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Vixen's Sphinx "go to" mount: for anyone looking to upgrade a favorite telescope to Go To capability, this computerized mount is a welcome addition to the marketplace.

IN THE PAST anyone contemplating the purchase of a computerized Go To telescope was presented with two choices: buying a complete system including a telescope, or purchasing a high-end computerized mount costing thousands of dollars and adding the telescope. But wouldn't it be nice if there were a reasonably priced Go To mount that could accept the telescope of your choice?

Yes, it would be nice, and that's why this new mount from Vixen is so attractive. It allows telescope enthusiasts to purchase equipment in ways that stereophiles have enjoyed for years: buying separate components, perhaps from different manufacturers, then mating them for a combination that best suits the customer. There is no requirement to use any particular brand of telescope tube assembly with the Vixen Sphinx mount.

I've long been a fan of Vixen mounts, and my previous tests of Vixen's Great Polaris (S&T: April 2000, page 56) demonstrated that mount's solid performance and superb tracking accuracy, despite its modest price. The older Vixen mount is still available and has an add-on Go To package (available as an option) called the SkySensor 2000, which I reviewed in the same issue. It works extremely well and has features that are unique in the Go To world.

Vixen has designed the Sphinx mount from the ground up as a Go To system. As a result, it is different from the Great Polaris, and it too has unique features.

Basic Mechanics

One particularly striking aspect of the Sphinx mount is its clean lines and tidy appearance. There are no protruding motor housings or gear covers. The Star Book controller attaches to the mount with a single cable. Stylish appearance alone makes the Sphinx a very attractive choice, but what really counts is how well it stands up in actual use.

Fitted with a 20-pound (9 kilogram) Celestron 91/4-inch Schmidt-Cassegrain tube assembly and a 10-pound counterweight to balance the tube, the Sphinx had a damping time of 5 seconds, regardless of whether the tripod legs were extended. This is on the borderline of being too shaky in my opinion. When I placed a set of vibration-suppression pads under the legs, however, the damping time was cut down remarkably, to just 11/2 to 2 seconds. This suggests that the Sphinx's tripod legs, which are made of aluminum channel (Vixen's #HAL110 tripod), though sturdy, could be improved to damp resonant vibrations better.

To be fair, my setup was rather heavy, since the mount is advertised as handling up to 22 pounds, which is reasonable for a small-aperture telescope such as a refractor or Maksutov in the 3- to 5-inch class, plus counterweights. (The Sphinx comes with one 4.2-pound counterweight).

When mated to a 90-mm short-tube refractor, for example, the mount's damping time was a very good 2 seconds. Adding Vixen's optional extender pier for extra eyepiece height (Vixen #MT-SX2516, $160) increased the damping time by about a half second. Using the antivibration pads cut damping times to about 11/2 seconds with the small refractor, with or without the pier, suggesting that the pads are a highly recommended accessory.

Locks on the right-ascension and declination axes allow the equatorial head to be swung about freely, but there are no manual slow-motion controls--any fine-tuning of the mount's position must be done electrically with the mount powered up. However, it isn't necessary to proceed through a full star alignment just to use the tracking motor, a welcome feature when you just want to make quick observations.

Basic Electronics

The Sphinx comes with an AC-power adapter, good for running the mount from 100- to 240-volt household current. An eight-battery D-cell pack is also supplied, but the Sphinx, as is true of most Go To mounts, draws so much power that the Star Book display died after only a few slews with either alkaline or NiMH rechargeable batteries, leaving the tracking motor running but the telescope effectively disabled. Users are on their own to devise an adapter cable for connecting the mount to a car battery or large multi-amp-hour power pack, which is how the Sphinx really must be powered in the field.

Watching a star at high power with an illuminated-reticle eyepiece, I could see that the Sphinx's periodic error in right ascension (due to mechanical errors in the worm-gear system) was a very low 20 arcseconds. Most of the error occurred during one to two minutes of the worm's 8-minute rotation period, leaving the star dead still for most of the rest of the cycle. This fine performance speaks of the Sphinx's well-crafted mechanics. It's a pity there's no electronic periodic-error-correction feature to improve the tracking still further, possibly eliminating the need for guiding in some imaging situations.

While the Star Book has a standard 6-pin modular jack, as supplied it doesn't function with an external autoguider. To use the autoguider feature you must upgrade to the latest software and also pay a fee (2,100 yen, or about US $20) via Vixen's Web site in Japan (it's in English). This will get you an activation-key code for your particular Star Book. You must also set several tiny dip switches inside the Star Book. Although well explained in instruction sheets, this seems a rather convoluted procedure for activating a feature that most mounts include as standard.

