Courtesy of
Sky & Telescope
May 1990

S & T Test Report


by Dennis di Cicco

     Lest anyone doubt that the electronic revolution has a firm grip on amateur astronomy, flip through the advertisements in this issue. The products range from simple electric focusing motors to sophisticated CCD cameras and autoguiders. And though computers themselves aren't touted, there's no shortage of companies selling astronomical software.
     This revolution hasn't happened overnight. Indeed, I've enjoyed watching it develop over the years. When I began reading Sky & Telescope in the early 1960's, the sole electronic gadget advertised was a transistorized drive corrector. What has impressed me of late, however, is not the extent of the revolution, but how rapidly it is gaining speed. No sooner does someone wonder aloud, "Wouldn't it be nice if?" then the item appears on the market.
     A perfect example is Celestron's Advanced Astro Master. In the September, 1988, issue, page 258, I reviewed the very impressive digital setting circles sold by several companies. Not only were they accurate and well made, but they were also the first to work with altazimuth as well as equatorial mountings. Furthermore these circles needed no alignment of the mounting. You just initialized them by pointing the telescope at two bright stars and pressing a button, and an internal microcontroller made all the necessary coordinate transformations to provide real-time readouts of right ascension and declination.
     The obvious wouldn't-it-be-nice idea was to couple these readouts to an electronic catalogue of stars and deep-sky objects, thereby making an extremely "user-friendly" CAT (computer-aided telescope). Viola, enter the Advanced Astro Master.
     In a nutshell, the Astro Master is a pair of shaft encoders linked to a 4-by-3-by-1-inch package that weighs 6 ounces complete with its 9-volt battery. The unit retains all the features of the earlier digital setting circles and combines them with four internal catalogues. It also has a stopwatch-type timer and a computer-assisted method for polar aligning an equatorial mounting. There is also a simple one-step method for aligning the Astro Master with the sky. The real power of the unit, however, is that you can access any object in the catalogues and have the Astro Master direct you to it regardless of the type of telescope mounting you are using or whether it is aligned.


     Here's an example of how the Astro Master works when attached to the Celestron Ultima 8-inch Schmidt-Cassegrain telescope that was reviewed in both the December and January issues. On a cold, clear winter evening I plunked the tripod and wedge down on sloping ground behind my house. I completely ignored polar alignment and left the polar axis pointed about 30 west and somewhat below the celestial pole.
     When the Astro Master was turned on, it prompted me to turn the scope until the declination circle (the mechanical rather than electronic one) read 0, after which I hit the "enter" button. This gave the unit's microcontroller a reference point where the optical and polar axes were perpendicular to each other. That done, the display automatically advanced to the "align star" mode. I centered Altair in the scope's eyepiece and selected Altair from the Astro Master's list of 30 alignment stars (the names appear on the display as you cycle through the list with the "up" and "down" buttons). I hit the enter button again. Now I moved the scope to Aldebaran, rising in the east, and repeated the procedure.
     All this took about 1 minutes, and I was ready to begin deep-sky observing. Andromeda was nearly overhead, so I switched to the Astro Master's Messier catalogue, entered the digits 31, and switched to the "guide" mode. Even though I was new to the Astro Master's operation, this took maybe 15 seconds. The unit instructed me to swing the scope west and north until the display's indicators zeroed (they counted down the angle that each axis had to turn as the scope was moved). Sure enough, the bright glow of M31 was nearly centered in the 30-mm eyepiece field.
     Off to the south I could see one of the companion galaxies. Pretending that I didn't know whether it was M32 or M110, I centered the object in the field and switched the Astro Master to the "identify" mode. Pressing two buttons was all it took to ask what was the nearest Messier object to where the telescope was pointed. Almost instantly the display responded with M32. One more button press and words scrolled across the display telling me that this was an 8.2-magnitude elliptical galaxy in Andromeda. A few more button pushes and the unit guided me to the other companion, M110.
     Remembering that there is a rather nice planetary nebula somewhere about a dozen degrees west of M31, I swung the scope to its approximate location and switched to the identify mode. This time the display replied that the nearest NGC object to where I was pointed was NGC 7640. That number didn't rind a bell, and, sure enough, one button press informed me this was an 11.5-magnitude barred spiral galaxy. It seemed interesting, however, so I switched to the guide mode and moved the scope as indicated. What an unusual splinter of light! I couldn't recall having seen it before, but it was certainly worth noting.
     I nudged the scope a little northward to where I thought the planetary would be and tried the process again. Success! NGC 7662 was a bit to the southeast, so I moved the scope as instructed and found the planetary almost smack in the middle of the field. All this was pretty impressive, especially since the scope wasn't anywhere near polar aligned and the motor drive wasn't even running. During the next hour or so I hunted down several dozen objects, including some old friends and a few new ones.


