28.8 - 1.9.2011 - Sedona and the light pollution ordinance (Part 2)

Date: 31.8/1.9.2011
Observing location: Sedona, Arizona, USA (1368 meters / 4488 feet)
NE Lim.mag: 7.0m (zenith)
SQM-L: 21.00 (zenith)
Background sky: 7 (good)
Seeing: 7 (good)
Transparency: 7 (good)
Weather: +26.0 - 21.0°C, humidity 28%, clear.

I had a mix of some interesting objects for the almost last night of the trip. I had forgotten all about IC 4628 in Scorpius so I started it that and the nebula wasn't difficult to spot as an E-W elongated puff  in a rich field of stars. I also humbled to sketch the large cluster Collinder 316, pretty much sketching everything I could see with the 40mm eyepiece (98' fov). Looking at the area after the fact, it seems like a big mess. Uranometria shows Cr 316 as a superimposed cluster surrounding Trumpler 24 with smaller clusters vdB-Ha 205 and Ruprecht 122 inside it. My SkyMap (updated cluster data of Dias et al.) shows vdB-Ha 205 with somewhat curious name of BH 205 and Ruprecht 122 as BH 202. It also includes three(!) additional clusters inside Collinder 316: ESO 332-08 (which I noted as a concentration of Cr 316), ESO 332-11 and ESO 332-13! Quick clean up is in order so vdB-Ha 202 = Ruprecht 122 (1975 vs 1967 discoveries respectively) and the ESO clusters are probable cluster candidates. And Uranometria was right, no surprise there.

NGC 6506 field

After all that, I was finally able to see the three open clusters that had eluded me on previous two attempts. The reason was obvious: I was looking for something far more brighter and obvious. These clusters, located just SW of Messier 8, are marked in Uranometria (Volume 2 / Chart 146) as Ruprecht 138, 138 and 169. However Ruprecht 138 = NGC 6506! There is a 4th grouping of stars in this area but it is merely an asterism (noted by Phillip Teutsch). SkyMap's cluster data shows Ruprecht 137 near by - just south of HD 164147 but there is no cluster here. It is located more to the south roughly at 18 00 17 -25 14 00. Luckily, MegaStar correctly lists the whole bunch.

Next in line was supposedly one tough trio: two Terzan globular clusters and a Palomar. I began with Terzan 10 as it was close enough to the previous clusters. I routinely sketched the field and with averted vision could a few times saw a very faint, tiny nebulous patch just NW from a roughly 12th magnitude star and I marked this as Terzan 10. Wait, back up! Is this the same globular cluster Barbara Wilson has failed to see with a 20 inch telescope? Yes, the very same. Then, is there even a remote possibility that I actually saw it from a location such as Sedona and with an 4 inch aperture? I'm not going to say I saw it just based on this single observation but I'm going to give you a few things to chew on:

1.) I sketched the cluster perfectly to the correct position @ 304x
2.) There apparently a very faint pair (~15th magnitude) of stars just in front of Terzan 10.
3.) Terzan 10 is placed way off in SkyMap (surprised, anyone?).

Terzan 5 field

With Terzan 10 in the (iffy) bag I moved to Terzan 5 which I can honestly say I saw. With such small aperture the field was dangerous none the less: I picked up 2 additional glows in the field when using low power. The most obvious is an asterism (Asterism 1 - I'm calling it the "Fake Terzan 5 group") of 8 stars between magnitudes 11.3 - 13.9 and it looked too good to be true and higher power showed it just as a group of stars. The other one was a group of 4 stars including a magnitude 11.7 and 11.9 double (Asterism 2). This too looked nebulous at lower power. Terzan 5 actually forms a triangle with these two and was seen as a very faint, very small glow just W of a 11th magnitude star.

And then came Palomar 6 (by the way, this globular cluster includes the planetary nebula JaFu 1 in case you didn't remember). Despite its apparent faintness (if you can call it that) it was easier to see than Terzan 5 and appeared as a faint, round smudge in a rich star field with dark nebulae. After a success with Palomar 6 I went on to sketch Palomar 11 - again. Then I got back to even more familiar territory with open cluster NGC 6604 which I saw more of a large cluster with two concentrations than the tiny 5' cluster listed as NGC 6604. I really took a liking with this field which includes the emission nebula Sh2-54 in the background. NGC 6818 showed a definitive ring structure and Messier 75 two "bars" extending from the bright core.