Selecting Your First (Good) Telescope

Introduction:

This note is to help people who have just been bitten by the Astronomy Bug to pick out their first good telescope. I say their first good one because many people first buy a "toy" scope for under a hundred bucks, with a rickety mount and blurry (often plastic) eyepieces. If that experience hasn't put you off the idea entirely, read on.

Selecting a telescope is similar to selecting a camera. Pretty much everyone has a camera, but there are sure a lot of choices! Digital vs. film (yes, you can still buy film cameras), 35 mm SLR vs. point-and-shoot, accessories galore, etc. But when you chose your camera you probably did not go through a lot of angst. If you just wanted to pull the thing out on Aunt Martha's birthday and take some snapshots, you bought a point-and-shoot. And you have probably been astonished at the beautiful images it takes (I know I was). You can even do some art photography with these; my brother has taken some fantastic sunset shots in Jamaica using a little pocket camera he has.

If on the other hand you wanted to be able to put on a 400 mm telephoto lens for bird-watching photography, or control shutter speed, aperture, and focus for those meaningful available-light portrait shots, you bought a top of the line DSLR with all the bells and whistles. Of course you paid ten times as much as for the point-and-shoot, but you can do a lot more. Also, admit it, you are a geek and you like dealing with the technical "flexibility" (complexity) of the DSLR.

And maybe you actually own both kinds of cameras!

Well, with telescopes the choices are similar. Do you want something you can pull out, put together in a moment, and point at a dark sky and see wonderful things? Then you're a "point-and-look" person. Just as with cameras, most people are better off starting with this style and trading up if they become more involved with astronomy. Or do you have some experience with technical hobbies and want "the best", a scope that has a computer that will find objects in the sky for you by name, follow their movements across the sky, and allow you to photograph through your telescope? This puts you in the "serious amateur" category, with the attendant price premium. And of course if your Astronomy Bug hit a vein when it bit you, you will probably wind up owning several different kinds of telescopes. I own three, which cost $250, $1800 and $1650. The expensive ones I bought used; new prices for them would be around $3000 each.

Executive Summary:

First buy a copy of the Petersen Guide book A Field Guide to Stars and Planets by Wil Tirion, Jay M. Pasachoff. and start reading it.

To point-and-shoot, get an Orion Short-Tube 80, or go deluxe and get a 6 or 8" (or 10", if you can lift it) Dob new from Orion (http://www.telescope.com). Get at least three eyepieces; a 25mm, 10 mm, and 5 mm. Don't buy cheap "Plossl" eyepieces, maybe even spring for one Nagler (http://www.televue.com). Be sure whatever brand of Dob you buy that it has a metal optical tube (not cardboard or "sonotube") Have a ball.

If you want to do photography, get an old C-8 (Celestron 8 inch Schmidt-Cassegrain) or similar off of astromart (www.astromart.com) with a clock-drive "equatorial wedge" or "German equatorial" mount; an old manual 35 mm film camera; and some Fuji 800- or 1600-speed color film for deep space, or 100-speed for the moon and planets (yes, Fujifilm does still make film) . Get at least three eyepieces; a 40 mm, 20 mm and 10 mm. Get one of them a Nagler. Start off with "piggyback" photography, where the camera is just clamped to the scope while looking at the sky through just the camera's own lens. The mount makes the camera track the sky, allowing several-minute exposures of the Milky Way, etc. Later, get a "T-mount" for your brand of camera and use the scope as a 2000 mm telephoto lens.

Toys

Any telescope advertising its magnification is a toy. A telescope itself does not have a magnification. It is the combination of the scope plus the eyepiece that makes the magnification. More about this below. And magnification is not the main purpose of a telescope anyhow.

Good (even excellent) telescope optics is generally cheaper and easier to manufacture than telescope mechanics like tripods and "mounts". If it doesn't stay pointed and it waggles in the wind, it won't be any fun to use.

