Chapter 10 - Mirror 3 (Nasmyth Mirror)



The Mirror 3 electronic unit controls the shifting of the observation service to the right hand or left hand platform of the telescope by rotating the Nasmyth system 180 degrees. It also allows the control of the M1 Sliding Cover, the M3 Baffle and the shutters on Focus A and B of the telescope.

10.1.1 General Specifications

The rotation of the Nasmyth system is effected by a torque motor. The two final positions are approached via an incremental encoder and fixed by an electro-magnetic brake.

The time taken for rotation from one focus to the other is between 30 and 60 seconds. Absolute reproducibility is 2,5 seconds of arc.

The M1 cover slides on the center piece of the telescope above the M1 and the Nasmyth mirrors. Its motion is effected by a 3-phase motor. The final positions are detected by mechanical end-switches.

The M3 Baffle slides up and down along the altitude axis of the telescope above Mirror 3. One on-off dc motor moves the M3 Baffle system. The two final positions are detected by two proximity sensors.

The two shutters on Focus A and B control the passage of the light into the adapters of the telescope. Their functions are carried out by the rotation of one on-off dc motor. The two final positions are detected by proximity sensors.

10.1.2 Functional Description

The four main functions carried out in the Mirror 3 electronic unit are under the control of a local unit. The local control unit includes two cabinets (M3-1 and M3-2) mounted on the center piece of the telescope; the M3 VME system is in the M3-1 cabinet.

10.1.3 Nasmyth Mirror Rotation

The rotation of 180 degrees is 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 during rotation of the Nasmyth structure. The two final positions are reached by reading out the expected data from the incremental encoder counter card.

The zero pulse of the incremental counter is used during initialization as an absolute position referred to the mirror position. Location at the final positions is effected by means of an electro-magnetic clutch. A vertical position interlock disables the shifting of the observation platform when the tube of the telescope is not close to the zenith position.

10.1.4 M1 Cover

The sliding cover can be closed or opened by means of an ac 3-phase motor and a mechanical two-drive system. Only the two final positions of the cover can be obtained through the remote mode. Intermediate positions are performed by means of special functions (local or test mode). Mechanical limit switches disable the command circuit at end positions.

M1 cover motion is interlocked with the M3 Baffle.

10.1.5 M3 Baffle

The baffle can be moved up and down by means of an on-off dc motor, a pinion and spindle drive system. The remote mode allows the upper and lower positions of the baffle. Intermediate positions can be obtained by special functions implemented in the electronic control module (local or test mode). Inductive proximity sensors disable the command circuit at final positions. A brake fixes the final positions reached.

An interlock based on electronic hardware disables the command circuit of the baffle when limit switches indicate that the M1 Sliding Cover is closed or is not completely opened.

Another interactive interlock disables the command circuit of the M1 cover when the baffle lower inductive switch is not activated. In manual or test operations, intermediate positions of cover and baffle both totally block computer control of these two functions. To cure this problem of interactive interlock, it is necessary to initialize the two systems to their end limits in manual or test mode.

10.1.6 Shutters on Focus A and B

The mechanical system for shuttering the focuses consists of two discs of electrically and magnetically isolated material with an eccentric circular hole in each disc. The two discs rotate in opposite directions via pinion drives. Two magnetic marks are mounted on the isolated discs in such a way that when one mark is detected by an inductive sensor, the shutter is closed. If both marks are detected, the shutter lets the light go through the adapter of the telescope.



10.2.1 Nasmyth Mirror

10.2.2 M1 Cover

10.2.3 M3 Baffle

10.2.4 Shutter Equipment



10.3.1 Nasmyth Mirror

Default Settings

Operating Sequence

PAFAULTI*NegatedInland power amplifier
IEERRI*NegatedHeidenhain EXE 702B
PACURIAnalog data 0VInland power amplifier
ILVPOSI*AssertedTelescope is more than 15 degrees further from zenith position, vertical position interlock module (M3-2 cabinet)
VANGLIAnalog data 2,5V +/- 0,1VSoftware vertical position of the tube, vertical position interlock module (M3-2 cabinet)
M3-1 TEMP40I*NegatedTemperature in M3-1 cabinet higher than 40 degrees C
M3-2 TEMP30I*NegatedTemperature in M3-2 cabinet higher than 30 degrees C
ILEMSTO1INegatedEmergency Stop push-button EMSTO1I is not released
ILEMSTO2INegatedEmergency Stop push-button EMSTO2I is not released

Release the M3 clutch (MTBRAKE*). Delay before next operation of 2 seconds.

