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Old 18-01-2018, 11:49 AM
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codemonkey (Lee)
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Join Date: Jul 2013
Location: Kilcoy, QLD
Posts: 1,765
Hey Bart,

Ray (Shiraz on here) once proposed the following formula to approximate the final FWHM:

SQRT(seeingFWHM^2 + scopeFWHM^2 + (2.35*guidingRMS)^2)

Playing around with these numbers should give you an idea of the impact of guiding on your final images. Seeing and guiding aren't really independent though but that should give a reasonable starting point.

One key thing is that RA and DEC RMS are similar. If they're different you'll get increasingly elongated stars / "motion blurred" images.

If RA and DEC are close in terms of RMS, as would be expected by a good mount, you will at some point become seeing limited (i.e. given enough focal length + aperture and small enough pixels). This now becomes a question of sampling... you don't want your sampling rate (arc-second per pixel) to be so high that you're consistently throwing away SNR chasing details you can't get due to your seeing.

The general rule is to view guide logs / charts in arc-seconds. Consider that your guide camera probably has different sized pixels than your imaging camera. This means that a 0.5px RMS guide error is meaningless without further information, because what you really care about is the magnitude of error relative to your imaging camera. If you set the guide software to show in arc-seconds then you can more easily interpret the results relative to your imaging camera, and hold more meaningful discussions with people since you're comparing apples with apples.

Cheers,
Lee
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