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The Riddle of the Sphinx

Posted by Chuck Zdeb   06/29/2004 12:00AM

The Riddle of the Sphinx
[ARTICLEIMGL="1"]For a variety of reasons, I was looking for an easily portable Go-to mount and was intrigued by the newly-released Vixen Sphinx. In essence, the Sphinx can be looked at as a Vixen GP-DX equatorial mount equipped with Go-to and controlled by a hand-held computer running a planetarium program. While using planetarium software on a laptop to control a Go-to scope is quite common, Vixen’s innovation is to integrate the computer and software with the mount. Why is this a good thing? Simplicity. A person could buy a used laptop and a good planetarium program, connect that to a substantially less expensive Go-to mount, and have more all-around capability. But going the laptop route requires things like supplying power to it, more cables to connect, more things to initialize, providing a table for it to sit on, a filter to keep screen brightness tolerable, and so on. Plus you get to disconnect and carry it all back in again at the end of the night. The strength of the Sphinx is simplicity: it allows one to easily carry out an assembled Go-to mount and computer in one trip. Attach the optical tube, plug in power, and you’re ready to do computer-aided observing. That concept was really attractive to me.

The Sphinx specifications are quite respectable: it has a 32-bit RISC processor and a 4.7” (120mm) 320x420 pixel full-color LCD display in the Star Book (Vixen’s name for the hand-held computer/controller), 22,725 objects in the database, an autoguider port, and a LAN port for upgrading the software (all implying that the Star Book has got some serious capability plus over-the-Web upgradability for future growth), 180-tooth worm gears driving it (implying smoothness and accuracy suitable for astrophotography), and the full-up weight of the equatorial head, tripod, and one counterweight is about 30 lbs (13.6kg), with a rated capacity of 22 lbs (10 kg). It also has some rather clever design features like the retractable counterweight shaft, fully enclosed servo motors placed to minimize the need for counterweights, and no external cables except to connect to power and the Star Book. It’s outstandingly light, clean, and compact, and can accept a wide variety of optical tubes. Additionally, Vixen mounts have long had a reputation as being well-built (the many clones don’t compare well), and Vixen’s SkySensor Go-to systems have been excellent performers. The old truism says that the best scope is the one that you use the most. The Vixen Sphinx seems aimed at being the mount you use the most. I decided I had to try the Sphinx.

After the Sphinx arrived and I had a chance to set it up, one can’t help noticing that it’s visually distinctive, having a sculptured shape painted bright white with translucent blue panels and covers. It wouldn’t look out of place in an Apple computer showroom. Quite a nice change from the predictable blacks, grays, and dull metallics of most mounts. Part of the clean look is that it doesn’t have setting circles. I don’t miss them. To me, putting setting circles on a Go-to scope is like printing logarithm tables on the backside of your calculator. The Sphinx does have raised black marks for initial alignment that are easy to use, even by feel in the dark. Very thoughtful. Unfortunately, like some other Go-to mounts, it doesn’t have manual slow-motion controls, which means that without power it doesn’t function at all. An eyepiece tray and polar alignment scope are optional. Why are these things included with fairly cheap mounts, but optional on fairly expensive mounts?

The Sphinx comes with a plastic battery case for use in the field, but probably would drain the specified set of eight alkaline D-size cells in a few hours. I got a Radio Shack “D” Adaptaplug connector to hook a power cord from the Sphinx to the accessory plug-in port on one of the ubiquitous 12V automotive jumper batteries. That has plenty of capacity to run the Sphinx, dew heater, and tube fan.

I mounted an Intes Micro 7” Mak-Cass (about 16 lbs (7.3 kg) with everything) on the Sphinx with the widely-used Vixen dovetail system. The scope clamped solidly into place, but balancing is a little vague since the Sphinx is very tight and does not turn very freely on either axis. I used two standard counterweights (4.2 lbs (1.9 kg) each). The 7” Mak/Sphinx combo is almost shockingly solid on the standard HAL 110 tripod, especially if you’ve used one of the clones of Vixen mounts. It’s even difficult to measure dampening time as it appears to be less than a second. It’s impressive how solid such a light mount can be, especially on the metal tripod at full extension. It’s definitely stable enough for astrophotography under this load.

My first night under the stars with the Sphinx didn’t go
well. The first problem was that the declination axis clamp (a shaft with a knob on each side of the mount) can be tightened from either side, but only one side actually tightens the clamp. The other side gets tight, but doesn’t lock anything. It’s a good idea to put a piece of tape or otherwise mark the “wrong” knob so you don’t inadvertently use it. Even if you are tightening it on the correct side, the tight motion of the mount can fool you into thinking the locks are tighter than they are. Failure to properly snug the clamps leads to drive slippage, which causes pointing errors and big frustration. Chalk this problem up to operator error.

The more serious problem I found that first night is that in standard form the Star Book is waaay too bright. Screen brightness and contrast are adjustable, but even at minimum brightness it just glares at you and wipes out your dark adaptation. Fortunately, Vixen sent me a filter kit that goes onto the Star Book screen to bring the brightness down to tolerable levels (I believe this kit is included with all Star Books now). However, the screen brightness is still somewhat problematic in that if it’s bright enough for my middle-aged eyes to read details on the screen, it hurts dark adaptation. This isn’t just a Star Book problem; it’s a general problem for any user of computers at the telescope. To put it in perspective though, even using a red flashlight to read paper charts hurts dark adaptation. One contributor in the case of the Star Book is that the pop-up menus have black text on a bright white background. White text on a black background might work better. With the filter in place, the brightness of the Star Book is acceptable in my moderately light-polluted suburban backyard. At a dark-sky site or for someone trying to reach the faintest fuzzies, it may not be.

