The Celestron RS-232 NexStar Serial Cable is one of those peculiar products which make me ask myself, "Isn't there some way I can do without this?" RS-232 is long in the tooth- to the point where it is starting to disappear from computers altogether in favor of the vastly superior USB. Yet, you have to get to this interface to attach one of Celestron's sturdy NexStar series telescopes to a PC. I have general information on telescopes and choosing one in my article on
Picking a Telescope.
Background Celestron has been making the NexStar series of computer guided telescopes for ten years, now. Variants include compact integrated fork mounts as well as larger German equatorial mounts. Some of the specific systems this cable is fielded to support include:
Small mobile single arm Nexstar series scopes such as the
NexStar 80GT and
NexStar 102 SLT.
Larger single arm fork mounts such as the
NexStar 5i,
NexStar 6SE, and
NexStar 8SE.
Full sized fork mounted telescopes such as the
NexStar 8 GPS,
NexStar 11 GPS,
NexStar CPC 800, and
NexStar CPC 1100.
Two series of telescopes not explicitly referred to as NexStar telescopes also use this cable, including the Advanced GT computerized CG-5 German equatorials and Celestron's top of the line CGE German equatorial mounts.
All of these have two features in common:
First, they use a computer hand controller to control the mount, which is the one recognizable piece across the NexStar series. This is the unit which controls the telescope as an observation system.
Second, they have dedicated computer controllers for the drives, so the assembled mounts is actually a network of three systems: The hand controller, and a controller each for the azimuth and altitude axes (I explain why this is important, later).
The NexStar hand controller has a phone-jack style plug in the bottom next to where the cord comes out, which is where this cable attaches. And, its reason for being is to make it possible to upgrade the onboard firmware for the latest handset, and by extension, the motor controllers. Alternatively, it can be used to direct the mount from a computer, for example, from a planetarium program like Starry Night or Equinox.
Description and Usage The NexStar serial cable has an old school standard RS-232 plug on one end and a 4 conductor phone plug on the other. The reason for the use of the RJ11 phone style plugs which appear commonly in telescopes (largely because they started networking components before computers were common). While this holdover is familiar to people who were observing in the 1970s, today it is quirky and antiquated, where many computers, including the HP I have used with this cable, no longer support RS-232, and RJ11 was never used for serial connections.
Therefore, one other $35 part was needed to attach to a NexStar computer using this cable: A USB to serial cable adapter. Since the main reason I got the cable was for updating the software on a NexStar SE mount, I will describe that use to give a better idea of its function. The serial adapter requires its own driver, so you need to find the driver and load it for your cable if you are going to use it. The network of separate computers in a NexStar system has the unusual feature where all three can have their motor controllers running on different firmware, independently of each other. In general, Celestron has only updated these occasionally, about every two years or so. The upgrades have typically been to introduce fixes to specific problems like drive noise, or having the mount incorrectly perform cord wrap, where it should keep track of where it is in rotation to prevent cords for power or camera from getting tangled by, for example, always slewing to the right.
The Celestron web site has the programs for updating the firmware on the different components. The cable plugs in to a phone jack hidden in the bottom end of the hand controller, where it is just possible to hang up the handset of the mount with the cable hanging out. In the case of the NexStar SE, I needed to update the hand controller as well as both motor controllers. However, only the latest version of the hand controller software, 4.13, allows the motors to be updated this way. Otherwise, a separate 6 conductor cable is needed to update the controllers via a different connector on the outside of the telescope (yes, this sounds wacko to me, too).
The Celestron software acts a bit like a wizard program, though it isn't a very smart one considering it asks you what port to use. So, it is off to Windows and the properties menu for My Computer to determine what port has what connected to it. In this case, it was lucky I had been required to get the USB serial adapter with its own driver, since it gave feedback for port 7 being in use with that part. At this point, the computer tells you to turn on the telescope and put it in update mode by a combination button press. It tells you what version of software is there, and gives you a directory to find something which sounds similar to replace it with. After downloading the new instruction set, the system slowly updates it to the hand controller, which flashes the specific memory locations being used as it goes over the next 15 minutes. The process is nearly identical for the motor controllers, with the difference of having two of them to update with the same program. Interestingly, the Celestron software, which seems nearly free of convenient automatic features to make it friendly to non gear-heads will refuse to allow you to install an incompatible set of hand controller firmware and motor control firmware, such as a CGE series hand control with NexStar SE motor controllers.
After completion, you can put the cable away for the next two years, unless you plan to drag a computer outside to give the hand controller commands via a computer planetarium program. I personally am not sure how useful that is unless you want to view temporary objects in the sky, such a planets or asteroids, where you can get an update on them online the computer built in to the telescope would not have. Since I personally have found the amount of equipment to be taken out is an exponential barrier to enjoying astronomy, adding a computer to haul out has been a low priority.
Conclusion The NexStar RS-232 serial cable
does make it possible to attach a NexStar driven telescope to a computer. With that said, the obsolescent interface means this cable must be used to make that connection, since more modern standard cables like USB are not supported. This lack of modern commonality makes this unique piece of cabling yet another single-purpose piece of proprietary equipment to have squirreled away in a drawer somewhere until you need it.
