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Old 28-11-2016, 06:13 PM
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Shiraz (Ray)
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Shiraz is offline
Join Date: Apr 2010
Location: ardrossan south australia
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Originally Posted by codemonkey View Post
There's some argument for sampling at 3x the possible resolution of the system if you don't want to leave any detail on the table, but like everything it's a trade-off.

Additionally, the diffraction limit isn't really a hard limit and changes depending on which formula you use. Dawes for instance gives you 1.14" for a 4" scope.

Jon Rista (posts on CloudyNights) uses the following formula to estimate FWHM:

FWHM = SQRT(Seeing^2 + Dawes^2 + ImageScale^2 + GuideRMS^2)

I'm going to make some (reasonable) assumptions and say we have typical seeing of 2", and a guiding RMS of 0.4", which means for your current (native FL) system, we get:

FWHM = SQRT(2^2 + 1.14^2 + 1.07^2 + 0.4^2)
FWHM = 2.56992218"

Keeping everything the same except switching to an (unobstructed) 6" aperture you would improve your resolution as follows:

FWHM = SQRT(2^2 + 0.76^2 + 1.07^2 + 0.4^2)
FWHM = 2.42538657"

And for the sake of illustration, were you to continue to use your 4" scope but go for pixels half the current size, you'd be looking at:

FWHM = SQRT(2^2 + 1.14^2 + 0.535^2 + 0.4^2)
FWHM = 2.39704506"

As you can see, the diffraction limit is not a hard wall, and you actually get (admittedly marginal) increase in resolution by going for even smaller pixels.

My 2c is that a 4" is fine for you, especially given that you primarily shoot nebulae.
maybe be a little wary of that formula Lee. It is a kludge that is good for getting an understanding of the underlying concepts, but the incorporation of a non-continuous sampling function (image scale), for which "FWHM" is almost meaningless and for which the addition of variances is not appropriate, means that it fails when you push too hard.

Last edited by Shiraz; 28-11-2016 at 09:58 PM.
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