Courtesy of
Sky & Space
December 1993


Digital Setting Circles Revisited

u Lumicon Sky Vector I
u Celestron Advanced Astro Master

by Peter Ward

Computerization of amateur telescopes is the way of the future, and some of that future is with us now, in the form of digital setting circles.  Peter Ward takes a look at three of the most popular models on the market.

 Digital setting circles

Digital setting circles are the electronic equivalent to the right ascension and declination dials found on many telescope mounts. They may be used in conjunction with a star chart to find objects in the night sky, in a manner similar to using latitude and longitude on a map.  The digital circles have the advantage of being much more accurate, are easy to read (even in the dark) and will work on almost any telescope. Current models have built-in computers which can retrieve information on thousands of objects, without having to refer to a star chart.

     When commercially produced digital setting circles for telescopes were first introduced to the amateur market, the name pretty much described what you got: a digital read-out for the right ascension (RA) and declination (dec.) axes. That was all, and to work correctly they had to be mounted on a perfectly aligned equatorial mounting. They were also fairly expensive. As a result, the market response was not exactly overwhelming.
     A few years later, US telescope manufacturer Celestron, introduced the first 'smart' digital setting circles. These compact units (made for Celestron by Tangent Instruments Inc.) had the amazing ability to correctly point the telescope no mater how poorly aligned it was. They had built-in databases to steer you to any object in the NGC or Messier catalogues, and could even identify an object if you were not sure what you were looking at.
     Times have changed, and while the basic functions of smart digital setting circles are much the same, the enhancement and changes make them worth looking at again. SKY & SPACE were kindly supplied with units from three manufactures: Celestron, Lumicon and Jim's Mobile. The models received covered the least expensive, mid-range and top end of the market.

Lumicon Sky Vector I

     At the least expensive end was the Lumicon Sky Vector I. This unit is aptly described as 'digital setting circle instrument with a small database'. The Sky Vector was shipped directly from Lumicon in a Styrofoam-chip-filled mailing box; the encoders and display were wrapped in 'bubble wrap' and the whole shipment easily survived any torments the postal service might have inflicted on it. The unit we tested was designed to fit onto a Meade Instruments LX-6 fork mount. Lumicon also supply mounting hardware for many other telescope brands, and this should be specified at the time of ordering.
     The Sky Vector's manual was very clear and comprehensive. Its 24 pages described all modes of operation, specifications and a printout of the database. The installation sheet was a little on the tricky side; I nearly missed it altogether, as I was expecting to find a sheet with at least a few schematic diagrams to help me along. Instead there was a printed sheet of instructions, which for the mechanically minded are easy enough to follow, but the addition of a few pictures could save a thousand words to the technically less able.
     The Sky Vector is unique amongst the three units tested, in that it has high-resolution encoders to measure the angular position of both RA and dec. axes. There are two types of optical encoders currently on the market, one with 2,048 steps per revolution (0.17 degrees per pulse) and one with 4,000 steps (0.09 degrees per pulse).
     The Sky Vector was also unique in not having intermediate gearing to rotate the encoder shafts. They mounted directly onto the RA and dec. shafts of the telescope. Accordingly, installation was very straight-forward. The shaft of the declination encoder easily slid into a machined collar screwed into the declination shaft, with the encoder itself held rigidly in place by a bracket mounted to the fork arm. The RA encoder was mounted in a similar fashion on the polar shaft of the mounting and held in place by a 'top-hat-shaped' collar. Installation took no more than about ten minutes, of a which a good portion was spent by myself looking for that non-existent installation diagram.
     Out under the stars the Sky Vector was rather like a humble motor car: perhaps not as fast as a Ferrari, nor as stylish as a Rolls Royce, but it still gets one from A to B inexpensively and reliably. To find its way around the sky, the Sky Vector must first be told what mounting it is fitted to. This can be any one of equatorial, German equatorial, Dobsonian or equatorial table mount. That set, you then sight on either one or two stars through the telescope, tell the Sky Vector their names, and welcome to the digital era of astronomy!
     What follows is akin to having a very experienced observer by your side, who will unerringly guide you to any of 56 stars or 58 non-stellar objects, plus the entire Messier catalogue. If two stars have been used for the initial alignment, the Sky Vector will give you a 'warp factor' or error read-out when moving from one object to the next. This is a measure of the how the calculated position of an object varies from its actual location, and typically should be less than half a degree.
     I should point out here a misconception regarding digital setting circles, and whether they should have 4,000- or 2,048-step encoders. The main factors determining the accuracy of digital setting circles are accurate initial settings and the mechanical accuracy of the mounting, not the number of steps the encoder can detect per revolution. Machining inaccuracies such as non-concentric bearings on polar shafts, less than orthogonal RA and dec. shafts, uneven fork tines, mechanical flexure in the mount, encoder gear backlash (or slippage), surface irregularities on Dobsonian platforms and so on will lead to errors far greater than the caused by calculation and rounding errors due to limited encoder resolution. Whilst it is true in theory that a 'perfect' mount will benefit from having higher encoder resolution, the reality is that warp factors of 0.2 to 0.3 were typical on all of the units tested here on a equatorial fork mounting, regardless of the type of encoder used. When the units were tested on a very-high-quality German equatorial mount, fitted with low resolution (2,048-step) encoders, the errors reduced typically to 0.1 ad frequently to zero.


