Chris McKitterick (mckitterick) wrote,
Chris McKitterick

Chris' telescope buyer's guide 2012.

A friend suggested that I offer some advice on buying a telescope as a gift for a first-time astronomer. Well, the holidays are fast approaching, so I thought he might not be the only one who could use such advice. Thus my first-annual Telescope Buyer's Guide!

Assuming that a telescope gift must fit a modest budget, my first note is a caution: Do not buy a Wal-Mart telescope! Also beware of "tabletop telescopes," because your view is only as nice as the mount is stable. Remember, if you're using 32x magnification, that also multiplies any vibrations by 32 times. Anything that costs less than about $200 will give you more headaches than pleasure, and ruin the young astronomer's feelings about this wonderful activity.

So, a few basic guidelines.

Optical Design

If purchasing a refractor telescope (one that uses a lens for the primary objective on the business end, with the eyepiece at the opposite end), start looking at one with an objective lens at least 60mm diameter (about 2-1/2"), preferably 90mm. Here's a nice example, the Celestron Omni XLT 102ED (that's 102mm aperture, and better-than-average ED glass):

Click the image to see this telescope's web-store page.

Refractors are simple to use but, at the low-priced end, give false color due to the primary-lens design. Achromatic (and apochromatic) designs reduce (or even eliminate) this flaw... but at a cost. Because they have no central obstruction in their light-path as with a reflector, they provide brighter, sharper images at smaller objective sizes. Here is a fantastic apo refractor that is also very portable, the Explore Scientific 80mm f/6 ED APO:

Click the image to see this telescope's web-store page.

Which is similar to the Meade ETX80AT (their refractor on the popular ETX robotic mount):

Click the image to see the OPT web-store page.

Reflectors vary a lot in design. The current most-popular style is the Schmidt-Cassegrain (SCT), which uses a mirror as primary in the rear of the tube that bounces off a small secondary mirror at the opposite end (which in turn shoots the light down the middle to your eyepiece at the rear), plus uses a sealed corrector lens at the business end (called the "aperture" - the diameter where light enters). A useful SCT starts at 90mm diameter (about 3-1/2") and can grow much larger. Here's a nice example at the small end of the spectrum, the Celestron NextStar 90:

Click the image to see this telescope's web-store page.

A similar 'scope is the Meade ETX90:

Click the image to see this telescope's web-store page.

Both are excellent starter instruments that easily track the sky and even have motorized, automatic go-to features, and are both affordable. I wouldn't go smaller than 90mm in an SCT. Get the nicest one you can afford! That's still portable, anyway; my 12" Meade LX90GPS weighs about 120 pounds, whereas an ETX90 only weighs about 20 counting the mount and everything. Here it is at sunset during the 2012 Venus transit of the Sun:

These are typically the easiest reflectors to use because they're so compact, and their shorter tubes make them even more manageable than a refractor. My 12" Meade LX90GPS (that's 304mm) would have been a nice university observatory instrument in the 1970s with its GPS locating and tracking and massive go-to database of astronomical objects, but now anyone willing to spend a few thousand bucks can buy one and enjoy hero views of the universe. The SCT optical design allows for cheaper mirror production, allowing you to get a lot more 'scope for the money.

You can find many other types of reflectors out there, but the Newtonian design (attributed to Sir Isaac) used to be the most popular, and provides the greatest light-gathering power for the dollar. It uses an open tube with a primary mirror opposite the open end, and a secondary mirror set at a 45° angle, which bounces the light into the eyepiece set at the top-side of the tube. Here's a nice example of a useful Orion AstroView 6" Newtonian reflector on a German equatorial mount:

Click the image to see this telescope's web-store page.

I wouldn't buy a telescope smaller than 6" aperture (152mm) in a Newtonian if you want the gift-recipient to enjoy it for more than a short time. Though a 4-1/2" Newtonian - a popular entry-level aperture - will provide lovely views of bright objects, it has very little resolving power (see below).

The Newtonian layout allows for the most-comfortable viewing positions and very stable mounts; to use a refractor or SCT at a comfortable viewing angle, you need to prop it up on a tall tripod, whereas a Newtonian can sit practically on the ground if the optical tube is long enough. On the other hand, it's also the least-compact design, requires occasional collimation to ensure the mirrors are aligned, and gets dirty inside faster. On the third hand, it's also the easiest to mount cheaply, because the heavy part sits low to the ground...


...thus was born the Dobsonian design, popularized by the sidewalk astronomer John Dobson in the 1970s. It looks like this Orion SkyQuest 6" Dobsonian (which uses the Newtonian optical design):

Click the image to see this telescope's web-store page.

