Transcript
Backing the DIMM Mount Off Its Hard Stops Tom Herbst, MPIA 18 March 2009 Scope The LBT DIMM consists of a number of commercial components, including a Software Bisque Paramount ME mount and a Celestron 14 optics tube. Occasional circumstances may drive the mount to one of its external hard stops, producing an error condition that requires intervention. This document explains how to back the DIMM mount off its hard stops. Full information about DIMM operation and subsystems appears on the LBTO wiki page at: http://wiki.lbto.arizona.edu/twiki/bin/view/Operators/DIMMOperatingProcedureAndManuals
The Problem The DIMM must point and track on objects that are not in the same look direction as the LBT. It therefore has an independent two-axis mount, located on a custom bracket on the upper right C-ring of the telescope (figure 1).
Figure 1: The LBT DIMM on the upper right C-ring
The two rotation axes of the DIMM are known as “left-right” and “up-down,” since they allow scanning left-right and up-down the telescope slit, respectively. As the LBT moves in elevation, the orientation of these axes changes, leading to some mathematical challenges in following objects. The tight space constraints prompted the installation of external hard stops, which prevent excess motion in the left-right axis (figure 2). If an error occurs, it is possible
that the Paramount will drive up against one of these stops. If this happens, human intervention is required to back the mount off and restart the software.
Figure 2: External hard stops (arrows) to prevent collisions between the DIMM and the LBT
The Solution This error condition can cause problems with communication with the mount. If you suspect a difficulty, the first step is to make a visual check of the mount. Place the LBT at horizon and move up to the DIMM location using the scissor lift. Is the counterweight shaft in contact with one of the two hard stops? If so, you must back the mount off the stop manually before proceeding. There are two knobs which lock the rotating parts of the Paramount against their respective drive gears. Figures 3-5 indicate the location of these knobs. Note that you will only be using the left-right knob to correct this error condition.
Figure 3: Location of the two axis locking knobs
Figure 4: The Up-Down axis locking knob
Figure 5: The Left-Right axis locking knob
The procedure for backing the mount off its hard stop is straightforward. You must disengage the worm drive from the worm gear (while steadying the Celestron telescope), rotate the mount off the stop, and re-tighten. NOTE: This procedure is also described on page 41 in the Paramount manual, available online in the operator’s wiki at: http://wiki.lbto.arizona.edu/twiki/bin/view/Operators/ParaMount
Here are the explicit instructions, reproduced from the Paramount manual… To disengage the worm for balancing, make sure you’ve secured the mount by grasping the counterweight shaft, and then turn the Balance Knob clockwise about 1-1/2 turns. The axis can now rotate freely. The worm will remain disengaged until you re-engage it by rotating the knob in the counterclockwise direction. Before letting go of the counterweight shaft, verify that you have completely disengaged the worm from the gear by very slightly moving the counterweight shaft. Any resistance to rotation means that the worm is not fully disengaged.
Note that the so-called “Balance Knob” must be turned clockwise to disengage and counter-clockwise to engage. This may be opposite to your expectation !
At this point, you should rotate the telescope assembly so that it is no longer in contact with the hard point. When the mount re-boots, it will seek its internal zero points, so you need not place the rotation axis at any particular location – at least 510 cm away from either hard point will be fine. Engaging the Worm To engage …the axis worm with the gear…firmly grasp the counterweight shaft with one hand while turning the Balance Knob counterclockwise until you feel the worm and gear make contact. Gently wiggle the axis using the counterweight shaft to be sure the worm is actually seated into the gear. Again, turn the knob counterclockwise to a full stop. Once the knob has been rotated counterclockwise to a full stop, rotate it clockwise about one knob click (that is, 1/10 of a turn). This holds the worm very close to the gear and does not allow separation if the mount is out of balance. It also does not force the worm against the gear, which can bind the servomotors. Note that internal spring plungers on the worm gears apply the actual force that holds the worm into the gear. That is, the balance knobs’ purpose is to disengage the worm from the gear and to keep the worm block assembly from disengaging from the gear. The Balance Knobs are not used to adjust tension between the worm and the gear. However, the Balance Knobs can be over tightened and bind the servomotors.
This completes the operation. The mount should function normally.
