|
Using exit pupil as your guide works for any sized telescope to give you maximum magnification. Whether you can utilise that magnification will depend a lot on your seeing conditions and the quality of your optics.
Exit pupil = eyepiece focal length / telescope focal ratio.
Images under 0.5mm exit pupil become too dim and eye floaters are common.
Therefore for a 200mm, f6, (1200mm f.l.) telescope to get a 0.5mm exit pupil you need a 3mm eyepiece. (x/6=0.5, therefore x=3). A 3mm eyepiece will give you 1200/3 = 400x on your telescope. This would be you maximum magnification.
Here's another example for an ED80 refractor, 80mm, f7.5, (600mm f.l.) telescope (ED80 refractor). x/7.5=0.5, therefore x=3.75. 600/3.75=160x. Therefore 160x is your maximum magnification for an 80mm telescope to give you a 0.5mm exit pupil. Note refractors can take a little more power as there is no central obstruction but the image really does begin to dim below 0.5mm.
A third example for a 400mm, f5, (2000mm focal length). x/5=0.5, therefore x=2.5. 2000/2.5=800x. Therefore 800x is you maximum magnification giving 0.5mm exit pupil.
And finally for a 200mm, f10, SCT, (2000mm f.l.). x/10=0.5, therefore x=5. 2000/5=400. Therefore 400x is your maximum magnification.
You can see from the above results that aperture / magnification = exit pupil but I like the first formula as you don't have to work out magnification first. Also works for minimum magnification as generally maximum exit pupil size of 7mm is aimed for. eg. 35mm eyepiece, f5 telescope, exit pupil = 35/5=7mm. This would be an excellent low power eyepiece for any f5 telescope under dark skies.
|