Here's my other telescope, a present for publishing my first book. It's a Meade Lightbridge 16" Dobsonian reflector, with a Newtonian optical design. Because it's so large, instead of a solid tube it uses removable struts. This makes it lots lighter and WAY more portable - so much so that this giant can fit into a small car:

Click the image to see this telescope's web-store page.

On the other hand, it's a pretty large and heavy 'scope, not something I'd recommend for beginners. But if you have the space to store it and the strength to carry around 60-pound components (the base, or the rear mirror-box), a large Dobsonian like this is far less expensive for the optical performance than any other design. Almost anyone, however, can get a lifetime of pleasure out of a FAR smaller 10" or 12" Dobsonian, at a lot less cost, too.

My second telescope was this Crown Optics 6" f/8 (that means the focal length was 8 times the diameter of the mirror, making the tube 48" long) Newtonian reflector on a German equatorial mount with clock drive (which required a REALLY LONG extension cord to track the sky):

The German equatorial design is a bit more cumbersome and a lot more expensive than the Dobsonian box, but it allows for precise tracking of astronomical objects. These days, you can get one with battery-operated clock drives to counter the Earth's rotation and even GPS-precision go-to and tracking.

My first telescope was a very simple and completely manual Edmond Astroscan, a 4-1/2" rich-field (meaning "low power") Newtonian on a ball-and-socket mount:

Click the image to see this telescope's web-store page.

It was ultra-portable, foolproof to use, and nearly indestructible, but really limited in what it could reveal of the heavens. I quickly moved on to the Crown Optics 'scope, wanting more sky and something with a clock drive. It didn't have GPS or go-to, but did track the movement of the sky (countering Earth's rotation), so objects stayed centered in the eyepiece for long-time viewing, star parties with lots of people wanting to look, or astrophotography.

German equatorial mounts do require some setup, whereas the GPS-driven go-to instruments make setting up for a night's observing much simpler. Not nearly as simple as something like the Astroscan or a Dobsonian-style telescope, but way more useful for beginners and experts alike.

What You Can Expect to See

I would recommend one of the simpler, smaller instruments for a first-time astronomer: One of the 90mm or 120mm SCT designs is great for super portability and ease of use. A 6" or 8" Newtonian on an equatorial mount will show you every single nebula, star cluster, and galaxy in the Messier catalog, plus lots more. It'll also show you every planet in the Solar System, though Uranus, Neptune, and Pluto require a good eye to distinguish from stars in the eyepiece. Reflector designs cost a lot less than equivalent refractors.

Want to mostly view the planets, the Moon, and other bright objects? A really nice refractor will give you the best images, and you don't need much light-gathering power. More important is the focal length: In this case, longer is better. You want a focal-ratio of f/8 or longer. SCTs are almost all longer than that, as are low-priced refractors and many smaller Newtonians.

Want to get really nice views of those same objects, be able to magnify them more? Then you need a bigger aperture for better resolution.

Want to go deep-sky, walk across galactic spirals and float through vast nebulae? Go with the biggest reflector you can afford, and aim for shorter focal lengths. They call these "rich-field" instruments, because the field is wider and brighter.

Oh, and if you want to study the Sun, you need dedicated equipment. My SCT uses a big mirror over the open end, which blocks most of the light. This is the cheap route, and provides nice views of sunspots. Optimally, you'll get yourself a dedicated solar telescope if you want to spend a lot of time with the Sun; I just got the older (now out of production) double-stacked version of this one, the Coronado SolarMax 40mm Hydrogen-alpha solarscope:

Click the image to see a similar solarscope, the Coronado PST 40.

Where a go-to instrument becomes worth the investment is hunting down dimmer and harder-to-identify objects. They can take a long time to find without a smart telescope pointing the way. On the other hand, like me you might enjoy the satisfaction of finding things yourself. If so, go crazy and get yourself a Dobsonian, which for the same money will reveal many more wonders of the deep sky. Typically, you get two or even three times the optics (diameter, thus light-gathering power) in a Dob than in an SCT or Newtonian, or many times that of a refractor.

A Note on "Power"

Cheapo supermarket telescopes will declare "600x POWER!" and such on the box. That is nonsense. Any 60mm refractor (or 120mm reflector) cannot give you that kind of magnification unless you're using it on a mountaintop, free of humidity and near the edge of the atmosphere. Useful magnifications are around 10x to 40x for most viewing; if you want to roam across the surface of the Moon, play around with different eyepieces to bring yourself "closer" (higher magnification, with worse resolution) or to show more of the Moon (almost no magnification at all, with sharp resolution). The Orion Nebula is stunning in my 12" telescope, but only visible at super-low magnifications.

