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Old 06-12-2008, 10:53 PM
Wavytone
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Wavytone is offline
 
Join Date: Jul 2008
Location: Killara, Sydney
Posts: 4,147
Yes, you've got it. 5 to 35 mm will be fine. At f/7 or f/9 pretty much any eyepiece will be fine, you could buy a cheap set from http://www.andrewscom.com.au/site-section-10.htm for example.

The other thing is to try to buy a parfocal set, so you don't have to refocus every time you swap one.

The eye pupil is the aperture of the iris of your eye when dark adapted. While optical textbooks suggest 7 mm for youngsters, as you get older this gets smaller. In addition, if there is some light around your eye may not be fully dark adapted, in which case its better to assuming a slightly smaller value than 7 mm. You can also measure your own eye pupil too.

The exit pupil of the telescope is the quantity ExP shown in the figure on this page
http://www.telescope-optics.net/eyepiece1.htm

Essentially, you have to get your eye positioned so that ExP coincides with the iris in your eye in order to see the full field of view provided by the eyepiece. Eye relief is the distance ER between ExP and the eye lens of the eyepiece.

If ExP is larger than the eye pupil some of the light will not enter your eye, in which case you're effectively seeing with less than the full aperture of your scope.

Last edited by Wavytone; 06-12-2008 at 11:16 PM.
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