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Vixen GP-DX Equatorial Mount Vixen Sky Sensor 2000 PC GOTO System


A common question on most newsgroups and eGroups (pardon me, Yahoo groups) as well as elsewhere on the Net is where can one find a good, reasonably priced EQ mount? Another equally common question is where can I get a "good, reasonably priced GOTO system?" (Is there an echo in here, Dave?)

Unless you have a Dobsonian telescope, you will need a mount to make your newly-acquired telescope function. To go with my ever-increasing stable of telescopes (they are, you know, kinda like peanuts, it is hard to stop after just one), which includes refractors and a newly-acquired Takahashi 250 Mewlon Dall-Kirkham, I have always preferred a simple altaz mount as my preferred mount. Easy to set up and take down, and if you get the right one, just as steady and smooth as the best fork or equatorial mount. (And, to show you how crazy I really am, I have a new Giro-2 XXL altaz mount coming to mount my 70+pound, 70+inch long 203mm f/9 refractor on as a 'quick grab' mount rather than its normal Losmandy HGM200 EQ mount.)

However, an altaz in normally not driven and requires frequent 'nudging' to keep the stars/Moon/planets/Sun in the eyepiece. So that leaves us with a fork mount (usually found with SCTs and some large binos) or an equatorial. And that brings us back to where I started this musing, is there a good, reasonably priced equatorial mount with, hopefully, an equally reasonably priced GOTO system? The answer to both is yes.

The Vixen GP-DX Equatorial Mount-- Basic Description and Set Up:

The Vixen GP-DX and its slightly 'lesser' brother, the GP mount, have been around for quite a few years. The "GP" stands for "Great Polaris" mount (the DX being the 'Deluxe' version of the GP, and having more features and with larger gears, etc.) The successors to the equally-well known Super Polaris mounts (SP and SP-DX) the primary differences between the GP and GP-DX other than minor cosmetics and the internal illumination of the polar scope in the latter is the weight carrying capacity.

Vixen rates the GP at 7kg or 15.4 lbs and the GP-DX at 10kg or 22pounds. (By the way, the latter rating seems realistic as Orion sells their 6" Mak-Newt on the GP-DX, and that OTA is 21 pounds.) The other major difference is the price. With the Hal-11 tripod (the heavy duty aluminum tripod used for both GP and GP-DX), and an 8.2 pound counterweight, the GP now sells for $799. The GP-DX (which the Orion catalog says has a head "38% heavier and boasting a beefier base for increased sturdiness" sells for $1199 with the same tripod and counterweight. These prices are up by about $100 since late 1999, when I got my first version of this mount and the Sky Sensor system discussed infra. The increase in these prices - and in the Sky Sensor price - follows general price increases at Orion lately.

(The GP without the tripod but on the Orion -nee Vixen - 'heavy duty' pier is listed in the latest catalog for $999, with the same package - head, same pier, no tripod - for $1499. Go figure, as it is the same pier. But since Orion in its infinite wisdom tells me that they are no longer offering the pier, and the one I got on sale was the last one, more on that little foible later in this review.)

Unpacking the mount head, you will find the typical grey-green Vixen color, and a surprisingly good heft and fit and finish. Is it a Takahashi or AP mount? No. Is it the 'quality' (if that is the appropriate word) one has come to expect from the Chinese EQ mounts that come with the lesser Orion, Celestron and other ubiquitous brands one sees today. A most emphatic NO! The fit and finish was very good, and the machining on all of the parts far better than its competitors from the other side of the Sea of Japan.

Set up was simple, with the manual for the mount fairly comprehensive, lacking only instructions on how to mount the internal illuminator for the polar scope. (Hint, you unbolt the face of the R.A. assembly, and then put the illuminator in, with the battery clips facing outward, using the longer bolts supplied to bolt the R.A. back on.) The mount ships already put together, needing only to be placed into the tripod or pier, the counterweight shaft mounted, and away you go. (Vixen supplies a set of the necessary Allen wrenches for assembling/working on the mount.)