Nevertheless, once the Sphinx was made auto-guider compatible, it worked very well with my SBIG STV autoguider. The STV successfully calibrated on the first attempt and then proceeded to happily guide the Sphinx. Initially the mount exhibited some autoguiding hysteresis in declination, but when you activate the autoguider function you also get an electronic declination-backlash adjustment. When I set this function to a midvalue of 55, the hysteresis seemed to settle down. The guiding rate is fixed and is independent of what slewing speed you have set for the manual buttons, a good feature.

The Vixen Go To System

To set up the Sphinx you start with a rough polar alignment and aim the telescope tube toward the western horizon (there are indicator marks on the mount's axes that provide the proper "home" starting point). Precise polar alignment is not required for the Go To system to accurately locate targets. From the home position you first command the mount to go to one of several dozen bright stars provided in a list (you must pick the star yourself as there is no "auto-align" feature that picks stars for you). You center the star in the field of your telescope and then press the Align button. Vixen recommends repeating this procedure with four alignment stars, two west of the meridian and two east of it. I found that using just two widely separated stars worked fine. Adding more alignment stars improves the pointing accuracy, especially in the area of the sky around the selected stars--up to 20 alignment points are possible.

The Star Book remembers the time and location (it stores only one location), so except for adding or subtracting an hour for daylight-saving time twice a year, or for moves to new time zones, there is no need to enter these standard data into the Star Book each time you turn the unit on. Very nice indeed--if there's anything a computer should be able to do, it's to remember the time.

The main instruction manual doesn't explain that the time-zone offset for North America should be set to a negative number (we are several hours earlier than Universal Time). That makes sense in hindsight, but I had one bad night fighting the Sphinx as it insisted on swinging in wild abandon around the sky until I realized my error. After updating the software for autoguider support and then entering my site information again, I had mistakenly entered 7 instead of -7 for my Mountain Time zone. A separate PDF information sheet posted on the Vixen North America Web site (but not included with the otherwise thorough printed manual) explains the settings for North American time zones. When you get it right, be sure to hit "Save Settings" to store the location.

As a check of pointing accuracy, I performed the recommended fourstar alignment, then slewed to 50 objects scattered around the sky. The Sphinx placed 25 percent of them dead center in the field of the 91/4-inch telescope, while the majority (65 percent) ended up within 1/2 [degrees] of the center, and just 10 percent ended up as far as 3/4 [degrees] from the center. This is excellent accuracy, as good as, if not better than, I've seen in most portable Go To systems. As an aside, Messier 52 in Cassiopeia proved to be the odd exception, since the Sphinx couldn't find it at all--a problem I traced to the Star Book, which has incorrect coordinates for this open star cluster.

The Sphinx accurately followed targets, keeping them in the field for hours at a time. When slewing around the sky, the mount flipped itself to the correct side of the meridian without a problem, avoiding a collision with the tube. However, a meridian line on the Star Book's sky-chart display would be helpful in letting you know if slewing to an object will force the telescope to flip to the other side of the mount.

Even with the onset of my Canadian winter, the Sphinx continued to perform well in below-freezing temperatures. While the Star Book's sky chart did smear as it scrolled across the display, when not in motion it was easy to read even on cold nights.

When powered through its AC adapter, the Sphinx is as fast as, if not faster than, most other Go To systems in its high-speed slewing, taking just over a minute to perform a worst-case move (going between close objects that are situated on opposite sides of the meridian). Indicative of its superb mechanical quality, it also centers targets quickly and positively, with no overshoot and without having to slowly creep up to the object. When slewing a short distance to a nearby object, the mount just makes a short, slow move. The Sphinx is also pleasantly quiet when slewing at high speed. On the other hand, it makes more noise than most systems when simply tracking, emitting a loud, though not annoying, whirring. A nice feature is that the slewing speeds automatically increase as you zoom the star chart out to a wide field and slow down as you zoom in to view a small area of the sky for fine pointing.

While the pointing accuracy was excellent, early in the testing with the original version 1.0 software I did encounter occasional nights when the Sphinx wouldn't find targets well. I could attribute this only to some poor combination of initial alignment stars. After I upgraded the software to version 1.2, the bad nights seemed to go away!

The Star Book can be updated by downloading software first to a computer (it needs to be a Windows machine) from the Vixen Web site. The Star Book then needs to be connected to the computer through a crossover Ethernet cable, the type used for peer-to-peer linking of a computer directly to another computer. (I got one at Radio Shack.) The process involves the input of arcane IP addresses, but by following the instruction manual to the letter, I made the upgrade smoothly. It resulted in an improved display with rapid refreshes of the star chart as the telescope slewed. The latest update also added Comet Machholz to the display. I hope Vixen continues to provide upgrades, as the principal shortcoming of the Star Book is that it lacks many of the standard features we've come to expect in Go To systems.

