mgoodfel wrote:"twinkling" spread over time
mgoodfel wrote:Why are stars different sizes in astronomical photos? I assume that at interstellar distances, they are all point sources. Yet in a typical APOD picture, you'll see widely varying sizes. I thought perhaps it was due to atmospheric effects spreading the stars over a larger area ("twinkling" spread over time), but you see the same effect in Hubble pictures. So I guess I just don't understand.
apodman wrote:geckzilla - If you zoom in on a star, the spaces between the concentric diffraction rings (actually not spaces but just dimmer than the rings and which may brighten in a time exposure) may contain many pixels. But as you zoom out, the distances between the rings may become equal to or smaller than one pixel (I haven't actually done the math on that, so I'm going out on a limb) in which case the rings are all combined into one blur.
bystander - I suppose in a perfect CCD there would be no spillage from one pixel to the next, and you could have a fully- or over-saturated white pixel surrounded by black pixels that would not brighten regardless of additional exposure time. I also suppose that in a real CCD there is some spillage among pixels both in the physical acquisition of the photons and in the conversion of impacting photons into a voltage, but I'm not the resident CCD expert so I can say no more regarding how much spillage there actually is.
mgoodfel wrote:So I think you are saying this: No, a star isn't "really" a point source, but since these telescopes aren't resolving the disk of the star, that's moot. The angle cut by the star is less than a pixel.
What is happening is that both bright and dim stars are being spread by this Airy Disk effect. Both the dim and bright stars spread over the same area, but since there's a minimum value that the camera (film or digital) can record, that chops off the base of the curve for the dim star, leaving a smaller image. And since there's a max value as well, the bright star is rendered as a solid bright circle, not a graded disk fading towards the edges.
mgoodfel wrote:I was trying to simulate a star field on my computer, and of course the first thing I did is sprinkle random dots. That immediately brought me up short, since it looks nothing like an APOD picture. I'll try it again with these cropped Gaussian shapes and see if that looks more realistic.
geckzilla wrote:Google isn't helping me this morning.
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