Omega Centauri, a Couple of Comets, and a Slew of Galaxies

 2024 March 16 (a.m.): Omega Centauri, a Couple of Comets, and a Slew of Galaxies

What do I do when it’s a weekend with good weather for deep-sky observing, but the First Quarter Moon is washing out the evening sky? Well, if I’m up for it, I just observe during the morning instead. 

Just after midnight on the morning of March 16th, I drove out to a dark site at Willow Valley Reservoir, in eastern Klamath County on the Oregon-California border. There is a light dome from Klamath Falls to the west, and a tiny light dome from Lakeview to the east, but overall this is a great site with wide-open horizons. The reservoir itself was full. Google Earth has a satellite image from 2017 that shows it flooding the access road. It wasn’t that high this time, but as I turned in to the parking area I stopped to make sure I wouldn’t be driving into the water. The Moon wouldn’t set until 2:18am, so I left my scope out for a while to cool down while I rested in the car. It wasn’t that cold, about 30 degrees F, but I wanted to conserve some warmth before a long observing session. 

After preliminary setup, my first target was Comet 62P/Tsuchinshan. This is a short-period comet that orbits the Sun in about 6.6 years. I’ve observed it on two previous returns, in 2004-2005 and in 2017. In 2011, it was also in the inner solar system, but on the other side of the Sun as seen from Earth, so it wasn’t visible. The 2023-2024 return was supposed to be one of the best, but due to bad weather and timing I didn’t get to see the comet at its brightest. (“Brightest” is relative; even at its best in late December 2023, 62P wasn’t visible to the naked eye.) By mid-March, it had faded considerably, but I still swept it up easily in my 10” Dob. The comet was diffuse with a slight central condensation that appeared to be elongated slightly west-east. In the way of diffuse comets, the coma appeared to fade out gradually with no distinct edge against the sky background. I thought I could detect a bit of elongation to the southeast, but this may have been an illusion caused by the lack of competing field stars in that direction. Every once in a while, the core seemed to pop out and it looked like there were jets to the southwest and northeast of the core. The coma was about 8 arcminutes in diameter. I estimated its magnitude as 11.2.

One of these fuzzies is a lot closer than the others ...

Comet 62P was located in Virgo, so there were a number of galaxies in the field. I would come back to explore those, but time was drawing near for another of my observing goals for the morning. For this act, I moved my telescope about 100 feet to a spot on the exit road with the best view of the southern horizon. I was going to attempt to see Omega Centauri from Oregon.

Omega Centauri is the brightest globular cluster as seen from Earth, and may contain 10 million stars. Unfortunately for Oregonians, it is at declination -47.6 degrees. Think of declination as celestial latitude, with the North Celestial Pole near Polaris at declination +90 degrees and the South Celestial Pole at declination -90 degrees. From latitudes north of 42.4 degrees north (okay, you can fudge it a bit with atmospheric refraction and negative horizons), the center of the cluster never rises above the horizon. My observing site at Willow Valley is at 42.01 degrees north, so the cluster skims above the horizon for a bit. As good as the horizon is at Willow Valley, it isn’t perfectly flat and level, and there is a distant low ridge of trees that the cluster needs to clear. The trick is to catch the cluster at its culmination, which occurred at 2:55am this morning. I struggled for a little bit to get oriented, but eventually figured things out, and Omega Centauri was clearly visible in my 8x42 binoculars, perched just above the tree line. I have seen Omega Centauri once before from this site, but I don’t remember looking for it with binoculars. I think I just swept the horizon with my scope. 

With Omega Centauri located, I got down in a fairly uncomfortable position to view it through my Dob’s eyepiece. Transparency of the air was very good considering the low altitude, but seeing (steadiness) was poor. The faint field stars near the globular looked like little fuzzy balls with no sharp focus, and I wasn’t able to resolve any of the cluster’s myriad stars. I’m also sure that I only saw the inner portion of the cluster. Its main feature was that it was oval, slightly elongated east-west, with a V-shaped lumpy area opening to the east of the core. This may have been formed from an unresolved chain of cluster stars or from a few foreground stars. At 104x, the Earth’s rotation made the cluster seem to scoot along the treetops (of course, the trees and cluster were upside down in my Dob, so it appeared below the trees in the field). By 3:15am, the cluster had dipped enough that it was disappearing behind the trees. 

