Synchronization Setup

The ASCOM Dome Control Panel can be used to synchronize the dome to your telescope; alternatively you may wish to use the POTH Scope and Dome Hub, which is also included in the ASCOM Platform. Any planetarium or other control software capable of interfacing with ASCOM can be used in conjunction with the ASCOM Dome Control Panel.

Programs such as MaxIm DL and ACP have built-in dome control capabilities. The setup for the dome geometry is very similar, but the details of the user interface will be different.

Open the ASCOM Device Hub, as described under Dome Setup. Click the Setup button, and click Select Scope.

Select the desired telescope, and then click Properties... to configure it. When you are done click OK.

You will need some measurements, which are all based on the intersection point between the RA and Dec axis of your telescope. The software uses this to calculate where on the dome surface the telescope is pointing, so it can line up the shutter with the telescope. The following information is needed:

Scope Position E/W - Distance from the telescope axis intersection to the center of the dome, in mm.
Scope Position N/S - Distance from the telescope axis intersection to the center of the dome, in mm.
Scope Position U/D - Distance from the telescope axis intersection to the equator of the dome, in mm. Note: The equator can be above or below the physical bottom of the dome.
Dome Radius - Radius of the dome in mm.
GEM Axis Offset - Distance from the axis intersection to the optical axis of the telescope. Is usually zero for a fork mount, but can be a significant distance for a German Equatorial mount.

Device Hub will get your latitude and longitude information from the telescope. Please make sure this information is set up correctly (if not automatic).

Getting the mount/dome parameters correct is critical for proper synchronization. It can be very difficult to tell which number is wrong if an error is made. If you find the telescope does not reliably align with the dome when Slaved to Telescope is checked in the main Dome tab, double-check everything, including the following:

Make sure you have the latest update to the ASCOM Platform.
Make sure the dome is not rotating backwards. If you click Home it should turn to the right as seen from inside the dome. If it is rotating backwards, reverse the wires to the motor.
Make sure the Tics Per Revolution in the Setup Dome dialog is set correctly.
Note that if the pulses from the encoder come too quickly, that could result in loss of tics; the recommended rate is one transition per second. If the optical sensor does not have the correct spacing, or it moves around during dome rotation, it could produce corrupted tics.
Use the GOTO button to command the dome to slew to 0 degrees, 90 degrees, 180 degrees, and then 270 degrees. Make sure the dome points due North, East, South, and West respectively. If the dome does not move exactly 90 degrees in each step, then your Tics Per Revolution is wrong. If the dome moves an appropriate distance but doesn't point the right way, your Home Azimuth is wrong. If the positioning is inconsistent check the alignment and spacing of the optical sensor.
Make sure you entered the radius of the dome, not the diameter. If you have the radius wrong, the telescope will not point accurately, especially when it is looking through the slit at an angle.
An error in the height of the mount pivot point will have a similar effect as an error in the dome radius. Remember, the pivot point is where the RA and Dec axes intersect.
If you have a systematic offset in a particular part of the sky, check the pier centering.
If the pointing is inaccurate when your telescope is way over to one side or the other of the GE mount, check the distance from the mount pivot point to the optical axis.

Once the telescope is synchronized, the dome will follow the telescope, whether it is slewing or sidereal tracking. The telescope position is read periodically (every 10 seconds for ASCOM Dome Control Panel), so the dome may move in increments if you move the telescope using the hand paddle, or if you are using control software that does not connect through the control panel.

To configure a telescope control program for dome operations, set it to connect to the ASCOM Dome Control Panel, instead of directly to the telescope. Now when a command is sent from the software to the telescope, the Control Panel will intercept the command before it is passed to the telescope. It will simultaneously command the dome to turn to the correction position. It will also report back that the telescope is slewing while either the dome or telescope are still moving. That way, if the dome takes longer to slew than the telescope, you will not get a photo of the inside of the dome!

Please see Normal Operation for instructions on using dome synchronization.