     The next unit I examined was the JMI NGC-MAX, manufactured under license by Tangent Instruments for Jim's Mobile in the USA. The JMI unit had the distinction of being the most expensive of the three tested. A hint of its premium quality was first gleaned from the packaging, which had a high density, die cut foam lining molded to fit the major components. The NGC-MAX was also supplied with the most comprehensive range of 'accessories' such as encoder covers for both axes, custom encoder connectors and a metal holder plus alkaline battery for the computer. The 14-page manual is supplemented with several other sheets including some diagrams and encoder ratio summary.
     Impressed with the packaging, I sifted through the installation instructions, which were clear, concise and included an assembly diagram with appropriate text. With the diagram in hand, I found I could attach the encoders without even referring to the text, and the whole process took no more than five minutes. Jim's Mobile can provide custom encoder mountings for just about any commercial telescope. I'd expect that the ease with which the encoders were fitted to the Meade mounting would be the same no matter which mounting is used.
     (Editor's note: Readers who wish to equip their Super Polaris, Great Polaris or DX mount with NGC-MAX encoders are out of luck unless they have a Polaris alignment 'scope. In order to save its Southern Hemisphere customers this added investment, Jim's Mobile Inc. offers a mock-up which substitutes for the alignment 'scope for $15 USD, postage-paid.)
     It was interesting to note that the NGC-MAX is supplied with a high-resolution encoder for the declination axis, but uses a standard 2,048-pulse encoder for the right ascension axis. The encoders were supplied with molded plastic covers, which apart from their protective role, made the final installation look very much like original equipment. Another elegant touch was the self-adhesive cable clips, which could be attached to the fork arms, and prevented cables from hanging loose, getting tangled, caught, twisted and eventually severed. Having personally used digital setting circles for some time, it was good to see inclusions such as these; if they were not supplied in the original package, necessity would have you buying a set (or something similar) after only a few nights usage.
     The initial setup and general use of the NGC-MAX computer is very similar to the Sky-Vector, the main difference being there are more initial alignment stars included in the database. With initial alignment complete it became readily apparent that the computer in the NGC-MAX was the most fully-featured of the three units examined. The large dot matrix LED display was very easy to read, and remained legible even at shallow viewing angles. The impressive database includes: 7,840 RNGC objects, the entire Messier catalogue, the planets, 2,852 IC objects, 360 'non-stellar' objects derived from 40 different catalogues, 369 double stars, 44 triple/multiple stars, 77 red and 333 variable stars. The user can also add up to 25 additional objects for storage in the database.
     One might ask, of what practical value is such a large database? The answer lies with the 'Grand Tour' feature of the NGC-MAX. By going to the 'IDENTIFY' menu you can ask for a search to find the nearest object to the current position of the telescope. The process can be further refined by specifying that only non-stellar or stellar objects, globular clusters, galaxies, open clusters, nebulae or planetary nebulae should be looked for during the search. You can also specify the limiting magnitude (or brightness), and search all catalogues by simply specifying 'ALL'. The NGC-MAX will update the search every two seconds, so if the telescope is moved while the 'IDENTIFY' feature is in operation, a new search will begin around where the telescope has been re-pointed.
     The 'Grand Tour' feature works so well, in fact, it leads one to the point of distraction. During a recent astrophotography field night at my local astronomy club's dark sky site, I found myself forgetting about acquiring images on film, and ended up searching for planetary nebulae due to the sheer ease and speed with which I was 'discovering' objects.
     The NGC-MAX also has a built-in RS232 port, through which encoder positions may be sent to the serial port of a personal computer. A small BASIC program to display encoder positions on a computer is listed in the instruction manual. This feature also allows a specialized version of The Sky astronomy software for IBM and compatible PCs to be run in conjunction with the NGC-MAX. With a personal computer, The Sky software (Level III option), and the NGC-MAX, the user can have all the catalogues listed above plus approximately 250,000 stars on a 'moving map' display.

