Friday, December 3, 2010

Buying Telescopes For Christmas

I have been asked this question before, so I thought that I would gather up my various emails on the subject and distill them into a blog posting.

One reader asked about a telescope for terrestial use--he had a very nice view from his office.

Keep in mind that under daylight conditions, and with the typical heat turbulence of daytime, you will probably find that few telescopes are going to be particularly useful above 100x.  However, even at the same magnification,  the image resolution goes up linearly with the diameter of the objective (the front lens on a refractor, the mirror on a reflector).  A 6" refractor will resolve quite a bit better of an image than 3" refractor.  If of equal quality, a 6" refractor will give at least three times the resolution of your 20x50 binoculars.  Of course, astronomical refractors (except for the very cheapies) are often much higher optical quality than binoculars.

For daylight use, I would strongly recommend a refractor.  Most reflectors aren't really adequately baffled for daytime use.  The few exceptions tend to be $$$$$$$$$ like the Questar.  Reflectors also tend to require a bit of adjustment to keep them optically aligned.  A refractor usually does a fine job for decades on end.

What you can realistically expect from an 80mm refractor (which is probably in about the price range that you are thinking of) is to see Saturn's rings, some of the brighter deep sky objects (such as the Orion Nebula, the Hercules globular cluster), Jupiter's bands and satellites, the phases of Venus, and more Moon than you can handle!

A few warnings.  There are a lot of short focal length refractors that are primarily intended for daytime use, and which can be used for some casual astronomical purposes, but I would not recommend them.  Example:

I would not consider any refractor with less than an 80mm objective as adequate for beginning astronomy.  If you go to Orion's web site you will find that you have a stack of choices: 80mm, 90mm, 100mm, 120mm.  The 120mm refractors start to gather enough light to pull in some of the deep sky objects that aren't even going to be visible with the 80mm.  Remember that light gathering power increases with the square of the increase in diameter, so the 120mm refractor will bring in 2.25x more light than the 80mm refractor.

Refractors are sold in one of three mounting arrangements:

1. As an Optical Tube Assembly (OTA), and usually these mount to a 1/4"-20 camera thread on an existing camera tripod.  Anything big enough to be useful for astronomy is going to be too big to put on a camera tripod without driving you crazy from vibration.

2. On an altazimuth mount, which is easy to use for daylight observation, and usable for astronomy--but barely.  You will find that you will be constantly chasing objects across the sky at higher powers.

3. On an equatorial mount.  Usually these include a motor that lets it track objects across the sky.  For astronomy, you really want this.

Back to the problem of short focal length.  The majority of the refractors in your price range are called achromatic; this means that there is some chromatic aberration, but except on bright objects, or at high power, this won't be a big problem.  See here for a more detailed discussion of the problem, and one of the solutions to it.  Here's the other, more expensive solution.

The shorter the focal ratio of an achromatic refractor, the more severe the chromatic aberration gets.  You will find plenty of f/5 and f/6 achromats offered; I would avoid these if you intend to use them for astronomy.  An achromat needs to be f/8 or more to get the chromatic aberration under control.  This is why refractors, especially the Japanese made refractors that used to be common, were typically f/12 or f/15.  Here's an example.

There are what are called "apochromatic" refractors as well.  These use exotic glasses to get rid of the chromatic aberration (or reduce it to a level that you have to really look to find)--and often with shorter focal lengths, such as f/6.  And they are priced accordingly.  Here's an example.

The upside of the apochromatic refractors is that they will often allow to use a lot more magnification, not just without color, but showing much more detail.  See here for a comparison test I did some years ago.  The result is that an 80mm apochromat will usually show more detail on Saturn or Jupiter than a 120mm achromat--because the apochromat will allow you to use far more magnification without the image getting fuzzy.  The downside is that an apochromat still gathers the same amount of light as a achromat of the same objective diameter.  An 80mm apochromat will do a GREAT job on Jupiter, the Moon, or Saturn, compared to a 120mm achromat.  But when it comes to deep sky objects (such as the Orion Nebula, or the Ring Nebula), where you need light gathering power, but not necessarily enormous magnification-- the apochromat is going to disappoint.

If keeping it below $400 matters, I would look seriously at this collection.

If you can go a bit higher, this set will definitely take into significant deep sky object territory.

Another reader asked a similar question.

First of all, telescopes tend to be addicting.  Bet you can't buy just one!

As a general rule, any quality refractor above 60mm aperture (or quality reflector above 4" aperture) will resolve Saturn's rings quite adequately--especially if the Moon isn't up.  (The Moon is a major obstacle because it is so bright.)  Most of the department store telescopes are pretty unimpressive--but oddly enough, the deficiencies are typically the eyepiece and the mount.  See here for a discussion of cheap vs. good.

