How do you Control a Telescope with C2A

C2A enables you to control a telescope through a serial port. Controlling refers to all the following functions:

For more details of the components of the hand controller interface, go to the section dedicated to telescope hand controller.

Native telescope interfaces which are supported by C2A are as follows:

In addition, C2A supports telescopes compatible with the ASCOM standard (see C2A has been validated with a number of ASCOM drivers, including the AstroPhysics ASCOM driver and Losmandy Gemini ASCOM driver.

Initial Configuration of your Telescope

To be able to use the telescope control function in C2A, you must have one of the models of telescope previously mentioned or an ASCOM compatible telescope. The connection is made through a serial port on the computer running C2A. To launch and configure the telescope control interface, go to the Telescope menu and select the command Hand controller…. Then follow these steps:

  • In the hand controller, click the button Options... The telescope options dialogue box is displayed.

  • In the first tab of the options box (Interface), begin by selecting the serial port to which your telescope is connected. Com ports present on the machine are automatically detected by C2A and added to the list.

  • Select the general kind of telescope you wish to use, that is to say a telescope which is natively supported through the serial port, or an ASCOM compatible telescope.


  • For a telescope controlled through the serial port, select the type of telescope connected in the Telescope controller type area. At this stage, 6 types of telescope are supported: native LX200, SkySensor 2000 PC, Celestron NexStar, Gemini Pulsar, AstroPhysics GTO, PicAstro and AudeCom controller.

  • For a telescope controlled by the ASCOM standard, click the Telescope Driver button and choose the exact type of telescope you want to drive. Important: You must already have installed the telescope driver on your machine. To do this, go to, and download the package “ASCOM platform 5” (which is the version available at in May 2011) or a more recent version, and install it on your machine.

  • Having installed the ASCOM driver, you can choose the telescope to drive. C2A displays a list of drivers installed on your machine, and you can set up the selected driver with the Properties... button in the dialogue box.

  • Staying in the ASCOM area, now click the Telescope options button. The allows you to specify the different rates of movement of the two axes of your telescope in accordance with the capabilities of the controller. In this dialog box, click the Update button for a list of valid rates returned by the driver. Note that not all telescopes allow this adjustment (in which case Update returns nothing in the area displaying speeds). If this is the case, do not input speeds as these will not be used. You can also specify in this dialogue box if you want to manage telescope parking and sidereal tracking (note, as before, these two functions may not be supported by all telescopes).

  • If you use an ASCOM focuser, click the check box Use an ASCOM focuser and choose the focuser using the Focuser driver button.

  • Click OK to close the options box.

  • Click the Connect button on the hand controller. If the connection is correctly established with the telescope, you will see that most of the components of the hand controller have become accessible, and the telescope’s current position will be displayed (in right ascension and declination).

  • Select the moving speeds Alpha And Delta on the right hand side of the hand controller. Three speeds are possible: Centring (low speed) Pointing (intermediate speed) and Slewing (high speed). For example, select pointing.  If the Alpha and Delta speeds are related (Same RA and DE speeds option checked) a single area of speed control is accessible, since Alpha and Delta speeds are defined as the same.

  • Press the buttons (Alpha+, Alpha-, Delta+ and Delta-) on the hand controller and watch how this moves your telescope. You can try different speeds. If the movements of the telescope do not correspond to the direction you want, go back to the telescope options dialogue box (the Options… button on the hand controller, go to the Commands tab and choose  Reverse Alpha commands or Reverse Delta commands.


  • The position of the telescope is continuously indicated in the C2A map by a green circle. You can re-centre the map automatically on the telescope position with the command Go to Telescope Position in the Telescope menu. A shortcut to this command is SHIFT+F3. You can also keep the telescope target always in the centre of the current view. To do this, just check the option Continuous target tracking in the Telescope menu. A shortcut to this command is SHIFT+F4.

When you press the arrow button (with the left mouse button) in one of the movement directions, the telescope will continue to move until the button is released. The button just to the left of the arrow lets you latch movement on (that is to say, movement will continue after releasing the left mouse button). Another click on the same button (or a click on the hand controller stop button) stops movement. The figure below shows the two types of buttons:

Automatic Slewing and Calibration of a Telescope

Automatically slewing the telescope towards a given object (the GOTO function) can be achieved in two ways:

Similarly, you can calibrate the telescope in equatorial (or horizontal) coordinates (that is to say, you can tell the telescope the location on the sky to which it is pointing). This can be done in two ways:

The figure below shows the two commands for slewing and calibration accessible through the contextual menu of the map (right-click on object):

The figure below shows the input area on the telescope hand controller along with the slew and calibration buttons:

Notice that you can specify either right ascension or hour angle to be entered in this input area.

Troubleshooting Potential Problems

Here are solutions to some problems you may encounter.

When I click the Connect button on the hand controller I get an error message.

  1. Make sure that the serial port declared in the options box (see above) is really the one connected to the telescope.

  2. Make sure that the telescope is powered up and reliably connected to your PC.

  3. Make sure that your telescope is supported by C2A (see list above).

I get this error message (for telescopes controlled through the serial port):

All requests for position and commands sent to the telescope are managed by a controller in C2A. If position requests are too frequent for the power of the machine running C2A, the controller saturates and causes this error message. To solve the problem, proceed as follows:

  • On the hand controller, click the Options... button. The telescope options dialogue box will be displayed.

  • In the options box, select the System tab.

  • First try changing the Position request value. Instead of 500ms, change to 1000ms, for example (the telescope position will be polled every second in stead of every half second).

  • If this still does not work, change the value of the Action queue (which corresponds to the time cycle of the controller). For example, use 50ms rather than the 100ms default value.

  • If you still get the same error message, try other values until the communication with the telescope works properly. Note that you do not have to re-enter these values every time, because C2A remembers them for the next session.


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