March 15, 2010: Short night at Lake Sonoma

Steve Gottlieb

On Monday night I met Carter Scholz for what turned out to be a fairly short evening (roughly two and half hours) at Lake Sonoma. Carter had e-mailed on Sunday that he was interested in observing and the CSC looked pretty promising so I quickly agreed to meet him. My van was still fully loaded with my astro gear from observing on Saturday night at Willow Springs (that observing report is still to come). My main concern was that I would still be recouping from Saturday night's frigid adventure (actually I was really quite comfortable while observing). Furthermore, I knew I would have to teach a calculus class at 8:30 in morning, but hey -- any good chance to observe is worth some minor inconvenience!

Unfortunately, by the time I took off at 4:00 in the afternoon the sky was looking pretty bad in the east bay, despite the NWS forecast of "mostly clear" and stayed "mostly ugly" until sunset (can't the NWS and CSC do a better job??). Carter and I, being eternal optimists, set up anyways and were actually rewarded when the clouds did thin out and allowed decent viewing through the winter milky way constellations. Because the transparency stayed pretty lousy in Leo, Virgo, etc., Carter and I both pointed our scopes in direction of Canis Major, Monoceros, Gemini and stuck with observing open clusters that showed up pretty well in fair transparency in that portion of the sky. By 11:00, though, the winter stuff was sliding down in the west and the transparency had not improved to make it worthwhile to start hunting galaxies, so we both called it an early night. By 11:30 I was out of the lot, back in bed at 1:00 and in OK shape when my alarm went off at 6:30 to get ready for work. Here are a few of the more impressive open clusters I logged.

M50
07 02 48 -08 22.6
V = 5.9; Size 16

18" (3/15/10): very bright, fairly scattered cluster, though contains a richer subgroup. About 125-150 stars were resolved at 175x in the central 10'-12'. The cluster has roughly a triangular or wedge-shaped outline, highlighted by an 8' string of stars oriented ~E-W on the north side (including mag 9.0 HD 52965) and a longer 10' string oriented NW-SE on the SW side that includes the brightest cluster star - distinctly orange mag 7.8 HD 52938 near the SE end of the string. Along this string are also four easy pairs of stars. There is a rich circular group of stars, ~5' diameter, a little north of center. On the east side of the cluster is an unequal double, h748 = 8.5/11 at 15". A faint, unequal pair of stars, BRT 392 = 11.5/11.6 at 4", is due west of the unequal pair by 3.5' at the edge of the rich group of stars. A number of stragglers extend out the cluster increasing the size significantly and a scattered group with some brighter stars that appears detached to the NW including mag 9.0 HD 52720.
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NGC 2355
07 16 59 +13 45.0
Size 9

18" (3/15/10): fairly rich group with ~70 stars in an 8'-9' diameter group, with the richest portion in the central 4'. On the north side of the central region is a 13" pair of mag 11/12 stars with a mag 13 star 16" south forming an easy triple. The stars are pretty evenly distributed and many are similar magnitudes, though a number appear to be arranged in strings that extend out radially from the center. The brightest member is a mag 10 star on the SE side. The cluster shares the field with mag 8.3 HD 56329 located 7' NNE.
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NGC 2420
07 38 24 +21 34.5
V = 8.3; Size 10

18" (3/15/10): this is a rich, pretty group at low power with roughly 80-100 stars in a 10' region at 175x. Higher power brings out the fainter members. The brightest mag 9.4 star is at the west end of the cluster and forms a large mag contrast pair with a 13th companion 12" south. On the N end of the cluster is an equally spaced, collinear trio with separations of 14". The brighter stars in the cluster are pretty evenly distributed. Many of the fainter stars are in a 2'-3' patch that follows the mag 9.4 star and which includes some unresolved haze. A couple of brighter mag 8-9 stars are in the field to the south of the cluster and also to the north.
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NGC 2506
08 00 02 -10 46.2
V = 7.6; Size 7

18" (3/15/10): at 175x, this is a beautifully rich 8' group of stars, roughly circular, with 80-100 stars mag 11-14 over unresolved background haze. The brightest two stars are at the west side and form a wide, 27" double. The richest portion roughly forms a "U" outline, open to the south, as if a chunk of the cluster was missing. The north side of the "U" consists of a string of stars oriented WNW-ESE (north of the two brightest stars) with a pair at the WNW end. At the ESE end of this string a few brighter stars form a right angle heading SSW and forming another side of the "U". A faint string of stars extends out of the cluster to the SE and another string extends out the WNW.
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M48
08 13 43 -05 45.0
V = 5.8; Size 54

18" (3/15/10): large, sprawling cluster that nearly fills the 67' field of the 31mm Nagler. The most distinctive portion, though, is a 20'x15' group near the center which contains roughly 75 stars, many arranged in strings. Most prominent is a fairly dense SW-NE string of brighter stars that cuts through the cluster with a nice double star in the center (h 2435 = 9.6/9.7 at 7"). A short chain of a few stars branches off to the south of h2435. Another stream of stars branches off this string to the SSW. A third elongated group of stars extends SW-NE on the south side, roughly parallel to main string, and is separated by a mostly starless lane. The cluster includes a number of mag 8-9 stars, with the brightest mag 8.2. At 73x, the main cluster is wedged between two very long streams of stars, roughly oriented NW to SE off both the north and south sides of the main group. These two strings of stars extend the total diameter to over 50'. On the NW side of the central group, a number of stars are scattered between these two streams including a few very wide pairs, but the SE end the two streams are well separated with a very few stars in between.