The mount does not come with motors, and the 8.2 pound counterweight is the only 'accessory' supplied.

In order to mount your telescope, you need to plan ahead. The Vixen dovetail system (a $14.00 part ordered extra) will allow most refractors or Maks, etc that use tube rings to work, if you have rings with a center hole that will accept a standard 1/4"-20 bolt. That dovetail comes with a pair of bolts, and as my TMB 100 in the Yang tube had rings with these center holes, they fit right on the dovetail. As you can see in the attached photos, there is more than one way to skin a cat. Using a dovetail designed to allow a TV scope to mount on the Vixen 'saddle plate', I was able to use not only the tube rings for the TMB 100 with the 114mm O.D. tube for the Tak FSQ-106 with the 'regular' Vixen dovetail, but I was also able to mount the standard Tak 102/106 tube holder to one of the holes in the TV adapter using the 1/4"-20 center hole in the Tak holder. That worked just fine with the 106 on the GP-DX and with the Tak Sky 90 using the FS-78/Sky 90 tube holder as well. (The new TMB100 f/8 in the German CNC tube, which uses a larger diameter OTA fortunately comes with rings that also feature a compatible center hole, as do most AP tube rings.)

Now for counterweights. The mount comes with a single 8.2 pound weight. The catalog does not show other weights as available, even though you need one using a scope that is much beyond 10 or 11 pounds. However, if you call Tech Support, and if they have any left (Orion is not real good about stocking a lot of misc Vixen parts) they may still have some of the 6.2 and 4.2 pound counterweights that Vixen makes and which are nicely finished and are a lot better for balancing most medium to heavy weight 4" scopes rather than a pair of 8.2s, which is overkill in many instances, and an unnecessary burden on the mount and the motors.

The pictures show the GP-DX set up on the Orion 'Heavy Duty' pier. Don't spend too much time looking at the pictures, since I was told that this is the last one Orion had in stock, and that is why they are shown as on 'SALE' in the latest (Spring 2001) catalog. The 'heavy duty' pier is really not all that heavy (28 pounds), it is 4" in diameter and 45" in height. Set up and movement is easy, and I am a fan of portable piers over tripods to begin with as I find them easy to set up and more stable in general as providing better dampening and less 'hassle' assuming you have a good surface to use them on. In 'mushy' or otherwise dubious footing, the supplied HAL-11 tripod (pictured in my Custom D Alt Az review also on this site) works quite well, and is far better than the lighter and unacceptable tripods Orion insists on shipping with its EQ and AtAz mounts other than the Custom D and the GP duo.

Since Orion is apparently no longer going to be carrying the Vixen pier, the pier Ken Dauzat (http://users.kricket.net/ken/) sells (4.5" diameter, 57" tall, and with optional accessory trays, etc) may be a more viable option. Indeed, had I seen that ad before I got the Vixen pier, I may have chosen that route - and may still do so in the future.

Setup, once you mount either the optional single or dual axis motors, or the far different, high speed stepper motors that come as part of the Sky Sensor 2000 PC kit discussed below, is a snap. Unpack the mount head and set up the tripod or pier. Attach the motors (unless you are going to be strictly manual in operation), plug in the battery pack (with the 'standard' motors, since the power supply of the Sky Sensor brings up a whole different issue, as also discussed below) and then adjust the altitude of the mount. Setting the appropriate altitude is easy, assuming you know or can find out your latitude (which is in most phone books, or you can check the Web) by simple turning the adjusting bolts on the front and rear of the mount base (best done before you put on the counterweight shaft, etc) and watch the well-marked pointer on the side of the mount. (The photos show my GP-DX set at 42 degrees.)

Then you put on the counterweight shaft, put on the appropriate amount of counterweights, mount your scope in the saddle plate, and then balance the mount. While this is a simple procedure, and you can skip it if it is something you already know how to do, here is a quick 'tour' for the novice which can help keep the mount and motors from not working properly if the scope is not balanced, and also prevent damage if you are not familiar with the procedure and don't want to put you OTA at risk. (I do this with the scope set up with the diagonal, a typical ep, and the finderscope, etc. mounted. Otherwise, you won't get a proper balance while the mount and scope are in use.)

