Astronomical Observing 101 (or before) Part 2

Some info on telescopes. The purpose of a telescope is to really to do two things, not just one.The first is to magnify the image which is the obvious one. The other is to increase the amountof light which reaches your eye. The magnification is a result of the focal length of the lenses involved. THe amount of light that reaches your eye is based on the size of the primary lens OR mirror.

Most people are familiar with binoculars and the designations listed. 7x35 are common field glasses or binoculars. They are 7 power (7x) which means that things look 7 times bigger when looking through them. They also have 35mm diameter lenses in the front (objective lens). They gather in more light than your eyes do normally. When using them at dusk for example, you can see more detail through them not just because the image is larger, but it is also brighter. For astronomical purposes, a 50mm objective lens will gather much more light than a 35 mm lens. Now a 10x power will enable you to see more (especially of planets and the moon) than the 7x binoculars. Obviously 20x would allow even more to be seen, but they are MUCH harder to keep steady if you are holding them in your hands. There are some devices that can be made or purchased that will allow one to use binoculars of high power with little or no movement. (A subject for the future?)

Now most refractors and binoculars have the objective lenses measured in millimeters and reflectors are usually measured in inches. Here is a table of approximate conversion. The area of the lense is a good indication of the light gathering effect of the lens/mirror. For example, going from a 4" to a 6" telescope increases the light gathering by over 2.3 times. Going from a 4" to an 8" increases it by 4.1 times.

35mm = 1.4" = 962 sq mm
50mm = 2" = 1963 sq mm
80mm = 3.1" = 5027 sq mm
90mm = 3.5" = 6362 sq mm
100mm = 4" = 7854 sq mm
125mm = 5" = 12272 sq mm
152mm = 6" = 18146 sq mm
203mm = 8" = 32365 sq mm
244mm = 10" = 46760 sq mm

There are basically two major types of (amateur) telescopes. The refractor and the reflector.

Both have benefits and drawbacks. There are a bunch of variations though. Some have to do with with the optics themselves and others have to do with the mount.

The refractor is like the old fashion spy glass type. At one end of a tube is a larger lens and the other end has a smaller lens which is what you look through. These are relatively expensive for the size of the objective and can be very expensive depending on the quality and type of lenses. In these there are two types, achromatic which can have a color halo on bright objects, more so on larger objectives which is why they are limited to a max of about 100mm but are relatively inexpensive (Orion has a 70mm with two objective lenses, mount and finder for only $150). An apochromatic has different glass and construction and can be found in larger sizes with less false coloration, but you will pay for it! (for example, Orion offers a 72mm apo with no lenses, mount or finder known as a tube only for $500, a 120mm version of tube only goes for $2,000!, a Takahashi 130mm tube only goes for over $6000!).

The reflector type scopes fall into (about) 3 categories.

The newtonian is what most folks think of as a reflector. These have a mirror at the bottom of a tube, a reflective mirror close to the open end in the middle of the tube that reflects the light out to the side where there is a mount for a lens (eyepiece) to look through.

The Schmidt-Cassegrains with a mirror at the back end of the tube, a lens combined with mirror at the front end that adjusts the lights, seals the end of the tube, and reflects the light back down through a hole in main mirror to a mount for the eyepiece.

The third type is the Maksutov-Cassegrain which is similar to the Schmidt-Cassegrain but has a slightly different type of front lens and reflective mirror.

Next time I'll look at the 3 types of reflectors and give some examples of the prices of some popular models (two of which I have plus one that is on loan to my brother).

More to come (?)