Caveat Emptor

Point and Look- the "Dobsonian"

A "Dobsonian" telescope is named after John Dobson ( http://tie.jpl.nasa.gov/tie/dobson/) an eccentric "sidewalk astronomer" who invented the mount (not the scope) to help popularize astronomy. The scope on a Dob is always a "Newtonian Reflector", a very simple design invented by Sir Isaac Newton! This is my clear choice for a first scope; inexpensive, stable, gives great views, easy to use. By far the most bang for the buck. No photography through it though, and it won't follow the stars across the sky, so you have to nudge it every minute or so to follow the stars' rising and setting motion.

Lots of people actually build their own telescopes (http://www.stellafane.com) , and the vast majority of these are Dobs. Orion
(http://www.telescope.com/) is one of the better manufacturers of ready-to-use Dobs.

The "mount" means the tripod or tripod-equivalent, the means of attaching the scope to the tripod, plus whatever provision there is for pointing the thing in different directions. The mount is the weak point of most bargain priced scopes. Even a great scope on an unstable mount that doesn't stay pointed where you set it and waggles in every breath of wind, will be terribly frustrating to observe with and will probably put you off the hobby quickly. Dob mounts are made of plywood or equivalent and they pretty much sit right on the ground (no tripod). This is actually good, because of the way a Newtonian Reflector works. That's why Dobson designed it that way.

When you buy a Dob you are putting all your money into the optics rather than the mount and computer controls. I advise a 6" to 10" Dob for a first scope. Aperture is king-get the biggest aperture you can afford, but don't forget that you also have to be able to lift the thing, and it has to fit in your car! A 10 inch Dob, brand new from Orion, costs $600. This is about the biggest scope that one man can reasonably handle alone. It weighs 58 pounds fully assembled, probably 20 lbs for the mount and 40 lbs for the scope. The tube is nearly a foot in diameter and about 5 feet long. Dob's can go out of optical alignment, but you can have them realigned for about $50, or do it yourself pretty easily.

Why is aperture so important? Simple. Most people think the purpose of a telescope is to magnify things. That is not really true. For several technical reasons, if all you did was magnify the image your eyes pick up you still would not be able to see much of anything in the sky. The purpose of a scope is to act as a "light funnel". Think of it- your eyes each have an aperture ( your "pupils") of about 1/4 inch when fully dark-adapted. That's all the collecting area you have for light from the stars, using the un-aided eye. With a 10" Dob, you collect all the light falling on a mirror that is 10 inches in diameter and funnel it into your 1/4 inch pupil. (The Dob collecting area is actually a little less than the full 10 inch mirror because of the "secondary mirror" inside the tube, but that's a detail) The amount of light collected is proportional to the area of the collecting aperture, so with the 10 inch scope you are getting about 10²/.25² = 1600 times as much light as with the un-aided eye. Now go ahead and magnify that, and you can see amazing stuff!

More Money, More Features

Tracking Mounts:
The only noticeable drawback of the Dob is its lack of "tracking". Everything in the sky rises and sets just like the Sun. That means the stuff you are looking at is constantly moving slowly across the sky. If you want to look at something longer than a minute or so, you have to move the scope. With a Dob you have to do this by nudging it with your hand. Actually, people do put computer controlled motion controls on Dobs but that's too complicated to discuss here.

The lack of tracking also means you pretty much cannot do decent photography through a Dob. The human eye is more sensitive than any photographic film. So while you can look in a scope and immediately see stuff, to photograph anything other than the moon or bright planets, you must use a time exposure of say 2 - 60 minutes. Obviously that isn't going to work if you have to nudge the scope back on target every minute or less.

Tracking mounts come in several varieties, which can be divided into two categories: "alt-az" and "equatorial". Alt-az (altitude-azimuth) tracking mounts must be computer controlled (usually the computer is built into the mount and costs nothing compared to the mechanics of the mount). They follow the stars alright, but in a complicated way that has to be digitally controlled. For technical reasons, alt-az tracking is still not suitable for photography- objects stay in the same place in the view but they slowly rotate because of the way the alt-az tracks.

Equatorial mounts can be driven by a single simple electric motor (powered by a 9-volt battery or other source), or they can be computer controlled. These track objects in a way that faithfully mimics their motion across the sky, keeping both the position and the orientation of objects fixed in the view. If you want to do photography you need an equatorial mount. Equatorial mounts can be either "fork mounted, with a wedge", or "German equatorial". There is no big difference between these that needs to concern us here. For fixed cost, fork mounts are somewhat more stable, but stability is mainly determined by the tripod, not the mount. An equatorial mount must be set up physically (leveled, pointed north) in order to work. With practice this takes 10 minutes for visual observing, more for long exposure photography.

