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Sky Commander vs. NGC-Max: A Digital Setting Circles Primer

DSC stands for Digital Setting Circle. It is a computer that connects to your telescope. It tells you what you are looking at, or where to move your telescope to look at a selected object. I have found that DSCs are invaluable for observing under Los Angeles skies. The light pollution here makes it impossible to star hop to most deep sky objects, such as galaxies, globular clusters, and open clusters. Indeed the light pollution makes it nearly impossible to find all but the brightest stars. Nevertheless, deep sky objects are accessible if you can just point your telescope in the right direction. DSCs allow me to point my telescope at these unseen objects and observe them.

I recently purchased a Sky Commander DSC unit, and I have had a few chances to use it this winter and spring. Last March I used it for a digital Messier marathon with my son. It was a lot of fun. I've owned an NGC-Max DSC for almost a year, and I've recently taken delivery of a Losmandy Gemini unit. I use the Sky Commander on a Teleport 14.5" f4.5 Dobsonian. Both were delivered in February. The NGC-Max previously resided on a Losmandy GM8 German equatorial mount (GEM) that I used with a Tele Vue 101 refractor. I now use the Gemini on the GM8. The Gemini is both a DSC and a "go-to" unit. Unlike the Sky Commander and the NGC-Max, it can move the telescope by itself.

The NGC-Max and the Sky Commander share a set of basic features: Guide to object; Search and identify, i.e., "What am I pointing at now?" mode; Customizable user's lists of objects; Limit searches by magnitude; Align and realign on object/star; Setup basic features. Both units support a variety of mounts, including German equatorials, forks, and alt-az (Dobsonian) mounts. Including encoders and cables, the cost for the Sky Commander is $390. The NGC-Max is $599. The NGC-Max also comes with a video and a mounting bracket for your tripod. The video is described as instructional, but it contained sales oriented materials. The mounting bracket is useful. The Sky Commander attaches with Velcro. You'll also need mounting hardware to attach the encoders to your telescope. Each telescope is different, contact your dealer for specifics. Sky Commander is manufactured by Sky Engineering. The NGC-Max is produced for Jim's Mobile, Inc. by Tangent. It belongs to a family of DSC units produced by Tangent for third-party vendors including Celestron's Advanced Astro Master and the Lumicon Sky Vector.

Physically, both units are similar. The Sky Commander is sealed against the weather. The keypad uses sealed buttons. Some people don't like the tactile attributes of sealed buttons. It doesn't bother me. The Sky Commander also has a built-in heater for dew removal, which works when the unit is plugged in to an external 12VDC power supply. Both units run on internal 9V batteries. The Sky Commander's display provides two lines by sixteen characters, in a yellow/green backlit, adjustable intensity, liquid crystal presentation. The NGC-Max's display provides a single sixteen-character line, in red LED dot matrix, also with adjustable intensity. Both displays are appropriate for their purposes. Both units can be upgraded by replacing the ROM chip inside the box.

The Sky Commander's LCD display can be sensitive to cold weather. If cold weather is a serious concern and you don't want to use the internal heater, then you may want to give more consideration to the NGC-Max. I do not live in a cold weather region but I have used the Sky Commander--without the heater--in weather that frosted my eyepieces, my eyepiece case, and my son. The LCD display had no problems in these conditions. (However my son did require a hot chocolate.) The Sky Commander's operating temperature is listed as 0 to 50C without the heater, and -25 to 50C with the heater.

Objects and Stars

Finding objects is simple. You select the item from a list, then hit a button to place the unit into "navigate" mode. Two counters show you how far to move your telescope in the four basic directions: up, down, left, and right. (Only for astronomy this movement is more properly called declination and right ascension.) You've located the object when both counters are zeroed.

The NGC-Max unit has the ability to search for both objects and stars. The Sky Commander, while it does the initial alignment on stars, does not have a catalog of stars to use in searching. This means that you cannot realign the Sky Commander on a star, but you can realign it on an object, i.e., a planetary nebula, galaxy, etc. It also means that you can't guide to a star with the Sky Commander. This isn't as bad as it sounds since the NGC-Max's star catalog is just about useless. Instead of using catalog entries like Polaris, Alpha Centauri, or Sirius, it uses ST094, ST571, and ST294, respectively. The hardcopy star catalog that JMI provides with the NGC-Max is sorted by ST number, not by constellation or star name. So you can't really find anything in it. I got around this by finding an electronic copy on the internet. Some good person had scanned the pages and converted the images to a comma-delimited file. I downloaded it into my Palm Pilot and now I can lookup the NGC-Max's cryptic star codes (See the Resources section below for the web address.) . So going to a star is possible with the NGC unit, despite itself. It is not possible with the Sky Commander.


