Tertiary Mirror Control

Contents

Tertiary Mirror Assembly Overview

A new tertiary mirror assembly consists of two mirrors, a convex miror for the SideCab receivers and a flat one for instruments at the second Nasmyth focus. Each mirror is mounted in an aluminum cage and driven laterally by a stepper motor through a lead screw. During normal operations, one of the mirrors is centered on the telescope beam. Or, for Cassegrain focus instruments mounted on the relay optics, both the mirrors are stowed.

Tertiary Mirror Controller Overview

Two stepper motors are controlled by a DCX-VM100 Modular Multi-Axis Motion Controller. The same controllers are used for the SideCab receiver optics and the secondary mirror (SMC1, SMC2 and SMC3). The controller is located in an enclosure underneath the SideCab. It communicates with the antenna computer over a serial line.

Software Overview

At the moment, the tertiary mirror assembly is under "manual" control.

UIP's TERTIARY Command

A new UIP command TERTIARY is used to move the mirrors to one of the pre-configured positions and to check the current configuration.

To configure the mirrors for the SideCab receivers and the second Nasmyth instruments, type

UIP> TERTIARY /SIDECAB
Will position the mirrors for the SideCab receivers.
UIP>

and

UIP> TERTIARY /NASMYTH2
Will position the mirrors for the instruments at the Nasmyth focus No. 2.
UIP>

respectively. For the Cassegrain instruments, type either

UIP> TERTIARY /CASSEGRAIN
Will position the mirrors for the instruments at the Cassegrain focus.
UIP>

or

UIP> TERTIARY /STOW
Will position the mirrors for the instruments at the Cassegrain focus.
UIP>

To verify the current status of the tertiary mirrors, issue the TERTIARY command without any argument:

UIP> TERTIARY
The mirrors are positioned for the instruments at the Cassegrain focus.
UIP>

The above command determines the current configuration by comparing the mirror positions with the pre-programmed values. There may be a problem if it returns the following message:

UIP> TERTIARY
The mirrors are not positioned properly.
UIP>

The TERTIARY command returns the following message if repositioning of the mirrors has not been completed yet:

UIP> TERTIARY
The motion controller is busy.
UIP>

Note: It has been observed that the encoder reading of the tertiary mirrors drifts by a few counts as a function of the telescope elevation. It may also be affected by the ambient temperature and the telescope azimuth. A mirror position error tolerance, for the purpose of determining the configuration, has been relaxed to +/- 10 counts (about 1 mm). This issue will be investigated further.

New SMC Program

An updated version of the smc program is available on the antenna computer for engineering purpose. There are a few differences from the previous version - move and stow have been added and load has been renamed to upload. A shell script smc4 automatically runs this new program for the tertiary mirror controller:

hau$ smc4
Opening SMC4... done
smc4> help
Commands may be abbreviated.  Commands are:

echo            exit            help            history         home            
lock            move            reset           send            status          
stow            tell            unlock          upload          
smc4> 

The following commands are most useful - status, tell, move and stow. The status command prints the current status of the motors connected to the controller:

smc4> status
Motor 1: status = 0x00080058 complete negative homed limit_left
Motor 2: status = 0x00080058 complete negative homed limit_left
smc4>

Note that when the mirrors are stowed they are at the left limit (of the motors). The tell command prints the target, absolute and encoder positions of the motors:

smc4> tell
Motor 1: target =      -64615, absolute =      -14615, encoder =        2094
Motor 2: target =      -36846, absolute =      -16187, encoder =      -29254
smc4>

Though not available for the tertiary mirror motors yet, the move command is useful for the SideCab receiver optics alignment. Finally the stow command stows the mirrors:

smc4> stow
Stowing mirrors... done
smc4>