Orwell Astronomical Society (Ipswich)
Observation of Sagittarius, 03 July 1987
Early-July 1987 was blessed with a rare period of fine weather with sunshine during the day and clear skies at night. Friday 03 July was a particularly clear evening: by 22:30 UT, the sky had darkened sufficiently for the Milky Way to be visible from Cygnus down towards Sagittarius. Sagitarrius is not a constellation with which I am particularly familiar, being so low in the south during summer months with skies that rarely provide good viewing conditions. However, this evening was one of those rare occasions: stars were visible, without optical aid, to as low as approximately 5° altitude!
I had intended to search for some of the deep sky objects in Cygnus and adjacent constellations high in the sky but the clear southern horizon was an opportunity not to be missed and I duly directed my attention low down to the south. I started scanning the skies using 10x80 binoculars and quickly found M16 in Serpens; this is an open cluster embedded in a gaseous nebula. It was also easy to find in my 7x45 finder telescope. In my main 250 mm reflector, approximately 50 or 60 stars were visible but there was no sign of the surrounding nebulosity. This was probably due to the background skyglow, clearly visible in a low power (80x) field in the main instrument. I then moved approximately 2° south, crossing the boundary into Sagittarius, to the beautiful, bright, diffuse Swan or Omega nebula, M17. I much prefer the Swan name for that is, indeed, a good description of the shape of the object when seen through a telescope. It is a bright nebula, at a distance of some 5700 light years, easily seen in the finder telescope as an elongated fuzzy patch. I found that the swan shape was just discernible in the 10x80 binoculars and was a truly beautiful sight in the main instrument. I next observed the small, open cluster M18, lying approximately 1° south of M17. After the glory of M17, M18 was a little dull in comparison, appearing as a faint smudge in the finder and binoculars, resolved as a cluster of 30 or so faint stars in the 250 mm reflector. In fact, most of the visible stars are background objects, and only the dozen or so brightest are true members of the cluster. The object is estimated to lie at a distance of 4900 light years.
Moving south approximately a further degree revealed M24, the Small Sagittarius Star Cloud. The cloud can be visible to the naked eye under ideal conditions, but I was unable to find it in anything less than the 250 mm reflector, in which it appeared as an extremely rich cluster with streamers of stars crossing the region. In fact, it is not a star cluster as such, rather a denser condensation of the Milky Way star field. It is a visually large object, roughly rectangular in shape, approximately 2°x1° in extent. Its large size mean that a low power, wide field eyepice is best for viewing it.
Scanning the surrounding area of sky with the binoculars, I could see to the east a bright open cluster in which a few stars could be resolved, and to the west a fainter misty patch. I concentrated on the brighter object first; it was M25, a bright open cluster at a distance of about 2000 light years with approximately 70 to 90 stars visible in the 250 mm reflector. The fainter object was M23, an attractive compact group of 70 or so fainter stars. M23 lies at about the same distance as M25.
I continued to move south and attempted to find M20, the Trifid nebula, a large, gaseous nebula surrounding a loose open star cluster, with a triple star near its centre. But by this time, it was after midnight UT and I had almost left it too late! The wall of my observatory prevents viewing objects at low altitude to the south; it is something that I have intended for some time to rectify, but have always postponed. The Trifid nebula was past the meridian and heading further south, and the aperture of the reflecting telescope was already partially obscured by the wall, so it was a race against time... The embedded cluster was visible as a large, loose scatter of stars, but the nebulosity was barely visible with only the bright region around the triple star being readily discernible. The faintest member of the triple star, at magnitude 10.5, was only just visible. The small, more condensed, open cluster, M21, was visible to the north-west in the same low-power field.
Approximately 1.5° south-west of the Trifid nebula, the Lagoon nebula M8 was easily visible in both the binoculars and the finder telescope. In the reflector, despite the loss of aperture thanks to the wall of the observatory; the bright nebulosity was clearly visible, divided into two by a dark region. Also easily visible was the small open cluster NGC6530, to the eastern side of M8. While observing so far south, to round off the observing session, I observed the the two nearby globular clusters, M22 and M28. The brightest of the two is M22: it was easily found in both the binoculars and finder and, in the reflector, was a beautiful sight, with stars resolved against a glowing background, right to the centre of the nebula. M28 was much fainter; invisible in the finder and faint in the binoculars. In the reflector, it appeared as a condensed glowing smudge with no resolution of individual stars. While observing, I had assumed that the lack of resolution was caused by the low southerly aspect and the loss of aperture due to the observatory wall. However, on later consulting "Burnhams Celestial Handbook", I found that the stars in M28 are magnitude 14 and fainter and so not capable of being resolved in my reflector.
By this time, it was early in the morning of 04 July and time to bring to a halt a very successful observing session of eleven deep-south Messier objects and one NGC object.
David Payne