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PC, Mac, Tablet and Smartphone Control of NexStar Telescopes

It is first important to understand that as the NexStar system evolved, three different control protocols were introduced. 

  1. The first NexStars on the market were the original NexStar 5/8 sold from 1999 to mid 2002.  These scopes had a silver optical tube and no connector ports on the top of the mount.  They also have a control protocol unique to them, so if a software does not state specifically that it is compatible with this mount, it most likely will not communicate. 
  2. Next on the market were the NexStar GTs in 2000.  The original hand control can be identified by a lack of the alignment option named Two Star Align.  The original hand control had a slight change to the control protocol from the NexStar 5/8 and thus introduced the second unique protocol.  There are very few software package compatible with this hand control.
  3. With the advent of the NexStar GPS series in 2001, Celestron introduced what would become their final (up to this time) control protocol.  The NexStar GTs received a hand control update in 2001, identifiable by the addition of the Two Star Align option, which used this same control protocol.  The original NexStar 5/8 were never upgraded but were replaced by the NexStar 5i/8i which also used this new control protocol.  All Celestron computerized mounts introduced since then also use this same basic control protocol although new commands have been added to newer versions of the NexStar control system through the years.

Additional Info

Software/Apps Compatible with Celestron Telescopes
Troubleshooting Serial and USB Connection Problems
Troubleshooting WiFi Connection Problems
Hand Control USB Cable
USB Drivers for NexStar+ and StarSense Hand Controls
Hand Control RS-232 Port Cable
PC Port Cable
Diagram by Larry McNish Depicting NexStar Cable/Port Connections
Article on Full Remote Control of NexStar Telescopes Contributed by Michael Wheeler
CPWI - Software to Replace the Hand Control with a PC
NexRemote - Software to Replace the Hand Control with a PC
Programming for NexStar Controlled Telescopes - Documentation and Examples

Most of the information you find here is the result of my work on NexStar Observer List, a PC control program for all models of NexStar scopes.  You can learn more and download your free copy here.

Software Settings

So, when configuring your external software/app, if it requires the selection of telescope type, look first for your specific model (NexStar SLT, CPC, etc.).  If your model is not available, try NexStar GPS or CPC.  If Celestron is the only option, select that.  Feel free to experiment, but be ready to press any of the arrow buttons on the hand control if the mount seems to be slewing out of control and it looks like the optical tube might strike the mount.

Wired Connections

PC and Mac software generally communicate with the scope via the port on the bottom of the hand control.  Current hand controls (manufactured since April 2016) have a USB port while older hand controls have an RS-232 port (looks like a phone jack).  For more information about the various ports and connectors on your scope, visit the Odds and Ends section of this web site.

Wired Connections for USB Hand Controls
Hand controls manufactured since April 2016 have a mini-USB port on the bottom of the hand control.  The correct cable to connect to this port is described in the link "Hand Control USB Cable" above.  This port is actually a connection to a USB to serial adapter inside the hand control.  As such, once you connect to it, your computer will have a serial port that your astronomy software will interface with.  Windows, Mac and Linux will all require a driver to create this serial port.  See the link "USB Drivers" above.  Once the driver is installed, a serial port, designated by a number, will be present on your computer.  Configure your astronomy software to connect to this port.

At this time, I know of no method of making a wired connection between a table/smartphone and the USB hand control.

Wired Connections for RS-232 Hand Controls
Hand controls manufactured prior to April 2016 have an RS-232 port on the bottom of the hand control.  The port looks like a tiny phone jack.  This connection is also called a 'serial port' connection.

