Re: Aperture vs Focal Ratio to determine exposure time
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dan kowall
It is important to remember that the number of photons collected from an object is solely dependent upon the aperture of the telescope.
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If you are going to 'compare' images of an object made with equal aperture telescopes, one with a lower f-ratio and one with a higher f-ratio then you should compare sky-limited subexposures. These two groups of subexposures will obviously have different times to become sky-limited but both will have buried the readout and photon noise and both will have collected equal numbers of object photons. dan kowall --------------------------------------------
On Sat, 5/12/18, 'Ray Gralak (Groups)' groups3@gralak.com [ap-gto] <ap-gto@yahoogroups.com> wrote:
.... I think Stan Moore states that you can down-sample such an image to 1/4 the size and get a similar S/N. Well, if you do the F10's image will certainly not be better because the light is spread over more pixels which results in more collective readout and photon noise. And, the image from the 10" f/5 has collected light from 4x the area of the sky, so it obviously collected a LOT more light even though the aperture is the same between the two scopes. So, for imaging *extended* objects (e.g., h-alpha nebula regions), I think f-ratio matters. No matter which sensor you use, I think that more light is collected *per*pixel with a faster f-ratio no matter what the aperture. I think that this results in less exposure time to get a usable image. Best regards, -Ray Gralak Author of APCC (Astro-Physics Command Center): http://www.astro-physics.com/index.htm?products/accessories/software/apcc/apcc Author of PEMPro V3: http://www.ccdware.com Author of Astro-Physics V2 ASCOM Driver: http://www.gralak.com/apdriver Author of PulseGuide: http://www.pulseguide.com Author of Sigma: http://www.gralak.com/sigma Posted by: "Ray Gralak \(Groups\)" <groups3@gralak.com> Reply via web post • Reply to sender • Reply to group • Start a New Topic • Messages in this topic (9) To UNSUBSCRIBE, or for general information on the ap-gto list see http://groups.yahoo.com/group/ap-gto Visit Your Group New Members 3 • Privacy • Unsubscribe • Terms of Use
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