how do I buy a telescope?

Question

I am interested in purchasing a telescope but I don't know what the specs mean ... i.e. Use (Astro, Terrest or both), Diameter (Mirror or Lens), Focal Length/ F Ratio, Magnification
(w/ included eyepieces), Highest theoretical magnification, etc. What is the better options to have? Thanks!

Answer 1

First, ignore magnification, a totally useless exercise. In telescopes, you can always get higher magnification by switching to a shorter length eyepiece. That will have the effect of changing a small bright fuzzy blob into a big dim fuzzy blob.

In order to make that blob less fuzzy, you need APERTURE, which is the diameter of the main lens or mirror. Aperture is everything, which usually means a reflector will give you more bang for the buck. A rule of thumb is that the highest useful magnification is about 100x per inch of aperture. You can compute the magnification by dividing the focal length of your primary by the focal length of the eyepiece.

The second most important consideration is the mounting. You want something as sturdy and steady as possible, preferably equatorial over alt-az, because that makes it easier to follow the object over time.

Focal length is the length between the lens or mirror and the focal plane. Longer focal lengths give higher magnification (and smaller fields of view) for the same eyepiece. Shorter focal lengths give lower magnification and larger field of view. F ratio is simply the ratio between focal length and diameter; it is more important in photography than in visual observation.

Answer 2

Diameter: This is the distance from one side of a circlular object to the other.

Aperture: This is a term for the diameter of a telescope's lens if it is a refractor, and the diameter of the telescope's mirror if it is a reflector.

Aperture translates into the light collecting ability of a telescope. The larger the diameter of the lens or mirror, the more light it can collect and the more you can see.

Focal length: This is the distance from the lens or mirror to where it focuses the light to a point.

Focal ratio: This one is a bit confusing to most people because of the way it is stated. Simply put, if you divide the telescope's focal length, by the aperture (remember, the diameter of the lens or mirror) you get a number which is the telescope's focal ratio. It's confusing because it is expressed as f/#, for example, if the number you get is 6 then it will be expressed as f/6. The confusion arises because a lot of people think f/6 means "divide the focal length by 6", but it doesn't.

Focal ratio basically gives you an idea of what you can expect as far as field of view, field distortion, and exposure time for astrophotography goes. For telescopes of the same aperture and design but different focal lengths (and thus different focal ratios) a smaller number in f/# means a wider field of view (and less magnification from an eyepiece), shorter exposure time, but more distortion around the edges of the field. A larger number means a narrower field of view (thus higher magnification from the same eyepiece), longer exposure time, less edge distortion, and possibly more detail in astrophotos. Neither is better than the other, it depends on what you prefer.

A telescope's magnification with any given eyepiece is expressed by: magnification = (telescope's focal length)/(eyepiece focal length)

The "maximum effective" magnification is the magnification you can use with a telescope that will still produce distinguishable views. Generally, the longer the telescope's focal length the higher magnification you can reasonably obtain.

It's generally best to find a telescope with a decent amount of aperture and a sturdy mount.

You can go to this website to learn a little more about telescopes.

http://hometown.aol.com/siriusbc/telescopes.htm

Answer 3

This is a big complicated subject. First, an astronomical telescope will show everything upside down. If you want to see things right side up, you need a terrestrial telescope. For more detail on the other subjects, see the source.

Answer 4

I agree with Keith P, but I think that rule of thumb should be 50x per inch of aperture, not 100x.

Astronomical telescopes have an inverted image - to make it the right way up for terrestrial use more lenses have to be added. This will always mean a poorer image.

Answer 5

Consider buying "astronomical binoculars" and a tripod to put them on before you buy a telescope. These are relatively inexpensive, easy to use, and can show many interesting things in the sky.

Answer 6

BEST OPTIONS ARE EITHER CASH OR CREDIT, YOU MAY BE ABLE TO USE A CHECK, SO TAKE A TRIP TO YOU LOCAL TELESCOPE STORE, OR A TELESCOPE SITE!

Answer 7

First, go to a store that sells them and ask them, then buy one.