Chapter 9 - Mirror 2 (Secondary Mirror)


The adjusting device for the secondary mirror serves to re-adjust the M2 mirror according to optical requirements during operation of the telescope. For this purpose, the Z re-adjustment of the axis direction of the mirrors is used for active closed loop focus control via the Shack on-line image analyser of the telescope. X-Y re-adjustment (normal to the mirror axis) serves as closed loop centering correction of the mirror system.

For the Z movement, two linear guiders are provided in the adjusting device. The X-Y movement is carried out via a pantographic mechanism with conically arranged guide spindles. Thus the X-Y movement is not in a straight line, but is around the centre of curvature of the M2 mirror, thus achieving coma correction without change in pointing.

9.1.1 General Specifications

X-Y Motion

Z Motion

The maximum lateral deflection of the plate support with the outer ring between horizontal and vertical position of the whole unit: 15 micrometre.

9.1.2 Functional Description

The X-Y and Z re-adjustment are carried out in the M2 VME system. As the three axes are not used at the same time, they are mounted in a multiplex mode, thus saving space and heat dissipation in the optical field.

9.1.3 X-Y Re-adjustment

The motion and positioning of the X and Y axes are obtained by the rotation of spindles. These motions are effected by a torque motor controlled in torque, velocity and position in a proportional integral-type servo loop system.

An incremental encoder delivers the actual position. The final positions are reached by reading out the position values from the incremental encoder counter card.

The zero pulse of the incremental counter is used during initialisation as an absolute position on the axes.

Final positioning is effected by means of an electro-magnetic clutch.

Limit positions of the range are provided by inductive proximity sensors. For security reasons, mechanical end-switches are available as final limit positions.

9.1.4 Z Re-adjustment

The focusing motion is performed by the rotation of a torque motor through a spindle. The Z re-adjustment is controlled in torque, velocity and position by a proportional integral loop. An absolute encoder provides the actual position directly to the VME system. Final positioning is effected by means of an electro-magnetic clutch. Inductive proximity sensors are used to fix the range of motion, and mechanical end-switches are available in order to disable the power amplifier.




9.3.1 Default Settings

9.3.2 Operating Sequence

PAFAULTI*NegatedInland power amplifier
IEERRIX*NegatedHeidenhain EXE 602 "X"
IEERRYI*NegatedHeidenhain EXE602 "Y"
LSXLOWI*AssertedMove X in opposite direction
LSXHIGHI*AssertedMove X in opposite direction
LSYLOWI*AssertedMove Y in opposite direction
LSYHIGHI*AssertedMove Y in opposite direction
LSZLOWI*AssertedMove Z in opposite direction
LSZHIGHI*AssertedMove Z in opposite direction
ILEMSTOINegatedEmergency Stop Pushbutton EMSTO has not been released
TEMP40I*NegatedTemperature in M2 electronic unit higher than 40 degrees C

Select a drive system (M2MTXSELO*, M2MTYSELO* or M2MTZSELO*). Note: only one system may be asserted.

Apply power to the Inland power amplifier (M2PAON70V0O*). Delay before next operation of 100ms.

Enable the power amplifier (M2ILENABLE*); the amplifier will energise if no end-stop condition exists. If the drive system is at the end-stop:

The required positioning is obtained by applying a velocity reference and reading the incremental encoder counter card, or the absolute encoder of the Z drive.

During positioning, check:

At these positions:

9.3.3 Reading Out The Incremental Encoder

The X-Y encoder system has a resolution of 4000 lines per revolution. With a value of 1mm linear spindle displacement per revolution, the theoretical resolution is 0.25 micrometre. The specification of 10 micrometre allows a tolerance of +/- 40 counting pulses.


After the command M2IERESETX* or M2IERESETY*, the X or Y spindle drive system moves until it finds an absolute position that is defined by the detected zero pulse ZP from the encoder.

The flag M2IEZEROX* or M2IEZEROY* indicates that the zero pulse position has been registered and must be the first read out. The relative position from the zero pulse can be read at any moment through the command M2IECSSELX* or M2IECSSELY*.

The activation of these commands registers the current counter data and must stay active until the end of the read out sequence; transfer of 4*8 bit counter values. M2IEDREADY*, common to X and Y drive systems, indicates that the 8 bit counter values are available.

The command M2IEBREAD* sent from local VME acknowledges to the encoder counter that it can make the next byte and the handshake M2IEDREADY* available. At the read out of the last byte, the command M2IECSSELX* or M2IECSSELY* is de-activated.

Post Initialisation

After initialisation, the X or Y drive system displacement is controlled by reading out the actual position through the incremental encoder counter card.

The range initialisation is set through the movement of the drive system, from one limit position to the other, and the calculation of the middle position.

9.3.4 Reading Out The Absolute Encoder

The Z absolute encoder system has a resolution of 1024 lines per revolution. The theoretical resolution for the spindle displacement is close to 1 micrometre. The specified range of +/- 20mm with a spindle displacement of 1mm per revolution forces the selection of the 64 turn absolute encoder.

The range initialisation is set through the absolute positions at the two limits (in binary code) and the calculation of the middle position.

9.3.5 Special Functions

The command M2OFF5V* allows the disabling of the 5V power supply to the encoders X, Y and Z, and all the inductive proximity sensors in the M2 unit when the X, Y and Z directions do not need to be adjusted during a whole night of observation.


