Thanks to stephenb for asking the question and kudos to astrospotter Mike for linking to Alvin Huey’s invaluable guides. (And scads more kudos to Huey for making them in the first place.) Huey is the first place I look when going after a new faint whatever. I zero in on positions using his charts, but I don ‘t always get a solid idea of what the object will actually look like. I believe he used a 25” scope when compiling the most recent editions of his guides. His 25” has 1963 sq in of collecting area. Your 18” has 1018 sq in, which is 52% of his light gathering area. Secondary size loss will likely be about the same % in both your scopes. Secondaries don’t mean much light loss but do mean contrast loss. At this point hard numbers begin to lose significance because of seeing, LP, how contrast-acute your eyes are, and how many extended, low-contrast objects you have in your mental image bank.
For globulars, after checking Huey I go to
Bill Harris’ catalog, where I compare the data for the new object, say Pal 2, with other objects with similar parameters for: (a) listed visual magnitude (Pal 2 = 13.0), (b) minus extinction (1.24 mag, which dims the visual to 14.25), and (c) the concentration class (which Harris doesn’t list but Wiki does). Pal 2 is Class IV. Classes I through IX mean I should see a stronger glow in the centre surrounded by a small, weaker halo. If a GC lists at Class X to XII, I know to not try for a white glow because the object will be quite spread out. Instead I look for an overdensity of stars at the faintest limits I can detect. Clusters that show no faint glow can be gorgeous little puffballs of tiny points.
A couple of other tricks. One, there’s a good way to estimate how large the faint fuzzy will be: the half-light radius, or r(h). This depends on the above-mentioned concentration parameter. Pal 2’s r(h) is a dismal 0.7 arcmin, or 42 arsecs. Two, check Harris for the Horizontal Branch magnitude (Pal 2 = 21.60). The red giant tip is usually 3.5 to 3.8 mags brighter, and red giants are the brightest stars in a GC. If you can see to mag 17.8 to 18.1, Pal 2 is in the realm of possibility. It will look like the dimmest planetary you’ve ever set eyes on, only dimmer. Best of luck.
Other GCs in Harris with parameters like Pal 2’s are NGC 6749 (Aquila), IC 1257 (Oph), the Pyxis globular, and the not-much-mentioned
FSR 1735 in Ara. It was only classified as a GC in 2007, doesn’t appear on any amateur lists, and yet its parameters indicate it should resemble Djorgovski 2 (Sco). It’s at RA 16 52 10.6, Dec −47 03 29, visual 12.9 but a half-light radius of just 20 arcsec and RGB at 16.3 to 16.5. Another Ara tougie. It's quite close to Westerlund 1, my favourite because it’s the most difficult of all. Source crowding is just awful in this region.
Once your eyes begin to smart from all the fruitless hopping, you can always console yourself with Ruprecht 106, the poor cousin to nearby Omega Centauri but a very rich cousin compared with the Pals and Terz’s. It is visual 10.9 with negligible extinction, HB 17.8 & therefore red giants from mag 14 to 15, half-light radius 1 arcmin (but looks to me more like 3.5 arcmin). I know I have a great night for seeing when Ru 106 resolves into a dozen stars with trembles of many more.
Running hard numbers is a lot less of a thrill than seeing a faint glow. But when it comes to toughies, numbers spell the difference between hesitant and sorta confirmed. Besides, what else is there to do on a cloudy night? Unto every clear night we dwell, a cloth of cloud doth swell.
(So I get a ‘pass’ for astronomy and a ‘dismal fail’ for ersatz Shakespeare, is that it?)
