Sunday, March 2, 2008

Telescopes: Getting started

In a comment to my last post, Summer asked about some suggestions for telescopes. Since other's may be interested too, instead of answering her by email I'll just post the information here.


There are 2 basic types of telescopes to consider. Refracting and reflecting. (There is a 3rd that combines the 2 concepts called Schmidt-Cassegrain.)

Refracting telescopes are the ones that people typically think of when they think "telescope." They contain a series of lenses stacked inside a tube.

This one is an example of a refracting telescope. Low cost refracting scopes can be good for looking at the moon, planets, star clusters, and near galaxies. High cost ones can offer greater clarity but come at a very high cost for good optics.

The downside of refracting scopes is size. They are long and often have a small diameter lens. Longer telescopes are little more cumbersome to store, transport, and set up. Small diameter scopes mean less light gets in so deep sky objects (or dim objects) are much harder to see.

A reflecting telescope uses a series of mirrors instead of lenses to bounce the image through the tube and focus it in the eyepiece. These types can be shorter without sacrificing focal length and offer a much larger diameter for capturing more light.

The downside to reflecting telescopes is the image gets flipped upside-down when you view it. So aligning objects in the eyepiece can be a little tricky at first until you get used to it. Left is right and up is down. (The trick is, instead of thinking about moving the telescope, think about moving the object. If the moon is too far to the left then instead of moving the telescope to the left, move the moon to the right.) So, a flipped image, not that big of a deal, unless you also want to use this telescope for terrestrial viewing. That is, observing animals or far-away objects on earth. A refracting telescope image will be oriented correctly.

Most reflectors have an open tube design that is subject to gathering dust, dew, or other junk on the mirrors that will degrade the quality of the image and may need to be cleaned if dirty. Cleaning these optics can be very tricky as it usually requires disassembly. And disassembly means reassembly which is even more tricky and requires special tools to align or collimate the optics. They all come with dust caps though, so keeping them covered when not in use and being careful outdoors to not use in a very dusty area will keep things clean for a long time.


If you want to gather a lot of light you need a large diameter lens or mirror. The larger the diameter the more light that gets collected and the brighter the objects. With more light you can see nebulae better and galaxies really pop. Also, the larger the diameter the larger the useful magnification. Meaning you get more detail at larger magnifications.

Focal length is used to determine how large the objects will appear through the eyepiece. The larger the focal length the larger the object. Eyepieces also have a focal length that works just the opposite. The "smaller" the eyepiece the larger the object. You can calculate the magnification by just dividing the telescope focal length by the eyepiece focal length. For example: a 700mm focal length will have a 28x magnification (28 times bigger than viewing with the naked eye) when using a 25mm lens (700mm/25mm=28x) or a 70x magnification with a 10mm lens (700mm/10mm=70x). A 350mm focal length would have half the magnification potential of 700mm with the same eyepieces (350mm/25mm=14x).

Motorized mounts

A motorized mount is great for a beginner. They typically come with a simple hand-held computerized keypad that can find objects for you. If set up accurately you don't have to know where Saturn or Andromeda are to be able to find them. They also can continuously move the scope for you to track the movement of the sky. (Yes, it's the earth that is rotating, but from the perspective of the viewer it's the sky that's moving.) And at high magnifications the objects can move out of the eyepiece very rapidly. The on-board computer can also take you on a sky tour and show you things you may not have previously known about.

However, motor mounts can really cause the price to jump. They also can take some time and finesse to set up. The computer needs to get everything aligned before it can accurately find objects and reliably track them. This can be fun or frustrating depending on your personality. The tripod needs to be leveled. Then the scope gets leveled and aimed north. Then you begin the alignment by entering the time and date, location on earth (nearest city or lat/long), and aligning to 2 stars. If done right and if the batteries are fresh it can make for a great night of viewing. But if done wrong or your batteries begin to die (and they do eat through batteries in just a few nights) it can quickly lead to frustration and packing things up early.

Telescopes without a motor mount do force you to learn the sky though, and that can be very beneficial to having a good time without the extra complicated setup or relying on battery power.

Some telescopes that don't come with a motor mount can sometimes be upgraded by buying the mount separately. This is not typical though. So if this is something you want to do to spread out the starting expense then be very careful to see if this is an option for the scope you are interested in.


The bare minimum accessories that I recommend for a beginner are:

25mm eyepiece (most scopes come with this)
10mm eyepiece
Barlow lens (this doubles the magnification of any eyepiece)
Lunar filter (cuts the glare and gives greater contrast)
A star chart, the very simple and cheap rotating type
A field guide to astronomy
A protective case for storage and transport (many scopes just come packed in styrofoam and cardboard and this is not suitable for constant use)
A flashlight with a red filter (so you can see in the dark without affecting your night vision)

There are many other filters available and can be bought individually or in a pack. Color filters are good for bringing out planetary, nebula, or galaxy details. A light pollution filter is good to filter out the wavelength of light typical to city lights. A solar filter is a must for any viewing of sun spots or solar eclipse. (Most filters screw on to the eyepiece, but solar filters attach to the outside of the telescope where the light enters. This protects the optics from getting damaged by the heat of the sun.)

If your scope can handle the higher magnification then a 3.5mm eyepiece is also good to have.

Purchasing accessories can really break the bank though, so keep them in mind when setting your budget.

Other notable things

I recommend sticking to well known names (Orion, Celestron, Meade, Edmund Scientific).

Make sure the eyepiece tube of any scope you buy takes a standard eyepiece size of 1.25" or 2". The 1.25" size is typical for most scopes and has more and affordable accessories.


  1. Bill, thanks so much for all the info. Wow, I asked the right person! We'll let you know what we get.

  2. Have you ever used a Retracting or Reacting telescope? I've had good experiences with both.