Two small telescopes side by side, labelled magnification × aperture: 10×50s magnify 10 times with 50 mm lenses. They make an excellent first instrument: wide field, upright image, both eyes in use. They show lunar craters, Jupiter’s moons and dozens of Messier objects.
Related terms
- Magnification
How many times the instrument enlarges the object: telescope focal length divided by eyepiece focal length. A 1,000 mm telescope with a 10 mm eyepiece magnifies 100 times (100×). More is not always better: past a limit the image becomes dim and blurred.
- Exit pupil
The small disc of light that leaves the eyepiece and enters the eye; its diameter is the aperture divided by the magnification. A 200 mm telescope at 40× gives a 5 mm exit pupil. Between 5 and 7 mm the image is at its brightest (nebulae); around 0.5 to 1 mm you reach the maximum useful magnification.
- Messier catalogue
The list of 110 deep-sky objects, M1 to M110, that the comet hunter Charles Messier published from 1774 so as not to mistake them for comets (the last entries were added later). It gathers the finest clusters, nebulae and galaxies visible from mid-northern latitudes. It is the ideal observing programme for beginners.
← The whole glossary