Einstein's Cross (HST)
by Rich Jakiel
Completed the Messier Certificate ages ago? Have you finished the Herschel "400" and found it "wanting"? Has hunting down 15th magnitude galaxies in Virgo become passe? You may be ready to take on the Ultimate Observing Challenge.
Sooner or later everyone reaches a point where this hobby can become stale, even boring. This often affects the more advanced observer, someone who has completed most of the "simpler" certificates and programs. Perhaps the best way to get out of the observing doldrums is to take on new and challenging projects. My personal projects include making drawings of interesting deep-sky objects and observation of structures in external galaxies (spiral structure, globular clusters and H II regions). Barbara Wilson has seen almost all of the globular clusters associated with the Milky Way Galaxy. Barbara, Larry Mitchell and Alister Ling like to hunt down obscure deep-sky objects that were previously "unseen". Tom Polakis likes to hunt down faint dwarf galaxies of nearby galaxy groups. Steve Gottlieb and Jack Marling have made and compiled observations on over seven hundred planetary nebulae. Other well-known deep-sky enthusiasts have pursued challenging programs around their particular interests.
But every list or program has its most difficult challenge. For example, someone working on the Messier list might consider the 11th magnitude M97 (Owl Nebula) as one of the most demanding to observe. However there are some objects that defy all attempts, no matter how large the scope or seasoned the observer. Many of these are relatively well known, and very few (if any) ever view these most elusive objects. The "Mount Everest's" of amateur astronomy, these seven objects ( the magnificently hard seven) comprise the Ultimate Deep Sky Challenge.
RA 5hr 39.1' Dec +28o 00' (2000)
Also known as Sharpless 2-240, this beautiful lacy nebula is a supernova remnant (SNR) in Taurus. It has been compared to the better known Veil SNR in Cygnus in delicate beauty. Before the late 1980's, this object was considered "unobtainable" due to its extremely low surface brightness. The use of nebula filters soon changed the rules of this game. In 1991, Alister Ling (Deep Sky 37, 32) armed with an O III filter and a 12.5-inch scope was perhaps the first person to view a few faint fragments of this elusive nebula. Even today this is considered a very difficult object to observe as it requires large aperture under very dark skies.
Typically, wide low power fields are required. Contrast is the key, since this object has an exceptionally low surface brightness. I was able to observe this system with a 15 cm, f/5 telescope at 29x. All I was able to see was a very faint oval "stain" measuring ~ 20' by 15'. A few observers have been able to detect the galaxy with instruments as small as 11 x 80mm binoculars.
Perhaps this object by itself no longer belongs to this select group of observing challenges. But Brian Skiff in the October 1996 DSO alluded to a much more difficult project - the resolution of the Sculptor Dwarf into stars! The brightest stars are around 18th magnitude, within reach of the largest amateur telescopes. The resolution of these faint dwarves will certainly push the skills of even the most seasoned observers.
Who's Afraid of Einstein's Cross
Observing Report by Jay Reynolds Freeman
RA 22hrs 40' 29.8" Dec +03 21' 30"(2000)
Einstein's Cross, or G2237+0305 is perhaps the best known gravitational lens in the northern sky. A distant quasar is "split" into four parts by a closer foreground galaxy. The gravitation field of the galaxy bends and refocuses the light from the distant quasar into four roughly equal components. This galaxy has a designation CGCG 378-15, a 14th magnitude system in the constellation of Pegasus. This galaxy is not very difficult to view even with moderate aperture but the quasar is a different story. The southern component A has a magnitude of 17.36, component B (northern) is 17.39 magnitude, C (eastern) is 18.43 and D (western) is the most difficult at 18.72 magnitude. Barbara Wilson provided much of the information on this object. The key to seeing "the cross" is a combination of large aperture, very dark skies and excellent seeing conditions.
RA 23hr 59.3' Dec +62o 27' (2000)
Abell 85 is a large supernova remnant (SNR) located in Cassiopeia. Originally catalogued by George Abell as a planetary nebula, it soon became evident that it was actually a SNR. It was also recognized as a radio source and acquired the alternative designation CTB 1. According to Brian Skiff, Abell 85 has relatively weak [O III] and H-Beta emission compared to the "traditional" supernova-remnant lines of H-alpha, [NII], and [SII]. For visual observers, the gain in contrast using [O III] and H-Beta filters appears to be minimal. Likewise, the digital sky survey reveals only a low-surface brightness object on the red POSS-1 image, while it was invisible on the blue light image. Skiff suggested that the SNR may not be a visual object.
