Thursday, January 4, 2018

Before Buying a Telescope

I’m certain that the total solar eclipse of 2017 will have tempted many in the US to have invested in telescopes. That’s wonderful for two reasons:
  1. Seeing the stars and planets through your own telescope is fun!
  2. There will soon be some amazing deals available for used telescopes in most parts of the country.

Now, forgive me for the negativity expressed in that second reason, but during the past year I’ve discovered that too many of us who impulsively bought telescopes have learned some hard lessons… and I wish I could have read the advice I’m about to give here before I invested in my telescope.

How often will you be able to use a telescope?

We often joke about the many treadmills and exercise bikes being used as clothing racks. Too many telescopes are doomed to join them. Why? Because of the weather!

Before investing in a telescope you need to come to grips with your local night sky. Telescopes can be fitted with special filters to block out much of urban light pollution, but nothing enables them to show you the night sky if it’s cloudy. I recommend that you first pay attention to how often your sky is relatively clear at night, especially during the hours that you’d want to be viewing the heavens. You might even start a calendar of clear nights that you’re having either where you live or where you might expect to drive to.

Where I live, since I bought my telescope last March, we’ve had 33 clear nights during those 286 days, which averages to about 1 in 9 nights. And given that sometimes there will be 2 or 3 good nights in a row, sometimes there will be 2 or 3 weeks without any good nights at all. Of course, now that we’re into winter there will probably be more good nights for viewing, but less time each night before hypothermia sets in…

What do you expect to see with a telescope?

The first thing you need to know is that it’s not the telescope that determines what you’ll see in the night sky, it’s your eyes. Human eyes work best with strong lighting; we can see more colors and for farther distances during a bright, sunny day than we can on a cloudy day or at night.

This means our eyes simply cannot see the colors through a telescope that pictures in magazines show us. Those are time-lapsed photos, taken with long exposure times with telescopes that track their targets across the sky. Those images are then manipulated with special software to bring out the colors that you’d see if you were much, much closer to those objects. Often special filters are also used to bring out particular colors produced by gases in the nebulae or other objects.

For example, here’s the Orion Nebula as seen with your eye through a telescope:




And here’s the same nebula as shot by an amateur with astrophotographic equipment and manipulated with software to provide color:


Beautiful, high quality images can be obtained with amateur-level equipment, but it will take both money and effort to get there.

What’s the best way to start out?

Binoculars, when used with a good tripod, are an inexpensive way of getting into astronomy. Good quality binoculars cost far less than a good quality telescope—all you need to add is a tripod (and an adapter) to stabilize the binoculars.

Do look for a pair with 10x to 20x magnification and have apertures of at least 50 mm (that would be written for example as 10x50). Some advantages to starting with binoculars:
  • They are portable and easy to handle
  • The image will look “correct” which makes it easier to find what you’re looking for
  • A $100 pair can deliver images rivaling those seen through much more expensive telescopes
  • They can also be used during the day for other purposes

So, which telescope should you buy?

Beginning astronomers often start out wanting just to see what’s up there. Eventually, though, they start wanting to share what they’re seeing by videoing or taking pictures. Both are fun, but the equipment required for each task differs in some important respects—there’s no such thing as the “perfect telescope” that can do everything very well.

Warning: Be prepared to ignore ads hyping 600-power telescopes (or even higher!). Why? Because from the surface of the earth you can’t use super-high-power telescopes and see anything worth seeing!

1.      Stars twinkle. Through our atmosphere, stars do seem to twinkle, to move about slightly, because our air is often turbulent. That’s why the most powerful telescopes are built for observatories on mountains like Mauna Kea in Hawaii, at 4,205 meters (13,796 feet). And those telescopes use advanced technologies with lasers and computer-controlled mosaic mirrors to minimize the effect of turbulence in the atmosphere above that altitude!

2.      Objects in the sky move, right? Well, you were told in school they appear to move because this old Earth is spinning—and that is true. So, when you magnify the image that apparent movement is also magnified and you end up having to keep the telescope moving to allow for it. In other words, if it takes 100 seconds for the moon to drift out of view at 20x magnification, it would drift out of view in just 20 seconds at 100x magnification.

3.      Much of what you’ll want to see doesn’t require much magnification anyway. For example, the Andromeda Galaxy stretches almost 6 times the width of the full moon (178 arc-minutes vs 30 arc-minutes).

Frankly, any resolving power over 300x will be wasted except on the clearest nights with extremely calm air.

Light matters!

With a telescope, what really matters is the amount of light that reaches your eye or camera. That’s the secret to seeing things many, many light years away. There are two ways to get more light to the eyepiece:

1.      Aperture—how much light is entering the telescope is usually a function of its diameter. Generally speaking, an 8 inch aperture telescope delivers about 4 times as much light to your eye or camera as a 4-inch one… and about 800 times as much light than normally reaches the typical 7 millimeter pupil of the human eye!

2.      Time—focusing on an image over time allows more light to strike the photoreceptors in a camera. In the olden days astronomers used photographic plates or film to produce their images. Today cameras take multiple images of a few seconds each and then use software to “stack” them.

Kinds of telescopes

Basically there are two kinds of optical telescopes:
  • Refractors are the type of telescopes we’re most familiar with in films and television, in which the focusing of the light is accomplished through multiple lenses and, sometimes, prisms.
  • Reflectors use mirrors to focus and bounce the light to the eyepiece.

Both refractors and reflectors have their strong points and are worthy of consideration.

What do you want to do with a telescope?

Answering this question will help you decide whether you want to spend a lot of dollars on a computerized telescope that does much of the work for you or just start with a small telescope and explore the night sky on your own, perhaps aided by a good book that will help you learn the stars and constellations. Some telescopes are better for astrophotography, others for just plain seeing what’s up there. You may even want to buy separate telescopes for specific purposes.

I strongly recommend that you visit friends who have telescopes or attend a few public “star party” hosted by local astronomy clubs. Look through their telescopes and pester them with questions until you’re comfortable that you can decide which sort of telescope you’ll want.


Whatever route you choose, welcome to the universe!


No comments:

Post a Comment