* Astronomy

Blob's guide to buying a telescope

Not all telescopes are suitable for astronomical use. Try to avoid the very cheap variety. Remember that this initial financial outlay is all you ever need to spend.

There are two main types of astronomical telescopes; a refractor and a reflector.
In a refractor, the light from a star is collected by a lens (object glass) and brought to focus. Here a second lens, known as the eyepiece magnifies the image. In a reflector, the light is collected by a specially shaped mirror and reflected back up the tube. Then the light is brought to focus and, as before, magnified by an eyepiece. This is the Newtonian form of reflector; there are other variants.

Refracting telescopes
With a refractor try to go for a large aperture - that is to say, the diameter of the (front) object glass, which is the light collector. The eyepiece does all the actual magnification. The bigger the aperture the greater is the amount of light is available to the eyepiece, and a high magnification can be used.

The maximum useful power for magnification is X50 per inch of aperture. That is to say, if you have a 3-inch refractor (object-glass 3 inches in diameter), you can use a power of 3x50=150. So the object will be enlarged 150 times as compared with the naked-eye view. You can, in theory, use a higher magnification - say x300 - but the image will be so faint that it will be quite useless. So avoid any advertised telescope that states magnification only.

Reflecting telescopes
With a reflector of the Newtonian types, the aperture is the diameter of the mirror. The rule of thumb is x80 per inch of aperture. Inch for inch the reflector is less efficient (because light from the target objects has to undergo several reflections, and over time the silver coating degrades). Remember that eventually you need to culminate the mirrors! This is not for the faint hearted... A cheap reflector will use a spherical mirror; a quality reflector will have a parabolic shaped mirror. There are various other patterns that use both mirrors and lenses, and these can be excellent. But, unfortunately, they are very expensive.

If you want to buy a telescope, don't pay a large sum for a refractor with an object-glass of 60-mm diameter, or a Newtonian reflector with a 4.5-inch mirror.

Small telescopes do not gather enough light to be really useful astronomically. The prices are, fortunately, quite cheap. The following gives a guide as to what to expect to pay for a suitably mounted telescope: 60mm refractor on an altazimuth mount from £100 (cheap scope, eg. Celestron Firstscope 60AZ)

3 inch refractor on an altazimuth mount from £199.98 (SkyWatcher Evostar 90 f/900 AZ3 )
3 inch refractor on an equatorial mount from £209.00 (SkyWatcher Evostar 90 f/900 EQ2 )
4.5 inch reflector on an equatorial mount from £149.00 (Celestron Firstscope 114 EQ )
6 inch reflector on an equatorial mount from £399.00 (Orion Europa 150)
8.75 inch reflector on an equatorial mount from £699.00 (Orion Europa 250 )

An altazimuth is a simple mounting, on which a telescope can be moved freely in any direction. An equatorial is more complicated, and means that only one motion is necessary (east/west). For astronomical photography, an equatorial is more or less essential.

It's full of stars
For observations of stars, aperture is again the essential factor; the more light you can collect, the fainter objects you can see. Consider, for example, the moons. With the naked eye, you can see the large mare, and a few large craters. Any telescope will show hundreds more.

I am not suggesting that a very small telescope is quite useless. Of course it isn't. What I am saying is that in my opinion, it is not satisfactory for anything but the most casual observation. Rather than spend a large sum of money (£100) on a very small telescope it is, I feel, much better to buy a good pair of binoculars.

Binoculars
They are defined by the apertures of the object-glasses (in mm) and the magnifications. Thus a 7x50 pair means a magnification of 7, with each object-glass 50mm in diameter. In general, I would recommend a power of between x7 and x10. With a higher magnification the binoculars become too shaky to handhold, and a mounting is necessary. The following gives a guide as to what to expect to pay:

9x63 Mini Giant Binoculars
Moderate, 9x magnification allows a wide 5.0° field and 7mm exit pupil. Extra-long 26mm eye relief. Good hand-holdability. Weighs 2 lbs. 10 oz. The lenses and BAK-4 prisms are fully-multi-coated and the aluminium barrels internally glare-threaded to eliminate internal reflections. Each features pop-down eyeguards and rubber armouring for a secure grip. Includes hard case, deluxe wide neck strap, and lens caps. Five-year limited warranty.

7x50 binoculars from £49 the best for young persons
8x30 binoculars from £30 lightweight, and can view the moons of Jupiter
10x50 binoculars from £49 ideal for the older person
12x60 binoculars from £200 Nice.. not recommended for beginner.
20x80 binoculars from £450 Heavy and expensive, needs a tripod stand, which costs about £50

Telescopes versus binoculars
Both have advantages and disadvantages. For a total beginner a pair of binoculars are recommended, and later on, he/she can purchase a small scope. A pair of binoculars takes no time to 'set-up'.

Of course never look directly at the Sun (or reflection) with any telescope or binoculars, even if you use a dark filter. This will almost certainly damage your eyesight. Have a look at these `before and after` pictures below, to see the affect...

Buying second-hand
What about second-hand telescopes? I would tend to recommend the refractors here...Remember that the silvering on the mirror, in a reflecting scope, only has a limited life expectancy.

So, watch out! A bad telescope does not always betray itself at first sight, and this applies to new telescopes as well as second-hand ones. A simple test on a bright star will easily show up the faults; the star should be point like and crisp. Try to get skilled advice if possible.

Making your own
You can, of course, decide to make a telescope yourself. Lens making and mirror making is probably best left to the professional. The cost of a ready made 6-inch mirror, for example, is about £100. The mounting is purely a problem of mechanics, but it is essential to know just what you intend to do before setting out.

Whatever you intend to do, I wish you luck, and clear skies.


-- Edited by Blobrana on Wednesday 4th of November 2009 08:46:17 AM