting diffraction-limited images at less than half an arcsecond is also rare. The longer the scope, the greater the magnification...very long scopes (when you add a 2x or 3x barlow, for example) are diffraction limited, but they magnify the spot so much that you can actually see the "waves", or the rings of the airy disk around the central star's peak (also only possible with truly excellent seeing...less than that, and the star jumps around and gets warped, so you still can't get a diffraction limited spot.) The central peak of a star might be 0.5", but the whole airy disk is still there an visible, so the actual diffraction-limited resolution is less than 0.5". With most telescopes, a diffraction limited star is larger than 0.5". With some of the best telescopes on earth, such as a Planewave or an RCOS, you might get on-axis diffraction limited spot at ~0.4", and off-axis (corner) diffraction limited spot at ~0.6". Those telescopes cost tens of thousands of dollars.
With a lens like mine, or your average astrograph refractors or RCs, your on-axis diffraction limited spot is usually going to be quite a bit larger than 0.5" in the absolute best of conditions. Any off-axis spots are going to suffer from some kind of aberration...astigmatism, coma, field curvature, etc. Corner spot size in many scopes can be quite large, and often looks like little comets or out of focus blur.