This will be a fairly lengthy report, of interest mainly to those who also like to view open clusters.
On September 26, 2006, I took my 8-inch SCT to the Cottonwood Springs Campground area of Joshua Tree National Park, California (elevation about 3,700 feet), to observe galactic clusters. It was a warm night -- short shirtsleeves weather -- under totally still skies. The seeing was generally very good and the sky appeared very transparent at the beginning of my observing session, though less so later in the night. The quality of viewing was only slightly affected during the early hours by the waxing crescent of the 4-day old moon, and the summer milky way from Cassiopeia through Sagittarius and Scorpius was plainly in view.
I started the session in Scorpius, first visiting my brilliant old friends NGC6231, M7 and M6. Although the constellation was already too low in the southwestern sky for several of my targets (NGC6249, 6259 and 6322), I did view five open clusters in Scorpius that were new to me (described below). I studied each of these targets with a 20 mm eyepiece, my favorite for open clusters, at 102x.
NGC6242 is a fine little cluster in which I counted 13 individual stars scattered among a hazy swarm of unresolved members. The texts I use (Archinal & Hynes and Luginbuhl & Skiff) rate this as a cluster of 23 stars within a diameter of 9 arcminutes with a total visual magnitude of 6.4
NGC6400 also offered me a swarm of mostly unresolved stars, peppered with a few dim individually discernible stars. Its appearance reminded me of some of the more extensive globular clusters, like M4 and M55, that do not have an obvious bright core; the similarity of NGC6400 to such globular clusters was enhanced by what appeared to be several "strings" of cluster stars radiating out from its circumference. This cluster is rated as containing 60 stars within a diameter of 12 arcminutes. I concluded that most of the cluster's stars were among those unresolved in my view of it, and this is consistent with its total visual magnitude rating of 8.8.
NGC 6416 is a fairly bright open cluster, with a total visual magnitude rated at 5.7, but I found it difficult to differentiate the swarm of stars comprising this cluster from the star fields of the milky way that surround it. My difficulty is apparently confirmed by the experts: Luginbuhl & Skiff states that "this cluster is hardly distinguishable from the field" (in a small aperture) and says its diameter is 30 arcminutes; while Archinal & Hynes says its diameter is 15 arcminutes; but both agree that NGC6416 contains 304 stars.
The other two Scorpius clusters I viewed this night, NGC6425 and NGC6451, are also situated among the star fields of the milky way, but both appeared to me to be in front of dark areas within those star fields (dark nebulae?), making them more obvious and identifiable.
As to NGC6451 I thus disagreed with Luginbuhl & Skiff which claims that this cluster does not stand out well from the rich background; I saw it as a fine swarm of unresolved stars, with a few dim individual stars sprinkled among them, of which I counted 16. The texts rate NGC 6451 as a cluster of 80 stars within a diameter of 8 arcminutes with a total visual magnitude of 8.2. It is one of the objects on the Herschel 400 observing list; I have seen it referred to as the "Tom Thumb Cluster."
NGC6425 is a triangular-shaped group of stars that appeared to me like a much smaller and fainter version of M39. Since that brilliant cluster in Cygnus is too extensive to fit within the field of view of any but the lowest power wide-angle eyepieces I have, I now know where to look if I want to see M39's equivalent without the hassle of changing eyepieces! The texts rate NGC6425 as 73 stars within a diameter of 10 or 15 arcminutes and with a total visual magnitude of 7.2.
One other open cluster that I viewed this night that was also new to me was NGC1342 in Perseus. This is a cluster of relatively bright stars, of which I counted about 25, in a box-shaped pattern within a fairly large area. The texts rate it as 99 stars (most of them obviously unresolved by me) within 17 arcminutes and with a total visual magnitude of 6.7.
I did not log my impressions of the many other objects I viewed this night that were already familiar to me, but they all appeared at their best as I peered at them. They included all of the Messier objects in Sagittarius, Capricornus an Scutum, as well as NGC457 and 7789 in Cassiopeia. I also viewed Uranus and Neptune (with a 6 mm orthoscopic eyepiece at 339x), and either saw or thought I saw Neptune's moon Triton about 8 Neptune diameters to the east of the planet.
