Tracking Slow - MOVE WEST!

Thursday, June 6, 2019 1:52 PM

Problem(s) Encountered:

No problems to report, although I did want to mention a few things I told Eric. There is starting to be a big pointing offset such that when I go to a new target, the guide star doesn't always appear in the field of view of the guider. You always need to move the telescope west and then the star will move into the field.
I've also noticed that the unguided tracking is not as good as it was. If you turn off guiding, you can watch a star drift.  To move it back, again you always have to move the telescope west. In both cases, there is a consistent sense of direction (move the telescope west). [Chornock]

I reported this same problem to Eric during my four nights prior to Ryan’s run.  At first I thought the RA track rate was not correct but the problem did not persist the entire night.  But when it did, even under closed loop guiding, I had to push the west guide button just to keep the star in the guide box.  And since the tracking rate was effectively worse at times the pointing became progressive worse.  It was good that we were just doing imaging because spectroscopy and slit alignments might have been very difficult.  Eric tightened the RA friction drive the next day and we did not have any more big issues that I noticed.  We did however have a RA drive coupling break the following night.  As Eric told me there are issues with the RA drive right now. [Wagner]


I haven’t yet convinced myself that this is related to the issues we are having with the RA drive roller walking out (south), but it does appear to be a pretty big coincidence.  I’m wondering if both issues are caused by a failing bearing (would likely be the lower of the two).  I don’t understand why a failure would be occurring as these bearings have been in for less than 2 years, but it might provide an adequate explanation.  That said, nothing we can do until the telescope is off-sky for an extended period.  I plan to disassemble the entire drive system this summer and in the least, inspect the bearings.  Likely we will change them out.I haven’t mentioned the slowed tracking rate to DFM yet but plan to shortly.  There is already talk of installing a 5/8” or 3/4” aircraft bolt & nut into the roller assembly as well as fabricating a new bearing retainer (prices pending).  This may also require a new roller be fabricated in the event that the existing one cannot be drilled.  This summer is shaping up to be a busy one…