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Old 05-05-2008, 12:59 PM
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ngcles
The Observologist

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Join Date: Dec 2006
Location: Billimari, NSW Central West
Posts: 1,580
Good Report

Hi Alex,

Good report mate -- a nice read much enjoyed.

Alex wrote:

"1: M104. The Sombrero Galaxy.

Viewed through a variety of ep's with and without barlows.
I found the best combination to be my 25mm with a 2x barlow, and also found myself itching for more apeture (already.. eeek!!!!)"

You've serendipitously stumbled across something important here -- the 2mm exit pupil "rule".

For small low-contrast objects (like galaxies, globulars and most planetary nebulae), as a rule-of-thumb, a telescope will produce best possible contrast when you use an exit pupil of about 2mm.

In this case 1200mm / 25mm ep = x48 magnification plus the 2x barlow = x96.

200mm aperture / x96 = 2.08mm exit pupil = best contrast!

Best,

Les D
Contributing Editor
AS&T
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