What this calculator answers
Before you buy an eyepiece or a camera, or before you plan a night, you want to know some simple things. How much will it magnify? How much sky will I see? Will the Andromeda Galaxy fit? Is my camera’s pixel size right for my telescope? The answers come from a handful of simple formulas. This page works them all out together and draws the result, so you can see the field rather than imagine it.
How to use it
- Telescope: pick a common telescope or type its aperture (the width of the main lens or mirror) and focal length in millimetres. Both are printed on the tube or in the manual. Add a Barlow or reducer if you use one. Typical seeing is how much the air blurs stars on an average night: 2 to 3 arcseconds for most backyards, and 1 to 1.5 at a good mountain site.
- Eyepieces: type each eyepiece’s focal length (the number on it, in mm) and its apparent field. That’s 50 to 52° for a standard Plössl, around 66 to 70° for wide-angle and 82° or more for ultra-wide. Add or remove rows as you like.
- Camera: pick a sensor or type its width, height and pixel size. Binning joins pixels together, which acts like bigger pixels.
- What fits in the view: type an object, such as M42, M31 or the Double Cluster. The picture draws the eyepiece view as a circle and the camera frame as a rectangle. A dashed outline of the full Moon (31 arcminutes) is shown for scale.
- Download everything as an image, PDF or Word file to keep with your kit.
What each number means
- Focal ratio: focal length divided by aperture. Lower (“faster”) ratios give wider, brighter camera fields.
- Light grasp: how much more light the telescope collects than a dark-adapted eye with a 7 mm pupil.
- Faintest stars: a typical limiting magnitude under a dark sky. Light pollution makes this a couple of magnitudes brighter.
- Resolution: the Dawes limit is the closest pair of equal stars the aperture can split. Real nights are usually limited by seeing instead.
- Power and true field: power = telescope focal length ÷ eyepiece focal length. True field = apparent field ÷ power.
- Exit pupil: the width of the beam leaving the eyepiece. 4 to 7 mm suits wide, faint objects, 2 to 4 mm suits galaxies, and under 1 mm suits planets on steady nights. Wider than about 7 mm wastes light, and much under 0.5 mm makes the view dim and soft.
- Image scale: arcseconds of sky per camera pixel, 206.265 × pixel size ÷ focal length.
- Sampling: for sharp, round stars you want roughly 2 to 3.5 pixels across a star’s blur. The gauge shows whether you are under, well or over sampled for the seeing you entered.
Worked example
A 150 mm f/5 reflector (750 mm focal length) with a 25 mm, 52° eyepiece gives 30× power, a 1.73° true field and a 5 mm exit pupil. That’s lovely for the Pleiades. With an APS-C astro camera (3.76 µm pixels), the image scale is 1.03″ per pixel, and the frame is about 1.8° × 1.2°. On a 2.5″ night that is well matched, and M42 with its surrounding nebula fits comfortably.
Questions
Why is a row greyed out? That eyepiece gives more power than the aperture can support, or an exit pupil wider than your eye. It still works, but it isn’t the best use of the telescope.
Is anything saved? Your eyepiece list is kept in this tab only, so it is there while you move between astronomy tools, and it goes when you close the tab.
Last updated 2026-11-07.