Own Galaxies

Not only can you gaze at stars using Stellarium, but whole galaxies, too. To do so, you need to integrate high-resolution telescope images, which you can find in the /usr/share/stellarium/nebulae/default directory. However, version 0.13 is only familiar with the default subdirectory, so you must make a copy of the high-res images in the new directory, ~/.stellarium/nebulae/default.

Online, you can find an updated compilation [8] to save in the new directory. The error messages in the logfile or in the terminal when launching the software point out that the software isn't able to assign some names correctly, with mismatches in some cases and missing files or file name extensions in others. At the time of publication, I still hadn't received an answer to my query as to whether some of the images might be protected by copyright.

Stellarium is already familiar with all large galaxies; however, it is missing those that only appear intermittently because of their distance from Earth. One example is the object SDSS J162746.44-005357.5. This object shows two galaxies that lie exactly on an axis with the earth but are several billion light years away from each other. This arrangement causes the front galaxy to bend the light of the back galaxy, in line with the general theory of relativity, forming an Einstein ring [9] (Figure 5). The front galaxy appears as a point, the rear one as a ring – here with a diameter of two arcseconds. For comparison, terrestrial telescopes cannot capture less than one arcsecond because of fluctuations in the atmosphere; however, the Hubble Space Telescope manages 0.05". Stellarium is already familiar with the object of interest. If you enter SDSS J162746.44-005357.5 into the search window, the program will guide you to the right place, although nothing is there yet to be seen. Stellarium doesn't do the search itself; instead, it asks the SIMBAD database [10].

Figure 5: Hubble image of Einstein ring SDSS J162746.44-005357.5. Image: NASA, ESA, A. Bolton (Harvard-Smithsonian CfA) and the SLACS Team.

Therefore, you have to place the image of the Einstein ring into the correct position and at the correct size in the Stellarium night sky. The basic procedure is the same as for the constellations; the "Fitting the Einstein Ring" box lists the specific steps. The main work this time involves determining the coordinates. A rough estimate is enough to demonstrate the principle: The diameter of the Einstein ring is two arcseconds. Figure 5 therefore approximately covers an extension of six arcseconds. According to SIMBAD, the coordinates of the Einstein ring are 16hr 27min 46.447sec right ascension, -00°53'57.56" declination.

{"imageCredits": {"short" : \
 "Einstein ring SDSS J162746.44-005357.5; Source NASA/ESA http://hubblesite.org/gallery/album/ exotic/pr2005032g/"}, \
 "imageUrl" : "EinsteinSDSSJ162746.png", \
 "worldCoords" : [[[-113.0556,-0.9001],[-113.0572,-0.9001], [-113.0572,-0.8985],[-113.0556,-0.8985]]] , \
 "textureCoords" : [[[0,0], [1,0], [1,1], [0,1]]], "minResolution" : 0.28,  "maxBrightness" : 16.0}

After converting the coordinate values to decimal degrees [11], the vertices of the image are the coordinates in the worldCoords line (in the boxout). The imageUrl line lists the name of the image file and maxBrightness the brightness. The value mag 17 would be correct. I chose mag 16. Stellarium suppresses objects with a magnitude smaller than 16 (higher numbers represent smaller magnitudes).

The data in the textures.json text file are in JSON format. Syntax errors such as comma or bracket mistakes will prevent all the images from being integrated, resulting in an error message in the terminal:

WARNING: Can't parse JSON description: "/home/rp/.stellarium/nebulae/default/textures.json"

You can check position and size of the added object by loading the Angle Measure plugin at startup (Configuration window | Plugins). After a restart, the tool will appear as an angle symbol in the bottom menubar. The diameter of the ring should be two arcseconds (Figure 6), if you have estimated the edge coordinates correctly. Figure 7 shows guide stars that can help find the tiny object.

Figure 7: Use the guide stars to find the Einstein ring.

Figure 6: If everything worked, you'll be able to find the Einstein ring as an image in the sky with the correct diameter.

Conclusions

Normal galaxies stretch out over many arcminutes. In the Einstein ring example, Stellarium illustrates the persistence required to find tiny structures in the sky like Einstein rings with a diameter of only a few arcseconds. Presumably, only very few users manage to come across the object by chance.

All told, it is pleasingly easy to familiarize yourself with the stars using Stellarium. By making a few small modifications, you can quickly adapt the software to include new environments and objects.