On September 26, 2006, I took my 8-inch SCT to the Cottonwood Springs Campground area of Joshua Tree National Park, California (elevation about 3,700 feet), to observe galactic clusters. It was a warm night -- short shirtsleeves weather -- under totally still skies. The seeing was generally very good and the sky appeared very transparent at the beginning of my observing session, though less so later in the night. The quality of viewing was only slightly affected during the early hours by the waxing crescent of the 4-day old moon, and the summer milky way from Cassiopeia through Sagittarius and Scorpius was plainly in view.
I started the session in Scorpius, first visiting my brilliant old friends NGC6231, M7 and M6. Although the constellation was already too low in the southwestern sky for several of my targets (NGC6249, 6259 and 6322), I did view five open clusters in Scorpius that were new to me (described below). I studied each of these targets with a 20 mm eyepiece, my favorite for open clusters, at 102x.
NGC6242 is a fine little cluster in which I counted 13 individual stars scattered among a hazy swarm of unresolved members. The texts I use (Archinal & Hynes and Luginbuhl & Skiff) rate this as a cluster of 23 stars within a diameter of 9 arcminutes with a total visual magnitude of 6.4
NGC6400 also offered me a swarm of mostly unresolved stars, peppered with a few dim individually discernible stars. Its appearance reminded me of some of the more extensive globular clusters, like M4 and M55, that do not have an obvious bright core; the similarity of NGC6400 to such globular clusters was enhanced by what appeared to be several "strings" of cluster stars radiating out from its circumference. This cluster is rated as containing 60 stars within a diameter of 12 arcminutes. I concluded that most of the cluster's stars were among those unresolved in my view of it, and this is consistent with its total visual magnitude rating of 8.8.
NGC 6416 is a fairly bright open cluster, with a total visual magnitude rated at 5.7, but I found it difficult to differentiate the swarm of stars comprising this cluster from the star fields of the milky way that surround it. My difficulty is apparently confirmed by the experts: Luginbuhl & Skiff states that "this cluster is hardly distinguishable from the field" (in a small aperture) and says its diameter is 30 arcminutes; while Archinal & Hynes says its diameter is 15 arcminutes; but both agree that NGC6416 contains 304 stars.
The other two Scorpius clusters I viewed this night, NGC6425 and NGC6451, are also situated among the star fields of the milky way, but both appeared to me to be in front of dark areas within those star fields (dark nebulae?), making them more obvious and identifiable.
As to NGC6451 I thus disagreed with Luginbuhl & Skiff which claims that this cluster does not stand out well from the rich background; I saw it as a fine swarm of unresolved stars, with a few dim individual stars sprinkled among them, of which I counted 16. The texts rate NGC 6451 as a cluster of 80 stars within a diameter of 8 arcminutes with a total visual magnitude of 8.2. It is one of the objects on the Herschel 400 observing list; I have seen it referred to as the "Tom Thumb Cluster."
NGC6425 is a triangular-shaped group of stars that appeared to me like a much smaller and fainter version of M39. Since that brilliant cluster in Cygnus is too extensive to fit within the field of view of any but the lowest power wide-angle eyepieces I have, I now know where to look if I want to see M39's equivalent without the hassle of changing eyepieces! The texts rate NGC6425 as 73 stars within a diameter of 10 or 15 arcminutes and with a total visual magnitude of 7.2.
One other open cluster that I viewed this night that was also new to me was NGC1342 in Perseus. This is a cluster of relatively bright stars, of which I counted about 25, in a box-shaped pattern within a fairly large area. The texts rate it as 99 stars (most of them obviously unresolved by me) within 17 arcminutes and with a total visual magnitude of 6.7.
I did not log my impressions of the many other objects I viewed this night that were already familiar to me, but they all appeared at their best as I peered at them. They included all of the Messier objects in Sagittarius, Capricornus an Scutum, as well as NGC457 and 7789 in Cassiopeia. I also viewed Uranus and Neptune (with a 6 mm orthoscopic eyepiece at 339x), and either saw or thought I saw Neptune's moon Triton about 8 Neptune diameters to the east of the planet.
NexStar 8i SCT, Orion 80ED APO refractor, Orion 120 mm EQ refractor, 3.5-inch Questar