Apply the power to the Inland amplifier (PAON70V*). Delay before next operation of 100ms.

Enable the power amplifier (PAENABLE*).

The required positioning is obtained by applying a velocity reference (600Mv for high speed and 150Mv for low speed) and reading the incremental encoder counter card. During positioning check:

At final positions:

Reading Out The Encoder

The Heidenhain encoder is an incremental type having a resolution of 0,02 degrees or 72 arc second (18000 lines per revolution).

The Heidenhain Evaluation and Interpolation Electronic EXE 702B increases the resolution by a factor of 25; i.e. 0,0008 deg or 2,88 arc sec.

The ESO Incremental Encoder Counter Card multiplies by 4 again; the global electronic resolution is then 0,0002 deg or 0,72 arc sec. Tolerance is 2,5 arc sec/ 0,72 arc sec = +/- 3,5 counting pulses.


After initialization of the incremental encoder counter card through the command IERESET*, the Nasmyth mirror rotates until it finds an absolute position that is defined by the detected zero pulse ZP from the encoder.

The flag IEZERO* 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 time through the command IECS*.

Activation of IECS* registers the current counter data and it must stay active until the end of the read out sequence; transfer of 4*8 bits counter value.

IEDREADY*, handshake to local VME, indicates that the 8 bits counter value is available.

After receiving IEBREAD* from the local VME, the incremental encoder counter sends a new handshake IEDREADY* indicating the next byte is available. At the read out of the last byte, the command IECS* is de-activated and initialized for the next data position transfer.

Shift From One Focus to the Other Focus

When the absolute position is known (initialization done), the rotation of the Nasmyth mirror is controlled by reading out the actual position through the incremental encoder counter card.

10.3.2 M1 Cover

Default Settings

Operating Sequence

SCLSCLI*Asserted (M1 Cover closed)Telemecanique end-stop, M1 cover card
SCLSOPI*Asserted (M1 Cover opened)Telemecanique end-stop, M1 cover card
SCMTPROTIAssertedSliding cover motor protection
BALSDNI*NegatedM3 baffle is not at lower position
BALSUPI*NegatedM3 baffle is at upper position
ILEMSTO1INegatedEmergency Stop push-button EMSTO1I
ILEMSTO2INegatedEmergency Stop push-button EMSTO2I

Assert the command SCOPEN* (open) or SCCLOSE* (close).

Read the limit positions SCLSOP* (cover opened) or SCLSCL* (cover closed).

Negate the command SCOPEN* or SCCLOSE*.

10.3.3 M3 Baffle

Default Settings

Operating Sequence

BALSUPI*Asserted (baffle up)Baumer sensor, M3 baffle card
BALSDNI*Asserted (baffle down)Baumer sensor, M3 baffle card
SCLSOPI*Asserted (cover opened)M1 cover is not completely opened
SCLSCLI*Asserted (cover closed)M1 cover is closed
ILEMSTO1INegatedEmergency Stop push-button EMSTO1I
ILEMSTO2INegatedEmergency Stop push-button EMSTO2I

Apply power to the M3 baffle module (BAPWON*).

Release the M3 baffle clutch (BABRAKE*).

Assert the command BAUP* (move up) or BADN* (move down).

Read the limit positions BALSUP* (baffle is up) or BALSDN* (baffle is down).

Negate the command BAUP* or BADN*.

Engage the clutch (BABRAKE*).

Switch off power to the M3 baffle module (BAPWON*).

