Dome Slewing performance has become progressively worse during recent sessions where hold-ups would occur during dome slews where the rotation drive wheels would spin at certain azimuth positions for 30-180s until they finally grip and rotation continues again.  In the last session (S1122, 2023-07-15) every slew during session suffered a significant hold-up.  The total of 22.7 minutes were lost during the session because of this problem , which is particularly significant given the very short observing period (short summer night).  Associated with the same issue Dome Parking failed when the operation was aborted by AstroMain program after the slew to the Park Position  (Az 90 deg) took more than 180s.    Other consequence of the slew hold-up is that the dome can gradually lose its azimuth accuracy, such that by end of session the dome azimuth can be out by 2- 5 degrees when Parked, meaning that the two halves of the Induction Charger aren't aligned and the Shutter Battery won't recharge following the session, unless an intervention is made to correct park the dome and resync the dome azimuth to the Park Azimuth (090 deg).

The left graph in charts below show the bad dome slewing performance from sessions S1122 & S1121 where majority of slews suffered from significant hold-ups.  This can be compared with the good slewing performance from the S1101 session (2023-02-22) where none of the 32 dome slews suffered from any significant hold-up.

The Observatory Dome has a tendancy to hold-up as the dome wall is not perfectly circular but is very slightly elliptical (this is due to a vital check being missed during erection of the observatory which is difficult to rectify now as the dome is securely bolted to its concrete base). This slight ellipticity causes the dome roof flange to be distorted as it passes over the wheels in the roof wall and can cause some 'semi-permanent distortion of the certain parts of the roof flange because of the long periods of time that the dome sits at its park position. The distortion leads to varying amount of tension between the drive wheels and the roof flange, and when the tension/friction is not great enough the wheels can spin on the same spot for up to 180-240s. There is sometimes a correspondance between the problem nights and nights with higher dew levels,  and tricks of wiping the roof flange from traces of grease, water & dirt can be sometimes be used to improve dome slewing performance. Taking out some of the distortion in the roof flange can be relieved by prior manual manipulation of the flange.   Attempting to tighten the drive tension knob generally fails to improve the situation, as the knob is pretty much as tight as it can go.

With usual tricks being unable to improve the poor performance in recent sessions, and with the problem now appearing to affect every slew made,  a fresh attempt was made today (2023-07-19) to increase the rotation drive tension. To do this we need to get access to a nut attaching the drive tension bolt to the metal plate that holds the single drive motor that turns the drive wheel against the inside of the roof flange, but that access is blocked by the half of the induction charger that is attached to the top of rotation drive unit.  The used steps to adjust the tension were as

• Loosen the two nuts that attach the induction charger to the rotation drive unit using an Allen Key. 
  (Note: it is not necessary to remove the associated nuts and indeed it is best not to remove them as they are very tricky to get them back on later. )
  
• Either i) take off the Induction Charger  (it has slots, rather than holes, that removal without taking off the nuts) or
  ii) rotate the dome by c. 5 deg so that the two halves of the induction charger are separated and then rotate the loosened charger backwards)
   
• With the tension bolt exposed adjust the nut on the plate that the dome motor is attached to increase its tension against the roof flange.
  
• Replace the Induction Charger in its normal position and tigten up two bolts using Allen Key.
• Tighten upTension Knob
• Test dome rotation

Photos below shows Rotation Drive Unit with one half of Induction Charger attached to its top. 
(One of the two nuts that has to be loosened to access inside of drive unit from the top is visible)

Dome Slewing will be checked in the next session (S1233)  to see if it has imrpoved or fixed the issue.

Update 2023-07-22

Good Dome Slewing performance in session S1123 (2023-07-22) , suggested that issue has been resolved (but in fact it hasn't as shown by later sessions)

 

Update 2023-10-25

The following session S1124  (2023-07-30) , again showed signs of the ongoing issue with 2 slews having significant hold-ups, indicating that the problem hasn't been totally resolved. 

Some later sessions e.g S1158 (2023-10-22) show that the problem can still be very bad at times, with 62% of slews suffering from significant hold-up and a total time lost to hold-ups of 45 minutes.

Issue was investigated on 2023-10-25 (see Investigation - Hold-ups during Dome Slewing, 2023-10-25) and concluded that Condensation/Dew acting as a lubricant reduces the force that the rotation drive motors can apply to the roof flange and is the number factor responsible.   It is proposed to apply Anti-Slip Tape to the inner surface of the roof flange to increase grip from the innermost drive motor wheel.