How does Comet ISON look through the telescope?

Posted by Michael Bakich
on Thursday, October 17, 2013

We get a lot of images here at the magazine, but not as many observing reports as you might think. The following one (which describes three separate viewing sessions) came from Wayne Johnson, aka “Mr. Galaxy,” who lives near Benson, Arizona.

This image of Comet ISON (C/2012 S1) shows a bright, condensed head and a lengthening tail. British astroimager Damian Peach captured the comet September 24 at 11h36m UT.
I have not seen many decent visual observations of Comet ISON, so I thought I would share a few observations I made of the object a couple weeks ago when it was just becoming bright enough to see in a telescope.

First observation from my backyard near Benson, Arizona
After at least six attempts for the past few weeks in Arizona’s indifferent monsoon weather, I finally and unequivocally found Comet ISON as a visual object in my 25-inch Dobsonian reflector. It was bright enough that I think it should be fairly readily seen in telescopes about half that size in similar climactic conditions.

This seems to have been a very difficult monsoon season here in Arizona for making observations of any sort, especially near the horizon. It hasn’t really been raining, but it has been humid. Most nights when I’m out trying to observe, I can hear the condensing water running off the metal roofs into my water collection containers. Mosquitoes have been making observing conditions distracting and difficult. I tried to observe the comet on September 11/12, but the clouds decided to roll in at about 2 a.m. and remain until dawn. The following night (September 12/13), it was the opposite — cloudy before midnight, then mostly clearing up the rest of the night. There was some distant lightning in the northeast and clouds threatening to roll in along the southeast horizon.

For my earlier observing sessions, I had been making 0.5°-wide finder charts using the Palomar Observatory Sky Survey images from the Hubble Space Telescope website. Since the motion of the comet has been about 0.5° a day, the chart size was a little inconvenient for multiple day tracking of the comet. So, I finally decided to download a 1°-wide finder chart (centered on 9h01m42s and 20°34', the three brightest stars seen on it are of about 8th magnitude and are plotted on the U2000 charts, for those interested) and was able to plot the motion of the comet in 12-hour increments.

The comet was found near the position I estimated for Sept 13.5 (when it’s 4 a.m. here in Arizona, it’s 11h UT or about halfway into the day). After locating the approximate field at around 3:45 a.m., I searched around the area for about 20 minutes and finally found a hazy spot that looked promising at 4:05 a.m. I knew there was a faint galaxy about 10' to the west of the calculated position of the comet, and it looked like the comet had passed by the galaxy earlier in the day. I was able to observe that the galaxy (maybe magnitude 14 and about 1' in apparent diameter) lies east of a distinctive small, flat triangle of faint stars and knew by then that the additional fuzzy spot I found was really the comet. While the galaxy was roundish with a small brighter middle, the comet had a pretty small, little brighter middle and appeared asymmetric with a general haziness (tail) to the west. Its inner coma was about two to three times larger than the galaxy with the tail about twice again as large in extension.

The nail in the coffin was the fact that I could detect motion in the comet after observing it for about 40 minutes until about 4:45 a.m., when the zodiacal light, which enveloped Mars about 2.5° to the south and reached up to about Jupiter, started to interfere. In that amount of time, I was able to see a generally eastward movement of about 2', or about the comet coma’s apparent diameter, with respect to several faint stars that I used as landmarks. The comet appeared to be about 1.5 magnitude brighter than the nearby galaxy, both of which I could fit in the field of view of my telescope (which, at 200x, gives about a 20' field). Although the sky was getting brighter, the fact that the comet was getting higher in the sky (out of the haze) made it easier to detect.

Second observation from my backyard
Since I was able to observe Comet ISON pretty readily with my 25-inch the night of September 12/13, I thought I would give it a shot with my basic 13-inch Dobsonian the following night (September 13/14). As expected, the task was more difficult. I used the finder chart I had generated from my previous outing and located the area where I expected to find the comet at about 4 a.m. local time (11h UT). After sweeping over the area for about 30 minutes using about 100x, I could not find the comet. The sky was starting to brighten with the zodiacal light, and I had about 30 more minutes of useable time to be able to find an object this faint. The comet was supposed to be fairly close and just south of a magnitude 8 star that I had used as a guidepost the previous night. I thought I had the wrong star, but retracing my steps and using a different star pattern to re-find the area landed me on the same star.

I decided to use higher magnification (200x), and, sure enough, there was the faint smudge of the comet! I had been scanning over it several times with the lower magnification and not seeing it. Higher magnification usually narrows the field of view, which makes the view in the eyepiece appear darker and helps make fainter objects stand out better. It worked in this case quite nicely, and the magnification was about the same as I used in my 25-inch when I located the comet the previous night.

In a nutshell, I found Comet ISON two nights in a row, a record for my Arizona observing! I located it at 4:30 a.m. and kept track of it for 15 minutes (until about 4:45 a.m.), when dawn light started to wash out nebulous objects. Since I didn’t track the comet for very long I noticed it only had a small amount of motion to the east (less than 1'). Since I was using a smaller telescope, Comet ISON had the same general appearance as the previous night but appeared about half as bright. I would estimate the size of the inner coma to be about 2' in diameter with a very slight extension to the west and total magnitude of about 13.

Early visual observations of this comet have been an interesting challenge, and I’m glad I finally had some success. The weather was fairly dry this night until about 4 a.m., when it started to dew up. The seeing was good, and the transparency was pretty good with some intermittent cirrus clouds.

My last look at ISON this lunation
On September 17, the Full Moon was nearly here, so I thought I would try to see Comet ISON one last time until the Moon moved out of the morning sky. The monsoons have mostly abated for a while, and it was nice to observe without everything being dripping wet. The mosquitoes were still a nuisance but not as bad as they had been.

The seeing and transparency were good for the near hour that I observed the comet, but it was a challenge to avoid the light of the nearly Full Moon setting in the west around 4:15 a.m.; the zodiacal light, which started interfering around 4:45 a.m.; and the inevitable dawn light, which obliterated the comet by about 5 a.m.

The comet’s appearance hadn’t really changed much in the week or so that I had managed to see it, but it did appear to be brightening ever so slightly. It wasn’t quite as much of a challenge to see it on the 17th in my 13-inch reflector as it was a few days prior. I could see the comet fairly readily with low power (80x), but it was much better with 200x. It appeared asymmetrically oval; the brighter preceding part to the east was the inner coma, and the fainter part (the tail) was to the west. I would say it measured about 1' by 3'. ISON’s inner coma was still pretty small and little brighter to the middle. I detected motion of about 2' in a generally eastern direction over the 45 minutes that I observed the comet. There was a very faint galaxy (that I marginally detected) in the field of view that the comet was moving toward. Mars served as a good (but moving!) landmark and lay about 2° to the south of the comet.

Clear skies,
Wayne (aka Mr. Galaxy) Johnson

Thanks, Wayne, for a great report!

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