Dome Rotations and Erroneous TCS Messages

Monday, November 23, 2015 10:24 AM

Problem(s) Encountered:

At the end of the night the dome encoders got lost and the dome started rotating and getting stuck, occulting the telescope (possibly similar to the  fugue state  mentioned in prior trouble reports).

The TCS stated (and continues to)  Dome occulting telescope and  Attempting to move with drives off  and  RA or DEC encoders not responding  (earlier it just said  Dec ). We manually moved the dome to home at the end of the night and put the telescope at zenith, but both may still be somewhat lost.

We tried to follow solutions from the archive, but nothing seemed to work. Since daylight was breaking, we didn''t investigate pointing, but the telescope was able to basically find zenith.


First thing I did this morning was to reset the telescope to zenith via the tilt sensors.  Declination was good, but HA read -0 1 1ish.  

Reported dome position was incorrect when we arrived today.  Dome was at the home position physically, but Dome Azimuth read something around 20.  Reset the Dome Azimuth encoder to 320 via the Setup menu on the xtcs GUI.  Moved the telescope around with the dome slaved to it.  Errors in dome position were almost immediate.  Values for the position would increment in both directions, unlike yesterday, but the slit was misaligned from the telescope.  Further telescope motions were accompanied by the dome turning in the wrong direction.  Never seen that before.  Stowed the telescope and performed a “Dome Home”.  Dome encoder went to 320 but dome was significantly off.  Moved dome manually back to the physical home position and reset the encoder once again.  Before switching the yellow “Auto Enable” switch back on, I power-cycled the dome controller box (red switch above the Auto Enable switch).  With power back on, I manually moved the dome 360º  in both directions.  Encoder agreed with physical position throughout.  With “Auto Enable” back on, more telescope/dome tests were performed.  The dome followed the telescope perfectly.  The dome also stowed correctly when switched from “Dome Free” to “Dome Home”.  Telescope/dome will be exercised more this afternoon, but right now, there just aren’t issues to fix.  

The “Dome occulting telescope" message is completely normal when the telescope is pointed to zenith.  This is not a problem and is detailed in the Telescope Manual, Section 5.6.4 (

The “Attempting to move with drives off” message typically occurs when the telescope is still trying to go to a position demanded via the xtcs GUI and the drives are powered down.  This can happen if the observer shuts things off before the move is completely executed.  One way to know a move is complete is to check the “Next Object” entry for Hour Angle.  If there are numbers, the telescope is still moving.  If instead, the display appears as * * * then the command is likely complete.  Aother way to know is that in the display on the lower-right, it will say “Move Complete”.  Hitting the “Stop” button in the xtcs GUI will also complete the demanded movement of the telescope.  An interesting aside that I noticed was the RA entry box on the xtcs GUI was tinted red this morning upon arrival, unlike the normal cream color.  Upon issuing my first demand to move the telescope, it went from red back to cream.

Ra & Dec encoders not responding is a red herring.  It is a known bug with the telescope, dating back to the olden days.  See:

It was also reported in the Run Report that "In addition, slightly after this problem started, the Dec encoder stopped responding and thus we basically lost tracking (stars appeared as streaks).”  I’m unclear what this means.  Did the encoder really stop updating even though the telescope was moving in declination?  Was it reporting inaccurately?  By “tracking”, do you really mean “guiding”?  As stated previously, everything with the telescope has been tested this morning and at no time have I seen any issue with the declination (or telescope at all for that matter).  The guider software also looks to be set up and running correctly.  More tests after lunch, but if nothing develops, there should be no addendum to this report.