After debating where to observe last Thursday night, Jim Shields and I decided to give Lake Sonoma a try after reading several positive postings on TAC the past year. Leaving at 3:00, I was expecting about a 90 minute drive from Albany -- this turned out to be a bit optimistic as there was quite a bit of afternoon traffic in pockets slowing the trip up 101. The drive, though, was very easy (especially since I was first considering the nausea-inducing Mines Road to San Antonio Valley) and the 85 mile trip took two hours up and 80 minutes back after midnight. We arrived just after sunset and located a large, flat parking area about a half mile before the camping turnoff with wide-open horizons in all directions except for a few trees at the north end. There were no other vehicles in the lot and other than an occasional passing car on the road, it was very quiet, calm, dew-free and surprisingly dark! Except for a light dome due south (perhaps 20 degrees up), the transparency was excellent (near mag 6.5 in most parts of the sky) and the seeing easily permitted 500x as the evening wore on. When I drove home, we experienced pockets of ground fog and the East Bay (and I assume S.F.) was completely socked in. This may have contributed to a darker than average evening at the lake.
After quickly throwing my 17.5-inch truss-tube dobsonian together and munching on a sandwich the sky looked invitingly dark and it was all of 6:30! I spent awhile on a few familiar bright planetaries, experimenting with different magnifications and filter combinations, trying to draw out as much detail as possible.
At 100x, NGC 40 appeared (unfiltered) as a slightly elongated, moderately bright disc surrounding a bright mag 11.5 central star. A slightly fainter mag 12 star lies 1.0' SW. I targeted this object because it is an unusually low excitation PN, with an OIII/H-beta ratio of 0.4. In most PNe, this ratio is runs from 10:1 to 20:1. This implies that on NGC 40, the OIII filter is not the best choice and as predicted there was a noticeable enhancement using the H-beta filter and it dimmed with a 2-inch OIII. At 220x, a star was intermittently visible at the SW edge and the PN was slightly elongated SSW-NNE. The best filter response using this power was with the UHC. The surface brightness appeared irregular -- darker around the central star and slightly brighter along the west and east side of the rim. At 280x, the faint star I had earlier noted was barely off the SW edge and PN was weakly annular with a brighter rim along the west and east side and a darker center. The SW and NE ends of the halo were clearly weaker, though. 380x provided a nice view with subtle irregularities in the interior.
Next up was NGC 1514 in Taurus. Again starting at 100x (20mm Nagler), this moderately bright PN displayed a round, 2' halo surrounding a prominent mag 9.5 star. This time there was an excellent response to UHC and OIII blinking and no response to the H-beta filter (In this case, the OIII/H-beta ratio runs at 12:1). Using the OIII filter, the surface brightness was noticeably uneven, with the NW quadrant of the rim clearly brighter. The SE side was also weakly enhanced while the center and ends of the minor axis were slightly darker. At 220x using a UHC filter, the halo appears nearly 2.5' in diameter. There was a small, darker "hole" surrounding the central star and the halo was clearly irregular with a brighter "knot" on the SE side, while the NW portion of the halo was brighter along the rim.
Back in 1980, I logged the beautiful double-shelled planetary NGC 1535 as "greenish", but in several subsequent observations with 13" and 17.5" scopes it has appeared blue to me. Has anyone noticed a perceived color difference based on aperture (or possibly aging eyes?). This PN has a wonderful double-shell structure which was easily visible at 100x surrounding the bright central star. The view at 380x and 500x was superb in the good seeing. The double shell envelope was very prominent with a bright inner ring ~20" diameter with a fairly sharp edge embedded in a fainter roundish halo roughly doubling the diameter. The inner shell is irregularly darker surrounding the central star (I've previously described this structure as "small dark gaps" in my observing notes).
The diminutive planetary, IC 418 in Lepus, has gained some notoriety lately because of reports of a unique pinkish hue. I've noticed this coloration twice, but the effect is pretty subdued to my eyes. At 82x, the mag 10.5 central star was enveloped in a very small round halo which appeared to have a slight reddish tinge at its edge. Once again, this is a low-excitation planetary (OIII/H-beta = 1.2) and using a H-beta filter, the halo brightened while the central star dramatically faded, leaving a more noticeable disc. At 220x, the prominent central star was surrounded by a well-defined 10" halo which partially "blinked" on and off by switching from averted to direct vision. At 280x, the small halo was possibly surrounded by an extremely faint envelope, but this could not be confirmed, 380x and 500x presented a superb view of the inner disc which appeared weakly annular.