10.3.4 Shutters on Focus A and B

Default Settings

Operating Sequence

ILEMSTO1INegatedEmergency Stop push-button EMSTO1I
ILEMSTO2INegatedEmergency Stop push-button EMSTO2I

Assert the command SHUTAOP*/ SHUTACL* to open or close the shutter on Focus A; SHUTBOP*/ SHUTBCL* on Focus B.

Read the status of SHALS1* and SHALS2* for Focus A; SHBLS1* and SHBLS2* for Focus B.

SHxLS1* asserted, SHxLS2* negated - shutter closed
SHxLS1* asserted, SHxLS2* asserted - shutter is open

Negate the command SHUTxOP* or SHUTxCL*.

Note: x represents A or B.

10.3.5 Special Functions

Nasmyth Mirror Velocity Loop Adjustment

In order to adjust the velocity loop, a velocity reference input is implemented in the electronic module vertical position interlock of the M3-2 cabinet. This allows adjustment of the velocity step response using an external function generator connected to VREFin when the system is in TEST mode.

M1 Cover

In parallel with the remote control through the local VME unit, intermediate positions of the M1 sliding cover can be obtained for test or maintenance purposes through manual control. This function is implemented on the M1 cover control module in the M3-2 cabinet. No interaction with the remote or manual control mode is possible as, when manual mode is not active, the system automatically remains in remote mode.

M3 Baffle

For local test and maintenance purposes, manual control of the M3 baffle is implemented on the M3 baffle control module in the M3-2 cabinet. Remote mode is always active if manual mode is not selected.

Shutters on Focus A and Focus B

Manual control of the shutters is possible through the electronic module M3 shutter control installed in the M3 VME chassis (M3-1 cabinet).

Vertical Position Interlock

This function is mainly implemented in the module vertical position interlock (M3-2 cabinet) in order to retrieve stored information of the telescope position independently of the vertical position provided by the altitude control system. The detection of an angle more than 15 degrees from the zenith directly disables the Nasmyth mirror control. The accuracy of this detection is directly linked to the accuracy of the mercury tilt switch and its mounting support.

An inclinometer is also mounted in the vertical position interlock module to interlock the Nasmyth mirror control through a software limit that has an angle from zenith less than 15 degrees.



The signals in the Mirror 3 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 M3 prefix for all Mirror 3 signals.

Note that the SC signals are included in this chapter as the M1 sliding cover is controlled by the M3 computer.

10.4.1 Power Amplifier Signals

PACURIAI*12Power amplifier current to motor
PAVREFOAO*12Velocity reference
PAFAULTI*DI*1PA fault indicator
PAENABLEO*DO*1Power amplifier enable (Note: disengage brake with MTBRAKE*)
PAON70VODO*1Switch on 70V to amplifier

10.4.2 Motor Signals

MTBRAKEO*DO*1Motor brake disengage

10.4.3 Incremental Encoder Signals

IEDCH-DIDI*8Data channel
IEDREADYI*DI*1Data ready handshake from incremental counter
IEBREADO*DO*1Data read handshake to incremental counter
IEZEROI*DI*1Zero pulse detected
IERESETO*DO*1Reset incremental counter
IECSODO*1Channel select, latches instantaneous value
IEUPO*DO*1Up count pulse to incremental counter (test only)
IEDNO*DO*1Down count pulse to incremental counter (test only)
IEERRI*DI*1Incremental encoder pulse shaping electronics error

10.4.4 Limit Switch Signals

No limit switches assigned to Mirror 3. Refer to Subsections 10.4.6 and 10.4.7 for sliding cover and baffle limit signals.

10.4.5 Interlock Signals

ILEMSTO1IDI*1Emergency stop button M3-1 pressed
ILEMSTO2IDI*1Emergency stop push-button M3-2 pressed
ILVPOSI*DI*1Telescope in vertical position, M3 rotation is possible

10.4.6 Sliding Cover Signals

All sliding cover signals are described in this subsection including the motor, limit switch and interlock signals. When a limit switch is activated, it is possible to move in the opposite direction, so an over-ride signal is not used.

