Quote:
Originally Posted by ghsmith45
Smaller f ratio means more coma, more critical collimation, a larger diagonal mirror, leading to some light loss and less contrast. It is also harder to make a short focus mirror, so for a commercial enterprise, a "good enough" f4.5 may not be as good as a "good enough" f6. A short focus scope is more portable and doesn't demand as much from your mount if you are imaging. Coma can be corrected with a coma corrector.
It also depends on what you look at most. For planets go as long as possible with the smallest possible diagonal to maximise contrast, for close equally bright double stars, the larger diagonal of a short focus job actually helps--light from the Airy disc is pushed into the first diffraction ring, so the Airy disc is smaller.
Field of view for visual is not affected. It depends on magnifying power and eyepiece--100 power with a 50 degree eyepiece will have the same FOV in both scopes. For imaging, the smaller f ratio will put more of the sky on your chip.
Everything is a trade-off, nothing is perfect.
Geoff
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The airy disk doesn't actually decrease in size with increasing central obstruction, but rather less percentage of light is concentrated inside the airy disk and more in the diffraction rings which translates into a loss of contrast.
The size of the disk only decreases as you increase the aperture and vice-versa.