I'm very partial to the Televue refractors, but they aren't cheap (made in the USA).  I've been very pleased with the quality of the telescopes that Orion sells.  This is about the cheapest real telescope that is likely to satisfy your needs.  I encourage beginners to look at refractors, not reflectors, simply
because reflectors require more expertise to keep collimated.  Most bad reflectors are not really bad--they just need proper adjustment.

I would also suggest that you look at and for used telescopes.  There are often some pretty decent deals for used telescopes in this area.  I've never found anyone pulling fast ones with stuff they sell at's still pretty much a bunch of astrogeeks buying and selling stuff.

Yet another reader asked:
I have a 7 year old son, and I was wondering if you could recommend a telescope for moon and stargazing.  We do not want to buy him the Hubbel for Christmas, but neither do we wish to buy a mere "toy." Can you recommend a decent one?
Orion Telescope Center does a good job in the reasonable, but not toy category.  (Most of what is sold in department stores is pretty deficient.)  I usually recommend that a good starter--especially for a child--is something like this.   Refractors are less fussy about alignment and maintenance.

It's enough telescope to be wowed by the Moon, see the rings of Saturn (although we are just about edge on for the next couple of years, so they will be a disappointment for a while), a few of the brighter nebula (M42), Jupiter's satellites and cloud bands (if the atmosphere here is reasonably stable--dependent on location).  In general, the more you go up in aperture, the better.  80mm or 90mm refractors show a lot--just make sure that you aren't buying anything that Orion calls "short focus" because those are generally better suited to terrestial use.

Any suggestions on good "Telescopes for Dummies" type references? I know photography, but the parts of a telescope are confusing for me and I don't want to make the same mistake a lot of people do when buying cameras (they spend too much on the body and not enough on the lenses!).

Again, Thanks a Ton,
In brief: astronomical telescopes consist of:

1. An Optical Tube Assembly (OTA).
         1.1 Refractors (big lens at the far end; eyepiece at the near end).    
         1.1.1 achromatic: flint and crown glass objective; unless f/10 or longer, usually have a lot of chromatic aberration on bright objects.
        1.1.2 apochromatic: exotic glass fixes the problem, but at an exotic price (usually: see this article)
        1.1.3 semiapochromatic: some refractors advertised as apochromatic are close, but not quite as pricey
   1.2 Newtonian reflectors (big mirror at the bottom of the tube, diagonal near the top of the tube, eyepiece where the diagonal reflects light to).
   1.3 Catadioptric (big mirror at the bottom of the tube, corrector plate at the top, eyepiece peers through a hole in the big mirror).
        1.3.1 Schmidt-Cassegrain
        1.3.2 Maksutov

For the same size aperture AND QUALITY, Newtonians are cheapest, followed by catadioptric, then refractors.

2. A mount.
   2.1 Equatorial mount: tracks objects across the sky using an electric motor.  Necessary for astrophotography.
      2.1.1 German equatorial mount: consists of two axes at right angles, with a counterweight at one end of declination shaft, and telescope at the other end.
      2.1.2 Fork mount: the telescope fits between the two tines of the fork.    2.2 Alt-azimuth mount.  Adequate for visual observing at low power; generally weighs less and cheaper than equatorial mount.  Sometimes come with go-to
         electronics to find objects.  Dobsonian telescopes are Newtonian reflectors on alt-azimuth mount.

3. Eyepieces.

4. Finder
   4.1 Traditional finder scope is a small telescope that you align with the main scope.  This is low power, and lets you find objects so that the main scope is pointing to them.
   4.2 Red dot finder: projects a red dot onto a glass.  No magnification, so pretty quick to use.

5. Camera adapters allow you to plug your SLR into the eyepiece mount.  These consist of three parts:

A bayonet type adapter that replaces the lens on your camera.  You turn and pull out your lens, and the adapter has the same interface to the camera body.  See here for an example.

There is another part that slides into the eyepiece tube on the telescope (either 1.25" or 2" inside diameter).  This is threaded so screw onto the bayonet type adapter.  For prime focus photography (where you are using the telescope itself as the lens of your camera), this is all you need.

A third part is called a teleextender tube, and it is threaded to fit in between the bayonet mount and the eyepiece adapter tube.  Here's an example all put together.  You put a telescope eyepiece in the teleextender tube, and now you can do what is called eyepiece projection astrophotography.  This gives you a lot more magnification, and a lot more frustration as you try to get a crisp focus (much harder than doing it at prime focus).

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