This interactive telescope calculator will answer commonly asked questions about a given instrument's performance when changing eyepieces and accessories.

Enter the 'scope's aperture in inches or millimetres (be sure to select the correct units!), its focal length in millimetres, the eyepiece focal length (also in millimetres) and eyepiece apparent field (typically 50-55° for modern eyepieces). If you use a Barlow lens or focal reducer, commonly encountered values may be selected from the pull-down menus.
Aperture of
mirror or lens		 Telescope
focal length		 Eyepiece
focal length		 Eyepiece
apparent field		 Barlow
lens		 Focal
reducer
mm mm

Here's an analysis of your 'scope's performance using the settings above:

Note: if the Eyepiece Exit Pupil exceeds the diameter of your pupil, some of the light gathered by the instrument will be lost. Also, a telescope's limiting visual magnitude varies with magnification, a factor our calculator corrects for.