Hi everyone. I've attached some helpful HP 1 finder & observation charts.
HP 1 is difficult in part because its surf brightness is <14.5. The listed magnitude of 11.6 is very misleading as it is spread out over the listed tidal radius of 8.1 arcmin. (Tidal radius is the boundary between the cluster's gravitational well and the Galactic well, in this case the Galactic well is the bulge surrounding HP 1; the Galactic arms and bar have little effect on the cluster compared with the bulge.) For me, a better measure of observability in any globular is the half-light radius, the diameter where half the light is concentrated. In HP 1's case that is 3.1 arcmin. That, too, is misleading due to the remarkably concentrated cluster. HP 1's core radius of 0.03 arcmin (1.18 arcsecs) due to binary clustering in a minute core. The attached images make this clearer.
If you are using a go-to, the coordinates are RA 17 31 05 Dec -29 58 54. I am limited to star-hopping with a laser finder, so find objects by triangulating from the crossing points of 3 imaginary lines from nearby reference stars. I use Michael Vlavsov's
Deep Sky Atlas printed on A-3 format paper plus downloaded DSS 20 and 5 arcmin field images that reach to ~mag 16.
Use the attached image 01 DSS 20' field to locate HP 1 in the field. Near the bottom of the image is an arc of six mag 12-13 stars 9 to 10 arcmins S; these show up much better in the eyepiece as a vivid 4-star chain with outliers. Above the cluster is a tent-shaped alignment of six 12-13 mag stars 5 to 7 arcmins N. Very close to the cluster core is a shallow triangle of three 13th mag stars 2 arcmins out. These are the stars to use pinning down the faint core. It is made easier by imagining the three stars as forming a Draco head with the cluster core as the nose. In my scope the core shows as only slightly fainter than the shallow triangle, ~13.5 to 14. The unresolved glow around the core reaches halfway to the triangle, roughly 1.5 arcmin, and is very fleeting. Even when the field is near-zenith about midnight here, it still requires a lot of patience and steady gazing. Once you do glimpse it, it will fade in and out of the seeing variations much more readily.
The rest of the images show the core concentration more accurately and will give you a better idea what to search for. Attached image 02 is 1.7 arcmins square. The 'ears' and 'neck' of the 3-star 'Draco' asterism is out of the field to the N in this image. You can see how tight the core really is. The numbers & letters on this image are from a study of nearby bulge-vs-cluster stars. The small lettered stars are blue horizontal branch stars, the cap letter stars are field red giants, and the numbered stars are cluster red giants. This is an IR image, which makes the red giants seem brighter than they appear in the eyepiece.
Image 03 shows the cluster in a B&W 2MASS field and the reversed field is from a VLT 8-metre IR study.
Glen, I will get back to you with the paper citations and references. You can start with a search for HP 1 on arXiv, IOS, and ADS. You can narrow the search with the date range 1997-2013 and use the names, Bica, Dotter, Minniti, Ortolani, Dotter, Barbuy, Bica, Davidge, Carollo, and Freeman.
Cheers, =Dana in SA