Photographing asteroids

The typical motion for a "bright" asteroid at opposition is about 15 arcminutes per day. Thus, with a 135mm or longer lens, you can see the motion in two frames on the same night, but this requires that you either stay at your dark-site for most of a night or that you go to the site twice. If you live out in the country, this won't be a problem, assuming that you don't mind staying up late. Also, if you live in an area with persistantly bad weather with only an occasional clear night, this is probably the way to go. Personally, with my 135mm lens, and the frequent occurrance of back-to-back (or at least 2 out of 3) clear or partly cloudy nights in the Rockies, I found that taking images on successive nights usually worked well for me. Although it makes it harder to find the asteroid on both frames, taking images several nights apart can still work.

This is best done with a fast lens for the greater light grasp (see the calculations page for more info). I've found that with a good 135mm lens wide-open at f/2.8 I can reach to magnitude 11.0 or better in a 2 minute exposure in a dark sky on ASA 800 or 1000 film, based on scans from the resulting 3.5x5 prints. I highly recommend scanning your images because it really does make it easier to find the bogey; if this is not practical, I recommend getting 4x6 or 5x7 prints to make things easier. Unlike normal astrophotos, I have no qualms about doing a lot of image processing to bring out the faint asteroids; specifically the "sharpen" command can be helpful as well as fiddling with the brightness and contrast. The length of the exposure isn't as important as the size of the stellar image on the frame. If your tracking isn't very good, a 2 minute exposure might work better than a 4 or 6 minute exposure (I've had this problem in windy conditions, too). Probably the best strategy is to take exposures at several different exposure times and then use the best image. I usually took two frames per field per night, usually a pair of 2 minute exposures, but sometimes a 2 minute - 4 minute pair.

If you want to do the brightest asteroids, don't be afraid to use a normal lens; it's just that you may have to wait a couple days before the motion is obvious at that image scale, and you may have more problems with the asteroid passing too close to a star to resolve the asteroid, especially with a fainter asteroid passing through the Milky Way. With fast film, you should be able to catch the "big 4" without any tracking when they are at their brightest.

However you do this, I highly recommend having a good star atlas like Uranometria 2000.0, or a good computer program, and plotting the ephemeride data for your targets. Suitable orbital elements and/or ephemerides can be obtained for all asteroids reaching magnitude 11.0 at least 90 degrees from the Sun from the Minor Planet Center. Once you plot the asteroid you can then find the nearest relatively bright stars that you can use to locate the field through your camera viewfinder. Once you have taken the images, use the plot to narrow the search down to the fraction of a degree where the asteroid is located, and carefully compare all the objects on the two frames.

Asteroid image pair gallery

Here are the eight asteroids I located using these techniques during the period Dec 1996 to Feb 1997 when I was doing this regularly. The field of view varies from frame to frame but is usually 40-50 arcminutes, and most of the images are pretty well aligned with north at the top. Notice that some of the frames have relatively poor image quality; for this work I can still use a frame without perfect tracking and also it's commmon to have a lot of wind in winter and some of these photos have been taken in 20-25 mph winds.

704 Interamnia

Photos taken 6:46UT on 10 Dec 1996 and 5:15UT on 11 Dec 1996. Asteroid at approximately magnitude 10.1, movement is from center of frame downward and to the right into the center of the asterism. The image center is near RA 3 28.5 and Dec +34 45.

22 Kalliope

Photos taken 6:57UT on 10 Dec 1996 and 5:24UT on 11 Dec 1996. Asteroid at approximately magnitude 9.9, movement is from left center to center. The image center is near RA 5 06.5 and Dec +27 40.

511 Davida

Photos taken 6:52UT on 10 Dec 1996 and 5:29UT on 11 Dec 1996. Asteroid at approximately magnitude 10.2, movement is from below center to above center. The image center is near RA 3 43 and Dec +1 55.

471 Papagena

Photos taken 5:12UT on 5 Jan 1997 and 4:43UT on 7 Jan 1997. Asteroid at approximately magnitude 10.2, movement is from the lower left to near the star above the bright orange star (this one is a little harder to see than the others). The image center is near RA 8 03 and Dec +32 00.

451 Patientia

Photos taken 5:33UT on 5 Jan 1997 and 4:43UT on 7 Jan 1997. Asteroid at approximately magnitude 10.8, movement is from below and slightly left of center to above and slightly right of center. The speck in the left center of the second frame is an artifact. The image center is near RA 8 18.5 and Dec +30 00.

324 Bamberga

Photos taken 7:10UT on 15 Jan 1997 and 9:53UT on 17 Jan 1997. Asteroid at approximately magnitude 10.4, movement is from above and left of center, to slightly above and to the right of center. The image canter is near RA 8 15 and Dec +29 55.

24 Themis

Photos taken 6:23UT on 8 Feb 1997 and 4:49UT on 9 Feb 1997. Asteroid at approximately magnitude 10.8, movement is from below and left of the central star, to just next to the central star. The image center is near RA 8 43 and Dec +19 25, on the southeast edge of the Praesepe cluster.

64 Angelina

Photos taken 6:42UT on 8 Feb 1997 and 5:05UT on 9 Feb 1997. Asteroid at approximately magnitude 10.6, movement is from the bottom center up and to the right. The image center is near RA 10 32.5 and Dec +7 20.


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File last modified: 06 December 2004

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