Rick J
(Postmaster)
12/02/12 02:17 AM
Re: Spot size, resolution and image scale?

Barry,
In normal seeing you'd be right. Pixel size should be such that typical seeing is 2.5 to 3 times that of the pixel's resolution. Your pixel is 1.1" which works well in seeing of 2.5" or worse. That's my situation. I can image with a 0.5" pixel (similar to your 2x pixel) but only one or two nights a year does that help and even then it is minor except for two nights in 7 years when it did help as I'd expect.

When you halve the pixel it gets one forth the photons so you need 4x the exposure to get sky limited subs. That's 40 minutes for me in my skies rather than 10 for 1" pixels. That means the mount has to track with twice the accuracy for 4 times as long. If that hurdle is cleared you then have shot noise. While a single 40 minute 0.5" pixel sub has about the same total noise as 4 ten minute ones at 1" per pixel it has twice the resolution of shot noise (but likely far less than twice the resolution of the object - often the same in fact). This makes shot noise more objectionable unless you reduce the image back to 1" where it would be the same. So you would want more total exposure time to reduce the shot noise to a less objectionable level. How much more depends on final image resolution.

Drizzle is useful when taking wide angle shots with a short focal length scope such that the seeing is better than the system can resolve. Often the case with say a 500mm scope with a STL-11000 camera with a 3.7" resolution in 2.5" skies. With a reasonably sampled image for DSO work it doesn't add anything more than simple dithering would if done in non unitary moves. So if the dither is 1.8 pixels rather than 2, dither does the job of drizzle in deep sky imaging as well as reducing noise when done with a sigma type stack.

For planetary work the Barlow is very useful. I like to run at f/25 (0.2" pixel) with my 14" when doing planetary imaging as it gives the software a lot more to work with but then I'm freezing seeing and using only the best of thousands of frames. A very different situation than DSO imaging.

Rick



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