Release the R.A. clamp (after first making sure the tube ring is FIRMLY closed on the OTA) and swing the tube over until the OTA and the counterweight shaft are both horizontal, holding the counterweight shaft FIRMLY while doing so lest it get away from you. It is then a simple task to move the counterweight in and out until the OTA remains in a stationary position when released (but keep one hand on the OTA as well, until all is stable!) If you have the one counterweight out to the extreme end of the shaft and it is still not balanced, add a second (the lightest possible) counterweight and proceed until all is 'comfortable' and it is now sitting with the OTA horizontal on one side and the counterweight shaft on the opposite side.

Once the mount and scope are balanced along the R.A. axis, swing the R.A. up until the OTA is on top again. Tighten the R.A. clamp and then loosen the Dec. clamp (again, one hand for the ship, er OTA and one hand for the clamp.) You then move the OTA up and back by backing off on the tube clamps just enough to slide the tube up and back (but not so much that the tube comes out and falls to the ground and makes expensive noises!) With the Dec clamp loose, the OTA will no doubt start to swing around the Dec axis. Move it up and back in the rings or tube holder until it too remains stationary when not being held any longer.

Your mount/scope is now balanced. While this seems like a pain, failure to do so is a major cause of complaints about the mount and/or GOTO system failing to work or track properly. Lack of balance also is a major source of damage to the mount and will quickly burn out the motors if not carefully checked with each use. (Last night, the FSQ and the 2" diag and some deep sky eps were in use. I balanced it with the 8.2 and 4.2 pound counterweights in the position shown in the photos, the single 8.2 not enough with the relatively heavy four element Tak and the heavy Nagler eps. The weight of the Naglers also resulted in moving the OTA fairly far forward in the tube holder. I left it up inside the porch last night in order to record the correct position of the balanced scope and mount in that configuration, removing only the eps and diag for the photos. It may look like it 'should' be further back in the tube ring, but the scope as you see it is as it was balanced the night before. The Sky 90 needed only the 8.2 pound counterweight, while the TMB 100 f/8 in the CNC tube used approximately the same setup as the FSQ, with the OTA and the counterweights positioned a little differently than as depicted with the 106.)

Sky Sensor 2000 PC - Basic Description and Comments:

The GP and GP-DX mount can be equipped with a single axis motor ($99) and a single axis controller (($119) or a pair of motors and a dual axis controller for $199. If you want to turn the GP or GP-DX (or, for that matter, the SP or SP-DX into a full GOTO system, you can buy the Sky Sensor 2000 PC system from Orion for $899.00.

The system consist of a pair of D.C. stepper motors capable of moving the mount over a range from 0.1X up to 1200X for guiding or high speed slewing. The 'brains" of the system are in a large hand controller (pictured) which connects to the motors and to a power source. The remaining pieces are the connecting power/communication cable, and a power cord and battery box of 8 D-cell batteries.

Let's stop right there. If you want to run this thing right, the D-cells won't cut it and won't last any longer than the only bottle of water at the end of a Marathon. You will need a large battery which will supply at least 3amps of power for an extended period and a 'car cord' adapter to connect it to the Sky Sensor. (WARNING- you must use a NEGATIVE tip adapter, which you can get from Orion or Radio Shack. The more common positive tip will not work and may well fry the system .) Or, you can use an AC adapter or an AC-DC inverter.

As you can see from the photos, I run mine using a AC-DC inverter I got from Jim Kendrick. It supplies at least 7 amps, and works for most of my DC needs from one of the sockets I had installed outside or from one near the observing areas we had built on the edge of the lake. With this system, the Sky Sensor runs very well and as designed.

The last part of the system is a RS-232 cable to connect the controller to a PC, which will let you supplement the nearly 14,000 object database it comes with The Sky or other software. I tried it last night with a laptop, and it worked quite well.

