Expert: Tom Whiting - 2/14/2011

Question
i! I just pick up a older Celestron Newtonian 114 mm 910 mm Telescope . Could you recommend the best Eyepieces for it, and is a zoom worth the price for this scope. I'm new at this, any Help will be appreciated . thanks sincerely John

Answer
Hi John,
so in plain English terms, you have a 4.5 inch mirror with an f8 focal ratio (910 mm focal length
divided by 114 mm mirror).
Best eyepieces are your 1.25 inch diameter barrel 30 mm eyepiece which will give you a reasonable field of view of slightly over 1/2 degree, with a magnification of ~ 38 X  (910/30).
A slightly higher magnification would not be out of line, say a 10 mm eyepiece, giving you 91X (910/10)... but at a much reduced field of view.
Your maximum effective magnification is 4.5 inches times 50 power per inch of aperture, or 225 power.  (Assuming all your lenses in the eyepiece and the mirrors are figured and ground to a very high degree of precision, say 1/10th wave. (1/10 wavelength of light, or about 2 millionths of an inch in both shape, and smoothness.) Any less quality than that, then you have to drop the 50 power per inch of aperture to a somewhat lower value, like say 30 per inch.)
(If you're not familiar with these terms, you have a long way to go...suggest since you're quite new, you punch up our astronomy club website at
http://www.velocity.net/~bwhiting
and read "Tom Whiting's Sound Advice for the New Observer"
Clear Skies,
Tom Whiting
Erie, PA USA
PS... I don't like zoom eyepieces, never used one. I've read that they are only 'perfect' for one focal setting, the rest being a 'compromise' from perfect focus. I don't like to compromise after spending thousands of dollars USD for my optical systems... unless things have changed from 20 years ago... you'll have to research it.

FOLLOW UP:
Oops... typo error; change 38 power to 30 power (910 mm/30 mm). Power is always the focal length of the telescope divided by the focal length of the eyepiece.
Tom