Even with the interest in comets brought about by the Shoemaker-Levy 9 collisions with Jupiter, I don't often get to share the experience of observing a comet. There are a number of factors that make comets less accessible to new skywatchers. First, we haven't had a bright comet in a while. Comets of 7th or 8th magnitude don't exactly jump out from the eyepiece, especially for someone who is not used to using a telescope. Second, comets tend to be brightest when they are near the Sun. This means observing right after sunset or, worse yet, right before sunrise. I know few people who will commit to getting up at 4:00 am just to (maybe) see a fuzzy patch. Finally, the same fleeting nature that makes comets intriguing means that there is only a limited observing window. A comet may be visible this week and gone the next, and with bad weather and moonlight one really has to be prepared to observe whenever possible.
So, my comet viewing is a lonely pursuit. A lone observer waits for the waxing gibbous Moon to set, as he prepares his telescope and finder charts. The temperature is about 15 degrees F, and an East Wind is keeping the skies clear but playing havoc with loose papers and 'scope covers inside the dome. The observer uses the telescope's setting circles to zero in on the first target, and the morning begins for real...
On 1996 February 1 between 1253UT and 1405UT (4:53-6:05 am, PST), I searched for the three bright comets visible in the morning sky. Here are my observations with a Celestron C-8 telescope from the James H. Karle Observatory at Lewis and Clark College in Portland, Oregon.
C/1996 B1 (Szczepanski)
This comet was just discovered a couple of days earlier, near the handle of the Big Dipper. By 4:30 am, it was nearly overhead, away from the worst light pollution of Portland. Still, I had heard that this comet was large and diffuse. Something every observer should know: If two objects have the same total brightness, the larger one will have a lower surface brightness and thus be more difficult to see. A diffuse comet of 8th or 9th magnitude could be a challenge. Using digital setting circles, I zeroed in on the place the comet should have been. Nothing. I made a few sweeps to either side and up and down. Still nothing. Then I remembered that I had the comet's position for 0 hours Universal time. It was now almost 13 hours, so to get the best position I would have to interpolate between today's and tomorrow's positions. I quickly did the math in my head and nudged the telescope in the correct direction. There! At the top of the field was one of the largest and dimmest smudges I had ever seen. I brought it into the center of the field, trying to shield my eyes from stray light to get the best view. It didn't seem to have shape or condensation. It was just...there. Not satisfied, I put a light pollution filter on a low power eyepiece. Sometimes these filters will improve a comet's view, sometimes not. The filter killed a few of the faint stars in the field, but the comet swam to the foreground. It was still a large, round, indistinct patch, but the southern part looked a bit more concentrated. I jotted down a few notes and made a sketch.
The comet
was sighted at 1253UT, using
the C8 at 71x. Near the zenith, the
naked-eye limiting magnitude was 5.4.
The comet appeared very diffuse,
with little condensation. With an Orion
Broadband Light Pollution Filter
and 67x magnification, the comet was much
more prominent and visible with
direct vision. There was a slight
condensation to the south, but no clear
nuclear region. I estimate the
total magnitude to be around 8.5, but the
comet appears much fainter
because of the low surface brightness.
Observed coma diameter was 11
arcminutes and there were no prominent
extensions. Degree of condensation
was 2 on the 0-9 scale.
One down, two to go.
45P/Honda-Mrkos-Pajdusakova
This comet promised to be diffuse as well. A periodic comet, it had already passed perihelion (closest approach to the Sun). It was headed in Earth's direction, so its apparent size was increasing rapidly. I missed seeing this comet before its perihelion passage, losing out to twilight glow and light pollution. Now I would get another chance. The comet was a short distance from the bright globular cluster M10. The previous day, an observer had posted on the Internet that the two objects were about the same total brightness. I was skeptical but hopeful nonetheless. I looked at M10 for a bit, then hopped around to its south.
This comet was a bit
difficult to locate
from a finder chart and ephemeris, due to its rapid
motion. I finally
pinned down its position. When I first saw it, at
1335UT, it looked like
a replica of C/1996 B1. As I observed it more
carefully, I noticed that it
was both brighter and more elongated. There
was a prominent nuclear
condensation to the SE, and the northern edge of
the coma appeared more
defined than the others. Eventually, I could make
out a tail extending 15'
to the WNW. I estimate the total magnitude to be
7.9. Observed coma
diameter was 9'. Degree of condensation was 4.
Although most comets develop tails when they are in the solar neighborhood, it is usually difficult to see these tails visually because of lack of contrast. Light pollution really complicates things. I considered myself lucky to even see this comet, and doubly lucky to see its tail. I pulled myself away from the eyepiece and prepared to hunt for the last comet of the morning.
C/1995 Y1
(Hyakutake)
I saved this comet for last. While it had a total brightness near that of the other two comets, other observers had noted that it was more condensed. Also, it was farther to the South and East in the sky, so I wanted to let it rise higher. It was a disappointment when I looked in the eyepiece and saw nothing. I re-interpolated its position from the ephemeris. Nothing. I swept the sky frantically before realizing that I had made an error of 1 degree in my math. A bit sheepish at forgetting how to subtract, I moved the scope a degree to the South and there it was.
As
expected, this was the easiest and
most condensed of the three comets.
At 67x with filter, it appeared as a
compact glow about 5' in diameter.
There was no sign of a tail, and
little elongation, but the N edge
appeared indistinct and fuzzy. I
estimate the total magnitude to be 8.4.
Degree of condensation was
6.
Twilight seemed to come suddenly, as just 15 minutes after I located the comet it started to fade. Dawn was approaching. I moved the 'scope to M13, the Great Hercules Cluster, and marveled at its thousands of glittering points. I then closed the dome and sought a warmer environment. Three comets in a night was a new record for me, but I expect I'll break it sooner or later. I probably won't see 45P until its next favorable trip sunward. As for the other two, the Moon is nearly full. I'll have to wait a couple of weeks for another shot, and that's only if the weather cooperates. Who knows? By the next clear, moonless night, there may be another comet or two for me to track down!
© Copyright Info