Focus motion issues

Monday, January 3, 2022 7:57 AM

Problem(s) Encountered:

Telescope focus crept again. Noticed it around 3:45 when the telescope was near zenith. It had increased 8 units. Couldn’t move it by any means. It quickly (within a couple minutes)  corrected itself, but then just as quickly started to creep again. Went just about as far before correcting itself once again after about 15 minutes. At this point I was at HA~-2.5, dec~56. Slowly started creeping up again shortly after that. Unlike last time, moving it to near zenith did not  reset  it. In fact, it got 10 units worse when I tried that. Tonight it didn't end up hindering observations, but it's definitely still an issue.


The focus encoder (Renishaw tape and read head) is extremely sensitive.  More so than I would expect.  The encoder head has a status light to indicate how well it is able to read the tape—blue, green, orange, red with blue being optimal.  I tried adjusting the mount for the Renishaw tape but could not get the light to go beyond orange.  Indeed it hardly ever left red.  I did add longer mounting bolts with washers and lock washers, which may shore up the system some.  I then pulled the read head and verified that it does at least work.  It does.  Visual inspection of both the head and the tape shows no obvious issues.  Next was to remount the head and try adjusting the mounting plate in an effort to optimize the position and angle orientation between head and tape.  This appeared to have the most success.  I was able to get the light to go blue, and remain so as I tightened things back up.  Next, I drove the telescope from far-east to far-west and checked again.  The light had gone from blue to green.  Back to far-east and the light returned to blue.  This was as good as I could get it.  I then adjusted to nominal focus (which was dramatically different from the position where I was working).  Unfortunately, after this extended travel, the light went from blue to green.  And nothing I did would change it back to blue.  I buttoned things up and moved around, verifying that I can at least adjust focus at demand.  It appears to be working.  
That said, it seems odd to me how sensitive this alignment is.  I cannot prove it, but it seems like motions <1mm cause the head to go out of alignment.  It just doesn’t seem like this is by design.  Until the very end, simple actions like lightly touching the rail with the tape would cause the light to go orange or red.  Thankfully by the end, I was able to get the system into a state where I could tap on the components and the light would remain green.  So that’s good at least.  Still, I have written to DFM to ask their impression as I do not have that much experience with Renishaw tape.  But with how sensitive it is, I am left wondering if there is something wrong with the head, the tape, or both.  

UPDATE:  I verified with the vendor that indeed the encoder appears to be working as designed.  I spent more time aligning and was able to get the indicator light to stay blue.  It’s been many days since doing that work and no further issues.  I suspect that alignment will be a task to be performed from time to time.