5 short tips on buying telescopes
There are many tips on the Internet on telescope purchase. These tips are from "old hands" and usually peppered with jargon. We want to explain in five short points what you should consider before buying a telescope. Without using too many difficult optical terms:
1. Telescope Aperture
The larger the diameter of the mirror or lens, the more mirror surface, the more celestrial objects can be observed. But also the resolution capability is increasing, i.e. the picture is sharper and more detailed. And last but not least: It can be higher enlarged. For each telescope the maximum useful magnification is twice the telescope aperture in millimeters. Enlarging beyond, the image gets darker and blurrier.
The mirror surface rises in the square of aperture. A telescope with a 6 " or 152mm opening, has a 4-fold mirror surface compared to a telescope with a 76mm opening
2. Lense or mirror (Refractor or Reflector)
Comparing the same price-level, a reflector offers more aperture than a refractor, that means you can see more clearly objects that are poorer of light, e.g. galaxies and nebula. Therefore refractors provide a better quality of reproduction. They are primarily used for the observation of the sun, planets or moons.
3. Focal distance
A longer focal distance is better for planets and moons, a shorter one is better for galaxies and nebula. A short tube also offers a larger visual field.
Very important: focal over diametre ratio (= focal distance divided by aperture, e.g. 1200mm / 200mm = f/6)
You need a lower focal over diametre ratio for deep sky observations (e.g. f/5), but the ratio should be higher for the observation of planets and moons (e.g. f/10).
Simply put: A long telescope is a planet device, a short one a deep sky device. Frequently, the golden middle path is available (ratio between f/6 and f /8).
It is very important that the mounting has a good stability!
There ar two kinds of mountings:
The azimuthal mounting, where two axis have to be handled simultaneously.Thus, it´s not so easy to follow objects with this installation.
The parallactic mounting is of advantage because it can follow celestial objects by only one axis.
A Dobson mount is perfectly suited for beginners because it is very stable. This azimuthal mount is handled on two axes. Therefore you need a little more exercise, but this solution is much cheaper. A subsequent purchase of a parallactic mount is always possible.
To beginn observing you need 3-4 eyepieces. Every eyepiece has its own magnification (= telescope focal lenght divided by eyepiece focal length, eg. 1200mm / 20mm = 60). More information about eyepieces are here: Okularseite.
For every telescope we recommend a moonfilter, which dimms down the moonlight.
Do you still have any questions?
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