SCOPENO*DO*1Open sliding cover
SCCLOSEO*DO*1Close sliding cover
SCMTPROTIDI*1Sliding cover motor thermal protection, motor switched off
SCLSCLI*DI*1Sliding cover limit switch closed
SCLSOPI*DI*1Sliding cover limit switch opened

10.4.7 Baffle Signals

All M3 baffle signals are described in this subsection including the motor, limit switch and interlock signals. When a limit switch is activated, it is possible to move in the opposite direction, so an over-ride signal is not used.

BAUPO*DO*1Move M3 baffle up
BADNO*DO*1Move M3 baffle down
BALSUPI*DI*1M3 baffle limit switch up
BALSDNI*DI*1M3 baffle limit switch down
BABRAKEO*DO*1M3 baffle brake disengage
BAPWONO*DO*1M3 baffle power on

10.4.8 Shutter Signals

SHUTAOPO*DO*1Shutter A open command
SHUTACLO*DO*1Shutter A close command
SHUTBOPO*DO*1Shutter B open command
SHUTBCLO*DO*1Shutter B close command
SHALS1I*DI*1Shutter A limit switch 1
SHALS2I*DI*1Shutter A limit switch 2
SHBLS1I*DI*1Shutter B limit switch 1
SHBLS2I*DI*1Shutter B limit switch 2

10.4.9 Miscellaneous Signals

M3-1TEMP40I*DI*1M3-1 cabinet over-temperature, 40 deg. C
M3-2TEMP30I*DI*1M3-2 cabinet over-temperature, 30 deg. C
VANGLIAI*12M3 vertical angle


FFLONO/acro011MTS1-13 B31LOWflat field lamp on
SCOPENO/acro021MTS1-11 B30LOWopen M1 sliding cover
SCCLOSEO/acro031MTS1-9 B29LOWclose M1 sliding cover
BAPWONO/acro041MTS1-7 B28LOWbaffle power on
BADNO/acro051MTS1-5 B27LOWmove baffle down
BAUPO/acro061MTS1-3 B26LOWmove baffle up
BABRAKEO/acro071MTS1-1 B25LOWdisengage baffle brake
TEMP40I/acro091MTS1-29 B23LOWover-temperature M3-1 cabinet
ILEMSTO1I/acro0101MTS1-27 B22HIGHemergency stop push-button 1
TEMP30I/acro0111MTS1-25 B21LOWover-temperature M3-2 cabinet
ILEMSTO2I/acro0121MTS1-23 B20HIGHemergency stop push-button 2
ILVPOSI/acro0131MTS1-21 B19LOWtelescope vertical
BALSDNI/acro0141MTS1-19 B18LOWbaffle limit switch down
BALSUPI/acro0151MTS1-17 B17LOWbaffle limit switch up
SHUTBCLO/acro0161MTS1-47 A32LOWclose nasmyth B shutter
SHUTBOPO/acro0171MTS1-45 A31LOWopen nasmyth B shutter
SHUTACLO/acro0181MTS1-43 A30LOWclose nasmyth A shutter
SHUTAOPO/acro0191MTS1-41 A29LOWopen nasmyth A shutter

PAENABLEO/acro0321MTS2-40 C32LOWenable mirror 3 power amplifier
IECSO/acro0331wire-wrap C31LOWincremental encoder channel select
IEBREADO/acro0341wire-wrap C30LOWincremental encoder data read handshake
IERESETO/acro0351wire-wrap C29LOWreset incremental encoder
PAON70VO/acro0361MTS2-32 C28LOWmirror 3 power amplifier power on
MTBRAKEO/acro0371MTS2-30 C27LOWdisengage mirror 3 brake
IEERRI/acro0411MTS2-54 C23LOWincremental encoder error
SCMTPROTI/acro0421MTS2-52 C22HIGHM1 sliding cover motor protection
SCLSOPI/acro0431MTS2-50 C21LOWM1 sliding cover open limit switch
SCLSCLI/acro0441MTS2-48 C20LOWM1 sliding cover close limit switch
PAFAULTI/acro0451MTS2-46 C19LOWpower amplifier fault
IEZEROI/acro0461wire-wrap C18LOWincremental encoder zero pulse detected
IEDREADYI/acro0471wire-wrap C17LOWincremental encoder data ready handshake
IEDCHI/acro0481wire-wrap A1HIGHincremental encoder data l.s.b.
IEDCHI/acro0491wire-wrap A2HIGHincremental encoder data
IEDCHI/acro0501wire-wrap A3HIGHincremental encoder data
IEDCHI/acro0511wire-wrap A4HIGHincremental encoder data
IEDCHI/acro0521wire-wrap A5HIGHincremental encoder data
IEDCHI/acro0531wire-wrap A6HIGHincremental encoder data
IEDCHI/acro0541wire-wrap A7HIGHincremental encoder data
IEDCHI/acro0551wire-wrap A8HIGHincremental encoder data m.s.b.
SHBLS2I/acro0561MTS1-55 A9LOWshutter B moving
SHBLS1I/acro0571MTS1-53 A10LOWshutter B open
SHALS2I/acro0601MTS1-51 A13LOWshutter A moving
SHALS1I/acro0611MTS1-49 A14LOWshutter A open