Computer controlled mounts almost always include a "go to" feature that allows you to select an object you would like to view, and have the scope automatically point to it. The computer program has to know where you are on the Earth (accurate latitude and longitude from your Road Atlas), what the time and date is, which way the scope is pointing at first, and whether the mount is level, in order to do this trick. So there is still some setup, though it is mainly entering data. I don't encourage people to use "go to" because I think learning the sky is one of the joys of astronomy. But each to his own.

The stability of the tripod is very important, particularly for heavier telescopes.

Other available features of interest for this discussion have to do with different kinds of telescope optics. I don't want to discuss optics here but I will discuss other kinds of scopes from a user perspective.

Other Types of Telescope Optics:

The Newtonian Reflectors that are Dobsonian mounted are optically optimized for relatively low magnification (60-200x), wide-field views of the sky. This is perfect for the "really cool stuff" you probably want to see- galaxies, nebulae, star clusters, etc. ("deep sky objects"). (Of course, to see these things, you need REALLY DARK skies. You must be at least 50 miles from Philadelphia or other major city; 150 miles would be better. Crisp winter nights with low humidity and a several day old high pressure air mass are by far the best.) These scopes are also "OK" for seeing Saturn's rings, one of the most spectacular sights in the sky, especially this year (2003); Jupiter's moons and clouds; craters on the Moon, etc.

But if you want to see those dust clouds on Mars, you are in a different game. Mars is a small planet and it is far away. You need some serious magnification (like 400x ) to get it big enough on your retina so that you can make out any surface features. But with 400x magnification, Mars will move across your entire field of view in under a minute, so you will have to nudge the scope every 30 seconds or so to keep it reasonably centered. And every time you nudge the scope, even a super-stable Dob mount will take a few seconds to settle down and stop vibrating. Bottom line is, Dob's are not so very good for high magnification viewing of planetary details. There are also technical reasons why the view through a Dob at high magnification tends not to be absolutely tack-sharp ("optical abberations").

The other two main telescope optics types sold for amateur use are "Refractors" and "(Schmidt)-Cassegrains". There is a tremendous amount of marketing material out there about these kinds of scopes, and Meade and Celestron particularly are selling the dickens out of the Schmidt-Cassegrains. The way I look at it is the following.

Refractors: For the ultimate in planetary viewing you definitely want a refractor. And for this purpose, Nature is kind- planets are BRIGHT, so you do not need a lot of aperture to collect light from them. That's good, because large aperture refractors are extremely expensive. For example a 6 inch Takahashi, optical tube only (no mount or eyepieces, some cosmetic flaws) is $3350 used on Astromart (http://www.astromart.com/viewad.asp?cid=202709). But the views! You can see ALL the ring components on Saturn, details of clouds and storms on Jupiter, etc. Magnification as high as 600x could be used on this scope under ideal sky conditions. I have a Televue (http://www.televue.com) Genesis, 4 inch refractor I bought for $1650, complete with mount, used, in perfect condition. Televue makes the best optics available. This thing is a pleasure. It weighs maybe 10 pounds and gives spectacular views of Saturn etc. However it is equatorial mounted, and the mount weighs about 30 pounds and takes half an hour to set up. The whole outfit (scope, mount+eyepieces, tripod) fits in 3 cases. Of my three scopes I use it the most. But because of the small aperture (4 inches) it gives disappointing views of deep space objects, even easy ones. On the other hand, refractors are rugged, never needing optical alignment. And they are perfect for photography because they almost always are equatorially mounted.

Schmidt-Cassegrain Telescopes: Schmidt-Cassegrain Telescopes (SCT's) (and variants Maksutov-Cassegrain, etc) are an attempt to get some of the advantages of a Newtonian (big aperture) into a physically smaller package that can work with higher magnification. In principle they are a good compromise, allowing you to have a large collecting aperture like a Dob, but still have the option of viewing planets at high magnification. If they are equatorially mounted (as opposed to alt-az), SCT's are very well suited to photography.

