Problem(s) Encountered:

 The problem with E/W tracking, reported earlier, returned about01:30 local time.

As a reminder, the telescope jumps 2-5 arcseconds to the west at intervals that can be as short as 12-15 seconds.  The guider cannot compensate, so the images are all elongated in the E/W direction and our slitless spectograms are not acceptable.

Once it reappeared this behavior manifested itself for stars in several positions on the sky.

Solution:

When this problem first occurred, my fix attempt was to cycle the MDC power and restart everything.  This appeared to work but I was worried in the back of my mind that the issue actually was with the RA motor encoder.  With the problem recurring last night, I swapped out the motor encoder today.  Testing now and all appears well.  This is a bit worrisome as I last replaced this encoder in January of this year.I’ve written to the vendor to enquire about the failed encoder and to express interest in purchasing a couple more spares as I am down to one unit left on the shelf.