     The Advanced Astro Master I tested was one of the first off the production line. It arrived in June, 1989, but was soon followed by a new read-only-memory (ROM) chip. This was the version 1.38 update of the unit's software. The 28-pin ROM contains half a megabyte of memory (including some 6,000 lines of computer code that form the operating instructions). I took 5 minutes and a small screw/driver to swap the ROM chips.
     This also offered an interesting perspective on the Astro Master. Because there are bound to be mistakes in such a complex piece of electronic razzle-dazzle (not to mention the huge electronic data base), the fact that updates are so easily made is comforting to anyone worried that the unit will soon become obsolete. I've been told that another ROM version with enhanced user features is in the works. These upgrades will be available to existing Astro Master owners at a reasonable cost (probably around $30).
     In addition to the tiny optical encoders that attach to each axis of the telescope mounting, the Astro Master arrived accompanied by brackets, gears, screws, bolts, and metal covers. These were the materials needed to fit the unit to all but the earliest Celestron 8 fork mountings as well as the Super Polaris German equatorial mounting. Because I installing it on the newer Ultima, most of this hardware was unnecessary.
     That's not to say installation was a snap, even though the Ultima appears to be the first Celestron telescope manufactured with the Astro Master in mind. I still had to swap the left and right fork arms a job decidedly more complicated than you might deduce from the one-sentence instruction in the Astro Master's manual. Furthermore, I had to drill a hole in the fork arm to attach one bracket. Installing the Astro Master on other Celestron telescopes requires additional drilling or disassembly. While none of this is especially demanding work, it will intimidate the mechanically faint of heart.
     Telescope makers should note that it would be rather easy to attach the Astro Master to just about any homemade mounting I've ever seen, whether altazimuth or equatorial. The encoder can be driven directly (1:1 drive ratio) by the telescope axes or connected with gears and the like as long as the drive ratio remains between 1:1 and 1:4. (This ratio is specified by the user during the one-time setup of the Astro Master's software.)


     Each optical encoder resolves 2,048 steps, or slightly better than 0.18. The right ascension is displayed to 1 minute of time, while the declination counts "1, 2, 3,..." which stands for 10', 20', 30', and so on. In addition to the tests mentioned earlier, I also set up the scope as an altazimuth and then used it precisely polar aligned. The Astro Master's microcontroller handled all three modes with ease, putting almost every object I searched for well within the field of a low-power eyepiece.
     The unit seemed slightly more accurate when the mounting was polar aligned. Rick McWilliams of Tangent Instruments, who designed and manufactures the Astro Master for Celestron, explained that the improved accuracy is the result of the simpler internal calculations needed for a polar-aligned mounting. This, in turn, allows using the maximum accuracy available from the shaft encoders.
     Overall, I suspect that the greatest source of error will not be the Astro Master's encoders or calculations but rather the mechanical limits of the telescope it is attached to. The mountings must have their axes precisely perpendicular to each other for the Astro Master to give optimum performance. Furthermore, especially with German equatorial mountings, it is important to have the telescope's optical axis perpendicular to the declination axis. Errors that arise from a failure to meet theses conditions are relatively small if the telescope is sweeping only one part of the sky on which the Astro Master is "aligned." They appear magnified when moving the scope across a large expanse of the heavens as when moving from the western to eastern sky.
     If you find that objects are not falling near the center of the eyepiece field, there is a simple solution. Center the telescope on any known object called up from the Astro Master's catalogues, switch to the "align" mode, and hit the enter button. The unit is now initialized on that part of the sky.