However generally I would not recommend a Schmidt-Cassegrain for a first scope. In particular, I have had very bad experiences with products from Meade Instruments, the biggest seller of SCT's. Possibly an old Celestron C-8 (on Astromart, under $650) would be good- these discontinued instruments were the darling of amateurs in the 1980's. I have one through Temple University which I have used on Jupiter at over 400x with fantastic results. If you buy one of these, be sure to get one which is "f/6.3" rather than "f/10", or at least an "f/10" with a "focal reducer". Otherwise it is like looking at the sky through a soda straw- very limited field of view makes finding things very time consuming and frustrating.

Telescope Magnification and The Sizes of Things in the Sky

This section is slightly technical, but it will help you a lot in finding things in the sky.

Magnification
A telescope itself does not have a magnification. It has an "aperture" (size of light collecting area, very important as discussed above) and a "focal length". This is a length (in millimeters or inches) that basically tells how much "lever arm" the scope has for potentially developing magnification. And the ratio of aperture to focal length is called the "f-number" of the scope. This number is very important optically but I will not discuss it at all here.

So what determines the magnification then? To look through a scope you must put on an "eyepiece" or "ocular". This is a small removable lens that goes where you put your eye. Without going into the optics of it, the magnification of a given scope-eyepiece combination is calculated by dividing the focal length of the scope by the focal length of the eyepiece. So, smaller (shorter) focus eyepieces give larger magnifications. Smaller focal length eyepieces are also more expensive. Especially for Dobs (which have rather short focal lengths) you need a very short lens to get high magnification for planets. The short one is where you should put your money- buy a Nagler for $200-250 used and you won't regret it.

Sizes of Things in the Sky
How big is the Moon? Just by looking, there is no way to tell how many miles across the Moon is. Key scientific developments in astronomy were held up for centuries because of this simple fact. It is related to the fact that you get no idea whatever how far away sky objects are, just by looking. The Moon could be one mile across and 120 miles away or 2200 miles across and 238,000 miles away (as it actually is) and you could not tell just by looking.

But we can definitely measure the "apparent size" or "angular size" of the moon as it looks in the sky. That's what determines how big it looks, and how much detail we can see on it.

The angular size is measured as an angle. (The angle between a pair of lines intersecting at your eye and tangent to the right and left edges of the moon.) The moon is just about half a degree wide. It is always this same angular size (excepting tiny variations due to atmospheric refraction and orbital abberations). Saturn's ring system is about 15 arc-seconds (.004 degree) wide, with considerable variations on timescales of several months due to the varying distance from Earth to Saturn. (Blame the common silly system of measuring angles on the Egyptians. A degree is 1/360 of a circle, an arc-minute is 1/60 of a degree, and an arc-second is 1/60 of an arc-minute or 1/3600 of a degree. Needless to say, physicists use a different, more logical system ["radians"]to measure angles.)

So Saturn looks REALLY small. If you are looking for it, you should be prepared for something tiny until you zero in and switch to the high-power eyepiece. Easy galaxies are a few to 10 arc-minutes in size (1/10 - 1/2 degree). Globular clusters (my favorite objects) are a few to 10 arc seconds in size, similar to Saturn.

And finally, how big is the patch of sky you see through the telescope ("actual field")? If you know this, and you know the size of the object you are seeking from the fine, extensive tables in your trusty Peterson's Guide, you will have an idea of how big it should look through the scope. This can be a big help in being sure you've found your object! The full answer is slightly complicated, but you get a good idea of the actual field in degrees by taking 90 degrees and dividing by the magnification of your setup. So at 100x magnification (2000 mm C8 with 20 mm eyepiece) you have 0.9 degrees of actual field. The moon will cover half the field of view. Saturn will be a speck, less than half a per cent as big as the field. You will barely be able to tell it has rings, if you have perfect eyesight. Put on your 10 mm eyepiece and the moon won't quite fit in the field any more. Saturn will be 1/50 of the field across and somewhat more recognizable.

Hope this helps.

Dark Skies!!