Real telescope "power" comes from three things:

1) Its ability to gather a lot of light and squeeze it into your eye. The wider the diameter of the objective lens or mirror - the greater its surface area - the more light it gathers. For example, a smallish 70mm objective has 100 times the light-gathering power of your fully dilated eye!

2) Its ability to collect detail. The greater the diameter of the objective, the finer the telescope's resolving power (limited by atmospheric seeing). On any given night, a larger telescope will provide more-precise views of objects and reveal more hidden detail than a smaller one. Up to a point. Dust, humidity, clouds, and wind turbulence all affect this. Observatory-scale telescopes are greatly limited by these things, which is why they live high atop mountains and use fancy software to correct the photographs they take.

3) Its ability to reveal minute detail. Here's where magnification comes into play. Remember, most of the time you don't use much at all - I almost never hit 100x, and can seldom use that much under crappy Eastern Kansas skies. However, the longer your focal length (length of optical path - twice the length of the tube in an SCT, or about the length of the tube in a refractor or Newtonian), the more detail you can find by increasing your magnification. Shorter eyepiece focal lengths give you higher magnification (you divide it into the telescope's focal length to find the x), but you can seldom use an eyepiece shorter than 12mm. My favorite eyepiece is a 32mm monster with huge field of view, because its images are so sharp. So the bigger your primary, and the longer your f/ratio, the higher USEFUL magnification you can get from a USEFUL size eyepiece.

Okay, that one might have gotten a bit complicated, but you see my point. Don't use advertised magnifying power as a selling point. Your 8x binoculars are about right for 40mm objective lenses; more power, and you couldn't hold the image still, and it would be blurry. Low power is best for almost everything.


People often forget that the eyepiece is just as important as the objective lens or mirror. Get at least two; I'd recommend a mid-power (14mm - 18mm) and a low-power (24mm or larger) for starters. Three is optimal, ranging from 12mm to 40mm or larger. Really short focal-length eyepieces are a pain to use and don't provide much benefit. Really long focal-length eyepieces get pricey, because they use large lenses.

Most beginners (and those wearing glasses while using a telescope) need at least 15mm of eye relief to see the entire field. This means the image formed by the eyepiece is visible from up against it out to 15mm away. The longer, the better.

Aim to get the widest apparent field of view you can afford. Low-priced but nice Plossl designs typically provide 50° or so AFOV. Really nice multi-element designs can provide more than 100° AFOV, providing a sort of "spacewalk" feel.

Finally, make sure it's both fully coated and multicoated. This means all the lenses are coated on both sides against stray light, damage, and so forth. A nice foldable eyecup is handy. Oh, and get yourself an eyepatch to cover the eye you're not using, because squinting reduces your ability to see very well and is fatiguing.

This is the updated version of the eyepiece I use most of the time:

Click the image to see this eyepiece's web-store page.

It's a 24mm focal length, so medium power. It has an 82° AFOV, which is HUGE. Even though it has a complex lens system, it gives sharp images (probably because it's fully and multi-coated). And it's great for everyone to use with 17mm of eye relief.

Here's a nice assortment of nice eyepieces available through OPT, a place I buy stuff from a lot. Sort by price to see how little or much you can afford. Remember, it's tough to go wrong by buying REALLY NICE eyepieces, because that's what makes all the difference. A great telescope's images can be killed by cheap eyepieces, and an assortment really enhances the experience.

Final Recommendations

If you have a big budget and want to get a 'scope that'll last a lifetime, consider a 120mm apochromatic refractor, an 8" or 10" SCT, or a 10" or larger Dobsonian. These all cost about the same - let your goals be the guide!

You can get a nice telescope at a lot of local optics shops, plus online dealers. My favorites are Oceanside Photo & Telescope (OPT) and Orion Telescope, both of which I've linked to frequently from the photos above. The two biggest manufacturers are Meade and Celestron, plus Orion.

Finally, remember that patience on the part of the user is the most-important element of a good night's observing. If your gift recipient is young or easily distracted, aim for easy to use, portable, and something that'll provide dramatic views.

In short, spend as much as your budget allows to get the greatest aperture, best mount, and nicest eyepiece assortment. Too small an aperture = dim, fuzzy views. Shaky or challenging-to-use mount = awful experience. Cheap eyepieces = nonexistent or crappy views. But if you go TOO crazy in size or weight, that can also kill portability.

Click the image to check out a star-party page.

I hope this helps!

Tags: astro-buyer's guide, astronomy, telescopes

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