Map asteroid shapes by video.
Have you ever dreamed of doing frontline solar-system research with a small backyard telescope? You actually can, even in an age when spacecraft study planets, moons, asteroids, and comet nuclei close up. One way is by making a particular type of measurement that citizen scientists can perform with very high accuracy using simple equipment: timing. In particular, timing the instants when the edge of an asteroid covers and uncovers a background star.
Thousands of asteroids await profiling, but the great space agencies are only going to send $100-million spacecraft to a carefully chosen few. That leaves thousands whose size and shape you can help map with a 3-inch scope and a few accessories that cost about $700 all told.
It's done by video. When asteroid-occultation campaigns began in the 1970s, timings were done by eyeball. You watched the star to be occulted and shouted when it disappeared, and again when it reappeared. A tape machine recorded your shout along with radio time signals playing in the background. If everything went exactly right, you could fix the time of the shout between the time ticks to within a few tenths of a second. Then you corrected for your estimated reaction time. It was barely good enough to get a useful size and shape for some asteroids, and it only worked if enough observers were lucky enough to be within the occultation track.
That was then. Far better is to record the occultation with a modern, ultra-lowlight astronomical video camera, and time-stamp each frame. This method reliably times events to within 0.03 second, more than ten times better than the eyeball method.
Predictions of occultation paths have also much improved, allowing observers to position themselves strategically. As a result, amateur asteroid mapping has blossomed.
But we need more observers! People used to think video occultation timings were only for techies with soldering irons and advanced computer-interface skills. No more. Described here is everything you need. And if you run into problems, there's an online community eager to help.
In addition to the telescope of your choice, you'll need the items shown on the facing page to successfully time occultations. They're numbered from the back of the telescope at left to the digital video recorder at the right end:
1. Knight Owl 0.5 Focal Reducer, $29.95.
2. Knight Owl C-mount, to the left of the Watec camera, $16.49.
3. Watec 902H2 Ultimate V2-inch Chip CCD Video Camera, $342.40.
4. The International Occultation Timing Association's custom-designed IOTA VTI v.3 GPS Time Insertion System; $292.00.
5. An old Canon ZR300 camcorder or lower found on eBay, about $50.00.
The Watec camera, the IOTA GPS time inserter, and the camcorder have power plugs that want 120 volts AC. You can get this in the field from a 12-volt deep-cycle battery and a power converter, which many observers already use to run their telescopes.
In the picture you see the Watec 902H2 Ultimate (not the Supreme, which is less sensitive) with its white power cord. The Knight Owl C-mount (similar to a T-adapter) sits next to the Watec camera. The Knight Owl 0.5 Focal Reducer is on the scope. You should use the Watec with no focal reducer when you're recording a bright object like the edge of the Moon during a grazing occultation. But for asteroid occultations, you want the focal reducer for two reasons. It enlarges your field of view, making it easier to locate the target star, and it increases the speed of your optical system and thus the camera's ability to record faint stars.
You need an RCA cable with a BNC adapter at one end to connect the Watec to the "camera in" RCA socket on the IOTA time inserter (the photo actually shows a KIWI GPS Time Inserter). The time inserter includes a GPS antenna with a magnetic end to attach to some high location. I set up my telescope next to the rear of my SUV and attach the GPS antenna to the SUV's rear door, which lifts high when open. This brings in a quality signal from GPS satellites in all directions.
The time inserter also has an RCA "recorder out" socket. In the picture, a black RCA cable runs from there to the Canon ZR300's yellow "analog to digital video cable" socket, by way of a small double-female RCA plug. The Canon ZR300 thus becomes a compact digital video recorder that you can hold in your hand to watch what the telescope is seeing.
As you record, the time inserter prints the time on every frame of your video within an accuracy of 0.03 second, if you're recording at 30 frames per second. When you're done recording, press the reset button on the time inserter and it will display your exact GPS longitude, latitude, and altitude. It does double duty for location as well as time!
That blink-of-an-eye time resolution is the heart of the system's scientific power. The greatest telescopes on mountaintops or in space can, at the very best, resolve an object in the asteroid belt to about 40 kilometers. Our little rig determines the position of an asteroid's edge in space to just hundreds of meters--hundreds of millions of kilometers away. Is there anything else an amateur astronomer can measure to 1 part per billion accuracy?
No space agency will ever spend what it would take to get closeups of a great many minor bodies of the solar system, from near-Earth objects to main-belt asteroids to comet nuclei, Trojans, Centaurs, and Kuiper Belt objects.
From his home near Boston, S&Ts Dennis di Cicco recorded the asteroid 160 Una occulting a previously unresolved double star on January 24, 2011. Watch his video at http://is.gd/unaoccn.
An Asteroid to Remember
Occultation astronomy is a team sport. But the team for every event is different, chosen by the predicted occultation track. For any given month, you can find more than a hundred shadow-track predictions worldwide mapped at https://is.gd/occlt.
I was part of a team last January that caught 115 Thyra crossing an 8.8-magnitude star in Cancer. The evening brought thin clouds and wind. Worse, from where I was, the star was partially occulted from moment to moment by tree branches waving in the wind! But my system saw through this noise. Once I got the correct star in view, I defocused it to prevent its bright image from saturating the pixel array in my CCD video camera.
I began recording a minute or two before the predicted time. Success! The star disappeared onscreen for several seconds. Later I stepped through the video, frame by frame, to find the precise times of the disappearance and reappearance.
Many on our team were successful. Six had positive observations; five recorded misses, helping to define where the asteroid wasn't. Combining all the data, we plotted the size and shape ofThyra's silhouette as then presented to Earth.
But such profiles are tantalizing. We could see the profile better if we just had a few more observations! That's where your efforts come in to play.--Tony George
Two Good Asteroid Occupations Coming Up
The medium-large asteroids 85 lo and 51 Nemausa will occult 7.5-magnitude stars high in the sky just a few days apart in late August and early September. That's brighter than usual for stars predicted to be covered by asteroids, and with asteroids this large, the stars could vanish for up to 7 or 8 seconds. The predicted paths here are probably correct to within a small fraction of their widths. (The second date in the title is the UT date.)
Even without video, these events will be interesting visually. And even if you can only report whether the star was occulted or not at your site, this could still constrain the location of the asteroid's edge.
Please note: Some tables or figures were omitted from this article.
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|Title Annotation:||OBSERVING: Celestial Calendar|
|Author:||Wilds, Richard P.|
|Publication:||Sky & Telescope|
|Date:||Sep 1, 2016|
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