A free planetarium program for amateur astronomers

It’s really quite remarkable what you can pick up for nothing these days. There are people out there who produce excellent quality, useful software absolutely for free. One such is Patrick Chevalley, the author of Cartes du Ciel (http://www.stargazing.net/astropc) – a planetarium program similar to The Sky or SkyMap Pro. However, instead of paying out 70 dollars (or the equivalent in pounds or euros) all you have to do is download a 15MB file.

Cartes du Ciel has few bells and whistles, but does the basics of sky display well. Best of all it’s free!

The key thing that a planetarium program does is to display star positions and some information on the star in question. Of course, there are other celestial objects as well as stars – galaxies, nebulae, comets, asteroids and planets – and Cartes du Ciel also displays those. There are 15 supported catalogs, including Tycho and the Guide Star Catalog, though only the Bright Star Catalog and SKY2000 are included in the base distribution. This is a reasonable approach since some catalogs are huge, and it does keep the distribution size down to around 15MB. But even with the two small catalogs, you can view 300,000 stars down to magnitude 9.

In common with other planetarium programs, you can zoom around and display various views of the sky from different directions using either the keyboard or dockable toolbars, the stars are nicely coloured and generally Cartes du Ciel does exactly what you’d expect if you’ve used similar programs. Is isn’t as nice graphically as The Sky; instead it has a simpler look and feel to it, more like SkyMap Pro.

There’s a basic telescope control interface (GOTO) via a serial port. You can either have the Meade LX200 style controls which are supported by a number of telescope control manufacturers or you can use the ASCOM standard interface. But that’s it – if you want to go further, then you have to roll your own interface. Now this brings me to one of the interesting things about Cartes du Ciel: not only is it free, but it comes with the source code (Delphi 6) under the GNU public license. Currently, this is only available for the next version (v. 3) now under development. I’m not sure how practical it is to go around tweaking the source code – it depends on how rainy it is outside , I suppose – but still, it’s there. Also, it has to be said that if you want a GOTO interface to, say, a top of the range AstroPhysics mount, then you probably aren’t going to be that interested saving a few bucks on free software.

I’ve tested the LX200 interface (not on an LX200, I might add) and it works fine. It does move the telescope to the star you’ve selected and the telescope’s position is reported correctly back to the program. However, the program itself doesn’t seem particularly well integrated into using a GOTO system. It’s really there as an add on if you want it.

One thing I missed was the ability to ‘rubber band’ a section of sky and create a new map window – that’s one of the real pluses of SkyMap Pro. However, with Cartes du Ciel one window is the limit, as it is in The Sky, incidentally.

So far, you might think that Cartes du Ciel is a competent but basic planetarium program. It works – and works well. But there are two features I really liked. The first is the Quick Locate feature. This displays a map of the entire sky (including those bits below the horizon) as a Mercator type projection. You just click on the star you want, and it is then displayed with its surrounding sky in the main window. I’m always flicking from star to star and I find this sort of linear map is extremely helpful. There is one slight problem, however – it’s quite easy to click on a star below the horizon. It would be very useful to indicate the visible stars on the Quick Locate map.

The Quick Locate feature is really quite neat. You can find a star quickly from the complete sky displayed as a Mercator projection.

The second feature I liked was the multiple eyepiece display. You can define the field of view of several (up to ten) eyepieces and display them on the map as circles all at once. You might find this useful when you’re trying to locate an object and then ‘zoom in’ on it with a sequence of eyepieces (I often seem to end up doing this). It turns out there’s a similar feature in The Sky (which I didn’t know about until I looked for it – fairly typical of The Sky: whatever you want is probably in there somewhere. But where?) and also there’s a less useful ‘finder’ in SkyMap Pro which is limited to three ‘circles’.

Zooming in on a target with a sequence of eyepieces is something I seem to do quite often. Cartes du Ciel makes this quite easy with its Eyepieces display

Compared to other planetarium programs, Cartes du Ciel has few bells and whistles, but it does do the basics very well. This is in many ways a plus. One of the problems with well established programs (and not just in astronomy) is that each new release suffers from ‘feature creep’. The authors need to add extras to encourage people to upgrade. To my mind, this doesn’t always lead to a better programs. For example, in TheSky you can alter the ‘fuzziness’ of the stars in the display. I just can’t think why I’d every want to do this: I don’t think it’s added much to the usability of the product. In contrast, the only thing I configured in Cartes du Ciel was the horizon colour – I simply couldn’t live with the default bilious green surround.

My one complaint about Cartes du Ciel is its help system. Unlike many open source systems, the good news is that it has one. The bad news is that it isn’t that great. Still, for something that works, is as good at the fundamentals as commercial programs and is free, I’m not going to complain very much at all. As Patrick Chevalley says “I feel that you should rather invest your money in buying a good eyepiece than to waste it on software!”. He has a point.

Dermot Hogan

September 2005