Finding Himalia

by Rick Scott

Himalia is the only moon other than the Galilean moons of Jupiter that is visible in an amateur size telescope. It is much farther from Jupiter than the other moons with an orbit around Jupiter that takes just over 250 days to complete. Himalia is small, only 170km in diameter with a visible magnitude of about 15. Because it’s so dim, the best time to look for it is when its orbit takes it as far as possible from Jupiter as seen from Earth. It also helps to have clean and well baffled optics.

The first time I tried to look for it was at the All Arizona Star Party in October 2001 around three in the morning. The sky was very transparent that night and I was able to see stars dimmer than I have ever seen with my scope before. More on how I determined that will come later in this article. A friend was at my scope sharing in the search for this elusive moon and was able to see these dim star too.

I normally use “The Sky” from Software Bisque for planning my observing sessions and in helping me find objects, but it only has the Galilean moons for Jupiter so I used Red Shift (version 3 at the time) which I also had on my computer. Red Shift is very cheap (about $40 on the street) and is excellent for solar system objects. Version 3 didn’t use the Hubble Guide Star Catalog (GSC) so it’s limited in the number stars it could show. I needed to have many of the dim stars in the area around Himalia to identify it, so I set up the two programs to display the same area of the sky at the same scale. By doing this I was able to use the ALT-TAB key combination to quickly switch between the two programs and see where Himalia was in the star field displayed by “The Sky”.

After locating the correct star field, it was possible to see something extremely dim quite close to the position where Himalia should have been, but it seemed dimmer than 15th magnitude in comparison to nearby stars. Clicking on the stars in “The Sky” showed that the dimmest stars in the GSC around this area was just brighter than mag 14, and the object near Himalia’s position was more than a magnitude dimmer. So now what do I do?

I did some follow up activities after that first search to find Himalia. The first thing I did was to pull up a photo of the star field from the Digitized Sky Survey (DSS) to look for any stars that we may have seen as the moon. And yes, there was a faint star in that position. So now I needed to identify that star to see if it was bright enough for us to see in my scope. I also purchased the latest version of "Red Shift" (version 4) because it uses the Guide Star Catalog (GSC) and therefore has many more stars to help in identifying the correct star field. To my surprise the new version showed Himalia in a different position as compared to version 3, the version I had at the All AZ Star Party. Now the true position was something I needed to resolve.

For my next step in solving this puzzle, I ran the Horizons Ephemerides program available on the JPL Solar System Dynamics web site and found that the position shown in the new version of Red Shift is correct. If you haven't figured it out yet, we weren't even looking in the right place! I still wanted to know how bright the star was that we thought was the moon, but it isn't in the GSC so I had to find another resource. It shows up in the DSS photograph, but that doesn’t provide me any data on the star.

“The Sky” can read the USNO database files from the Naval Observatory Flagstaff Station, so I went to their web site and downloaded the 250MByte file that has stars down to the 21st magnitude for the area of the sky where Jupiter was that night. It took about an hour and forty-five minutes to download the file. After setting up the computer to display the USNO database, I used another great feature of “The Sky”. “The Sky” can display a photograph overlaid on its computer generated star field to help in identifying objects. I used this feature to overlay the Digitized Sky Survey photograph on top of the USNO stars. The USNO database was made by scanning the Palomar Observatory Sky Survey (POSS) plates and it was obvious from the computer display that the database has a star for every star that could be seen in the photograph. This is a truly amazing tool available for our use and should not be overlooked.

Using this setup on my computer, I was able to find the star in the USNO database at mag 16.5 which, for me at least, agrees well with the difficulty in seeing the star. This is the faintest star I’ve seen in my scope and shows how good the sky was that night. It’s just too bad we were looking in the wrong place. I also checked a recent Sky & Telescope article on the moons of the outer planets. The map they had for Himalia also agreed with version 4 of Red Shift and the Horizons program. After doing this research, I believe that someone using nothing but the Sky & Telescope article will never be able to tell the moon from the field stars.

So now I knew what resources to use in planning future attempts to find Himalia (or any other faint obscure solar system object). The following are the steps I used to plan my follow-up observations:

1. Use the JPL Horizons Ephemerides program to get the J2000 astrometric coordinates and the apparent coordinates for the time I was planning on observing Himalia.

2. Download the Digitized Sky Survey photograph centered at the apparent coordinates.

3. Add Himalia to the database for “The Sky” using the J2000 astrometric coordinates. “The Sky” will then precess the J2000 coordinates to the proper location for the observing time entered in the Horizons program.

4. Overlay the photograph with the USNO database in “The Sky”.

5. Use “The Sky” to see exactly where in the photograph Himalia will be for that observing session.

6. Mark up a printout of the photograph with the location of Himalia and magnitudes of various field stars for use at the eyepiece.

This system has worked very well for locating where to look and to have star magnitudes to check the limiting magnitude for that observing session. Checking the dimmest stars I could see allows me to determine if Himalia could be visible or not for that night.

With the system now developed, I looked for Himalia at the December 2001 local star party, the January 2002 and February 2002 deep sky parties, and at the 2002 Sentinel-Schwaar star party. I was just able to glimpse it in February and got a positive observation at Sentinel, but it was sure dim. This observing project taught me a lot about the resources available and how steady and transparent the atmosphere has to be to find such an elusive object. Now it’s your turn, I’d love to hear if you can find it and how small a scope you can do it with. I used my home-made Lurie-Houghton scope with an aperture of 9.8 inches.

Here are the web site addresses for the on-line resources I used:
JPL Horizons Ephemerides program
Digitized Sky Survey
USNO Database
Software Bisque (“The Sky”)
Maris Technologies (“Red Shift”)