Notes from Under Sky

Selecting an Atlas

What are the factors influencing choice of atlas?

Someone asked:

What is everbody's favorite Star Atlas that is either considered the de facto or that they just like and use for most of your star/DSO plotting?

I know you didn't say "what's the best one for me" but I happen to think that's a more interesting question than "what's the most popular" so I insist on answering it. :)

Let's just consider book atlases. I've revised my thinking about this a little bit. I used to say that it primarily depends on how you navigate the night sky. Now, I think a second principal factor is how you intend to use it. Partly, this change has to do with the increased availability of GOTO telescopes. I'll explain that in a moment.

Do you, for example, intend to use the atlas to star-hop? If so, what do you star-hop with? In my experience (and that of others), hopping is most successful when what you see in the sky matches what you see in the atlas. If you star-hop by sighting down the tube or with a red-dot finder like a Telrad, then you need an atlas that shows all the stars you can see with the unaided eye. These are the so-called mag 6 atlases, and they include the Cambridge Star Atlas, the Bright Star Atlas (both by Wil Tirion), Norton's Sky Atlas, Edmund's Mag 6 Atlas, and so forth.

If, on the other hand, you star-hop through an optical finder, or you observe with a pair of binoculars, then you need an atlas that sees as deep as you can through those instruments. Sky Atlas 2000.0 (now in its second edition) is the leading choice here—it shows about 80,000 stars down to magnitude 8.5. If you have a larger finder or a larger pair of binoculars, you may prefer a slightly deeper atlas, such as Uranometria 2000.0. This atlas is currently in revision for its second edition, but it should be out by late 2001. Another possibility is the Herald-Bobroff Atlas, which has six different series of charts, including an all-sky series down to magnitude 9.

If you star-hop through any kind of telescope at all (in dark skies), then you'll probably want an even deeper atlas. The choice here is the Millennium Star Atlas, which goes down to magnitude 11.5. Even this atlas doesn't show as many stars as can be seen by even a small telescope under dark skies, but it does show you enough to navigate by. This atlas goes for about $250 U.S., so at this point, many amateurs go for a computer planetarium program, which can often print out charts for navigation.

Now then…

What if you use it primarily to select DSOs to look at? If you have a GOTO scope and you only need stars to align your scope. Then it depends on the aperture of your telescope, and it's a little less clear-cut. My personal feeling is that Cambridge and the like include most of the objects that can be seen without undue difficulty with a 4-inch or smaller telescope under dark skies; Sky Atlas 2000.0 goes up to a 6-inch scope; and Herald-Bobroff and Uranometria 2000.0 go up to perhaps an 8-inch scope. Millennium doesn't actually contain that many more objects than HB or Uranometria, but the magnitude limit is much more consistent with Millennium. All of those atlases contain the necessary alignment stars, so it's really only the selection of DSOs that should concern you in this case. (Of course, it's not a bad idea to learn the sky while you're at it!)

Copyright (c) 2001 Brian Tung