The signals in the Mirror 2 subsystem are divided into the following groups that indicate where the signals originate (inputs to the computer) or where they have to be routed (outputs from the computer):

The above abbreviations are used as the second prefix after the standard prefix M2 for all Mirror 2 signals.

9.4.1 Power Amplifier Signals

PACURIAI*12PA current to motor X, Y or Z (1 common PA)
PAVREFOAO*12Velocity reference to X, Y or Z
PAFAULTI*DI*1PA fault indicator
PAON70VO*DO*1Switch ON 70V busbar voltage to amplifier

9.4.2 Motor Signals

MTXSELO*DO*1Select X drive system (motor, tacho, brake)
MTYSELO*DO*1Select Y drive system (motor, tacho, brake)
MTZSELO*DO*1Select Z drive system (motor, tacho, brake)

9.4.3 Incremental Encoder Signals

IEDCH-DIDI*8Data channel (common for X and Y positions)
IEZEROXI*DI*1Zero pulse detected for system X
IEZEROYI*DI*1Zero pulse detected for system Y
IEDREADYI*DI*1Data ready handshake signal (common for X and Y)
IERESETXO*DO*1Reset incremental encoder counter X
IERESETYO*DO*1Reset incremental encoder counter Y
IEBREADO*DO*1Byte read handshake signal (common to X and Y)
IECSELXO*DO*1Channel select X
IECSELYO*DO*1Channel select Y
IEERRXI*DI*1Channel X pulse shaping electronics error
IEERRYI*DI*1Channel Y pulse shaping electronics error

9.4.4 Absolute Encoder Signals

AEDCH-DIDI*8Data channel
AEDCH-DIDI*8Data channel
AELATCHODO*1Latch data channel value
AEUP/DOWNODO*1Up counting rotation direction

9.4.5 Limit Switch Signals

LSXLOWI*DI*1Low limit detected, system X
LSYLOWI*DI*1Low limit detected, system Y
LSZLOWI*DI*1Low limit detected, system Z
LSXHIGHI*DI*1High limit detected, system X
LSYHIGHI*DI*1High limit detected, system Y
LSZHIGHI*DI*1High limit detected, system Z

9.4.6 Interlock Signals

ILENABLEO*DO*1Enable amplifier and disengage selected brake
ILOVERRO*DO*1Override interlock
ILEMSTOIDI*1Emergency stop pushbutton

9.4.7 Miscellaneous Signals

TEMP40I*DI*1M2 cabinet, overtemperature 40 deg. C
OFF5VO*DO*1Incremental encoder + limit switch power off (5V)


AEDCH-D08I/acro001MTS1-15 B32HIGHabsolute encoder high byte lsb
AEDCH-D09I/acro011MTS1-13 B31HIGHabsolute encoder high byte
AEDCH-D10I/acro021MTS1-11 B30HIGHabsolute encoder high byte
AEDCH-D11I/acro031MTS1-9 B29HIGHabsolute encoder high byte
AEDCH-D12I/acro041MTS1-7 B28HIGHabsolute encoder high byte
AEDCH-D13I/acro051MTS1-5 B27HIGHabsolute encoder high byte
AEDCH-D14I/acro061MTS1-3 B26HIGHabsolute encoder high byte
AEDCH-D15I/acro071MTS1-1 B25HIGHabsolute encoder high byte msb
AEDCH-D00I/acro081MTS1-31 B24HIGHabsolute encoder low byte lsb
AEDCH-D01I/acro091MTS1-29 B23HIGHabsolute encoder low byte
AEDCH-D02I/acro0101MTS1-27 B22HIGHabsolute encoder low byte
AEDCH-D03I/acro0111MTS1-25 B21HIGHabsolute encoder low byte
AEDCH-D04I/acro0121MTS1-23 B20HIGHabsolute encoder low byte
AEDCH-D05I/acro0131MTS1-21 B19HIGHabsolute encoder low byte
AEDCH-D06I/acro0141MTS1-19 B18HIGHabsolute encoder low byte
AEDCH-D07I/acro0151MTS1-17 B17HIGHabsolute encoder low byte msb
PAON70VO/acro0161MTS1-47 A32LOWpower on 70V busbar voltage to PA
MTXSELO/acro0171MTS1-45 A31LOWselect X drive system
MTYSELO/acro0181MTS1-43 A30LOWselect Y drive system
MTZSELO/acro0191MTS1-41 A29LOWselect Z drive system
OFF5VO/acro0201MTS1-39 A28LOWincr enc + lim switch power off
AEUPDOWNO/acro0211MTS1-37 A27HIGHup counting rotation direction
AELATCHO/acro0231MTS1-33 A25HIGHlatch data channel value

9.5.1 I/O Boards Jumper Settings


9.6.1 Drawing List

Mounting Plate Layout
Absolute Encoder Interface
Cable Analog Interface
VME Chassis Cabling
Cabling Overview
Functional Diagram
Electronic Unit Junction Box
Electronic Unit Mounting Plate 1
Electronic Unit Mounting Plate 2
Electronic Unit Mounting Plate 3
Electronic Unit Mounting Plate 4
Electronic Unit Mounting Plate 5
M2 Cabling


9.6.2 Data Sheets