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As a terrestrial photographer for some 30 years prior to taking on astrophotography, I had great difficulty in reconciling the fact that there a different set of rules governing the imaging of stars. For extended objects such as nebulae or landscape, the traditional rules apply, but for stars it is different.
I haven’t attempted the math, but I believe that even the closest star would in reality be like a point source, occupying less than a pixel on any of your imaging systems. What we observe is the spread of the light caused by dispersion in the cosmic dust, the atmosphere, optical components, and our eyes. Stars with more brightness appear bigger through our imaging systems because of the wider dispersion of their light, not by their physical size.
This all becomes combined and confused with the other phenomena we observe such as nebulousity, and glow caused by scattered light reflected on cosmic dust.(extended objects) Gathering light caused by these effects obeys the same laws as for terrestrial photography. Ie, a lower F number for a given field of view, means more light gathered per unit of time.
I think this clicked for me when I was (as I usually am) viewing viewing in polluted skies. A combination of high thin cloud and usual smog. I compared views of the owl cluster in my 60mm Tak I was utilising as a finder scope and in the 18” Dob. Through the sky mush, with the Tak, and same FOV, I could barely make out the two brighter stars in the cluster, but with the extra aperture of the Dob, I could make out quite a few of stars. The conclusion being that the aperture of the Dob increased my star to mush brightness ratio considerably when compared to the smaller Tak. The background mush obeyed standard photographic laws with which most terrestrial photographers are familiar, but the star brightness changed as a result of larger aperture.
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