It should be no surprise that a virtual universe of stars is out there on the web. All the modern catalogs professionals use—stars, galaxies and other deep sky objects, and even some classical ones—are available from the French CDS repository, Centre de Données astronomiques de Strasbourg. There you’ll find catalogs of every kind, web-based interfaces for visualization, bibliographies and references to the astronomical literature. The VizieR catalog service includes the venerable Hubble Guide Star Catalog (15 million stars to about 15th magnitude, version 2.2), the Hipparchos-Tycho 2 (a million stars to 11th magnitude), US Naval Observatory’s A2.0 (500 million stars, also available in a "lite" version of 50 million stars), UCAC-2 (highly accurate positions for astrometry, magnitude 10 and fainter), and the most complete catalog available, the B1.0 database. While they are free for the downloading, you’ll need a fast connection, a good ftp client, and plenty of patience in addition to huge chunks of hard disk space. In total, USNO's A2.0 consumes over 6 GB (that's 11 CD-ROMs) and the B1.0 includes over a billion stars and occupies 80 GB. It is available only on-line.

So how do you turn those gigabytes into a facsimile of the sky? There are numerous star charting and planetarium programs—many free themselves—on the web, and here’s what I look for in astro-charting software. The program should have an easy, quick install and a small footprint, occupying little disk space. The program should launch quickly and have pull-downs menus, toolbars or hot buttons that work intuitively—no manual need, but good help files are a plus. Setting your location and the time must be straightforward, and you should be able to scroll and zoom instantaneously. The moon, planets and sun should be optional, and minor planet and comet elements need to be updated online. Most importantly, it should come with enough stars and basic deep-sky catalogs, and searching and finding any object should bring it to the center of the screen alongside an info-box.

Lately, I’ve found three to be especially useful: Cartes du Ciel (and the Virtual Moon Atlas), C2A for Windows, and HNSKY. All are freeware, written and supported by European developers. You can install whole deep stellar catalogs or just download faint stars “as needed” for the field of view on your screen. If that's not enough realism, you can insert deep sky images from the Digitized Sky Survey from the Palomar and UK Schmidt survey plates. It is even possible to incorporate your own astrophotos or sketches into the "sky" on the screen. Although it’s not quite like looking through the eyepiece, it is impressive nonetheless. For the opposite experience, try the Stellarium sky simulator. This program is definitely a work-in-progress, but it produces an intriguing wide-angle simulation of twilight with custom horizons, has twinkling stars and meteors, light pollution and simulated dark adaptation. Be sure to check out the graphical figures representing of the mythological constellations.

Although I can’t claim to be expert in any of these programs, I know that they can help plan observing sessions and aid in navigating around the sky. Take your pick: there are many more available from which to choose. Although they can take a bit of getting used to, charting programs are great aids for enjoying all those free stars—and the price is definitely right!

Much more information and recommendations on star catalogs can be found at the USNO and Project Pluto web sites. Moondark is written by Doug Miller, published at the Moondark web site, and printed in the Delmarva Star Gazers' Star Gazer News. This document was last revised on 29 January 2006. Text and images copyright © 2006 by Douglas C. Miller, All Rights Reserved. This material may not be reproduced in any form without prior permission.