Alex: Allow me to clarify a bit what makes for good pointing. There are half a dozen sources of flexure, polar misalignment, orthagonality, and other error that will make systems have pointing error. This is also true of professional observatories (except they usually nail the polar alignment). There are a number of error terms that are standardized in the industry, they include MA, ME, NP, IH, DF, and a few others. Type that series letters into google and you'll get some astronomical engineering hits. Software Bisque sells a laptop system which will also do this analysis.
Gemini compensates for these sources of pointing errors by taking the alignment stars and building a statistical model of the mount. The first few alignment stars get you "most of what you need," and you can build a super-refined model using a hundred stars (or forty, or sixty, you can choose) all over the sky for an observatory set-up. But most of the time when we say "build a model" we are talking 3, 5, maybe 10 stars being fed into the computer, depends on the person and the application.
As it happens, you can get Gemini type accuracy with an Argo Navis computer, which allows for a multi-star alignment that will increase your pointing accuracy. You can talk to Gary Kopf at Wildcard Innovations. (send him an email) There is also an Argo Navis Yahoo! group. Software Bisque, Gemini, and Argo Navis do not use the same program to achieve their results, but they are all conceptually similar and are addressing more or less the same variables. The gemini system is a bit more "opaque" to the user, you just zap stars and it figures out what to do; Argo Navis requires a few button clicks to put is compensation model, which they call TPAS, to work. There are other systems in use including CGE and rumor has it that AP will introduce a similar system one of these days.
If you want bullet proof reliability on a mount you can use and abuse, that's the G11 with steppers. but 95% of imagers, or 98%, prefer Gemini. I use Argo Navis but I don't image.
With Argo Navis you can get about 5 arc minute pointing accuracy on a G11 equipped with 8192 tic encoders. You could get down to below 2 arc minutes with 10000 tic encoders. (you would disucss these options with Gary Kopf)
Gemini and Argo Navis are your logical options to improve pointing accuracy. Gemini will slew automatically, which is nice, and the servo motors have a smoother motion. The steppers are more rugged for throwing into the car and for "set it and forget it" characteristics in extreme temperature changes. If you got to a real cold site, for example, Gemini may stall unless you adjust the worm gear. Argo Navis has a better user interface than Gemini.
But all things considered, most Losmandy sky photographers are using Gemini. Since I switched to Argo Navis pointing accuracy is no longer an issue in my observing life. There is one functionality Gemini has that Argo Navis does not. Gemini can be set to compensate the *tracking* (not just the pointing) for polar alignment error. This would be useful for longer exposures, but you would still get slight field rotation (say you were ten arc minutes off true N in your polar alignment). You have to perfect your polar alignment to eliminate field rotation. Argo Navis can't fix that for you because it is not connected to your drive (which can be a good thing from other points of view). But field rotation for someone who is 10 arc minutes off true N isn't going to be a huge issue.
Both gemini and Argo Navis will correct pointing for polar misalignment, and both have systems to help you improve your polar alignment.
I would suggest that you review your setup procedures too. The following things can make you lose accuracy as the night progresses:
a. failure to torque down those azimuth lock screws on the deck of the mount head--if the mount moves, your pointing will go off
b. failure to tighten the altitude second altitude screw so that mount is tightened down in altitude after you have done polar alignment--it will shift very slightly as you use it, if you don't
c. some people don't have the dsc box set for the right number of tics....often off by a little....you'll be either 4096 or 8192 on most configurations
d. observing on some surfaces such as concrete can allow for twisting motions of the scope to turn the tripod. Not a problem on grass or rough surfaces.
In addition to polar alignment, the single biggest source of pointing error on systems that don't "model" is the accuracy of the right angle of the optical axis to the actual mechanical axis of the mount; the optical axis should be at right angles to the dec axis. (This is "NP" in professional parlance, non-perpendicularity) If you're using a Newtonian and rotating it in its saddle this situation is pretty hopeless; I don't know of any system that will give you good accuracy with a Newt unless the optical axis is real spot on. You could use a Newt and stay accurate, you'd just have to be sure not to rotate it.
Refractor and cat owners on the other hand used to shim their mounts to improve the orthagonality. I have done this procedure more than once but I now consider it a stone age technique. All the good systems will compensate for this mechanical error mathematically (except for Newts that are rotated in the rings).
regards
Greg N
"Scope will get you through times of no money better than money will get you through times of no scope." --Freewheelin' Franklin
