[QUOTE=skysurfer;
When a lower f/ratio were better than a high one, regardless of the absolute aperture, as some claim,
[/QUOTE]
nobody is claiming that - aperture is fundamental, but it is not the whole picture
Quote:
Originally Posted by multiweb
Like a fisheye lens exposed to all the sky yeah but I just can't visualise how rays that are not near parallel to a closed tube optical axis will hit the mirror at the end of it, with baffles as well, etc... The few that hit the primary sideways would they even bounce to the secondary or miss it altogether?  |
agreed, the angles are typically not large for a telescope (generally within about a degree of parallel to the axis). The point is though that they are definitely not parallel from different parts of the sky and the same mechanism as in JAs widefield example applies. Each point in an image is illuminated from an individual part of the sky and the ray bundle that forms it is not parallel to the ray bundles that form all other points in the image. Telescopes and fisheye lenses form images in essentially the same way - just over vastly different field angles.
The amount of light that gets into a pixel is determined by two things:
how wide each ray bundle is before it is focused by the optics (determined by the aperture) and
how much sky is included in each ray bundle (determined by the angular extent of the pixel).
Cheers Ray