Comet seeker
In astronomy, a comet finder is a compact, powerful telescope with low magnification . These properties make it suitable for the systematic search for comets . Meyers Konversationslexikon from 1895 describes them as telescopes with a lens aperture of about 10-15 cm with a short focal length ... and a large field of view.
In a systematic search for two- dimensional celestial objects - be it comets, nebulae or other extensive objects - the aperture of the lens is important on the one hand , but even more on preserving its surface brightness as much as possible . Therefore - by using an eyepiece with only low magnification - the exit pupil of the telescope should be selected so that it is only slightly smaller than that of the dark-adapted eye (between about 8 and 6 mm, depending on age). This also makes the telescope field of view large, which is always advantageous with search programs. A wide-angle eyepiece is also useful - but not necessary .
Historical comet seekers
Most historical and also modern telescopes that can be used as comet finders have an aperture ratio of 1: 6 to 1:10; the best magnification follows from 5–6.5 mm of the exit pupil. With an 8 cm / 60 cm refractor, the magnification would be about 12 to 16 times.
The multiple comet discoverer Jean-Louis Pons (1761–1831) used the “large viewfinder ” of his refractor for his search campaigns , which apparently had a large opening and a field of view of 3 °.
The Fraunhofer's comet finder of the Kremsmünster observatory from 1825 with an achromatic lens of approx. 7 cm aperture and approx. 60 cm focal length. Two eyepieces with 10 or 15 times magnification result in an unusually large field of view of 6 °.
The Archenhold Observatory in Berlin has a very powerful comet finder . The refractor has an opening of 25 cm (a "ten inch") and a focal length of 160 cm. A magnification between 30 and 50 times would make sense here. The Urania observatory in Vienna even has three comet finders: two from Zeiss with 6 cm or 8 cm aperture and 70 cm focal length, and an instrument with 14/130 cm that can be set up as a transit.
Binoculars and binoculars
If there are no special requirements for stability or mounting , a large, powerful binocular is a suitable, easy-to-use replacement for a regular comet finder. Binoculars available on the market such as the legendary Wachter Gigant 14 × 100 mm, which are also available at Celestron or Fujinon as 20x80 mm, are a good choice, but must be propped up or guided on the ball joint when observing . A slightly smaller version (e.g. 16 × 70 or 12 × 60) can just about be used hands-free, but the light intensity is lacking for weak objects.
A much cheaper (from ~ 70 €) and more manageable alternative, especially for beginners, is binoculars 7x50, which are also well suited as twilight glasses (e.g. for hunters). Of course, the range and magnification are lower, but some nearby comets with low surface brightness are even easier to find in it. The more commonly offered 10x50 binoculars are less suitable.
Double telescopes with an aperture of 100 mm, on the other hand, require a regular altazimuth mount, which makes them much more expensive and difficult to handle. A telescope with a binocular eye socket is only slightly inferior to a binocular telescope. In both cases, an eyepiece that deflects by 45 ° is advantageous, with which one can look into the zenith without fatigue . The largest comet finders available on the market have apertures of 15 to 20 cm and focal lengths around one meter, but already cost around 3000 euros. Those from Zeiss have dimensions between 8/50 cm and 11/75 cm.
It is also important to have a high-quality coating of the lenses and a good anti-reflective coating on the tube in order to increase the contrast in the sky, which can best be assessed against backlighting . If the tube apertures are not sufficient for this, the anti-reflective coating at critical points can be improved with a matt black lacquer.
Mirror telescopes available on the market are less suitable because 1) their focal length is too long and 2) the secondary mirror causes vignetting, which also reduces the contrast.
Sources and web links
- Specialist books by Rudolf Brandt , Günter D. Roth , Astronomie-Lexikon
- Stars and space , monthly column on observation practice
See also
Individual evidence
- ↑ on the subject see also night sky
- ↑ Manfred Wachter, Precision Mechanics and Optics. In: http://www.astrotech-hannover.de . Elmar Remmert, 2007, accessed in 2007 .