Highest Useful Magnification

Thargor

Ive got a Celestron Astromaster 130 EQ which is a 5" reflector with a 650mm focal length. I have a 10mm and a 20mm eyepiece and a 3X Barlow which someone already told me gives me this:

20mm = 32.5x so w/ 2x barlow = 65x w/ 3x barlow = 97.5x
10mm = 65x so w/ 2x barlow = 130x w/ 3x barlow = 195x

Red magnifications are unuseable apparently so 97.5X is the best I can hope for, but in the manual and various sites it says the highest useful magnification is 306X. What do I have to do to get up to this level of magnification? Ive read the sticky at the top of the page but I just dont get the relationship between the eyepiece and the magnification.

Redshift

The maximum useful magnification for your telescope is probably calculated at about 60 X per inch of apperture but this is a best case scenario and would require exceptional seeing and dark skies the like of which you would rarely encounter.
on a good clear night with a quality 5mm eyepiece I expect you would be able to get 130 X anything higher is going to be pushing it most of the time.

Remember a barlow comes with a performance penalty particularly if it's a cheapo one that came with the scope. The more glass there is in the light path, the dimmer and harder to focus the image becomes so try and avoid them where possible.

If you must use a barlow get a good one like a televue powermate, also quality eyepieces make such a difference but they are not cheap from £150 to about £500 each for really good ones.

Don't get obsessed with high magnification, I do most observing at low to medium power enjoying crisp wide field images, sometimes less is more.

Edit to get the magnifcation for a particular eyepiece you divide the focal length of the telescope by the focal length of the eyepiece
Your Telescope is 650mm with a 5mm eyepiece you get 650 / 5 = 130 X Magnification

Thargor

Thanks for that Redshift.