Asteroid hunting

A senior research project

My name is Adam stein. I am a senior graduating with a self-designed major in planetary science, which combined geology and astronomy courses. Over the course of the past year I have taken images of the sky and identified asteroids on those images for my senior research project. The goal was to be able to find at least one new asteroid by the end of the school year.

Before this could happen I researched how to report findings. The organization responsible for tracking, cataloging, and evaluating data on all asteroids is the Minor Planet Center. The Minor Planet Center operates at the Smithsonian Astrophysical Observatory under the auspices of the International Astronomical Union.

Before anyone can report data on new asteroid discoveries that person must obtain an observatory code from the Minor Planet Center. To obtain an observatory code the observer must show that he or she knows how to conduct the searches accurately enough to produce useful data. The observations need to be accurate to within one second of time and to within about one second of arc in right ascension and declination. Also, an asteroid must be observed at least three times on each of two nights.

To conduct the observations I used a Santa Barbara Instruments Group ST-8E CCD camera attached to a Meade LX200 0.2m f/10.0 Schmidt-Cassegrain telescope with a focal reducer attached to yield an effective focal ratio of f/6.3. A laptop running CCDSoft by Software Bisque was used to control the camera and focuser and to acquire and store the images. An Optec TCFS electric focuser was used in conjunction with CCDSoft to obtain precisely focused images.

The area of the sky in each image is 0.69 degrees by 0.46 degrees. Just for scale, this is an image of the moon taken under the same conditions as the asteroid images. The moon subtends an angle of about 0.5 degrees.

The Moon

As you can see the images do not cover much of the sky. However, most new discoveries occur along the ecliptic - the orbit of the earth projected onto the sky - which limits significantly the area to search. Also, most new discoveries with comparable equipment have magnitudes of around 16 to 21.

Once the images were taken, CCDSoft in conjunction with another Software Bisque program called TheSky was used to align the images and blink them. Blinking the images is a process wherein the images are aligned and then shown in a slide show fashion. If the images are aligned properly the stars will not move as the slide show progresses but any asteroids in the images will move in a straight line. This is how asteroids are identified.

Note: If the images and accompanying text below are not formatted well, please increase the width of your browser window.

These images show the contrasting motion of the stars and the asteroid 526 Jena:

These are some of the other images of known asteroids, 407 Arachne, 492 Gismonda, and 526 Jena respectively, taken to obtain an observatory code. The arrow is pointing to the asteroid.

407 Arachne

Magnitude 13.21

60 second exposure

Limiting Magnitude 15.00

Arachne

492 Gismonda

Magnitude 13.33

300 second exposure

Limiting magnitude 17.60

Gismonda

526 Jena

Magnitude 15.71

60 second exposure

Limiting Magnitude 16.80

Jena

The original images are not as clear as the ones above. They contain noise and artifacts that come from a number of different sources. The first is dark current noise, which is thermally generated inside the camera. Cooling the CCD chip can alleviate some of the noise but not all of it. By creating a dark frame, which is a closed-shutter image exposed for the same time and at the same chip temperature as the actual star field exposure, and then subtracting the dark frame from the star field image, the effects of the dark current can be largely eliminated. Also, there are already a small amount of electrons associated with each pixel on the CCD, even for a minimum-duration exposure. This problem can be corrected by taking a bias frame, which is a minimum-duration closed-shutter exposure. Finally, nonuniform illumination on the CCD chip caused by the optics and variation in the sensitivity of different parts of the CCD chip cause the sky background to appear nonuniform and complicate accurate brightness measurements. This can be corrected by taking a flat field image, which is an image of a uniformly bright field. In practice, this was achieved by stretching a white T-shirt over the front of the telescope tube and exposing during daylight without changing the focus or camera orientation from those used in the nighttime asteroid images.

These are examples of dark, bias, and flat field frames taken for an image reduction:

Dark Frame Bias Frame Flat Field frame

CCDSoft can perform the reduction process on multiple images at once. This is a screen shot of CCDSoft conducting a reduction on a folder of images:

These are before and after images of a star field that has been reduced:

Before After

Besides this more in-depth process one can also simply change the contrast and brightness, or create a negative - it is easier to distinguish a dim black speck on a white background than a dim white speck on a black background- for a fast analysis of the images.

Original Adjusted Negative

The observatory code, H69, was given to me so I can now report new observations from Perkins Observatory. Unfortunately no images I have taken contain any new asteroids. I was limited in the number of images I could take because of an unusually high number of cloudy nights. However, I am continuing my research after I graduate and it is only a matter of time before I find one.

I enjoyed this project immensely and I learned an amazing amount of information during it. It was very time consuming, but worth it. I have no doubt that if a future student conducted the search, with better luck in the weather, they could discover an asteroid during the school year.

I would like to thank first Dr. Robert Harmon for advising me, and spending countless freezing nights teaching me how to use the equipment. I would also like to thank the Department of Physics and Astronomy for the use of the equipment. Finally, I would like to thank Mr. Tom Burns and Mr. Gary McCool for their help while I conducted my research at Perkins Observatory.