How To Pick a Telescope
Advice from an astronomy Nut on What Not to Buy When it Comes to Telescopes
I see this over and over again, especially around the holidays, special astronomical events etc-the bargain telescope with all the bells and whistles just waiting for you to purchase at your local department store. Well I'm here to tell you think before you buy! Most consumers have no idea what they are buying when it comes to telescopes, and what you don't know will take away your joy and your hard-earned money.
There are many bad name-brand telescopes, and some not-name brand that are what you really want to enjoy stargazing, especially if you have kids. Above all, it's important to remember-you get what you pay for! Don't go cheap! It's like putting your $100 on the green spot on a roulette wheel!
What kind of telescope should you consider, and what type of mount should you get with it?
The 3 main telescope optical designs on the consumer market include (1) the refractor (the oldest design), the reflector (Sir Isaac Newton yes!), and variations of what is called catadioptric, or occasionally "mirror lens". There are advantages and disadvantages to each design: Refractors come in many varieties, sizes and lengths, and depending on the optical design can be fairly inexpensive or incredibly expensive. These telescopes are composed of a large lens at the front called an objective lens, and generally have at least 2 elements designed to minimize chromatic aberration, which can cast purple or yellow fringes at the edges of of whatever you are looking at. In the old days before the exotic glass that exists now, designers reduced this problem by making a very long focal length (thus the very long tubes of some of the old telescopes). Most of the higher quality new designs incorporate what is called "Extra Dispersion" glass, which helps to produce what is called "color free" images, or apochromatic (often abbreviated as APO). Unfortunately this can make an evenly modestly sized refractor very expensive. The main advantage to the refracting design is the very high contrast images it can produce, making it desired for planetary observation. If you live in a city, where the sky is washed out by the lights and all you can see is a few bright stars and the planets, a 3-4 inch (80-100mm) medium focal length refractor might be just the telescope for you.
If you want bang for the buck, it's hard to go wrong with a 6-10" reflecting telescope. This design came from Isaac Newton, and incorporates a primary parabolic mirror and a 45 degree angle secondary mirror suspended in the optical tube directing the image to a focuser mounted on the side. This telescope can produce excellent images nearly as good as a high end refractor (provided the primary mirror is made accurately), and the price per inch of optical diameter is much less. If you live outside the city, or want to travel to a darker site to observe, a dobsonion mounted reflector would be my choice. Some of the disadvantages of the reflector include inferior images produced is the optics are not made well, can be awkward to use due to the location of the focuser, and equatorially mounted versions need a good quality mount to be usable. Almost all of the professional observatories around the world use some variation of the reflecting telescope!
Finally there is the catadioptric telescope, which simply means it incorporates both mirrors and lenses in it's design. The primary reason to own this telescope is it is much more compact than either the refractor or the reflector. You can with this design have a very large diameter telescope in a short tube length, making it in most cases portable. The design originated in different forms with Bernhard Schmidt (Schmidt Camera), James Baker, and in a different form from an optical designer named Maksutov (a Russian) and a variation from John Gregory, an optical expert, musician, and general renaissance man who lived in the town I live in now (Dripping Springs). The modern version which incorporates a thin "corrector plate" in the front holding a convex mirror and the primary mirror in the rear was brought into production by Alan Hale. You know this well known telescope company today as the Celestron Corporation. In general, the difficulty and precision in building this telescope requires very tight tolerances in the design and manufacturing. There are very good examples of this design and some very optically poor examples. Amazingly today Meade, Celestron and other manufacturers build consistently good quality telescopes of this design. Disadvantages: Optically inferior to the best examples of reflectors and refractors on the market; require precise collimation of the optics to provide good images; prone to dew collecting on the front corrector plate or meniscus. On the plus side is they require a smaller mounting due to the reduced weight; are very good for astrophotography; and can be optically very close to the other two designs if it has high quality optics.
