Quote:
Originally Posted by FlashDrive
What do you regard as the ' cut off ' focal length between High Mag ... and Low Mag
eg ... would 20mm and up be regarded as ' low mag ' ...anything under that you start to get into the ' high mag ' eyepieces
What would you regard to this....
Col...
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Lowest = 6mm exit pupil. The eyepiece focal length required is thus 6mm x the focal ratio of the scope. For example an f/7 refractor, this means a 42mm eyepiece. With UWA eyepieces however another limit comes into play - the diameter of the field stop - or eyepiece barrel (if there is no field stop). The result is that for a 2" barrel, a 38mm UWA eyepiece with an apparent field of view of 70 degrees will be limited only by the barrel, and a longer eyepiece is pointless.
Highest depends on the type of scope. For Newtonians and SCT's, 1X magnification per mm of aperture. Hence for say and f/5 Newtonian, 5mm.
For an achro refractor or Newtonian with a small secondary, maybe 1.5X per mm of aperture. This means an eyepiece of focal length = F/1.5, where F is focal ratio of the scope.
For a good APO refractor or very good maksutov this can go as high as 2X per mm of aperture for planetary or double stars when the seeing permits (which is rarely). For example for an f/15 maksutov in excellent seeing, 8mm is pretty much the sweet spot.