Observing Report: NGC 7331 Group and NGC 2419
by Douglas Bullis
Site and Equipment
These tests took place at a 6,730 ft. dark site in New Mexico. Transparency averaged 4 to 5 (on a scale of 5). Seeing was 2 to 4, though many times a 4 night improved to 5 starting around three hours before dawn. The light dome of Santa Fe rose about 20 degrees high in the NW, and Albuquerque’s reached 15 degrees high in the SW. No observations below 50º elevation were recorded. Non-visual entertainment was provided by an astonishing number of coyotes checking in with each other from dusk till dawn.
The test scopes were a 127mm f/6.3 Antares, Burgess 127mm f/8, and an Anton Jaegers 150 mm f/5. A 127mm lens has a throughput of 307x a naked eye exit pupil of 7 mm. The Jaegers’ throughput is about 365x. Objects viewed with the refractors were cross-checked against an Intes MK61 Mak-Newt and a 210 Tak Mewlon. These scopes are kept scrupulously clean, with practically no dust on the optics. The Antares and Jaegers dewcaps and interiors also were lined with Protostar flocking, including the baffle edges; the Burgess was satisfactorily blackened as is.
Eyepieces, like the scopes themselves were middle-budget: Astro-Tech ED2 Paradigms, Spears-Waler, Burgess/TMB, and Baader Hyperion.
Method
The test objects were selected to gauge the richest detail in extended objects and faintest attainable magnitude in stars. Two popular visits for most everyone were chosen for this report: the Deerlick Group in Pegasus and NGC 2419 Lynx. Throughout the entire test period seeing conditions required very long observing sessions per object—twenty to forty minutes patiently awaiting every last glimmer of detail at powers ranging from mid-50s to low 200s. The tests comprised multiple observations on many nights under varying seeing conditions. Each night’s tally was recorded a star-by-star tick chart, so that eyeball-to-eyepiece concentration was interrupted as little as possible. A mark went down each time a faint detail was glimpsed and the sumtotals tallied later. The apparent angular size of extended objects was estimated by the angle it subtended in a triangulation using a star adjacent to the object and two more distant ones whose separation was known.
Deerlick Group
The grid-like linearity of galaxies NGC 7331, 7335, 7337, and 7340 plus two pair of stars on either side of 7331 make the group a good test of contrast and seeing limits. At mag 10.3, NGC 7331 is one of the more attractive near-edge-on spirals in the sky. About 4’ to the west (left here) are two stars of mag 10.3 and 11.2. A third co-linear “star” below them (to the south) is the demanding elliptical galaxy NGC 7340 at visual mag 13.9. In between are NGC 7335 at 13.6 and 7337 at 14.6. Two convenient reference stars lie on the opposite side of 7331, USNOA2 1200-19344308 at 13.5 and 19344923 at mag 14.

NGC 7331 Deerlick Group. All objects with listed magnitudes (decimal points removed) above 14.2; were observed during the course of these tests; “vm” = visual magnitude. For larger image see http://tinyurl.com/ye9yp7t . For detailed further information on the Deerlick objects, see the NED database http://tinyurl.com/ygdrrq8 ; SIMBAD http://tinyurl.com/yemgr24 ; and HyperLEDA http://tinyurl.com/7331d . Paul Alsing provides a very useful chart on his Pegasus database http://tinyurl.com/alsingpeg
The 127mm Antares could pick up NGC 7335 using averted vision. In the 127mm Burgess the stars were slightly sharper on a darker field, and 7335 was slightly fuzzier. The 148mm Jaegers brightened the fuzz in 7335; 7340 was clearly diffuse and not a star. NGC 7337 appeared as a nonstellar wisp above a mag 11 foreground star, looking rather like a very faint NGC 7008.
There is a good stellar-limit test near 7331 in the form of a ternary asterism of mag 13.4 and >14 stars on the other side of 7331 (two recorded here). The Jaegers and Intes could see the two brightest, even though they are supposed to be beyond a 150mm aperture’s limit. More surprising is that the mag 13.5 star was frequently glimpsed in the Antares and Burgess. Perhaps the fact that this observing site’s green-level skies render M33 visible with averted vision and occasionally with direct can explain why such faint objects are attainable. (At various times, other naked-eye transparency tests were M52, M35, M46, the Rosette, and the marvelous 5.8 vm open cluster NGC 2477 in Puppis.)
NGC 2419 (Lynx)
This globular has a certain exotic appeal due to its status as an intergalactic wanderer—NGC 2419 is the 5th most distant globular cluster and one of the 6 extreme halo globular clusters. At magnitude 10.8 it is not hard to spot (it shows faintly in my 80mm Orion ST finder). Its interest as test object is the pentagon of mag 13.0 to 14.8 field stars surrounding it, whose positions are certain with respect to the GC. Patience rewards, but ever so slowly: observing times for this object averaged 25 minutes each on 11 different nights.

