Buying Your First Telescope

 I have put together these postings before but my go-to place for first telescopes (Orion Telescopes) became a "gone too." going bankrupt and leaving a lot of customers in the lurch on both unfilled orders and warranty work (sometimes warranty work on scope already back with them).

So, you want to enjoy the majesty of the night sky's interesting objects.  First, what types of telescopes are sold for amateur astronomy?  What is your goal?

Goals

For amateur astronomy, you want to gather as much light as you can, magnify the image contained in that light.  This magnification is less important than the big box store telescope boxes (up to 1000x!) suggest.  Resolution is how sharp and detailed an image you will see.

Telescopes need a mount to hold them in place.  At 100x, even a fraction of an inch of movement will move Jupiter several million miles out of the field of view.  Mounts are either alt-azimuth or equatorial.  

An alt-azimuth mount allows you to move the telescope in altitude (up and down) and around (azimuth).  Think of a camera tripod.  For many beginners, such mounts are adequate at low magnification to enjoy the splendors of the Moon:

and for fleeting moments Jupiter, Saturn, and deep sky objects like the Orion Nebula:

If your expectations have been dashed by these photos, let me explain that getting good photos through a telescope is very hard.  None of these photos captures well what you will see.  Saturn's rings will generally be sharp and clear against a distinctly pale yellow planet.  The Moon's craters as small as a hundred miles diameter will be amazing.

If you are only casually interested (often the first step to an unhealthy addiction), you may not need an equatorial mount.  You will immediately notice when you first look at any object in the sky that it is moving!  No, you are moving.  Earth is rotating at about 1000 miles/per hour at the equator.  As the Earth, you, and your telescope move, what you are looking at will seem to move away rather quickly, especially as magnification increases.  An equatorial mount tracks objects across the sky.  Some have a manual control where you turn a knob to keep up.  These are clumsy to use; you really want one with a clock drive.  Once you have an object in the eyepiece, it will stay there until someone bumps into it with their face.

Equatorial mounts are produced in many forms: the only two you will see commercially are German equatorial mounts so named after Wilhelm Herschel (William after he moved to England to be royal musician before discovering Uranus).  These are often sold separate from the telescope.

Fork mounts are in most cases sold as part of a complete telescope:

The other equatorial mounts are part of telescopes you cannot afford:

Equatorial mounts are always the most expensive part; usually more expensive than the telescope, several times as much for the highest quality mounts.  You are paying for stability for when someone bumps the eyepiece or a really big truck drives down your street.  (Yes, I have seen this cause motion.)

The tradeoff is usually weight versus stability.  My sophisticated and intelligent readers know that I am fumbling through machining a small equatorial mount made of carbon fiber composite which will have no tradeoff at all except for cost and time spent making it.

Telescope Types

There are three major categories: refractors, reflectors, and catadioptrics.

Refractors

These are what most people think of when you say telescope.

There is a big lens at the front end (called the objective).  There will be a focuser at the back where you insert any of a number of eyepieces.  The larger the objective, the brighter the image, and the finer the resolution of the image.  Brightness increases with the square of the diameter change.  A 6" diameter objective gather 4x the light of a 3" objective.  Resolution increases linearly with diameter: a 6" objective will resolve details half the size of a 3" objective. This is why every amateur astronomer develops aperture lust. The same is true for scientists:

Palomar 200 inch mirror.

Refractors are usually described by the diameter of the objective.  Common sizes: 50mm, 60mm, 70mm, 80mm, 100mm.  I would consider the 60mm the smallest size to consider for a beginner.  This will show you the Moon in breathtaking detail; the satellites of Jupiter; the rings of Saturn on a night with stable air; the brightest deep sky objects such as the Orion Nebula and the brightest star clusters.  As I mentioned above, I used to link to Orion's website.  B&H Photo & Video is primarily a camera store but they also sell telescopes.  I bought a Televue-85 from them several years ago and I have respect for their integrity.  (I cannot say the same for some of the crooks in New York City whose ads you will find in camera magazines.)  You may have a hard time placing orders starting Friday evening or on Jewish holidays.  They even give their computers the day off!

