[Screwdriverone (Chris)]

Hi Nick,

It depends a lot on the seeing conditions (turbulence and transparency of the atmosphere) but generally its about 2 x your aperture which in your 8" = 200mm case, about 400x magnification.

Now 8" scopes are either 1000mm or 1200mm focal length (on average, if its a newtonian reflector) so the eyepiece you need to get to 400x is going to be a 2.5mm (1000mm FL) or a 3mm (1200mm FL) and at this small EP FL size, the eye relief will be small (need to get REALLY CLOSE to the glass to see anything) so you are better off getting say a 6mm eyepiece and a 2x or 2.5x barlow to get to that magnification level but retain the eye comfort of the larger eyepiece.

Generally, if you have a 1000mm FL scope, a 10mm eyepiece (which gives 100x mag) and a 2.5x barlow (gives total of 250x mag) will be about as far as you can really go in most conditions. To work out the magnification of an eyepiece, divide the eyepiece FL into the scope FL ie. 1000mm / 10mm = 100.

For example, the SMALLEST EP focal length I have (my scopes are 1000mm(5" reflector), 1000mm(8" reflector) and 1500mm (12"dob)) in my collection is an 8.5mm Pentax which gives me a magnification level of 117x or 176x. If I want to bump that up, I slot in my 2.5x barlow to get 292x or 440x (on the big dob). Now on the 12" dob, the theoretical limit is 600x but I have never got that as its too much and the image also gets too dim.

Hope this helps, I tend to ramble on a bit, but I thought I may as well answer as many following questions at the same time.

Cheers

Chris