If your PC/Mac does not have a serial port (serial ports are very rare on today's computers) you will need an adapter. The one you will most likely use is a USB to RS-232 serial adapter ($20 to $40). Note this should be USB to RS-232 serial, not USB to TTL serial.  Some computer stores stock USB to RS-232 serial adapters but not all work well with Celestron's scopes.   I recommend you purchase Celestron's own USB to serial adapter (Celestron part number 18775) or an adapter that specifies it uses either a "Prolific" or "FTDI" chipset.  You then connect from the serial port/adapter to the hand control with the "Hand Control RS-232 Port Cable" at the link in the upper right of this page.  Another option is an all-in-one Celestron-specific USB serial adapter and cable combination from Astro-Gadget (see this link) or at Amazon (see this link).  This goes directly from a USB-A port on your PC/Mac to the jack on the bottom of the hand control. 

Connecting any USB adapters/cables will require loading a driver; Windows/MacOS generally does this automatically but you might need to download a driver from the adapter manufacture's website.  Once properly installed, the USB adapter will create a serial port, designated by a number, on your computer.  Configure your astronomy software to connect to this port.

Wired connections to a tablet or smartphone are pretty much limited to Simulation Curriculum's SkyWire and their SkySafari application.  This adapter is only compatible with specific iPhones, iPods and iPads.  See their website for more details.

Before smartphones, many people owned a PDA ('personal digital assistant', Palm Pilot, Pocket PC).  If you have one stuffed in a drawer somewhere and want to try it out, PDAs require a serial "sync" cable or a serial port CF card which is used to connect to the Hand Control RS-232 Port Cable.  When using a CF card, the standard hand control RS-232 port cable is used.  When using a serial "sync" cable, you will need a null modem and gender changer to use the hand control RS-232 port cable or a custom cable.  Details of these cables are explained in the link "Hand Control RS-232 Port Cable" above.

Be sure to check out Larry McNish's diagram linked above for various cable connections to the RS-232 hand control.

Wireless Connections

Wireless control takes two basic flavors - Bluetooth and WiFi.

Bluetooth RS-232 serial adapters connected to the RS-232 port on the bottom of the hand control (the serial port hand controls, not the newer USB hand controls) take the place of the standard control cable and can work with a wide variety of software/apps.  There are several generic adapters on the market for $40-$60 dollars though they are not designed specifically for this purpose and some may simply not work.  But, since Bluetooth is supported by Windows, Mac and Android devices, this is a very attractive option.  (I am unaware of any iOS/iPhone/iPad app that communicates with serial ports, but the WiFi option is available for iOS.)  Essentially, the Bluetooth adapter presents itself to your computer/tablet/smartphone as a serial port.  Thus, any astronomy software or app that can communicate with a serial port (those designed for the standard cable connection) can potentially use this configuration.  If you decide you would like to experiment with Bluetooth adapters, you will want your Hand Control RS-232 Port Cable to be fairly short to allow the adapter to hang from the bottom of the hand control.  Also be sure the adapter has a power source that works well for this purpose, such as an internal battery.

One Bluetooth adapter designed specifically for telescope control is Simulation Curriculum's SkyBT (as of Apr 2020, seems to be unavailable).  SkyBT is an adapter specifically designed for Android which will also work with their software products for the Mac.  This option is tested with their SkySafari software but may work with other software.  It is only compatible with the serial port hand controls (not the newer USB hand controls).

Another Bluetooth adapter with documented support for telescope control is the StarBT from Astro-Gadget.  StarBT is an adapter specifically designed for Android which has been tested to with the SkySafari Plus/Pro and Stellarium apps.  It is only compatible with the serial port hand controls (not the newer USB hand controls).

Another Bluetooth adapter with documented support for telescope control is from U.S. Converters.  It requires an external power source, but is less expensive than SkyBT.  They document Android use here and Windows PC use here.  It is only compatible with the serial port hand controls (not the newer USB hand controls).

The second method of wireless control uses a WiFi connection similar to your home router or the Internet service at your local coffee shop.  WiFi adapters for your scope are only compatible with a limited selection of software/apps.  There are a few possibilities in this arena. 

