|
An RFT, Richest Field Telescope, is one which has an exit pupil that matches your pupil's maximum dilation. If the telescope exit pupil exceeds the maximum dilation of the observers eye, only a portion of light from the scope is transmitted to the eye, the rest is wasted and it's as though you're using a smaller scope.
Note:
1. It has nothing to do with whether it is a refractor or reflector and nothing directly to do with the f ratio other than some f ratios are impractical.
2. The maximum pupil dilation varies with age of the observer and at a given age, varies between individuals. so one persons RFT is not necessarily another's.
3. The scope's richest field view is always a very low power. Therefore you won't see much difference in the views through an achromat vs APO. APO performance shines at high not low magnifications.
HOW?
Look up the average pupil dilation for your age group (see graph) or measure it.
Multiply the exit pupil by the scope f ratio to get the eyepiece focal length required for Richest Field(RF).
examples:
A 60 year old has a pupil dilation of 5mm and a 10"f6 newt.
5mm x f6 = 30mm eyepiece required for RF.
A 20 year old with a 7mm pupil dilation using the same scope needs
7mm x f6 = 42mm eyepiece for RF.
If the same 20 year old was using an f10 scope, a 70mm eyepiece would be required - this is impractical.
However the circumstances for a 75 year old with a 4mm pupil dilation are very different - 4mm x f10 = 40mm eyepiece for a richest field.
cheers
Joe
|