Chew's Ridge (MIRA) on Aug 23

by Steve Gottlieb

Two weekends ago, Dennis Beckley and I decided to head to Chew's Ridge, avoid the Mars' crowds and revisit the legendary seeing at the crest of Tassajara Road (5000+ ft) in the Ventana Wilderness near the Mira observatory. Dennis and I had both observed there a couple of times (my first time was 19 years ago) and had been impressed with the sub-arcsecond seeing (average has been measured at 1.1"). Long-time planetary observer Joel Goodman also joined us.

A few years back I made arrangements to observe inside the observatory gate with astrophotographers Chuck Vaughn and Robert Bickel. The drive takes nearly 3.5 hrs from my home in Albany with the last several miles on a beat-up washerboard road (if you've been to Tassajara Hot Springs you know what I'm referring to) -- only to discover that I had left my struts in the garage! Needless to say, this time I was more careful packing.

Dennis had made arrangements so we would be able to get inside the locked gate but this proved unneccesary as the gate was open and advertising a "fire" exhibit at the MIRA Observatory. We set up just off the road below the unmanned fire lookout tower, figuring this would be a quiet night. Not quite. A dozen or so observers from the Santa Cruz area rolled in as it was getting dark and took over the surrounding hillside.

For the first several hours of darkness, conditions were superb and I was easily using 500x to resolve faint globulars and planetaries with my 18-inch Starmaster. A few examples:

N6426 17 44 54.7 +03 10 13 V = 11.2; Size 3.2

18" (8/23/03): at 160x, appears fairly faint with an irregular triangular outline, 2.5' diameter. There is only a weak concentration though the surface has a patchy, irregular appearance with a few faint stars superimposed. At 435x, the brightest resolved star is at the NW edge and a few others resolved halo stars along the western side are collinear. The slightly brighter core is offset east ofthe geometric center and just resolved into several extremely faint stars at moments. A total of up to 10 mag 15 to 16 stars are resolved at moments.

N6535 18 03 50.6 -00 17 49 V = 10.6; Size 3.6

18" (8/23/03): at 435x, appears fairly faint, ~3' diameter with an irregular outline, and just a weak concentration. A trio of mag 13-13.5 stars are easily resolved on the west edge and the middle star has two close, mag 14.5 and 15.5 companions. With careful viewing about a dozen extremely faint stars sparkle over the central glow, often popping in out of averted vision. At 538x, the cluster barely breaks up into a swarm of extremely faint stars in steady moments.

N6539 18 04 49.7 -07 35 09 V = 9.8; Size 6.9

18" (8/23/03): at 323x appears fairly faint, round, pretty diffuse with only a broad, fairly weak concentration. A mag 12.5 star is off the NW side and mag 13 stars are off the SW edge and further off the SE side. A couple of mag 15 stars are resolved between the two brighter stars on the west side. At 435x, the surface brightness is quite mottled and seems on the verge of resolution but only one or two extremely faint stellar sparkles are intermittently visible.

N6760 19 11 12.0 +01 01 50 V = 9.0; Size 6.6

18" (8/23/03): at 323x this globular appears moderately bright, round, nearly 4' diameter, broad concentration to a slightly brighter 2' core. A half-dozen stars are resolved around the periphery with several of these on the SE and E side. With avertion vision a few additional stars sparkle over the center. At 538x, 10-12 stars are resolved around the edges of the halo and the core is very lively and on the verge of resolution.

N6905 = PK 61-9.1 = PN G061.4-09.5 = Blue Flash Nebula 20 22 23.0 +20 06 16 V = 10.9; Size 47"x37"

18" (8/23/03): beautiful planetary at 320x. The faint central star is visible continuously with direct vision. At 538x, the central star is easily visible continuously. The interior seems unevenly lit and there appears to be a very slightly darker "hole" to the north of the central star. Bracketed by a mag 11 star off N edge and a mag 12 star just off the S edge and the planetary is slighly elongated N-S in the direction of these stars.

Later in the evening, the seeing became softer and the relative humidity rose. Still, transparency was excellent and I took a look at a little-known but huge HII region in Cassiopeia -- Sharpless 2-157. This was the last in a small observing project of Wolf-Rayet shell nebulae (best known objects in this class are NGC 6888 - Crescent Nebula and NGC 2359 - Thor's Helmet).

Sharpless 2-157
18" (8/23/03): this is a beautiful object with a 31 Nagler and OIII filter, reminiscent of one of the sections of the Veil as viewed in a small scope. The main N-S shallow arc, convex to the east, spanned a large percentage of the field. At the south end, the strip widens and is less distinct although it contains the bright knot LBN 537 surrounding a mag 11 star (easy without filter). A much fainter western section (roughly 20' W) also extends N-S, passing through a scattered group of stars. At the south end it appears to merge with the eastern branch.

17.5" (10/17/98): surprisingly easy, huge HII region at 100x with OIII or UHC filter. Appears as a faint, curving graceful arc, very elongated N-S and bowed out on the following side. Extends north and south of open cluster Mrk 50 which is off the west side. Appears up to 35'x8' in size with the brightest section (near Mrk 50) showing subtle structure. The bright knot Sh2-157a = LBN 537 is situated beyond the southern end of this arc. The illuminating star is HD 219460 = WR 157, the brightest member of oc Markarian 50 and this object is listed as a probable Wolf-Rayet shell in ApJ, 252, 1982.