ASCOM is a standard developed for interfacing computers and telescopes. From the ASCOM web site:
The ASCOM Initiative is a loosely-knit group of developers and astronomical instrument makers that work together to bring vendor-independent and language-independent plug-and play compatibility between astronomy software and astronomical instruments on Windows computers. ASCOM stands for the Astronomy Common Object Model.
Developers of astronomical software can use the ASCOM standard to allow their software to talk to any kind of telescope that includes an ASCOM-compliant driver, eliminating the need to develop a separate interface for each popular type of telescope out there. The remainder of this page describes the ASCOM driver I wrote for my digital setting circles project.
This ASCOM driver allows the use of a variety of commercial and homebrew passive encoder-based digital setting circles systems with any Windows software that supports the ASCOM standard. This driver works with either alt-az mounts (like a dobsonian) or equatorial mounts. Equatorial mounts do not need to be accurately polar-aligned (although an accurate polar alignment will likely lead to a more satisfying observing experience).
This driver will not work with equatorial platforms.
(Not all of these systems have been tested, but all should work with this driver.)
- Celestron Advanced AstroMaster
- Dave Ek’s Digital Setting Circles
- David Chandler Co. Deep Space Navigator
- Discovery Digital Setting Circles
- JMI NGC-MAX
- Lumicon Sky Vector
- Mountain Instruments Star Pilot
- Nova Astro Micro-Guider III
- Nova Astro Micro-Guider 5
- Software Bisque BBox
- Tele Vue Sky Tour
Setup and Configuration
1) If you have not already done so, run the installer for this driver to install it on your computer.
2) Consult the documentation for your preferred astronomy software package (e.g. TheSky, Cartes du Ciel, Hallo Northern Sky) for instructions on accessing the ASCOM Chooser dialog box. The ASCOM chooser looks like this:
3) Select “Tangent-Compatible Digital Setting Circles” from the drop-down list, and then click the Properties button. The Setup dialog box will appear:
4) In the Settings dialog box, enter the following information:
Name: a name for your telescope (optional)
Description: a description of your telescope (optional)
Mount: the type of telescope mount you’re using (alt-az, German equatorial, or other equatorial) (optional)
Focal length: the focal length in meters (optional)
Aperture Dia: the diameter of your telescope’s aperture in meters (optional)
Aperture Area: the unobstructed area of your telescope’s aperture in square meters (optional)
Interface type: the type of encoder interface between your computer and your telescope (required)
RA/Azimuth Ticks/Rev: the number of encoder tics (in quadrature mode) traversed as your telescope makes one complete turn on its RA/azimuth axis (required)
DEC/Altitude Ticks/Rev: the number of encoder tics (in quadrature mode) traversed as your telescope makes one complete turn on its DEC/altitude axis (required)
Serial Port: the serial port to which your encoder interface is connected (required). The Serial Port field can be edited if necessary. Some bluetooth drivers don’t write their port names into the registry correctly, which can result in the names appearing incorrectly in the list of ports. If this happens to you, manually edit the name so that it is correct.
Reverse Encoder Counts?:
Altitude: checking this box causes this software driver to reverse the rotational sense of the encoder counts returned for the DEC/altitude encoder. This box should normally be left unchecked. Check this box only if you cannot obtain an accurate alignment any other way.
Azimuth: checking this box causes this software driver to reverse the rotational sense of the encoder counts returned for the RA/azimuth encoder.
Test button: clicking the Test button will guide you through a test of your encoder system to determine whether the Azimuth checkbox needs to be checked. The test is not foolproof, so if you cannot obtain an accurate alignment using the recommended settings, you’ll need to experiment to find the correct settings.
Latitude: the latitude of your observing location, as degrees, minutes, and seconds. Optional, but required for accurate calculation of azimuth, altitude, and sidereal time.
Longitude: the longitude of your observing location, as degrees, minutes, and seconds. Optional, but required for accurate calculation of azimuth, altitude, and sidereal time.
Elevation: observing site elevation above sea level, in meters.
Click the OK button when all settings have been entered. The settings will be saved for subsequent sessions.
Alignment and Use
Connecting to your telescope will result in the driver guiding you through the alignment process:
1) First, you’ll be asked to move your telescope so that its declination (for an equatorial telescope) or altitude (for an alt-az telescope) is approximately zero:
Note that extreme accuracy is not required. After pointing the telescope as instructed, click the Continue button.
2) Next, you’ll be asked to move your telescope so that its declination (for an equatorial telescope) or altitude (for an alt-az telescope) is approximately ninety degrees:
Again, extreme accuracy is not required. After pointing the telescope as instructed, click the Continue button.
3) Next you’ll choose two stars to use for the alignment process. First, you’ll be presented with the following window:
First, select a constellation from the drop-down list. Then click on one of the five-pointed alignment stars to select that star for alignment. Next, point your telescope at that star, centering it in the field of view. Then click the Continue button.
4) Repeat the process in the last step to select your second alignment star:
After you’ve selected your second alignment star and pointed your telescope at it, click the OK button to complete the alignment. At this point, your telescope should be aligned to the night sky, and your astronomy software should reflect the coordinates at which your telescope is pointing as you traverse the sky.
Although slewing a telescope usually implies that a motor is moving the telescope, this driver implements slewing functionality by simply providing visual feedback as you manually push your telescope tube toward the desired target. Typically, you would initiate a slew in your astronomy software by selecting an item in the display and then choosing the Slew command. After you do so, you’ll see the following window appear:
The numbers in the window will change as your move your telescope around in the sky. To slew to your intended target, push the telescope around until the numbers each go to zero. Then click the Close button. You should now be pointed at your desired target.
Sync’ing your telescope is the act of telling the driver that your telescope is actually pointed at a certain object. Typically, you’d do so in your astronomy software by selecting an item in the display and then choosing the Sync command. After you do so, you may observe that the telescope position indicator in the display shifts slightly.
The Sync command is useful for making minor corrections to the driver’s alignment data. No telescope alignment will be perfect across the entire sky, and you can use the Sync command to reduce the errors in parts of the sky where the initial alignment is not so good.
If the driver indicates that the telescope interface is not present, make sure that the correct serial port was specified, and that the interface is powered up and ready to go.
If the alignment is poor, first double-check the encoder resolutions that were entered. Also, for most hardware, make sure that the hardware was initialized with the correct resolutions. After that, you may need to vary the settings of the Reverse Encoder Counts checkboxes, experimenting with different combinations until you achieve a good alignment. If all else fails, leave a comment below describing your question or problem.