The instructions that come with the Sky Sensor to show you how to install it on the GP or GP-DX are very, very good, but they are short as the job is simplicity itself. After first stating that the motors for the non-GOTO system are not compatible with the Sky Sensor, you basically remove some of the covers for the R.A. and Dec. and then mount one of the two gear wheels supplied, one on the Dec slo mo control shaft and one on the R.A. slo mo control shaft. The motors with the exposed gear wheels are then bolted to the Dec and R.A. assembly.

This is where patience is necessary. My friend Ron Wodaski warned me that the key to getting this system to work right is to carefully line up the gears on the slo mo shaft and the gears on the motors. Since this is done by securing the motors with a single Allen bolt and using one of the supplied wrenches, getting them squared off and flush against each other took a bit of patience, and several tries, shifting the gears on the slo mo shafts up and back and then securing the motors several times until they were perfectly meshed.

Another hint here. If the mount is at its normal altitude for the North American continent, the rear adjustment screw for the altitude gets into the way when you are trying to put the longer of the two motor fastening bolts through the base of the mount. It is best to drop it down to an almost horizontal angle, mount the motor, and then return it to its proper altitude. Since doing that is quite easy, this will avoid a lot of frustration trying to do it the way they show you in the picture in the instructions. (Never did figure out how they got that picture, unless the man shown had REALLY tiny hands - and a shorter Allen wrench )

Once the motors are in place, you take great care to not snap a prong off the cheesy plastic covers they give you to cover the motors (the only real 'cheap out' aspect of this combination), cover the motors, plug in the cable, connect the computer/controller, and connect a power source.

That is it. Even a nitwit like me, with a little patience and using the tools they supply, can have the mount up and the system installed in under an hour.

Initial Set Up and First Use:

Getting the completed GP-DX/Sky Sensor up and working is as simple as the mechanical set up. You can use the mount as a conventional driven non-GOTO or DSC system. All you need do is polar align using the excellent and supplied polar alignment scope and it functions using the directional buttons on the controller. (See Ron Wodaski's article on how to polar align and use this scope at www.wodaski.com . He does an excellent job along with detailed photos of the polar scope and settings.) You can also use the mount in the AltAz mode without using the GOTO function.

However, since using the GOTO is the point of the exercise, that set up is easy. You need not polar align at all. Just turn the system on. The first time you power up it will ask you for the time, date and your location. The lat/long for your location can be found in many sources. I am a recreational boater, and have a couple of hand held GPS units at home with me for the Winter. I simply turned one on while I was out next to the mount the first evening, and punched it in precisely for the location of the mount. (And while I don't know if this added to the accuracy of its pointing later, I am sure that it didn't hurt it either.) This has to be done only once, as the system will remember all of this data, and you don't need to re-enter the information unless you are observing from another site.

After the initial set up the manual itself walks you through the basic procedures in well-written (for a GOTO system coming from a non-English speaking company) language. I read it the night before my first use, and did a few 'dry runs' in the daytime or on cloudy nights while I was waiting for the skies to clear. When they did, the basic procedure is simple.

The controller (which is easy to read at night and not as prone to fading out in cold weather as others) tells you to place the scope in the "initial position' (with the OTA pointing sideways and to the right in relation to the base of the mount and the counterweight shaft as near to vertical as possible.) Note that the proper orientation in this position is to have the Dec motor on the outside. (Yes, when I finished up the night before, I took the eps and diagonals off and then replaced the 106 in the saddle plate without re-orienting the OTA. The pictures DO show it backwards, with the Dec motor on the inside. Well, at least I didn't do that when I was actually observing!)

Once the scope is in the intial position, and you have the polar axis roughly to the North, you slew to one of the listed alignment stars with the GOTO key and that brought me pretty close to where I wanted to be (within the field of the 22 Nagler, and relatively close to the center.) Like most other similar GOTO systems, you use the directional keys to center the object in the eyepiece. (At this point, if you have set the speed too high, it is easy to adjust the slew or guiding speed by just using the "-" or "+" buttons on the key pad.) Once you have achieved a satisfactory centering, hit the ALIGN key.