Speaking of catalogs, with the glaring exception of a star catalog, the Sky Commander has a very thoughtful selection of 9,000+ objects in separate lists: Messier, NGC, IC, Barnard dark nebula, Berkeley open clusters, Collinder open clusters, Melotte open clusters, Trumpler open clusters, UGC Uppsala General Catalog of Galaxies, Saguaro Double Star catalog (but no single stars!), the planets, and user-defined objects. The Sky Commander documentation states that the 9,000 object total counts only unique objects. Many of the objects could have been counted twice for advertising purposes, i.e., M1 is also NGC1952, but they were not. The NGC-Max contains a 12,046 object database. Separate catalogs are provided for Messier, NGC and IC objects to 16th magnitude, planets, and user-defined objects. An additional 928 stars, and 386 non-stellar objects including 19 black hole candidates and quasars are included as ST- and NS-numbered objects. These objects are not readily accessible due to the difficulty of cross-referencing their real names with JMI's new names in the hardcopy catalog provided with the unit.


You must align a DSC before you can use it. This is simply a matter of pointing the telescope at a star such as Sirius, and selecting "Sirius" from the computer's list. Once the unit knows where one or two stars are, it can calculate the position of all other objects.

The Sky Commander does a one or two star alignment for German equatorial mounts, and a two star alignment for Dobsonians. Both units require you to place your telescope in an orthogonal position prior to aligning, but the Sky Commander does not have this requirement for Dobsonian mounts. For Dobsonians, the NGC-Max simply requires either an initial vertical or horizontal position for the telescope tube. Both units require GEM telescopes to start from the orthogonal position--the telescope has to be placed perpendicular to the mount. (This bit of telescopic yoga sounds impossible, and it is more easily explained by looking at JMI's illustrations than in words.)

I have not used the NGC-Max with a Dobsonian, but I've found that the alignment on my Losmandy GEM, which required an orthogonal initialization, was never accurate enough for me. The NGC-Max's documentation states that alignment errors can come from inaccurate starting positions. Maybe it is just me, but when using the NGC-Max I generally had trouble finding an object without using my widest field-of-view eyepiece: about 4.5 degrees. I've had more success with the Sky Commander's two star alignment on my Dobsonian, but we are now comparing an NGC-Max GEM alignment to the Sky Commander's Dobsonian alignment and that probably isn't fair. Nevertheless, JMI's web site has a publication stressing the importance of an accurate initial orthogonal position. This is not required with the Losmandy Gemini DSC, and its accuracy is good.

Dobsonian Considerations

Both units work with a Dobsonian mount that has been placed on an equatorial platform. What is an equatorial platform? When your Dobsonian is placed on an equatorial platform it will track the stars the way a clock driven GEM tracks the stars: automatically--but only for an hour or so, then it needs a reset. Both the NGC-Max and the Sky Commander units allow you to turn off the DSC's sidereal clock, enabling the equatorial platform to do the sidereal tracking. It is a menu option in the Sky Commander, and "ET" mode on the NGC-Max. If you can't turn off the sidereal clock on your DSC, then you can't use your DSC unit on an equatorial platform. When the platform has run its course you can reset both the platform and the Sky Commander (via a menu item) to avoid doing another star alignment--a simple solution elegantly applied. The NGC-Max does not have a reset feature. When you reset the platform with the NGC-Max you'll need to perform another star alignment. I've used the reset feature and, much to my surprise, it works. Locating objects is accurate after the reset. No realignment is required.


Support is an interesting issue. Victor, of Sky Commander, answers his support phone from 7 pm to 10 pm EST. This may be good, since you can speak to him at night when you are using the unit. Victor is slow about answering e-mail, but he does respond.


In conclusion the Sky Commander has an advantage on Dobsonian equatorial platforms with its reset feature. The NGC-Max has an advantage due to its cold-proof LED display and its star catalog, but this advantage is greatly diminished by the catalog's use of cryptic ST and NS numbers instead of common star names. The Sky Commander's inclusion and fine handling of popular star cluster, double star, and galaxy catalogs more than makes up for the exclusion of single stars. Additionally the Sky Commander is weatherproof and it has a heater which will remove dew as well as provide heat for the LCD display. The NGC-Max's LED display will probably never need heat, but you'll have to wipe off the dew yourself.

Personally, I give the edge to Sky Commander for three reasons: The Sky Commander is about $200 less expensive; The Sky Commander contains separate catalogs for deep sky objects, retaining the original catalog's object numbers; and the Sky Commander's Dobsonian reset feature means that I don't have to realign the DSC after my hour of equatorial table time is over. This saves four or five alignments in a typical observing session. If these issues are not important to you, then I think you will be happy with either computer. Both will do the job and both have many happy owners.

May 2001. Los Angeles, CA, USA.
All comments apply to Sky Commander V3.38 and NGC-Max V3.52.


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