Quote:
Originally Posted by Icearcher
...want to do some lunar viewing, what eyepieces would you suggest I look at
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Hi Chris, yes there is a rule of thumb:
1. For a good quality refractor (an ED doublet or triplet APO) the highest useful magnification is usually M = X2 per mm of aperture, in your case 140X.
Focal length / magnification = eyepiece focal length,
in this case 420/140 = 3mm.
2. There is a tradeoff between sharpness vs more magnification which to some extent is dependent on your vision. For most observers this is around X1 per mm.
Using the same as above, this gives 6mm.
3. These are not exact, so basically any eyepieces in the range 3...6mm should do nicely.
For what its worth I have a quadruplet 70mm APO, f=475mm and this can show Uranus as a neat little disk with a 4mm eyepiece, at 118X. Moon is excellent with that magnification.