Types of Refractors

Oh dear, more complexity?  There are two types of refractors suitable for the amateur: achromats and apochromats.  Glass lenses break light up into its constituent wavelengths.  This page has some lovely illustrations, which I will not steal.  The longer the focal length, the better it is.  This produced in the 17th century some hilariously Rube Goldberg telescopes:

John Dollond, a British optician invented the achromatic telescope in the 18th century.  Achromats combine two different types of glass attempting to bring all the colors back together at the eyepiece.  Some are pretty good at it. None are perfect.  You will always get some color fringing on objects.  The brighter the object, the more fringing.  This is not just cosmetic: all those colors on top of each other will impair resolution.

The next step are apochromats.  Originally made from fluoride glasses, they bring everyone back together so well that you will never go back to achromat, if you can afford an apochromat.  I have two apochromat refractors.  They are optically gorgeous.  

One is mechanically not so nice; it started as an achromat.  Then I upgraded it with a piece of wonder optics that came out of the collapse of the Soviet Union.  A now unemployed Ukrainian who had been part of the Soviet  military-industrial complex developed an ingenious corrector that turned a ho-hum achromat into a flawless apochromat for about 1/4th price.  (He seems to have gone silent; he may be building wonder optics to defeat his former employers.)  See here for a review I wrote.

If you are just a beginner, a long focal length (f/10 or f?15) achromat is likely the best choice.  The equivalent apochromat will likely delay you getting into this hobby, unless you hit the Powerball or have rich relatives.

Reflectors

Unless you know an amateur astronomer, you may not recognize this:

The mirror at the bottom of the tube is the objective.  All the same rules about size, resolution and brightness apply.  Because there is that mirror in the middle of the tube, reflectors are seldom as bright or high resolution as a refractor the same diameter.   But you can afford an 8" Newtonian reflector.  You probably cannot afford an 8" refractor, even an achromat.  Also reflectors are naturally apochromats.  There is no glass through which the light passes.

Two Types Again

You will see references to Newtonian reflectors and Dobsonian reflectors.  They are all Newtonian reflectors.  Dobsonians are named after John Dobson, a Buddhist monk in San Francisco who wanted a telescope.  (Was that on your bingo card for today?)  He figured out a way to make one from very cheap parts, including use of a very simply ingenious alt-azimuth mount.  

Soon, the craze dragged telescope makers in.  You can buy a startling large aperture Dobsonian at a price barely imaginable in 1968, when I first wanted a telescope.  The downside is you need to get good at adjusting position as the sky turns.  Up a little, over a little.  Use low power eyepieces with a large field of view and this is not that hard.  There are tracking devices called equatorial platforms that will make it track across the sky.  I am not taking that tangent right now.

Traditional reflectors sit on equatorial mounts. Prices reflect this complexity and ease of use.

Catadioptric

These are telescopes that combine a big piece of glass and a mirror in a surprisingly compact package.  As you might expect the price reflects this.  They are usually spectacularly well made.  There are many variations depending on what fiendishly clever optician figured out to make it: Schmidt-Cassegrain, Maksutov, Bird-Newtonian.

Eyepieces

These go at one end or the other.  These magnify the image to a useable size.  Magnification is calculated by dividing focal length of telescope by focal length of eyepiece.  2000mm/25mm=80x.  Your telescope will come with at least one eyepiece, sometimes several.  Cheaper telescopes cheap out on cheap eyepieces.  I will post about eyepieces if anyone asks for information.

UPDATE: Along with other vendors linked here AgenaAstro provides a range of telescopes from reasonably priced beginner telescopes to serious amateur products with price tags to match.

Televue is an American maker of excellent but pricy refractors.  Not absurdly so, but only in the beginner category because you finally made the last payment on your jet 

Astro-Physics is a premier money no object maker.  You put down a deposit years in advance of delivery.   People have been known to sell their spot in line although the company frowns on this.  They are expensive enough that even I have to draw the line on this side.