At this point the display will show you the data for the first object (which can be the Moon or Jupiter, etc if you aren't sure which star to start with.) You hit ESC and that takes you back to the Object selection menu. Selecting an object as far away from the first as possible is preferred, and the second slew was more accurate than the first. You are now two-star aligned, and if you want, that is all that is normally necessary. Now it is time to start observing, with all of the 'normal' GOTO functions (tour, go to by name, catalog name or number, etc) available to you.

It is really as simple as it sounds. I found accuracy after even a two star alignment to be quite good. By the end of the first evening, objects near either of the alignment stars were coming up almost dead center in the eyepiece. Objects in the more remote (from the points of initial alignment) were normally within the field of view of the Naglers I was using and (in only one of two instances) did I need to look 'up and around' to find anything.

Slewing speed was almost whatever you wanted it to be, and the system a lot less 'finicky' as I was led to believe. Given the light pollution in this area, the Messier and Caldwell objects (you can key in a number of 'user' objects, and I do like the Caldwell objects) were a cinch to locate.

My tests were done with three scope, the Tak Sky 90, the Tak FSQ-106 and the TMB 100 f/8. My initial review was for visual use only, so PEC etc will have to wait for an update. However, I found that this mount was very stable with all three of these refractors, the latter with large eps and diagonals, and with the Extender Q hanging off the back of the Tak 106 and the AP Binoviewer and a pair of Tak LEs in the TMB. The mount handled all of this very well, and the Sky Sensor tracked very well indeed once I was on my chosen target. On several evenings, this allowed me to have a relaxed and enjoyable 'graze' through the M36-38 group, slew over to M-45, and then spend some time looking at Jupiter. When the Moon was up, the Sky Sensor's list of objects on the lunar surface (which I really didn't need as a confirmed Lunatic) was reasonably accurate and kind of fun to play around with. All in all, an easy system to work with, and one that allows you to observe, not 'hunt and peck' during your limited time of good observing.


I want to spend more time with this system, and record more details about its capability to locate the tougher objects and evaluate its long term potential for CCD work. Ron Wodaski was very impressed with its potential as an imaging platform, and I respect his sagacity in this area far beyond my own limited experience. As for the GP-DX mount itself, it is well built and reasonably priced. Coupled with the Sky Sensor system, it is a good, reasonably priced, and well thought out package.

That brings us to my last point, what else is out there? The closest competitor IMHO is the Losmandy GM-8. That mount, with a supposed 30 pound capacity and WITH the motors included as standard, is $1399.00. (Street price.) So, with the standard motors about the same price. The Losmandy is an excellent mount, even if I think the 30 pound rating may be a tad on the high side. Suffice it to say, assuming a mount is available (and that is not always a sure thing, as they are not always on hand) I would rate the basic GP-DX and the GM-8 as fairly equal.

The GOTO version of the GM-8 is $2449.00street price, with the GOTO retrofit $1595.00, also street price. I haven't seen the new Losmandy GOTO system (I have one on order for my G-11 and my HGM-200) so I can't compare that to the Sky Sensor just yet.

So the GP-DX/Sky Sensor and the GM-8 GOTO would be the natural competitors. What else is out there in GOTO mounts of this type? The AP 400 GOTO is substantially larger and more expensive. It is also a mount that is 'wait listed' but then also one that is excellent, with a GOTO system that is the model for all others. Outside the box we are searching in now. Takahahsi? Their GOTOs are just coming out, although their EM-10 dual axis driven mount is a gem but one that is more expensive by a considerable margin.

The GP-DX is a fine mount, and coupled with the Sky Sensor 2000 PC an easy to use and (so far) reliable and enjoyable GOTO system at an 'entry level' price.

As always, thank you for taking the time to read my comments. I will continue to use this mount over the next few months, and will see how it compares to my AP 400 GOTO using comparable OTAs, gauging accuracy, ease of use, and the other factors that go towards evaluating a good mount and observing system.


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