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An Amateur Star Geezer’s Account of His Comet ISON Recovery Imaging
Guest blog post by Bruce Gary, 2013.08.14, 18.0 UTTrying to recover Comet ISON (C/2012 S1) was the last thing on my mind when our local astronomy club member, Doug Snyder, asked local amateurs to join his project in trying to be the first to recover the comet. I like challenges, even though I’d never observed a comet with any scientific goal before. Since Doug has a comet discovery in his resume I assumed his challenge might be feasible, so I signed up.
The only problem was that my principal telescope, a Meade 14‐inch, needed repair (can’t afford such things), so I had to rely on a backup Celestron 11 ‐ inch telescope – also in a dome. That was the first glitch, because the C11, as I refer to it, has an obstructed view of the sky below 11 degrees, and the comet’s observing window was ~25 minutes long when it was at about 6 degrees elevation. That meant it was going to perform like an 8‐inch aperture telescope. Other local amateurs had apertures of 14, 18 and larger, so my goal was a “long shot.”
The next glitch happened when I got the wrong coordinates for the comet from the JPL Horizons web site. There are two ways to request coordinates, apparent and astrometric, and this site had always remembered my past request for astrometric (J2000) – so I didn’t bother to check this. I also got coordinates from the MPC, and these differed from the JPL ones. In fact, for some mysterious reason the MPC ones were also wrong; I still haven’t figured that out. So as I opened the dome at 4:15 AM on August 12 I had two coordinates that differed by 11.5 ‘arc, and both were wrong by 5’arc in opposite directons – but I didn’t know it! Since my little ‘scope has a large FOV, 32 x 22 ‘arc, I was able to include both incorrect coordinates in my carefully positioned pointing.
I started an automatic imaging routine using 20‐second exposures. When the sky brightened 15 minutes later I switched to 10‐second exposures, then 5‐second ones. After the sky was too bright for observing I took sky flats, closed the dome and took darks and bias frames. After stacking a few images I counted ~ 116 stars and proceeded to check with a couple planetarium programs (TheSkySix and C2A) to identify each star to see if it “belonged” there. Only one smudge didn’t belong, and it was in the center of my image, midway between what I had mistaken for JPL and MPC predicted locations. After some more stacking I saw a what looked like a tail! I did a quick photometry using a nearby star with an APASS r’-band magnitude, and found that the putative comet was about the right brightness.
Semi-convinced that I had imaged the comet, I e-mailed JPL’s Don Yeomans to ask if he had any updated coordinates, and if not I had an update observation. Don’s a polite fellow and he let Richard Miles reply to me that my measured coordinates were exactly where the comet was expected, according to both the JPL and MPC ephemeris. I then returned to the JPL Horizons site and noticed my error, changed to astrometric and got a coordinate listing that agreed with my measured position. I even measured the its motion to add to my confidence that this was the comet, and I found that the movement during my 24-minute image set exactly matched the comet’s.
I created a web page, and Doug notified CIOC, and soon I was getting e-mails showing more interest in my magnitude than the comet position. It took me awhile to realize that I had info on the comet’s possible future brightness, so I began a re-analysis of magnitudes using techniques that I had refined over years of photometry of exoplanets, asteroids and other targets. I claim to be an expert in all-‐sky photometry as well as differential photometry, so I relished in the task of including all kinds of precautionary techniques for reducing systematic errors. My final result, using 7 nearby stars with APASS r’-band magnitudes, was essentially identical to my quick result: r’-band magnitude = 14.20 ± 0.07, which corresponds to a V-band magnitude of ~14.35 (assuming a solar-‐like spectral energy distribution). My analysis of magnitude versus photometry aperture showed that my large apertures were capturing almost all of the coma. This meant that I should be able to compare my V-band magnitude with predictions for “total” magnitude. The JPL Horizons brightness model listed a total magnitude of 13.43 for August 12. Darn! That meant the comet was ~0.9 magnitudes fainter than expected. No wonder others, more knowledgeable on comets, were focusing on my magnitude.
I should say that I’ve never done differential photometry at elevations as low as 6 degrees before, so maybe there’s a systematic error that I’m unaware of in my result. I really want someone else to confirm, or disconfirm, my comet brightness. I’m urging the other amateurs in the local club to hurry up and observe the comet with a 20-‐inch telescope that the club uses. Surely, other amateurs, and some professionals, are trying to observe the comet; especially now that my measurement is questioning whether this will be “the comet of the century.” Time will tell, and this new comet fan can’t wait to learn what’s going on with Comet ISON.
Bruce Gary's ISON recover web page
Bruce's photometric analysis of Comet ISON
This guest blog is posted on behalf of Bruce Gary, who recovered Comet ISON after it had spent several weeks hidden behind the Sun.