by Jay Reynolds Freeman
This combination is light, portable, and fun. With it, I like a Vixen 8-24 mm zoom eyepiece, with a field of view nearly two degrees across at 24 mm focal length, and most of the magnifications I need for deep-sky observing elsewhere in its range. I had a Herschel-400 survey under way with this telescope when I bought the NexStar. I had been using a good-quality hand-operated altazimuth mount, but it was awkward and annoying close to the zenith. The NexStar mount makes it easy; I can log scores of Herschel-400 objects in an evening without getting tired or vexed. I work that fast even though many of these objects are so faint in a 70 mm aperture, that I need charts even with computer pointing, to find out exactly where in the field to look for the target.
Computer control was particularly useful at a recent educational event. I was at Lick Observatory, helping as a docent with a college class that had come up for a tour. The class was small, so there was plenty of time for staff astronomer Elinor Gates to show us half a dozen objects with the 36-inch refractor. Alas, it cannot work close to the horizon, so that night, we couldn't look at the Moon or any bright planets: They were all too low when the group was there.
People wanted to see the bright stuff, so I set the NexStar / Vixen 70 up west of Lick's main building in early twilight, aligned as soon as I could see two stars, and picked up Jupiter, Mercury, and Saturn low in the west. Jupiter was obvious, and Mercury was visible to the naked eye if you knew where to look, but Saturn was so low I could not see it at all, even in Lick's clear sky. The NexStar found it easily. The tour had to leave by midnight, and Mars wasn't high enough for the 36-inch by then, so I moved my setup around the building to view its tawny disc. Even with just 70 mm and 70x, we could see markings. The Moon rose about then, so I swung the telescope to it and showed craters while Luna was still orange from horizon obscuration. It's not often that a tiny telescope can compete with a Great Refractor twelve times its aperture, but those students were really glad to see the Moon and planets.
The lightweight tripod that came with my NexStar 8 -- newer models ship with a heavier one -- continues to be adequate. The secret to using it is to install the accessory tray, tighten everything down, be sure the legs are fully spread, to take up any remaining slop in their joints, and avoid touching the mount. I observed with the 70 mm OTA one evening, in a 20-knot breeze, from the lee of my car, and was not bothered by wind. I had a set of Celestron's excellent vibration dampers with me, but forgot to use them, and did not miss them.
I have adapted three more OTAS to the NexStar, but have tried only two in the field so far. My Vixen 55 mm f/8 fluorite, Refractor Red, points at the zenith in balance, with no counterweight, and looks really cute on the mount. My Vixen 90 mm f/9 fluorite weighs six pounds, so a counterweight for it might overload things. I will use it for objects less high than the zenith. My Intes 150 mm Maksutov-Cassegrain weighs as much as the NexStar 8, so I don't counterweight it, and it won't reach the zenith, either; I am still playing with extension tubes and star diagonals, but at the moment, it hits the mount at elevations higher than 60 to 65 degrees.
As many people have suggested, it helps to minimize pointing error if you know that when the software asks you to level the tube, what it needs is the telescope optical axis set at right angles to the azimuth axis. I haven't gotten around to making reference marks to help with this operation, but I can often eyeball it accurately enough to reduce average pointing error below ten arc minutes. Celestron should rewrite their manuals and software prompts to tell users what the system really wants, and ship telescopes with reference marks in place.
Furthermore, though I do not have enough experience to be sure yet, I think I have figured out a bit more about NexStar pointing errors. To begin with, I seem to get more satisfactory alignment -- less pointing error when finding objects -- when I align by the two-star method, instead of using the default full alignment. Perhaps that wouldn't be so if I entered latitude and longitude with high precision, and took great care to level the drive base when I set up, but the two-star method is quick and easy, so I have little cause to use the default.
I have also noticed that after centering an object and pressing "align", the drive stops for five or ten seconds while the NexStar tells you alignment is satisfactory and asks you to turn off the star pointer. I think rotation of the Earth during that interval is not accounted for: The telescope does not immediately recenter the object aligned on, which drifts during the no-drive period. Drift during (say) ten seconds with drive off is at most 2.5 minutes of arc, but NexStar pointing is too sloppy to afford losing even this small increment of accuracy. I help deal with the problem by using Polaris as one alignment star; it drifts very little...
I suspect that more secrets remain, to getting around the software deficiencies of this otherwise pleasant little mount. For example, as presently equipped, with the new finder discussed below, my NexStar 8 OTA balances slightly tail-heavy on the mount, and it doesn't seem to drive as accurately that way as it did before I added the weight aft. I intend to check that out.
I bought a DC / DC adapter, that plugged into an cigarette lighter fixture and provided user-selectable polarity and output in the range 1.5 to 12 volts, with a whole lot of different output plugs ganged together. That let the NexStar work with several different lead/acid batteries I regularly use, but the adapter died a smelly electrical death after only a few uses. A simple plug-and-cord works just as well, but I did not happen to have one with the right polarity. I do now.
I have long thought that unit-magnification finders are worthless junk, so I bought a finder bracket for the NexStar 8. I have an old Parks 8x50 finder which takes 1.25-inch parts; it has enough focus travel for an Amici 90-degree diagonal, which gives an unreversed view, like star charts. Normally I prefer straight-through finders (which also do not reverse the view), so I can keep both eyes open and use the finder as a reflex sight, to find the start point of a star-hop. But the NexStar points well enough to put things in an 8x50's field of view, so I will forgo reflex for comfort. I use the finder when I want more than minimum magnification on the main telescope, so as not to take time to swap eyepieces, for the NexStar cannot get an object into a field much smaller than a low-magnification view.
It works very well -- matching the view on my charts with the views through a non-reversing finder is easy. The drill is to look up the chart pages while the NexStar is slewing, find how the object sits in a recognizable pattern of stars, then step to the finder and use the pattern to center the object. I spent part of one night handily finding and separating double stars down to about 8th magnitude, while an eight-day Moon restricted naked-eye limiting magnitude to three or four. There were plenty of stars visible in the finder field, but getting the finder pointed to the right field by star-hopping would have been very difficult in such a bright sky.
I bought JMI's hard case for the NexStar 8; it is expensive, but the grab-and-go virtue of such a telescope is compromised if you have to fuss and baby an exposed OTA for transport. The case has space enough for many accessories, too.
I may make more reports as I learn more about the NexStar 8, but that is probably all for a while. The bottom line is that the NexStar 8 mount, despite vexing software problems, is a versatile and lightweight computer-driven mounting, and that the Schmidt-Cassegrain optics in my particular NexStar 8 are very good.
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