Everything is upside-down through my Dob ...

Another southern deep-sky object near Omega Centauri is the peculiar galaxy NGC 5128, home of the radio source Centaurus A. NGC 5128 is about 4.5 degrees further north than Omega Centauri, so I’ve seen it numerous times from southern Oregon and northern California. Usually, it looks like a washed-out “C” with a gap in it. But this morning’s good transparency allowed the galaxy to reveal more of its nature. The large dust lane looked like an actual dust lane, separating two parts of an elliptical galaxy. Even though I was only seeing the inner core of the galaxy, it was still an interesting sight.

Moving my telescope back to its original location and centering Comet 62P again, I made notes on the galaxies visible in the same eyepiece field. 

NGC 4380: A nice galaxy, extended ~2.5:1 SE-NW. Nearly stellar nucleus and oval core, possibly barred. Faint arcs suggest a counter-clockwise spiral pattern.

NGC 4390: Looks elliptical, also elongated SE-NW. Condensed to a core, fading out evenly to the edges.

NGC 4320: Has a nearly stellar core; otherwise it is very faint with a small halo only visible with averted vision. A star of similar brightness to the core is just to the southwest and is confusing. No detail in the halo.

NGC 4325: Elliptical, condensed to a core. Oval NW-SE with no other details but halo visible with direct vision.

I made a quick dash along some of the other galaxies that were reasonably nearby, noting the pair NGC 4424 and the high-surface-brightness NGC 4417 for future observation. NGC 4442 had a very high surface brightness. NGC 4178 was very elongated with structure, and I’d like to do a deep observation. I also couldn’t pass up a quick sweep of the core of the Virgo Cluster, but I didn’t sketch any additional objects until I arrived at my second comet of the morning, C/2021 S3 (PanSTARRS). I realized later that I should have checked up on C/2023 A3 (Tsuchinshan-ATLAS), which should have been 12th magnitude and likely would have been easily visible under the conditions, but I didn’t prepare a finder chart for it so I will have to catch it another time. C/2023 A3 is likely to become bright in the autumn of 2024. If it performs well, it could be similar to C/2020 F3 (NEOWISE).

Back to the present. How good were the skies at Willow Valley Reservoir on the morning of March 16th? Well, they were decent, but well short of the best I’ve seen at this site. Sky Quality Meter (SQM-L) readings clustered around 21.5 magnitudes per square arcsecond at the zenith. Limiting magnitude estimates through star counts in several regions averaged 6.8. I could occasionally make out the Gegenschein near the head of Virgo, but it didn’t stand out. The Milky Way showed nice extension but wasn’t shockingly bright. 

In 2021, on April 8/9 and 9/10, I experienced a couple of nights at Willow Valley that seemed otherworldly by comparison. SQM-L readings were around 21.8, and limiting magnitude was 7.1-7.2. There was much better contrast in the Milky Way on those nights, and part of the Zodiacal Band was visible. 

I’m not sure why conditions weren’t that good this morning. Sky transparency seemed good, with no horizon haze. I didn’t notice bands of airglow, either. Perhaps there was more moisture in the air, or airglow that wasn’t apparent. In the end, we get the conditions that we get and make the best of them.

Comet C/2021 S3 (PanSTARRS) was in western Aquila, and predicted to be about magnitude 10. A couple of large open clusters, NGC 6633 and IC 4756, were in the general vicinity. I star-hopped over to the comet with the Dob. The comet showed a milky white coma that was small and well-condensed to a stellar nucleus, and a tail that was readily visible and stretched for about 15’ to the WSW. Some subtle linear structure was visible in the tail. The coma was about 1.5 arcminutes in diameter and I estimated it at magnitude 9.3. A Western Screech-Owl was hooting in the near distance. 

I finished observing the comet at around 5:45am, as morning twilight began to overtake the sky. I grabbed a nap in the back of my car, did a little birding around the reservoir, and then headed back to town.