Quote:
Originally Posted by Merlin66
The narrow light cone associated with slow systems, >f8 puts less demands on the eyepiece optics. Less eyepiece elements and less curved surfaces are required to give aberration free, well almost, performance.
Faster optics <f5 put far more onus on the eyepiece to be able to cope with the very steep light cone. This requires more complex glass shapes, spacing and usually more elements to give acceptable performance.
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Hi Ken,
That's true only to a certain extent. The shallower light cone certainly is a lot more forgiving for cheaper eyepieces and will enable cheaper eyepieces to work reasonably well in that system. On that basis a lot of aberrations from the eyepiece itself are eliminated. So its only the inherent telescope aberrations that come into play
It's worth remembering however that some of the telescope aberrations are inherent on the radius of curvature of the primary optic. For instance SCT's have a lot of field curvature because it is a function of the ROC of the primary optic which with most SCT's is fast at F2 to F3. This is the reason most imagers use a field flattener when imaging with an SCT.
The point of my post is that you can't make an overall assumption that because the overall F Ratio of the telescope is slow at F10, that it will essentially be aberration free.
You can make a reasonable assumption that cheap eyepieces will generally work better in an F10 SCT, than they will in an F5 Newtonian, but they may still be a long way from aberration free.
Cheers
John B