Last updated on
4/11/2006

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Sky chart software is used to plot a specific region of the night sky visible from your particular location (longitude and latitude) at a particular time of the year.

What Sky Chart software isn�t all that useful for:
Sky chart software can give you an overall picture of the constellations visible at your location for a specific time of the year, but a planisphere is a better tool for that purpose.  It can also indicate the objects visible in a specific region of the night sky but it isn�t really useful for providing a list of all the good objects you may want to see at any specific time of the year - especially when you�re starting out, observing lists are a better tool for that purpose.

What Sky Chart software is very useful for:
Sky chart software is most useful for hunting down faint objects or objects not contained in your scope�s database, and confirming that you have actually seen them. There are a number of faint objects that are fun to track down but because their brightness is at the limit of what typical (i.e. affordable) amateur scopes can make visible you need a plot of a very specific region of the night sky to locate the object or to confirm that you have actually seen it. Here are a few examples:

• Pluto All the planets except Pluto are resolved into a disk by a typical amateur scope, so there is little doubt that you are seeing the planet you think you are seeing. Pluto however is on average a 14th magnitude object which is around the limit of what a typical amateur scope can expose, and appears like a star (only the Hubble Space Telescope has resolved Pluto into a disk). If you want to confirm that you have seen Pluto you will need sky chart software to be sure you haven�t confused it with a star.
• Quasar 3C273 In Virgo, Quasar 3C273 is a 13th magnitude galaxy that is 2 billion light years away and is the most distant object visible in typical amateur telescopes. When the light you see in your scope from 3C273 left that galaxy two billion years ago, the primary form of life on Earth was bacteria (the Proterozoic part of the Precambrian era).  At 2 billion light years away, the only reason you can see Quasar 3C273 at all is because it has a luminosity 2 trillion times that of the Sun or 100 times that of the entire Milky Way galaxy.  It is moving away from us at 1/6 the speed of light; this object is not something you will see in your every-day experience, and is worth looking for.
• Lalande 21185 Red dwarf stars are a common type of star in the overall palette of the universe, but because they are dim very few (only those close to Earth) are visible from Earth.  Lalande 21185 (in Ursa Major) is, at magnitude 7.5, the brightest red dwarf visible in the northern sky and the fourth closest star to the Earth.
• Luyten's Star Luyten's Star is another red dwarf and is the 22nd closest star to the Earth.  Located in Canis Minor, it is a magnitude 9.8 star and because it is so close to us it has a very high proper motion.

Most sky chart software can also be used for controlling a GoTo scope - when you make the appropriate connection between a computer and a GoTo scope you can aim the scope by moving your mouse in the sky chart software at the computer, which is totally cool.

Most sky chart software applications are commercial products that can tend to be expensive (due to their limited audience and consequent limited sales opportunities to recover the cost of developing the application) but they can be very helpful observing aids; one of the most well-respected sky chart applications is �The Sky� by Software Bisque.  But another of the most well-respected sky chart applications is free.  Patrick Chevalley, a Swiss amateur astronomer, wrote Cartes du Ciel (Sky Charts) and makes it available over the Internet.  (If you have a broadband connection you can download the entire 15 megabyte file with most of the fundamental sky object databases included, but he also has it separated into parts so downloading it via a modem connection is feasible.)  It is a great sky-charting application and I highly recommend it.

Sky chart software applications comprise two components - the charting software and a group of databases that list the name, RA, and Dec of a group of objects (stars, nebulae, etc.).  The basic download of Cartes du Ciel includes a basic set of sky object databases but if you do choose to use Cartes du Ciel, Patrick Chevalley�s web site also provides a number of additional databases you may add to the basic application.  I recommend that you add the Tycho 2 star database and the Variable Stars ("Variables") and Multiple Stars databases; the Boss General Catalog (GC number), Bonner Durchmusterung and Southern Durchmusterung (BD number), Smithsonian Astrophysical Observatory (SAO number) and Henry Draper catalog number search indices; and the Barnard (dark nebulae) and Caldwell Objects databases.  If you want to use Cartes du Ciel to control your scope you�ll also need to download and install the ASCOM Platform.

If you have a Celestron U2K scope, I have created a version of the U2K�s Non-Stellar Objects database that can be loaded into Cartes du Ciel as well - download it here:

U2K-NSObjects-CduCCatalog.zip

Download this file and follow the directions in the Install.txt file to install it in CduC.  You will then be able to use CduC�s search function to locate an object in the U2K�s Non-Stellar Objects database, such as Quasar 3C273. The U2K�s Non-Stellar Objects are in fact generally included in the other Cartes du Ciel databases I mentioned above, but they tend to be easier to search for if you use the U2K�s Non-Stellar Objects database.

If you have a Celestron NexStar scope, Mike Swanson has created an excellent freeware application called the NexStar Observer List (NSOL) which contains objects not in the NexStar databases and allows you to drive your NexStar scope to these objects. It is available from his NexStar Resource Site.

Note that Christian Legrand has teamed with Patrick Chevalley (the author of Cartes du Ciel) to produce the Virtual Moon Atlas, a free software application to help your lunar observing by displaying and identifying detailed lunar features.

Dimming Your Computer Screen
If you use a computer with Sky Chart software to drive your scope, to preserve your night vision you�ll need to make a neutral-density filter to fit in front of the computer�s screen - you need to make the screen far more dim than is possible with the computer�s internal screen brightness controls.  Here�s what works for my laptop: I cut a piece of 1\8� thick plexiglass to fit over the front of the screen, and covered both sides of it with the black film sold in auto parts stores for darkening limosine windows (the film I used has a visible light transmittance of 5%).  Two layers of this film - one on the front and one on the back of the plexiglass sheet - gets my laptop screen dark enough that it doesn�t ruin my night vision.  You may need to experiment with your computer to see how many layers of film you need, but be sure that you�re checking the result in the dark with your eyes dark-adapted - you�ll be surprised how dim the screen really needs to be.
 

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