     The heart of the Astro Master is its data base stored in four catalogues. The first contains 363 stars, including multiples and noteworthy colored stars. Accessing objects from this catalogue isn't easy since it's done by a star number you have to look up in the manual. For example, you can only access the beautiful, colored double star Gamma Andromedae by calling for star 29. Once you call this number, however, a single button press will scroll information across the display telling you that the star is 2.2-magnitude Almach and a colorful double.
     Another of the Astro Master's catalogues contains all 110 Messier objects and correctly lists M40 and M73 as asterisms, M102 as a duplicate observation of M101, and M91 as the spiral galaxy NGC 4548 in Coma Berenices, which is the widely accepted candidate for this once-missing Messier object. The largest of the Astro Master's catalogues contains 7,840 deep-sky entries from the Revised New General Catalogue of Nonstellar Astronomical Objects by J. Sulentic and W. Tifft.
     Lastly there is a "user" catalogue that holds up to 25 objects. These could be positions of planets, asteroids, comets, or perhaps selected variable stars. To make optimum use of this catalogue the user must enter the positions in equinox 2000.0 coordinates. The information is stored in a special nonvolatile memory that is retained when the power is turned off and even when the battery is removed. McWilliams says that this memory can be over-written (changed) at least 4,000 times without damage and, by one estimate, can be changed 100,000 times.
     It was impossible to check the accuracy of every entry in the Astro Master's catalogues, but only a fool would expect them to be free of errors, especially in early versions, of the software. Indeed, as Murphy's law would have it, the very first object I tried to find with the unit, NGC 246 in Cetus, had the wrong sign on the declination.
     The only other outright error I found was that the nebulous knot NGC 604 in the outer arm of M33 was listed as a galaxy. But there were also inconsistencies. such as the descriptions of M32 and M110 being very different depending on whether they were called from the Messier catalogue or from the corresponding entries in the NGC catalogue. Little things like this ought to be ironed out in later releases of the software. I certainly don't feel these error should delay a purchase of the unit.
     The instructions that were shipped with the Astro Master were pretty good, especially considering this was an early model. Some seemed a bit confusing, but a few minutes of pressing buttons while reading the instructions cleared up any questions I had.
     At first I was leery of having only four buttons to control all the Astro Master's features. I thought this would be especially annoying when entering numbers, since it means you would have to scroll through the digits 0 to 9 to get to the number you want. I assumed that entering a four-digit NGC number would be a pain. But in practice, the scroll up and down buttons make this task very efficient, especially in the dark. Once I got the hang of it, it took only seconds to enter any number. Furthermore, the large, well-separated buttons were easy to operate even while wearing mittens!
     Both Jim's Mobile Industries and Lumicon sell variations of the Astro Master with mounting hardware to fit telescopes other than Celestron's. These include the fork-mounted Schmidt-Cassegrains by Meade and most Dobsonian designs. Both companies also have versions of the device that use shaft encoders with twice the resolution of those I tested. My guess is that these would have the most noticeable improvement only on telescopes with extremely accurate mountings as mentioned above.
     Ever since the first CAT was introduced a few years ago, I've listened to countless arguments for and against computer-aided telescopes. No doubt the debate will continue. I see valid points on both sides, but people always seem to take extreme positions. Where are the moderates in this debate?
     I look at CAT's as electronic catalogues to be used right at the telescope. They have given me a chance to check out objects that I'm sure I would have ignored while I was sitting at a desk planning my night's observing with conventional charts and catalogues because they seemed too small, too faint, or too far from an easy guide star. And every time that little voice in my head gets too loud telling me I'm not being a "real" amateur, well, I just reach down and turn off the Astro Master. It's that easy to go back to the one-on-one relationship I've had with the stars for nearly 30 years now.