Optec Alt-Azimuth Derotation Server

What is De-Rotation and Why is it Needed?

While alt-azimuth mounted telescopes are wonderful for visual observing, they present a unique challenge for astrophotographers. In order to capture images longer than several hundred seconds the camera must be de-rotated during the exposure to compensate for the rotation of the image plane. This rotation rate is not constant over time but can be calculated based on the right-ascension and declination of the object, the longitude and latitude of the observer, and local sidereal time. With the use an alt-azimuth mount, an Optec camera field rotator, and the Optec Alt-Azimuth Derotation Server, anyone from the amateur level to the professional astrophotographer can easily capture high quality images using an alt-azimuth mounted telescope.

The graphic to the right represents the structure of Optec Alt-Azimuth Derotation Server and how it interacts with the ASCOM client application and physical devices. The Optec Alt-Azimuth requires an ASCOM client application. The client app connects up to the rotator shim which in turn connects up to the actual rotator driver. We call it a shim because it does not actually control the physical rotator as a normal driver would. The rotator shim has two functions: it passes commands and data between the client app and the actual rotator driver, and it makes de-rotation move requests to the actual rotator driver. The telescope shim functions in a similar way as the rotator shim. The client app connects up to the telescope shim, which in turn connects to the actual telescope driver; any ASCOM telescope driver can be used. The most important part of the Optec Alt-Azimuth Derotation Server is the arrow that points from the telescope shim to the rotator shim. This arrow represents the passing of the current altitude and azimuth data from the telescope to the rotator shim. The rotator shim uses this data when performing the de-rotation rate calculations.

The Alt-Az Server Status window will always be open when the server is running and can be used to monitor the progress of derotation. The windows displays the drivers currently selected, the current Altitude and Azimuth of the telescope, Position Angle of the rotator and current derotation rate. The Status window also allows one configuration value called “Steps per Adjustment”. In some cases the communication latency between the PC and the connected devices is too great for the derotation server to run at it’s optimal speed. If this is the case you will see the Missed Steps number increase. If this occurs regularly, increase the Steps per Adjustment property by one. This means that the derotation server will wait longer to make a derotation move but that the move will be larger. The net rotation over time will be the same but with less frequent adjustments. If you change the Steps Per Adjustment property value you must press the Store Value button for the change to take effect.

Alt-Az Derotation Server Software Installer:

OptecAltAzServerSetup(2.0.0) .zip Windows 32-bit  (release date:  2017/11/07) Installer program for Optec AltAz Derotation Server and Control application.  ASCOM Field De-rotation local server application for use of the Pyxis rotators in Alt-Azimuth telescope configuration.  Provides de-rotation capability within range of 10-degrees to 89-degrees altitude. AltAz Server requires ASCOM Platform 6 Microsoft .NET Framework 3.5. Pyxis LE Firmware v1.03 or above required. Pyxis 2" Firmware v2.03 or  above required. Pyxis 3" firmware v.3.41 or above required. Software copyright 2017 - Optec, Inc.

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Optec AltAz Derotation Server