The first option is Celestron's WiFi solution.  The NexStar Evolution and the Astro Fi scopes have built-in WiFi.  Celestron sells their SkyPortal WiFi Module (part # 93973) which is compatible with many of their other mounts - check their website to see if yours is supported.  The SkyPortal WiFi Module plugs into an Aux jack on the scope and may require an "Aux splitter" (also available from Celestron - part number 93919).    In fact, I generally use the splitter as an extender on most mounts to allow the WiFi module to hang loose rather than providing a hook for things to get caught on. The Evolution, Astro Fi and SkyPortal WiFi Module are only compatible with the SkySafari and SkyPortal apps on Android and iOS.  This is a unique solution as SkyPortal/SkySafari and your smartphone or tablet completely take the place of the physical hand control - you actually perform the alignment and all further operations inside the app itself.  The only other software compatible with Celestron WiFi are NexRemote and CPWI (Windows PC only - see the links in the top, right of the page) and SkySafari 6 for Mac computers.  In all cases, the software/app takes the place of the physical hand control and the alignment is completed in the software/app.

Astro-Gadget offers a clone of Celestron's SkyPortal WiFi Module which they call AuxFi.  It works just like Celestron's offering (and thus the details in the last paragraph regarding compatible software apply) but AuxFi is significantly less expensive.  See Astro-Gadget's website for details on mount compatibility.

The following solutions require the WiFi adapter be connected to the bottom of the hand control and the software/app can control the scope after you complete an alignment with the hand control.

Simulation Curriculum offers SkyFi III.  SkyFi III connects to either the RS-232 port on the bottom of the serial hand controls or the USB port on the bottom of the newer hand controls - check their website for telescope compatibility.  SkyFi III is compatible with SkySafari on Android/iOS and SkySafari and Starry Night on Mac/Windows. 

Astro-Gadget offers their StarFi WiFi adapter.  StarFi is specially designed for Androis and iOS and has been tested with the SkySafari Plus/Pro and Stellarium apps. It can also be used on a PC through a "TCP-COM" bridge application (refer to their website).  It is only compatible with the serial port hand controls (not the newer USB hand controls).

Software Bisque supports connections from their software, TheSkyX, with a device named "WiSnap WiFi to RS232 Adapter".  Documentation on the use of TheSky is here.  The device doesn't seem to be available at their website but can be purchase from the manufacturer - Serial IO.  It is only compatible with the serial port hand controls (not the newer USB hand controls).  Most other Windows software should be able to connect via the WiSnap using AstroGeeks' ( COM2TCP software though configuring the adapter and COM2TCP could be a significant hurdle.

ScopeCom allows WiFi connections from Android, iOS, Mac and Windows devices to any of the serial port hand controls (not the newer USB hand controls).  On Android and iOS, connections may be limited to SkySafari, though other software may work.  On Mac and Windows computers, any software that can communicate via a wired serial connection should work fine.

Putting It All Together

With SkySafari using the "Celestron WiFi" connection method or with SkyPortal, NexRemote or CPWI, you will first perform an alignment in the software/app itself.  For all other software, you must first perform a regular alignment with the physical hand control before attempting to control the scope with the external software/app. 

Start your astronomy software and the computer should be ready to communicate with your telescope. Some programs have a menu item or toolbar button to establish communication while other programs are ready to talk immediately after starting; refer to the documentation for your program if this is not readily apparent. The software/app may require some configuration settings such as setting the type of scope and the communication port the scope is connected to.  A good test for communication is to direct your program to retrieve the telescope's coordinates, or, if it is planetarium software, the crosshairs on the screen should follow the telescope as you slew around. The next test would be to use the program to direct the scope to GoTo an object. If the coordinates match within a few arcminutes, then all is working properly. If the telescope doesn't move at all, check the LCD panel on the hand control for a slew limit warning - the object you requested might be outside your established Slew Limits.

If things don't seem to be working as expected, refer to the 'troubleshooting' articles at the top of the page.


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Michael Swanson
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