Re: Aperture vs Focal Ratio to determine exposure time


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.
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 &#92;(Groups&#92;)"
<groups3@gralak.com>




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