Old 18-10-2009, 04:35 PM
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Arc seconds per pixel.

I was wondering if someone could shed a bit of light on the "arc seconds per pixel" story. What is bad, what is good, what is better, how to calculate?
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Old 18-10-2009, 04:39 PM
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This is a good read
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Old 18-10-2009, 09:08 PM
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How good are your skies? Seeing will greatly impact the arcsec/pixel formula. Oversampling has its merits, but only under the right sky conditions.
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Old 21-10-2009, 10:09 PM
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My images are moderately under sampled, that being said, Im happy with the images... Under sampling can cause blocky looking stars when one star covers less than one pixel.. the star will appear square and unnatural.. My seeing here rarely gets better than 3 FWHM, my current resolution is 3.35" per pixel. I would think then that with the average of 3 FWHM seeing, having a better resolution would likely not make too much difference to the quality of my images.. If however I regularly experienced 1FWHM seeing, I would want a camera with smaller pixels that would result in roughly 1" per pixel.

If I was setting up what I'd consider a perfect imaging system, I would aim for between 0.7~0.9 arc sec per pixel.. Some would call that grossly oversampled for most conditions.. but I say, better to have it and not need it than to need it and not have it...

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Old 22-10-2009, 07:01 AM
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As far as mechanically calculating it for your scope/camera combination, Rod Wodaski's CCDcalc does the job:


As Jase says though, I guess that seeing conditions will render the result to be variable.
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Old 22-10-2009, 09:15 AM
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I've been puzzled by this recently - how exactly do you measure your seeing? Is it just by eye or is there a technique that establishes whether you have 2 or 3 arcsecond seeing?

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Old 22-10-2009, 06:02 PM
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I suspect you measure the FWHM width of the star - the size of the area that a single star covers on your sensor, to 50% of maximum brightness.

IE, if you have:


The FWHM width would be 3.

000001393100000 FWHM would be 1 (or 1.7 or something)

You then multiple that number of pixels by the width in arc seconds, and you get your effective resolution limit in arc seconds. Divide that number by 2, and you get your maximum effective sampling resolution.

Quite a few apps out there will calculate the FWHM size, as it is also used as a metric to establish how good your focus is.
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