Guide to the new MDM Hiltner Rotator

A new rotator motor and associated electronics were installed in April and May 2011. The upgrade was necessary to enable the precise (0.01 deg) rotator offsets from the control room that are required for efficient slit mask alignment for OSMOS. A new hand paddle was also installed. Click here for commissioning photos and further details. The rotator may now be operated from the dome or via a GUI from the control room.

Safety Rules for Operation

Please follow these two rules when operating the rotator:

  1. Watch the cables during all moves greater than a degree
  2. Only move the rotator when the mirror covers are open
The cables WILL get caught on various protrusions when the telescope is at certain hour angles and declinations. Please keep in mind that a safe rotation angle at one telescope orientation may not be safe at another. The mirror covers must be open to prevent rubbing between the covers and telescope baffle, which can deposit metal particles on the mirror surface. There are switches that define the maximum and minimum safe rotator angle, as well as prevent operation when the mirror covers are closed.

Dome Operation

The new motor may be operated by a new hand paddle. The hand paddle is attached to the telescope fork as shown in this picture:

The paddle has two buttons that are labeled for clockwise (CW) and counterclockwise (CCW) rotation. These directions are with respect to an observer looking up at the mirror cell. There are a pair of limit switches that confine the rotator to angles between -85.7 degrees and +96.3 degrees and are intended to help protect the various cables that extend between the rotating and non-rotating parts of the telescope. If you encounter one of these limits, motion in that direction will stop, although the paddle will still function in the opposite direction. These rotation limits correspond to the APPROXIMATE safety limits for the cables. Observers should nevertheless watch ANY large moves from the dome floor. The rotator angle may be read from the TCS monitor. There is one on the dome floor (normally off during observations) in addition to the one in the control room.

Control Room Operation

The rotator may also be operated with a GUI called rotator.tcl. The GUI may be opened via the Telescope Control menu (TBD). The GUI only allows relative offsets from the current position, and not absolute offsets. The size of these offsets are limited to a maximum of +/-1 degree. Larger offsets should be initiated from the dome floor while watching the cables (see the warning above). The GUI will report an error if larger moves are requested.

The rotator GUI accepts offsets in degs, arcmin, and arcsec. Most users will use the default units of degs, even for multi-slit alignments with OSMOS (which require on order 0.01 degree relative precision). Enter the value of the offset in the input box and press the SEND button to initiate the offset. To change the units, select the current units and type the new one.


The rotator is not responding to the GUI or hand paddle
Check that the electronics box is powered on and that the mirror covers are open.

Moves are much smaller than expected
Check that the units are correct on the GUI.

The GUI implies a move has occurred (SEND button turns yellow briefly), but the rotator does not actually move (do not hear the brake, for example)
This is an infrequent fault of still unknown origin. The remedy is to press QUIT on the GUI, turn the power off on the electronics box on the mirror cell (see the image below), wait 15 seconds, turn the electronics box back on, wait 10 more seconds, and finally restart the GUI. Repeat if necessary.
Here is a picture of the rotator electronics box. Note that this is an old picture that shows the hand paddle next to the box, whereas it is now located on the telescope fork as shown above.


The MDM rotator upgrade was generously funded by the NSF grant to build the OSMOS instrument and by the Ohio State Department of Astronomy.

Return to: [ MDM Observatory | MDM at OSU | OSMOS User's Manual ]

Updated: 2011 Dec 8 [pm]