Beta version of Charon

I've just added support for Tycho-2 to Charon. I have hopes that this will make it possible for many more people to get the sort of high-precision astrometry needed to (for example) compute asteroid occultations. It may also be of help in photometry, though I'm not so sure of that yet.

Doing this required me to include a good-sized chunk of previously untested code. So for the moment, the new version can be downloaded from here. Eventually, it will be moved over to the 'main' Charon page. None of the code I added ought to affect the 'normal' operation of Charon (i.e., with catalogs other than Tycho-2.)

Requirements for use: Aside from downloading the above CHARON2.ZIP file, you'll need the Tycho-2 data itself. If you have a very fast link, or are extremely patient, you can get the data from Erik Høg's site. The file in question is about 500 MBytes, however.

Those without fast Internet access or without patience can get the data on a CD-ROM instead. People in North America should contact Sean E. Urban, at the USNO. People in Europe should contact Erik Høg, at the Copenhagen University Observatory. People in the rest of the world... well, I'd try USNO first. I simply asked, and the CD was sent at no charge.

Using the data in Charon: This consists of two steps. First, you have to tell Charon, on the command line, where the data is to be found. Second, you have to go into Charon's settings menu and tell it that you're using Tycho/Hipparcos data. To accomplish this, run Charon as 

charon -Gd:\data\tyc2.dat -=

Replace 'd:\data\tyc2.dat' with the actual path to the data, of course. If you've downloaded the data, the full path specifier will be necessary, since (as far as Charon knows) you could have put the file just about anywhere. CD-ROM users are more fortunate; if you specify the drive letter, Charon should figure out where the data is, and the following would work to tell Charon, "The Tycho-2 CD-ROM is in drive E:" 

charon -Ge -=

The '-=' part tells Charon to immediately go into the Settings menu, so you can switch over to "Match to Tycho/Hipparcos", select the image, set the target name, and so on.

At that point, Charon should be able to load the image and match it to Tycho-2 data. The main thing you'll notice is that the RMS errors have dropped quite a bit, and that you're matching to far fewer stars than was the case in GSC or Ax.0.

Some important notes:

  • If Charon fails to match to Tycho-2, try making sure everything is set up correctly by running again, omitting the '-G' switch and reverting to use of the GSC or Ax.0.
  • Tycho-2 has about 2.5 million stars. This is about 1/8 the number of stars in GSC. Don't be surprised if images that match wonderfully to GSC or Ax.0 fail with Tycho-2, simply due to a lack of matchable stars. In testing this, I tried a whole slew of images with 18' square fields of view. About one in four failed due to a lack of sufficient reference stars. In some other cases, only three or four stars matched (you get a warning message when this happens.) Some of the others matched ten or fifteen stars.

    • With GSC, the catalog authors deliberately aimed at a consistent density of about 500 stars per square degree. Tycho-2, on the other hand, just got as many stars as it could; it's dense near the Milky Way, sparse far from the galactic equator. You may find that you get a match every time in Sagittarius, but only rarely in Virgo.

  • Tycho-2 provides only VT and BT data (V and B magnitudes on the Tycho system, which is close to, but not quite, the 'standard' Johnson system.) Charon is bright enough to convert these to Johnson V and B magnitudes, and can use these to estimate an R magnitude; set the 'photometric band' in the Settings menu to one of those three bands, and Charon should react correctly (and does, as best I can test it.)

    • However, much of that "good-sized chunk of previously untested code" I mentioned above is involved in exactly these conversions. (The curious should look at the file COLORS.CPP, in the 'basic astronomical calculations' code.) I'm pretty sure I got it right, but there's a reason I call this 'beta' code and post it here instead of on the main Charon page.

    In some situations, color conversion will be impossible. For example, if BT-VT < -.2 or BT-VT > 1.8 (i.e., very red or very blue stars), you can't compute Johnson V or B. For some faint stars, only one of BT or VT may be given, and again, this prevents determination of Johnson V or B. In still other situations, the Johnson B-V may be less than -.23 or greater than 1.95, in which case a Johnson R can't be determined. If Charon can't get the band you asked for in the 'settings' menu, it will leave the magnitude in another band and will use that star only for astrometry, and not for photometry.

    Similar concerns apply to Ax.0. This dataset provides R and B magnitudes; if both are available, Charon can compute an 'inferred' V magnitude. But again, if the band you want isn't available or can't be computed, some other band will be used.

    To make this apparent, the magnitude band for each catalog star is now shown in the legend. Next to the star ID and magnitude (in green), the band is shown as the usual single letter. (VT is shown as lowercase 'v', BT as lowercase 'b'.)