Wired Connections Between Scope and Laptop

In Part 1 of this series, I discussed some of the reasons why you might want to use your laptop or tablet to control your NexStar, and went over some of the options for how you can accomplish that. In this part of the series we’ll get a laptop set up to control your NexStar using the Stellarium software package through a wired connection between the scope and the laptop. With Stellarium, you’ll be able to point and click to easily slew your scope to any object you’d like. Stellarium also is capable of showing a ton of information about that object. It also provides a very pleasing and easy-to-use interface. We’ll go through the process from start to finish. Even if you decide to use a different connection type (WiFi or Bluetooth), the instructions for Stellarium will remain pretty-much the same.

The fundamental requirement for controlling your NexStar with a computer of some sort is, of course, establishing communications between the two. As far as I know, a wired connection can only be done between a computer with a serial port or a USB port and the scope–tablets are not supported for wired connections (but let me know in the comments below if you know of a way).

USB or Serial?

Depending on the age of your NexStar, the bottom of your hand controller either has a port that resembles a phone plugin, or it has a mini USB port. If you have the hand controller with the USB port, all you need to do is plug the right cable into that USB port and then the other end of the cable into a USB port on your laptop.

My hand controller is the older serial type. The serial cable plugs into the port shown here.

On the other hand, if your hand controller is older, it probably has the serial port (that looks like a phone plugin). Your scope should have come with a cable that has what looks like a phone plug on one end and a 9-pin serial connector on the other. Since practically no modern laptops have serial ports built-in these days, you’ll need a USB-to-serial converter cable.

The NexStar serial cable.

It used to be that USB-to-serial converters were kind of hit-or-miss in quality, but that doesn’t seem to be as much of an issue these days. Still, if you need to buy one, I’d steer away from cheapies on Ebay and spend a few extra bucks to get something from a reputable vendor. Amazon tells me that I purchased this one back in 2012, and it’s worked well for me ever since. What I like about the converter I bought is that it has ten feet of cable, making it much handier. I think a good piece of advice is to look for one that uses the FTDI chipset, which will be plug-and-play on Windows 7 and 10.

My USB-to-serial converter cable.

So, if you’re using the serial cable, plug one end into the hand controller, and plug the 9-pin end of the USB-to-serial converter into the other end of the serial cable. Then plug the USB end of the converter cable into your laptop.

Figuring Out the Serial Port

Once the connection is made (and the scope is turned on if you have the USB version of the handpad), the laptop should detect the NexStar as an additional serial port. All you need to do is to identify the name of the new port. To do so, open the Device Manager on your laptop (find it in the Control Panel on Windows 7, or by right-clicking the start button in Windows 10).

The Device Manager display. Look for the “Ports (COM & LPT)” node.

In the tree view of devices that is shown in the Device Manager, find the “Ports (COM & LPT)” node and expand it. You should see at least one port listed there. It may be listed as something like “USB Serial Port (COM3).” What you need to know is the part in parentheses–that’s the port name that you’ll need to specify to Stellarium in a later step. If multiple ports are listed and you’re not sure which one you need, unplug the USB cable and then plug it back in. The port you need will disappear and then reappear in the list.

In this scenario, we’re going to use Stellarium on the laptop to control the telescope, so go ahead and download and install that if you haven’t already. Stellarium has built-in support for Celestron telescopes, so there’s no need for any add-ons or additional software. There are several other options out there, as well, but you’ll need to explore those for yourself if Stellarium isn’t your preferred solution.

Aligning the Telescope

One thing about Stellarium (and other laptop planetarium software, as well) is that it expects your telescope to be already aligned before connecting to it. This means you need to use the hand controller to run through the alignment process before allowing Stellarium to connect to your scope. I usually use the built-in SkyAlign method, but there are other methods as well. One method that’s handy for use while you’re still indoors testing things out is the one-star alignment process. While SkyAlign and most of the other alignment methods will fail if you’re not really accurately pointing at celestial bodies (difficult to do when you’re inside), the one-star alignment works just fine without much accuracy.

If you’re indoors and want to use the one-star alignment process for testing purposes, select that option from the main menu on the hand controller, and then use Polaris as your one alignment star. You can probably slew your scope to roughly the location of Polaris even from indoors (and that’s all the accuracy that’s needed), so it works well for testing purposes.

Configuring Stellarium

Once your NexStar is aligned, it’s time to connect Stellarium to the scope. The first thing that needs to be done is to activate and configure the built-in Telescope Control plugin. So start up Stellarium, and then move your mouse pointer toward the bottom left corner of the Stellarium window (or the screen if Stellarium is in full-screen mode). You’ll see some toolbars appear on the left and bottom parts of the window.

Click the wrench to bring up the configuration window.

In the toolbar along the left side, look for the wrench symbol and click on it to bring up the configuration window (F2 works, too).

The Stellarium Configuration window.

Along the top of the configuration window, click on Plugins.

The Plugins window. “Telescope Control” is near the bottom of the list.

Scroll through the list of plugins that appears and click on the one that says Telescope Control. The first thing to do is to make sure that the checkbox labeled “Load at startup” is checked. Then click the Configure button.

The Telescopes window will list any telescopes you’ve configured in Stellarium.

Click the Add button to add your scope to the list. Up will pop a window allowing you to specify the details for your particular telescope.

The window for adding a telescope to Stellarium.

In the top half of the window, check the button that says “Stellarium, directly through a serial port. In the bottom half, give your telescope a descriptive name in the Name field. Then scroll down to see the remaining telescope settings.

Set your serial port and telescope model in the Add a New Telescope window.

Set the Serial port to be the same as the port you determined when you first connected your scope to your laptop, and then choose “Celestron NexStar (compatible)” from the list of Device models. Once you’ve done that, click the OK button.

Your telescope had been added to the list.

Connecting and Controlling the Telescope

Now you should see your telescope in the list. Its status is Stopped, however. That means Stellarium hasn’t connected to the scope yet. So click the Start button to connect.

Stellarium has now connected to your telescope.

If you’ve set things up correctly (and your telescope is turned on), the status of your scope should now change to Connected. Note the number of your telescope (if it’s the only telescope listed, it’ll be 1), because you’ll need to know that when you start to command the telescope from Stellarium. (Note the instructions at the bottom of the Telescope window).

Now close the Telescopes window and the Configuration window to get back to Stellarium’s main display. Click and drag with your mouse to slew the Stellarium display to the part of the sky that contains the last star you aligned on (Polaris, if you used the one-star alignment technique I described above). You should see a circular marker around that star, which indicates where Stellarium thinks the telescope is pointing.

The pointer indicates where the telescope is pointing.

Now click on a nearby star, and then press Ctrl-1 (or whatever your scope number is). Your telescope should now slew to that star. That’s it–you’re all set!

Stellarium Commands

Stellarium has a very limited number of commands for your telescope. The Ctrl-# command slews to the currently-selected object in the Stellarium window. The Alt-# command slews your telescope to the center of the current view in Stellarium. The Ctrl-0 (zero) command allows you to manually enter coordinates to which you want to slew your telescope.

I’m by no means an expert with Stellarium yet–it’s filled with options and capabilities to explore. I’d encourage you to explore those capabilites for yourself. If I stumble across anything I think is particularly interesting or useful, I’ll be sure to mention it in a future post.

In Part 3 of this series of posts, I’ll go over how to use WiFi to connect you telescope to your laptop. Stay tuned!

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