NGC 2419, 10.3 vm. All stars above with numerical magnitudes (decimals removed) were multiply observed during the test. For a more detailed image see http://tinyurl.com/yfh2xzy . NGC 2419 has one of the most fascinating color-mag diagrams of all MW globulars; for this and other astrometric details, see http://tinyurl.com/y9twlqx . For details of 2419’s red-giant dominated photometry, see Natali, G.; Pedichini, F.; Righini, M., “BVI CCD surface photometry of the globular cluster NGC 2419” in Astronomy and Astrophysics, vol. 248, no. 2, Aug. 1991, p. 426-429. For more on 2419’s interesting place in the Milky Way’s morphlogy, see Harris, William E., et. al, “NGC 2419, M92, and the age gradient in the galactic halo”, Astronomical Journal v.114, p. 1030-1042 (1997). For readers keen to keep up with 2419’s latest doings, see http://tinyurl.com/ya8mbs6 .
NGC 2419’s trapezium of mag 13 to 15.5 stars resembles the outline of Auriga. The ISTAR-equipped Antares and Burgess easily brought in USNOA2 1275-07006422 (13.9) with averted vision, and mag 14.3 USNOA2 1275-07004126 with a challenging-but-gratifying mix of rarity, ephemera, and certainly. In this cluster, patience is less a virtue and more a duty. The Jaegers picked up the entire pentagon, whose faintest direct-vision member, USNOA2 1275-07006422, was 13.9. The 14.8 field star USNOA2 1275-07005609 on the edge of 2419’s halo was seen quite frequently—and as often with direct vision as averted—despite it’s being 0.9 magnitude fainter than the 13.9 limit calculated by http://www.cruxis.com . (Parameters: clean optics, 1.2mm eye pupil, 3.5mm eyepiece @ 234x, lim. mag 6.5, 70º elevation, estimated atmos. extinction 0.3 mag.) The most difficult objects were the 14.2 & 15.5 pair on the right side of the image. This pair was ephemeral at all times, glimpsed fleetingly in averted vision as an elongated patch, but four times as a clear pair of distinct stellar points during the cumulated 6.6 hours of observing. All the above sightings were confirmed multiple times using the Intes Mak-Newt and Tak Mewlon.
The 14.8 halo-edge star provided a boundary object to gauge against the triangulation reference mentally drawn by extending straight lines from the 8.1 and 10.7 stars at the upper left of the picture (2.1 arcmins separation) across the 7.2 mag HD 60771 lying 4 arcmins from 2419’s core. The narrow isosceles thus formed can be used to estimate the diameter of the faint halo as seen through the various telescopes. The halo’s core would brighten and its disk fatten from approx 0.5 to 1 arcmins apparent diameter as the eyepieces and scopes were changed. (NGC 2419’s astrometric diameter is listed as 4.1 arcmins). Subjectively, in moments of good seeing the core simply appeared more “there.”
Some of the stars cited above are fainter than the calculated seeing limits for the apertures of the scopes. This was not out of the ordinary: in earlier observation notes of, NGC 7006 in Delphinus, the 152mm f6.3 Intes could pick up the pair USNOA2 1050-19061377 and -190961997 at mag 14.15 and 14.2. Another example: the two 127mm refractors brought in USNOA2 1275-07004126 (13.5) inside the planetary disc of NGC 2348 adjacent to M46.
The phenomenon of seeing well beyond theoretical limiting magnitudes occurred too often on too many objects on too many nights to be chalked up to the Starcebo Effect—”I see it because I know it is there.” (Also known as “delusions of glimmer.”) Perhaps air as turbulent as that above the dark site, where seeing was usually 3/5 and transparency 4/5, a transient lensing effect can occur which concentrates a particular stellar wavefront across the full air column of a scope’s aperture into an energy density high enough to end up at the eyepiece seeming brighter than the object actually is. Throughout the entire observing period, field stars shimmered through the eyepiece in the same tremulous manner as starlight reflecting off a zephyred pond.
Conclusions
There were rewards to this exercise beyond knowing the faintest fuzzy in a field of four. One reward was comprehending just how difficult it is for scheduling directors of major observatories and the HST to allot the most efficient amount of time needed to attain a specific limiting magnitude or spectral line. (And pity for the poor Ph.D. candidate whose long-awaited observing slot has just been swallowed by a New Mexican snow squall.)
This test occasioned many waits of five-plus minutes to confirm the sighting of a single star at or below the aperture limiting magnitude. This led to the second reward: getting familiar with exceedingly small patches of sky. One can grow rather affectionate toward a circle in the sky 15 or so arcminutes in diameter when it becomes as familiar as one’s children. The third reward is the joy of subtlety: a detail glimpsed for a few fleeting instants across the span of half an hour can acquire the tang of suspense and reward one has while watching the last at-bat in a no-hitter or a 60-yard pass caught with only one cleat left on the grass inside the end zone. Visual astronomy looks past the beauteous face of the Mona Lisa to notice the even more beauteous weather that was going on that day.
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