I usually try to include an edge-on from the Flat Galaxy Catalogue (FGC) in my observing list (the "Integral Sign" galaxy was listed in January's "Off the Beaten Path"). This catalgoue resulted from a systematic survey of the Palomar Sky Survey (POSS), searching for edge-on galaxies at least 40" in diameter and with a major:minor axis of 7:1 or greater. This night I decided to take a look at UGC 2082 = FGC 317 located at 02 36 17.4 +25 25 20 in Aries. This galaxy has an integrated blue magnitude of 13.7, but with dimensions 5.4'x0.8', the surface brightness is only 15.2, typical of low surface brightness UGC galaxies. At 100x I picked up a ghostly streak, ~4'x1' oriented NW-SE with little or no central concentration. This large, low surface brightness edge-one was difficult to view at 220x although it could be held steadily with averted vision. At this power it appeared ~4.0'x0.8' with a very weak broad concentration. A mag 10.5 star 4' NW is collinear with the major axis and there are several additional mag 10 stars in the field.
Perhaps the most challenging object of the evening was Hickson Compact Group (HCG) 29 at 04 34 43.3 -30 32 39 (2000). This is a linear chain of 4 dim galaxies with a total length of only one arcminute! This challenging group is easy to located -- just 11' west of 4th magnitude 52 Eridani (Upsilon 2), but nothing was visible at 100x. It finally took 280x and averted vision to record a marginal sighting of the brightest member (HCG 29A) which was intermittently visible as a 10" knot just 20" NE of a mag 12.5 star. This galaxy was missed by the comprehensive catalogues based on the Palomar Sky Survey (UGC, MCG, CGCG), so just detecting this one member felt like an accomplishment. A number of the entries in the Hickson catalogue have members with anomolous redshifts (such as Stephan's Quintet) and in this case, the three fainter members (blue magnitudes only 17.0-18.4) have redshifts over twice that of HCG 29A.
Most of the evening was spent picking off nondescript galaxies in Eridanus and northern Fornax, but viewing several in the field always adds interest. This southern equatorial belt was surveyed in the 1880's at Leander-McCormick observatory at Charlottesville, Virginia with a 26-inch refractor and their discoveries contains many of the most challenging NGC entries. To further frustrate the situation, their published positions were only given roughly (to the nearest time-minute of RA), so often modern catalogues and sky-plotting software give contradictory identifications. This was the third time I've visited the NGC 1561 group (04 23 01.1 -15 50 45)-- each time picking up a few more faint members. This evening I finally logged all six galaxies in the field including NGC 1561 through 1565, as well as IC 2063. All of these galaxies appears 30" or less in diameter, so 200x or higher is very helpful.
Later in the evening Jim and I took a close look at naked-eye NGC 2264, better known as the "Christmas Tree" cluster. It also contains the Cone Nebula which I believe is often misidentified. William Herschel's discovery description from 18 Jan 1783 simply reads "Double and attended by more than 30 considerably large stars" but a reobservation in 1785 mentions "Some pB stars ... are involved in an extremely faint nebulosity which loses itself imperceptibly." John Herschel also described a "star 5.6 magnitude enveloped in a nebulous haze."
The whole field surrounding the cluster is weakly nebulous but Herschel's description probably applies to brightest area of nebulosity, easily visible in dark skies ~10' SW of S Mon (brightest star at the base of the tree) which includes a group of three brighter stars -- but this is *not* the Cone Nebula. At the south end of the "tree" is the double star Struve 954, a mag 7.1/9.6 pair at 13" separation. There is much weaker nebulosity surrounding this pair which was also readily visible at 100x. The "Cone" is not this faint HII region either, but rather a dark finger of dust which extends south of the double, protruding into the weak glow. On this night we both noted a slightly darker "hole" just to the south of Struve 954 in the position of the Cone although the nebulosity is very subtle here and we could not detect a sharp "edge" to the finger which shows up well on deep images.
As we packed up, we both gave a thumbs up review to the site and plan to return in the near future.