Shortly after Skiff's informative posting on sci.astro.amateur, two German observers published an extensive catalogue on galactic nebulae in the January 1996 issue of the Webb Society Quarterly Journal. Andreas Alzner and Ronald Stoyan observed the nebula at 70x using a 50cm (20-inch) telescope equipped with an O III filter. Though listed as "very faint", their success clears the way for more observations by serious amateurs. For those seeking a difficult challenge, here are a few important statistics:
Center of Abell 85: 23hr 59m +62.5 (2000)
Diameter: 35'
Brightest Segment: 23hr 57.5' to 00 00.5 at 62.2 Dec (2000)
These numbers were originally posted by Brian Skiff on sci.astro.amateur. Good comparison photos can be found in van den Bergh et al. 1973 (ApJSuppl 26, 19) and on page 55 of the Sky and Telescope 1971 monograph The Planetary Nebulae by Lawrence Aller.
Observing Report by Barbara Wilson
RA 17hr 54.5' Dec -24o 09.0'
Located deep in the Sagittarius Star Cloud, UKS 1751-241 is a highly obscured globular cluster. Listed at 17.3 magnitude and less than 1' across, this is considered by many as the most difficult globular cluster in the sky. According to Malkan et al., 1980 paper "Infrared Studies of Globular Clusters Near the Galactic Center" ApJ 237 432 the "UKS 1751-241 is completely invisible on the Palomar O (blue) and E (red) Survey prints." The globular cluster was discovered on infrared plates taken by the 1.2 m UK Schmidt telescope, visible as a hazy ~ 16 magnitude patch. Brent Archinal of the US Naval Observatory confirmed its difficult nature by hunting for it using DSS and IRAS surveys. No convincing image was found! Nor could he find any published images of it.
Lately, it has been imaged by CCD at George Observatory. So there is hope that this object can be imaged and perhaps even observed by enterprising amateurs. Even with this mildly encouraging news, its location in a dense star doesn't help much, either. Big aperture, very dark skies and accurate star charts are the best means to tackle this "Holy Grail" of globular clusters. (much of the technical information presented is courtesy of Barbara Wilson).
Hickson 50 is by far the most difficult of a generally tough catalogue of galaxy groups. There are 5 tiny members of this extremely distant galaxy group. The brightest member, "A" registers at a grim 18.4 magnitude, far beyond the reach of all but the largest amateur scopes. Members "B", "C", "D", and "E" are magnitudes 18.5, 19.3, 19.2 and 19.7 respectively. Amazingly, Barbara Wilson and Larry Mitchell have spotted this elusive little group with a 36-inch scope under the pristine skies of the Texas Star Party. Nonetheless, this group pushes the "edge" of what can be observed and is one of the most difficult objects in this list.
RA 18hr 33' 20.3" Dec -23o 57' 52" (1950)
If any object is impossible to see, this small planetary nebula comes close! Located deep in the heart of the great globular cluster M22, this object has defied every observing attempt. And yet, there may be some hope.
This most unusual planetary nebula was first recorded in the IRAS Point Source Catalogue (1985). It is a member of a rare class of halo planetary nebulae and bears certain similarities with Pease 1 (K 648) the only other PN in a globular cluster. This peculiar object lacks the characteristic H and He emission lines, with only [O III] and [Ne III] present (1991, ApJ 379 168 ). There is a substantial amount of dust accounting for the distinct infrared emission. Measuring 10" x 7", the estimated magnitude is < 15, though the lack of visual confirmation suggests it may be considerably greater. The PN forms part of an optical triplet only 1' from the core of M22. The other two "components" are faint 14 and 15 magnitude stars separated by less than 2 arc-seconds (1989, ApJ 338 862).
Before attempting this object, try Pease 1 in M15. It is visually at 13th magnitude and located several arc-minutes north of the globular's core center. Alister Ling (1990, Deep Sky 32 36) gives a good write-up on how he finally succeeded in locating this PN.
Unfortunately, our little PN in M22 is a much more difficult customer. Many serious observers have attempted with apertures up to 25-inches with NO success. I have tried this several times, the last at the Chiefland Starfest in 1996. Using a detailed finder chart of M22 and Vic Menard's superb 20-inch f/6.2 dobsonian we gave it our best for almost an hour. Using magnifications over 400x, and an O III filter we just could not pick out this PN from the haze of unresolved stars in the core. And that's the paradox. Its emission is the strongest in [O III] lines, but this type of filter is best used at lower magnifications. However, to separate the nebula from the extremely dense stellar background high power is required. The O III filter "mushes" out the very faint stars turning it into a diffuse background haze. Perhaps a very large scope ( > 30-inches), coupled with very dark and steady skies an observer may be able to ferret this nebula. A sliding filter bar with both UHC and O III filters would be an invaluable tool.
Of course there are other challenges awaiting the serious observer out there. But not all require huge aperture to see. Next time, try to locate the Fornax or Sculptor dwarf galaxies with a richfield scope. Or just hunt down all the Messier objects using just 50mm binoculars. With some patience and skill, you'll soon find out what is your observing "envelope".