All of the above means very little if the telescope is not mounted on a good quality mount. There is nothing more frustrating when trying to see the faint bands on the surface of Jupiter at 300x if you have a shaky mount that never stops moving around! This is one of the reasons I believe that the first telescope someone owns should be a 6 to 10" dobsonion mounted reflector, an 80-90mm APO refractor on an alt-azimuth mount, or a 6 to 8" schmidt cassegrain with a good quality GOTO mount.
All of the the above telescopes recommended will give good service and allow you to see most if not all the faint "fuzzies" in the sky.
Here's a Better View of How the Optics differ - 3 Basic types of Telescope Designs
The picture below shows the optical layout of the refractor, reflector and cassegrain telescopes
A Very Nice Dobsonian and an Extremely Nice Refractor
2 examples of the higher end of consumer telescopes-you need some deep pockets to own these babys!
A Large Equatorially Mounted Reflector
Here's a very large classic Newtonian reflector mounted on a heavy duty equatorial mount. Equatorial means the mount is designed to follow the motion of the celestial equator, so that only on motion is needed to keep a celestial object in view.
What I DON'T Use For My Observing and Astrophotography - This is NOT a suitable telescope in my opinion-the mount is inadequate!
I, like most avid astronomers are constantly looking for that best combination of optics, mounting, accessories, eyepieces, cameras etc. But to be sure bigger is not always better. When choosing a telescope, the most important question to ask is where and how you will be using it. If you live in the city, and need to go out of town to stargaze, a 12 Schmidt Cassegrain or 6" Refractor on a massive equatorial mount will be worthless for you, and ultimately a waste of your hard earned money. If you are just starting out, it's hard to beat a 6 or 8" dobsonian mounted reflector. These telescopes are portable, and more importantly have ample light gathering capabilities.
The main problem with this telescope is it is under-mounted, and will exhibit excessive shakes and vibration in use. It would be much better to splurge for a CG-4 or CG-5 type mount, which will have better stability and damping of the shakes and wiggles.
Another item of importance is the focuser and eyepieces. Usually a marginal focuser and eyepieces are included. I recommend a 2" focuser with a wide angle (25-30mm) and a mid power (10-12mm) eyepiece. Remember that any telescope can go to extreme maginification, but useful magnification is a function of both the size of the telescope objective (and quality), and of the quality of the atmosphere (called seeing).
Planetary viewing is the most critical test of a telescope's resolving power-and this is where a mid size refractor can excel if made well. Even a 90mm to 100mm refractor can show a plethora of detail on Jupiter and Saturn when the seeing is ideal. Of course part of the challenge to see small details on the planets depends on training your eye to see fleeting detail that comes and goes from second to second.
DON'T Buy a Telescope Mounted Like This One!
This is what I call a dust collector-you buy it, get frustrated trying to use it, and then it sits in the corner of a room in your house collecting dust! Don't waste your money on something like this! A little research will yield good results, and a happy newbie to the amateur astronomer ranks. More importantly, you are trying to gain interest in this hobby from your walking hormone of a teenager, so it's important that you get something that will deliver the goods: AKA, the night sky the way it is supposed to be viewed.
While it's good to have slow -motion controls, this mount is inherently imbalanced, and you will constantly be fighting to keep it from flopping over as you try to move from one celestial object to another.
The popular trend in mounts is the GOTO computerized mount. These can be either your best friend or your worst enemy depending on the ease of use. I know several people who bought the cheap version of this mount and could never get it to GOTO ANYTHING!
If you do want to buy a telescope with this mount, Celestron's NextStar is easy to use, and Meade's new LightSwitch is unbelievably easy to use. But for me, being old-school, sometimes I like to just randomly point my scope to a portion of the sky or work on my Star-Hopping skills.
And remember, these GOTO mounts require electricity to operate, whether through batteries or ac adapter-if you are out in the middle of nowhere with your scope and the power dies, the mount useless at that point.
If money was no object for a quality equatorial mount, I'd purchase either a Astro-Physics or Takahashi GOTO version. The quality of these mounts is next to none, but they will set you back many greenbacks!
