As some of you may know, I have recently been experimenting with the capability of our observatory equipment to accurately measure the position of objects in the telescope camera images. Things outside of our solar system don’t change position much, but that still leaves hundreds of thousands of things to measure! These are the asteroids and comets within the solar system that orbit our sun.
The IAU (International Astronomical Union) Minor Planet Center has orbital information on 482419 objects. This includes almost 12,000 named asteroids and comets. Some which are of special interest to us here on Earth are the NEO or near Earth objects. Great efforts are being made to look for these and determine their orbits.
With the telescope and software at the Harken Observatory we can join in the action. As a trial, I set out to image and measure the position of some know asteroids and compare them to those calculated by the Minor Planet & Comet Ephemeris Service: [www.cfa.harvard...]
The telescope was configured with the 0.63 focal reducer and the Starlight Express SXV-H9C camera. A Meade DSI-2 was configured as a guider on the on the 80mm APO refractor. The camera was setup to “bin” pixels 2 x 2 such that each effective pixel covered about 1.69 arc-second. Each exposure would be five minutes long.
The first asteroid attempted was named (195) Eurykleia. It is a 14th magnitude object originally discovered in 1879. Below is an animated GIF of two images separated in time by about 28 minutes. These images were cropped to the common aligned area.
Animated image field containing (195) Eurykleia mag 14.3 asteroid
Eurykleia is the reasonably bright object jumping back and forth near the upper right corner of the image. Its measure positions at times indicated were:
2010 April 17.094988UT RA 07 53 50.38 Dec 26 40 48.0 (Harken Observatory measured)
Date UT R.A. (J2000) Decl. V
2010 04 17 021647 07 53 50.3 +26 40 49 14.3 (Minor Planet & Comet Ephemeris Service)
This is pretty good agreement! We are able to achieve this accuracy because the Maxim DL5 software is capable of calibrating the image information (star patterns) to a know database which is the Hubble Guide Star catalog. Once the image has been “plate solved”, an astrometry position cursor may be moved about over the image to indicate the position of an object in the cross-hairs.
We tried this again on a fainter object. (4929) Yamatai is a 17th magnitude asteroid. It is the rather faint object jumping back and forth near the middle of this animated GIF.
17th magnitude asteroid (4929) Yamatai
Harken Observatory measured position: RA 10 04 21.47, Dec 14 36 56.6
Minor Planet & Comet Ephemeris Service position:
R.A. (J2000) Decl. V
(4929) Yamatai 10 04 21.5 +14 37 00 17.0
The movement over about 28 minutes was about 6.3 arc-seconds.
I have to confess that (4929) Yamatai was NOT the intended object for this exposure. I was looking for (1820) Lohmann, but the camera field of view missed this object. I also missed finding (4024) Ronan. As I was moving between 5 different objects and taking guided exposures, I was not re-solving the plate position and re-syncing the telescope pointing to the actual position. I had expected that the telescope pointing was sufficiently accurate and would be able to move back to the same position a half hour later. My thoughts were that the field of view was about 20 x 14 arc-minutes and I could not miss. I was wrong.
I learned something very important for this work when measuring object positions. The astrometry showed told me that the field of view was only about 1.3 RA minutes across. Each RA minute corresponds to 15 arc-minutes. Each RA second corresponds to 15 arc-seconds. Even though I knew that each RA hour angle corresponded to 15 arc-degrees, I had not understood all the implications! Because the earth is rotating, the RA pointing accuracy is critical. It is very important to plate solve and sync the telescope to the image field and then move the telescope on to the final target coordinates to ensure that the object will be near the center of the FOV.
This confusion made for an interesting evening of “comet chasing” for a number of us at the observatory last Saturday night. If you scroll up and look at the first animated GIF in this article, you can’t help but notice the fuzzy blob jumping back and forth a large amount near the upper middle of the image. When I first saw this, I measured its position and checked back with the Minor Planet Center database. Was this a known comet? No! It was not a known asteroid either. Also, if you look just below the bright star, you will see another faint ball moving back and forth. What was that? Again, it was not any known object. I came to the observatory that night with an interpolated track for the rapidly moving fuzzy object. Might we have snagged a new comet??
Now I don’t think so. I was tricked by something and another good lesson was learned. First, a truly fast moving object would have formed a streak on the image as long as the exposure duration was. A five minute exposure should have created a streak of about one fifth of the overall distance that the object moved in 28 minutes. I think the key to unlocking the mystery is in the bright star of the field. Possibly some kind of internal optical reflection had occurred. Anyway, the correct thing to do would have been to re-image the exact same star field to see if the “ghost-comet” re-appeared. I expect that it will. What about that dimmer object below the star? Was it actually moving in the reverse direction as the others? Probably not!
As for that interpolated track prediction, it should have been calculated based on the proper relationships of RA minutes to arc-minutes. Simple interpolation was probably not considerate of multiple body orbital relationships. Our desperate attempt to re-calculate the position last Saturday night (although was exciting and team building) was not fruitful. Lesson two is to always try to take a preliminary look at the images. If you see something unusual, look for reasons that these could be artifacts. If you really think that you have imaged a comet unexpectedly, take a few more images in a time series to help with the orbit estimation.
One last inclusion for this article. There is a NEO asteroid YU55 that has just made a close pass. On April 19th it came within 1.4 million miles of the Earth. That’s pretty close for a rock that might be up to 800 feet wide and moving at 8 miles per second. Don’t worry though! Scientists have pretty much calculated that the risk of an impact from this body is zero. But since we knew it was coming, we took a few minutes to try and image it on the night of April 17th. This is what it looked like. Note the real trail of light over the 5 minute exposure time.
Near Earth Object YU55 at 2010-04-17 03:28:07 UTC
As always, your comments and questions are welcomed!
Enjoy! Clear Skies!
Randy B
