Well, I'm back from Mexico and looking through my first attempts at some rudimentary astrophotography using the OGPS-1 and my K5-II.
There's good news and bad news:
The bad:
-The tourism explosion in Cancun has spread all the way to south of Tulum, so the while the area in which I was staying might have had truly dark skies as recent as ten years ago, the recent addition of mega-resorts has tarnished it. I had better viewing conditions than in the suburbs of Cincinnati, but not by much... it takes just a few mega-resorts to make a lot of light pollution.
-The tracking results with the OGPS-1 provided mixed results. I won't go into all the details, but the tracking was sometimes steller (like when using the 70mm f/2.4), mediocre (like when using the Vivitar 13mm f/2.8), and poor (like when using the Tamron 28-300mm @ 300mm). I'm sure a lot of the issues can be attributed to user error, so I'll have to see if I can improve the tracking results with practice.
The good:
-I was able to see some Messier objects with the naked eye on a few nights, which was new for me. It's nearly impossible to spot them near Cincinnati.
-I got a few decent (for a beginner) images using my OGPS-1 and the 70mm f/2.4.
ISO 800
100 sec
f/3.2 (to minimize purple fringing)
Cropped a lot, removed purple fringing and edited curves in PS