The Star Book

The Star Book's electronic chart is a great concept, but I feel that Vixen's implementation is likely to disappoint many observers. The display shows only 109 deep-sky objects from the Messier catalog. This means you cannot use it to find non-Messier targets--the very thing most printed star charts do best. The only time you see an object from the NGC or IC catalog on the Star Book display with its label is when you enter its catalog number, then execute a Go To command. Thus, you already need to know what NGC or IC objects might be of interest in order to display them. The Star Book is not as friendly to beginners as it could be.

Surprisingly, the Star Book lacks the following: any packaged tours of the sky's best targets, any double or variable stars, and any ability to slew to entered right-ascension and declination coordinates. In the case of this last item, the Star Book does constantly update its display of coordinates as you move the telescope, so it is possible to locate a position by slewing toward it, but this is hardly a convenient way to do it. There is no lunar tracking rate and no Park or Hibernate mode so that you can restart the Sphinx for Go To operation without redoing a star alignment. The only information supplied about an object is its brightness, type, and size--and even that appears only briefly in a dialog box when you initiate a Go To command. You cannot click on an object and call up information about it at will, and once at a target the Star Book provides no further information about it--no text to describe what you are looking at, for example. The Moon is shown but not its phase, the Galilean satellites of Jupiter are not depicted, and there is no information about rise/set times of any objects.

To be fair, Vixen makes no promises that the Star Book will perform any of these functions. Nevertheless, they are useful features that we've come to expect as standard in Go To scopes and basic star-charting software.

Vixen does claim a database of 22,735 objects, but the specifications state that 17,635 of them are stars, with just 5,000 being deep-sky objects. Stars cannot be called up by any catalog number--you must find the star on the chart and scroll the display to it. Even so, only stars down to about magnitude 6 are plotted, far fewer than the promised 17,000 stars, and none are labeled (no Bayer letters, for example). The only stars accessible from a list are two dozen used for alignment. Stars fainter than 6th magnitude do not appear as you zoom in on the chart, limiting the Star Book's utility as an electronic atlas. However, Vixen has told me that future upgrades will enable the full complement of 17,000 stars to be displayed.

In short, I was expecting the Star Book to provide at least the modest capabilities of planetarium software running on a PDA or hand-held computer, but not so. I really wanted to like the Star Book, as the prospect of having an electronic atlas at the telescope without any need for a separate laptop computer is very appealing. But the shortfalls in the Star Book's current software make it far less useful than it should be.

Recommendations

Despite these criticisms, the Sphinx is a well-engineered, lightweight mount. It tips the scales at just 26 pounds (11.8 kg) for the head and tripod without the counterweight. Its low periodic error and autoguiding capability make it ideal as a portable astrophotography mount.

While the Star Book's software deficiencies diminish the Sphinx's attractiveness as a platform for a visual Go To scope, the fact remains that the mount finds and tracks objects with wonderful precision. It is solid and visually appealing, and it performs superbly in all the critical basic functions that a mount should. This is truly an innovative and hot product that can be improved through software upgrades that I trust Vixen will provide.

Alan Dyer is a contributing editor of Sky & Telescope and is coauthor with Terence Dickinson of The Backyard Astronomer's Guide (available from Sky Publishing).

WHAT WE LIKE:

Excellent stability yet lightweight

Excellent pointing and tracking accuracy

Works well with autoguiders

WHAT WE DON'T LIKE:

Star Book has limited software features

Overly bright screen display

Obstructed polar-alignment scope

Vixen Sphinx Mount

With tripod, Star Book controller, one 1.9-kilogram counterweight, one-year warranty

US price: $2,200

Vixen North America 32 Elkay Dr. Chester, NY 10918 845-469-8660 www.vixenamerica.com
SPECIFICATIONS & MEASUREMENTS *

Vixen Sphinx

Weight (head only)          15.4 lb (7.0 kg)

Power Requirements
Voltage                     12 volts DC
Slewing current             2.18 amperes
Tracking current            570 milliamperes

Slewing speeds
Maximum                     2.5[second]/second
Minimum                     15[second]/second

Periodic error              20[second]

* All values measured by Sky & Telescope.

S&T RATINGS

Sphinx Go To Mount

Go To Functions       * * *
Mechanics             * * * * 1/2
Overall               * * * *

***** Sensibly perfect. No meaningful improvements possible.
**** Any shortcomings will go unnoticed in normal use.
*** Defects noticeable but do not seriously affect performance.
** Defects noticeable during normal use--performance compromised.
* Defects so severe that the equipment is virtually unusable.

Ratings are intended to convey performance compared with equivalent
equipment and should not be used to predict the relative performance of
instruments having markedly different features or designs.


Bottom-Line Summary:

Handsome, well-engineered, and well-made German equitorial Go To mount with novel Game Boy-like hand controller. Despite limited software features, the Sphinx is a stable platform for visual work and astrophotography.
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Title Annotation:test report
Author:Dyer, Alan
Publication:Sky & Telescope
Article Type:Product/Service Evaluation
Date:Jul 1, 2005
Words:2879
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