How to use it
Type in the two numbers printed on your telescope: the aperture, which is the width of the main lens or mirror, and the focal length. A box marked "130/650" means 130 mm across and 650 mm long in focal length. Then add the focal length of the eyepiece you are holding, which is written on its barrel, and pick a Barlow if you are using one. A Barlow is a small lens that goes in before the eyepiece and multiplies the power, usually by two or three.
The page works it out as you type, and the address in your browser changes to match, so you can send the answer to someone in your club.
How to read the answer
Magnification is how many times bigger things look than with your eyes alone. More is not always better. Past about twice the aperture in millimetres, the picture gets bigger but no sharper, and on many British nights the air is too unsteady to go even that high.
True field is how much sky you see at once. We also give it in Moon widths, since the full Moon is about half a degree across: a field of 2 Moon widths shows the whole Moon with room around it.
Exit pupil is the width of the beam of light coming out of the eyepiece. Around 4 to 7 mm gives bright, wide views of star clusters and galaxies. Around 1 to 2 mm suits the Moon and planets. Much wider than 7 mm and some light misses your eye; much smaller than half a millimetre and the view goes dim and blurry.
Resolving power is the closest two equal stars can be and still show as two. It comes from Dawes' limit, a rule worked out by the Victorian astronomer William Dawes from his own observing.
How it is worked out
All of this is standard telescope arithmetic. Magnification is the telescope's focal length divided by the eyepiece's. The true field is the eyepiece's apparent field divided by the magnification, which is the usual estimate; real eyepieces come out a few percent either side. The exit pupil is the aperture divided by the magnification. Dawes' limit is 116 divided by the aperture in millimetres, in seconds of arc.
We checked the sums against a published telescope. The Sky-Watcher Skyliner 200P is listed at 203 mm and 1200 mm, f/5.91, with 406x as its highest practical power. The calculator gives the same f/5.91 and 406x, and 60x and 96x with the 20 mm and 12.5 mm eyepieces that come with it.
What to point it at
The Moon is the easiest first target, and the Moon phase finder shows what shape it is tonight. The terminator, the line between light and dark, is where craters look sharpest. Even at low power, Jupiter shows its four big moons as points of light, and our guide to Jupiter's moons explains how they change places from night to night.
Written by Craig Fearn, Aphelion Prints. Last updated .