Celestron Advanced Astro Master

     The last of the three units examined was the Celestron Advanced Astro Master. Packaging was similar to the Lumicon unit. Copious amount of 'bubble wrap' kept the contents safe inside their cardboard container. Celestron supply encoder mountings and gears for their fork and Super Polaris mounts only. As a result the Advanced Astro Master was the most difficult of the three to set up on a third-party mount. Even mounting the Astro Master on early Celestron fork mounts requires some modification to the fork tine. Later model Celestron 'scopes are modified at the factory, making addition of the Astro Master a quick and simple 'bolt on' process.
     The 61-page instruction manual was a tour de force as manuals go. Indexed with clear diagrams, concise step-by-step instructions and a selected list of 'showpiece' NGC objects made the process of installing and learning how to use the Advanced Astro Master a snap.
     I was pleased to see inclusions such as a cover for the dec. encoder, yet puzzled as to the absence of an RA encoder cover. Similarly, while velcro fasteners were supplied to hold the Astro Master to the mount, there were no fasteners to keep the cables from interfering with the movement of the mounting. I was also concerned with the possibility of the pins on the RA encoder being damaged inadvertently, when the cable is removed for storage. Care also had to be taken when installing the encoder cables to ensure that the male and female connectors were correctly aligned. Incorrect alignment could bend, and eventually break the pins on the encoder (I could also level similar criticisms at the Lumicon Sky Vector).
     The Advanced Astro Master and NGC-MAX bore a remarkable resemblance to one another. This should come as no surprise as they are both made by Tangent Instruments. That is not to say, however, that there were not some significant differences. The encoders used by Celestron are the standard 2,048-pulse type on both axes. More importantly, Celestron use a different read only memory (or ROM) chip. The latest version, 3.15, is a significant upgrade from the earlier 2.1 version we tested last year (Jan/Feb 1992 issue, page 22). The display has been upgraded to a dot matrix LED, providing slightly larger characters, and is more legible than the older segmented display, especially at shallow angles. There is even an RS232 socket. Sadly however, it does not function as there is no buffer chip fitted to the circuit board within the Astro Master. The database consists of 7,840 RNGC objects, the entire Messier catalogue, the planets, 25 user-defined objects, and 241 'interesting, red, multiple or variable stars'. To many observers this list would be more than adequate. (There will also be those who will not be satisfied until the entire Hubble Guide Star Catalogue is in the ROM!)
     The alignment procedure is identical to the JMI and Lumicon units, with the expanded list of 30 alignment stars being available. Use of the Astro Master's 'computer' was essentially the same as the JMI unit. What came as a surprise was the inclusion of the 'Grand Tour' feature in the latest ROM version - totally undocumented! In this instance the tour was more of a 'tourist highlights' due to the smaller database, but still very adept in updating the display every two seconds while in the 'identify' mode, which could then be used to guide the user to the nearest galaxy, planetary nebula, open or globular cluster and so on. With this latest software update, the Advanced Astro Master proved to be a solid 'all rounder' giving excellent value for money. While to some extent we are comparing apples to oranges here, it would be fair to say the price of each unit tested reflects the level of performance to be expected by the user. The Lumicon Sky Vector I while basic in function is every bit as accurate as the Astro Master and NGC-MAX. Similarly, if you desire a premium package the JMI NGC-MAX is simply one of the best units available. Celestron's Advanced Astro Master while priced at a similar level to several mid-range units available from both JMI and Lumicon, provides a good level of functionality for a modest price.
     Common to all three is the ability to quickly locate objects in regions of the sky in which even experienced 'star hoppers' would be lost. If there is a problem with any of any of these units, it would be the despair that would set in should the batteries fail before the end of a night's observations!