[Wavytone]

Hi Sasha,

Borrowing an earlier reply, with all lengths in mm here are the relations:

Focal length of the scope = F = 900mm
Eyepiece focal length = f
Aperture of the scope = A = 90mm
Maximum pupil of your eye when dark adapted = a, which we will assume is 6mm (in average conditions).

Focal ratio = R = F/A = 10, or alternatively F = RA
Magnification = m = F/f = RA/f
Exit pupil of the beam from the eyepiece = e = A/m = f/R

Lowest useful magnification occurs when the exit pupil = the eye pupil, i.e. e=a. Or, when choosing your lowest power eyepiece f = Ra, which for your scope is 60mm. Your scope is quite small however, so there not much point trying to squeeze an eyepiece longer than the 25mm one you already have.

For a person with normal vision, the maximum useful magnification occurs between 1.1X to 2X per millimetre of aperture. For refractors and f/15 Maksutovs it's 1.5, possibly 2X if you have truly superlative optics. For SCT's and Newtonians, 1.1X to 1.5X. This is the highest magnification at which you should just be able to discern the diffraction pattern around bright stars. At higher magnifications, objects get dimmer and fuzzier and you won't see any more detail.

So for your refractor the maximum useful magnification is m = 1.5A or about 135X. The eyepiece focal length required is f = R/1.5 or perhaps even R/2, which suggests an eyepiece in the range 6-7mm is the shortest useful one.

Given that you have 10 and 25 mm, I'd look for one around the 6-7mm mark and perhaps another about 15mm. Since the scope is f/10, pretty much any eyepiece will work well enough so there really isn't much point splashing out a lot of money for posh ones.