GOTO Mounts from Amazon
Stars over the Prude Ranch 2010
Time lapse I took of the Milkyway rising over Ft Davis TX Prude Ranch during the Texas Star Party of 2010
A Much Better Choice of Refractor Telescope Here
If you are looking for a highly portable, and great quality telescope, I'd recommend something like the Astrotech shown here. A 3 to 4 inch ED glass objective, nice focuser and a stable, easy to use mount is perfect for the grab and go telescope which works good for daytime use too. Astronomics.com and Anacortes (buytelescopes.com) are 2 excellent resources for telescopes. Now you will have to purchase the optical tube and mount separately, but the nice people at these stores will be glad to help you. No, I don't work for them or get any commission-I've used both of the stores with superb service. Orion Telescopes (telescope.com) is another fine place to shop with great service. Other quality names include Takahashi, Astro-Physics, Stellarview, William Optics (I own a 90mm Megrez from him), Televue, TMB.
Eyepieces and other Accesories
An often overlooked part of the beginner's telescope are the quality of the eyepieces. Most of the department store "bargain" telescopes come with very poor quality eyepieces. At the very least make sure that they are 1 1/4" focuser diameter, and if the telescope comes with a 2" focuser even better. Fortunately these days there are a plethora of good to great eyepieces to chose from. If money is no object, it's hard to beat TeleVue for the quality. They have the "Ethos" design which gives 100 degree apparent field of view! Which brings me to another point-for viewing pleasure and the "spacewalk" effect go for the wider field of view eyepieces. I'd suggest as a minimum of 60 degrees FOV (field of view) especially for your low and mid power items (12-30mm).
As a quick lesson on eyepiece design and use, the magnification of a telescope is determined by the focal length of the telescope divided by the focal length of the eyepiece. For example, lets say you have an 8" (or 200mm) diameter schmidt cassegrain. Most of these would have a focal length of around 2000mm. A normal wide angle eyepiece for this telescope would have a focal length of 25mm. Dividing 2000mm by 25mm gives a resulting magnification of 80x. Cutting the focal length of the eyepiece to 12.5mm gives a magnification of 160x.
Many of the new eyepieces are designed to give a wide FOV which not only gives a great view through the telescope, but makes it easier to find objects especially when using the "Star Hopping" method for finding deep sky objects. But generally speaking these eyepieces have increased optical abberations as compared to a 50 degree FOV Plossl design. If you spend most of your time planet and moon watching, a planetary type eyepiece designed to maximize resolution and contrast may be the better choice.
My recommendation for the novice starting out is to purchase a good quality 25-30mm 2" diameter wide angle eyepiece for low power work (provided your telescope has a 2" focuser), and then get an 8 to 10mm mid to high power eyepiece. Finally add a good quality Barlow lens, which is designed to double or triple (depending on the design) the magnification of the eyepiece in use.
Telescopes I Own (or Owned)
What you see here are a couple of telescopes I own or owned-a 6" Astro~Physics apochromatic (color-free) refractor, and a Meade 12" Schmidt Cassegrain folded optics telescope. They are both on Celestron equatorial mounts.
I've been through the gamut of telescopes in my life, small to big. I've built several telescopes, including grinding my own mirror for an 8 inch reflector when I was in high school. I actually won the regional science fair my senior year for building that telescope!
My final advice is to do some research-don't get caught up in the magnification hype, or the flashy appearance of department-store telescopes. The 3 main items of importance when buying a telescope are: 1) quality Optics, 2) a stable mounting for those optics, 3) and good quality eyepieces. It will be immediately apparent the first time you use your telescope if it doesn't meet the basic items above.
I recently sold the 12" Meade and the mount that the refractor is on in this picture.
My C-14 on CGE Pro Mount - My current setup
I have nice and dark skies where I live now, so I decided to cast me an 900 lb pier and mount my latest telescope on it. It is a Celestron C-14 schmidt-cassegrain with a very large German equatorial mount. I'm still working out the tracking on it, but it works very well so far.