Setting up and aligning the Sphinx is much like most other non-GPS Go-to scopes. Point it north and get it more-or-less level, set the initial position, power up and set lat-long and time (first time only), then select an alignment star either from a menu or (unique to the Sphinx) from the star chart display on the Star Book. Do a Go-to, center the object in the scope, do an Align, and repeat on the next object. I always used a reticle eyepiece when aligning. With an eyeballed polar alignment (no polar scope or built-in polar alignment routine), the Sphinx generally puts the first alignment star in the finder or close to it, gets the second star into a low-power eyepiece, and after the third star will nearly always put objects well inside the field of a TV Radian at 150X (approx. 1/3 degree diameter), even when slewing across a large stretch of the sky. Up to twenty alignment points can be entered, though it worked well enough that I rarely entered more than six. Aligning on a nearby object will let the Sphinx locate a target almost dead center every time. No complaints about the Go-to capabilities of the Sphinx. Tracking also seems quite accurate, though limited by my ballpark polar alignment.

Using the Star Book controller is fairly intuitive, very much so if (like my kids) your hands are permanently curled from using a video game controller. The Star Book isn’t nearly a replacement for Sky Atlas 2000 or planetarium software like The Sky, Cartes du Ciel, etc., but has enough capability that you can probably leave the fancy charts at home on many nights. The Star Book has two modes, Scope Mode and Chart Mode. In Scope Mode, you can press the buttons to directly slew the scope while the Star Book displays on the chart where the scope is pointing and also has a numerical RA-Dec display on the screen. The display allows selection of RA-Dec or alt-az charting; I thought the alt-az is more intuitive. You can zoom the chart in or out through several steps, from a very wide field showing a wide swath of the sky, to an up-close view of a small corner of a constellation. The slew speed of the scope is controlled by the zoom selected, wider view = faster, narrower view = slower. The higher speeds are mildly noisy, and slews end with a chime to tell you it has arrived. Chart Mode allows you to scan the star chart without moving the scope, and also allows you to pop up menus with planets, Messier objects, NGC/IGC objects, famous objects, etc., to directly select them. Interestingly, the menus selectively list only objects above the horizon. The biggest drawback of the Star Book for me is that it doesn’t display any star names or designators on the star chart. This is especially annoying when selecting alignment stars since, despite having been an amateur astronomer for several decades, I couldn’t tell you where to find (for example) Schedar or Mesarthim. The next biggest drawback is that neither does it show any non-Messier deep sky objects on the chart unless you currently have that object selected. Displaying at least some of the brighter NGC objects would be a big plus. There also isn’t a capability to do direct entry for RA-Dec coordinates, though you can slew in Scope Mode to reach an unlisted object of interest. The Star Book deep-sky database goes down to objects of 14th magnitude (it displays stars down to about 7th magnitude), which is a reasonable visual cut-off for the scopes of 8” and smaller that the Sphinx is best suited for.

Using the Sphinx is fun. After the ten-minute set-up and alignment, you notice that, for example, the Big Dipper is well-placed for observing. You know where to find M51 and M81 and 82, but are drawing a blank on what else is there to see in that part of the sky. Have a look on the Star Book, and do Go-tos to M63, M97, M101 and M106 (since NGC objects are not displayed, you have to plan ahead for them). Doing something like cruising through the Coma Cluster really lets the Sphinx show its stuff. You can select a galaxy from a pop-up menu or by placing the crosshairs on it, press Go-to, and the scope slews there, with the Star Book showing the neighborhood around where your scope is pointed. Curious about other Messier objects visible in the field? Identify them from the Star Book. Want to have a look at something else you see on the Star Book chart? Move the crosshair and hit Go-to. You simply look down at the Star Book, click as necessary, then look back in the eyepiece. I experienced no glitches or hangs or any other software problems. For casual backyard observing, it works very well.

Unfortunately, at the time of this writing (June 2004), one can’t say that about the Sphinx regarding astrophotography. The current software revision does not yet support an autoguider, periodic error correction (PEC), or backlash compensation. If you’re adventurous enough to try manual guiding without PEC or backlash compensation, you’ll find that the drives have quite a bit of overshoot when manually controlled. It’s moderately annoying when centering objects during visual use, but would be catastrophic for someone trying to smoothly guide a long exposure shot. Astrophotography will have to wait until a new software revision is available.

As it exists today (June 2004), the Vixen Sphinx is a highly portable, very solid, and accurate Go-to mount for visual use. The Star Book is a convenient aid and makes the Sphinx especially well-suited to teaching beginners their way around the sky. The things it does, it does well, but the Star Book software would have to be called unfinished. It’s clear that the Star Book software is intended to be readily upgraded, and it’s worth noting that the flaws (other than the too-bright display) are curable through software upgrades. The Sphinx is a great concept that has lots of potential, but it’s not there yet.

Click here for more about the Sphinx. -Ed.