Moving the Swing Arms

Last update 30 April 2010 by John Hill

Changing the telescope swing arm configuration can be done using MCSPU or OSS. This latter one is much easier, however, it is advisable to keep the swing arms page on MCSPU open to monitor the process as well. This updated version emphasizes on using the OSS gui. The MCSPU command line procedure is preserved below.

Caution: This procedure covers moving the swing arms and rebalancing from a KNOWN balanced configuration. Before moving any swing arm with an UNKNOWN balance configuration, be sure the auxiliary drives are ENGAGED (beyond the scope of this procedure)

Moving the Swing Arms using OSS

Move to Zenith and Insert Stow Pins

1. Turn on HBS if it isn't already on.

2. Ready the Elevation axis if it isn't already ready.

3. Move the telescope to the zenith using MCS: click on Slew with argument 90 degrees.

4. Engage (insert) stow pins. For that, on MCS bring up the Elevation sub-GUI and click on Stow Pins IN for left and right.

5. Rest the AdSec shells. AOS > Rest, or AdSec Control GUI > Rest. Verify that the statuses in AOS and AdSec Control GUI are "Safe" and "Shell RIP" respectively.

Reconfigure the Swing Arms

6. On OSS, click on Swing Arms either right or left to bring up the sub-GUIs. Both sub-GUIs can be started at the same time.

7. Turn the motor drive power supply on. For that, click on Motor Power On on the Swing Arms sub-GUI, and wait a couple of seconds before power is completely on.

8. Click on Retract or Deploy depending on what configuration you need. The retracting/deploying process should take about 5 minutes per swing arm to complete. You can click commands for multiple swing arms to queue them ahead. Note that only one swing arm at a time can move on each side of the telescope.

9. Once the configuration is done, turn off the motor power supply off on the swing arms sub-GUI by clicking on Motor Power Off.

Adjust the telescope balance

Be sure to verify the balance before you retract the stow pins.

10. Adjust the dynamic balance of the telescope. For that, go to the dynamic balance page on OSS. 10.1 Clear the errors of the pump drive line by clicking

Clear Error Y

.9.2 Do the dynamic balance by clicking

Reconcile Y

and monitor the process until the line of the accumulator shows a low value (below 20 Kg*m).For assistance, ask the telescope manager or engineering manager about it.

10a. Set the AdSec shells.

11. Verify that the telescope is still precisely at zenith (and not resting on the stow pins). If the telescope is resting on the EL stow pins, EL does not equal 90 (i.e. 89.5). Can use "elslewtohold 90 o" in mcspuDisplay to get off the stow pins. Verify that the elevation drive torques are normal (mcspuDisplay. Actual: +~40%, ServoError/rmsTrackErr: ~-40%).

12. Retract the stow pins.


13. Use the IIF GUI to update the instrument authorization for the new configuration and instrument.

14. If changing telescope sides, use the TCSGUI to stop PCS, select the new side, and restart PCS.

15. Check that primary mirrors, secondary mirrors and tertiary mirrors (as needed) are ready in the new configuration.

Moving the Swing Arms using MCSPU - This is the old version of the process using the engineering interface. (still valid though)

Start MCSPU as needed, and move the telescope to the zenith.

elslewtohold 90

Turn on HBS if it isn't already on. Ready the Elevation axis if it isn't already ready.


Insert the stow pins.

From MCSPU, push the stow pins in.

leftstowpin in

rightstowpin in

Make sure the swing arm drives are not locked out.

The lockout key and reset button are in the upper right tree house.

Turn the motor drive power supply on

on MCSPU rsa or lsa page type

powerSA right on


powerSA left on

Select the Swing Arms you want to move.

There are 4 swing arms for each primary mirror:

Adaptive secondary (AdSec)

Prime Focus (PF)

Tertiary Mirror (TM)

Mirror Cover (MC)

You are most likely to need to move the Prime Focus swing arm. To select it, type the following command on MCSPU:

selectSA right pf


selectSA left pf

Now you can move the swing arm using the following commands on the MCSPU window;

retractSA right pf

this will start retracting the prime focus right swing arm.

deploySA left pf

this will deploy the prime focus left swing arm.

When you are done with your work at the swing arms, turn the motor drive power supply off

on MCSPU type

powerSA right off


powerSA left off

Be Sure to Get the Balance Adjusted BEFORE you retract the stow pins.

For that, on MCSPU go to the Dynamic Balance Page by typing


then clear the errors of the pump drive line by typing


then do the dynamic balance by typing


Check the elevation servos and verify that telescope is still precisely at zenith


Verify that the Elevation drive torques are nominal.

Retract the Stow Pins

Use the MCSPU, to push the stow pins out.

leftstowpin out

rightstowpin out

-- JohnHill - 01 Jan 2010
Topic revision: r22 - 14 Sep 2018, JoshuaWilliams
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