PAVREFO/aio0OUT-1TS2-1, A32-C32velocity reference
VANGLI/aio0IN-0TS2-5, 6 A1-C1M3 vertical angle
WINDI/aio0IN-2TS2-?, ? A3-C3wind speed
PACURI/aio0IN-4TS2-3, 4 A5-C5mirror 3 motor current
TESEI/aio0IN-9TS2-?, ? A10-C10tube temperature
TEM1I/aio0IN-12TS2-?, ? A13-C13M1 temperature
TEEMI/aio0IN-13TS2-?, ? A14-C14EMMI temperature

10.5.1 I/O Boards Jumper Settings

ACRO 9481J1, J3, J5, J7Installed Enables internal pull-up to +5V of the I/O lines
J2, J4, J6, J8InstalledEnable internal threshold reference of the input
J17Base address:Base address:
board 1; 5-6 installedffff1000 hex (/acro0)
board 2; 1-2 and 5-6 installed ffff1400 hex (/acro1)
J19InstalledShort supervisory and non-privileged access
J20OmittedSYSFAIL is not asserted after reset
VMIC,VMIVME-3111, analog I/ODefault setting is: analog inputs; 16 differential, -10 to +10V
analog outputs; single-ended, -10 to +10V
J1InstalledGround P3 return
J2InstalledGround P3 sense
J3Omitted5V full scale input
J41-1 installedBipolar analog inputs
J52-3 installedAuto input gain adjust
J6OmittedUnipolar analog inputs not enabled
J7, J8OmittedOutput test is validated
J9, J10, J11OmittedP3 inputs enabled
J121-2 installedBipolar analog outputs
J13Omitted20V full scale output range
J151-2 installed20V full scale input range
J17 to J24OmittedDifferential input
J27 to J34OmittedDifferential input
J25Omitted2.5Vdc reference unused
J26Installed10Vdc reference used
J35InstalledGround P2 sense
J36, J37Base address:Base address:
board 1; J36: 0, 1, 2, 3, 6 and 7 installed ffff30000 hex (/aio0)
J37: 0, 1 and 2 installed
J37-3Omittedshort non-privileged access
J382-3 installedAuto input zero adjust
J39InstalledGround analog outputs return
J40Omitted5Vdc reference unused
J41OmittedExt. trig not connected to P2 ground sense
J421-2 installedExt. trig rtn connected to P2 ground sense


10.6.1 Drawing List

M3 Motor Control
- Functional Block Diagram 25-04-89CS-E-1457
- M3 Motor Control Chassis 09-10-89CS-E-1515
- M3 Vertical Position Interlock M3-2 Unit 18-06-89CS-E-1535
- M3, M1 Sliding Cover Control 25-04-89CS-E-1508
- M3-2 Unit 26-06-89 CS-E-1514
- Limit Switch Connections 11-01-89CS-E-1627
M3 Baffle Control
- M3-2 Unit 09-10-89 CS-E-1662
- Baffle Cabling 09-10-89CS-E-1669
- Baffle Control Board CS-P-1662
- Chassis Cabling Overview 27-06-90CABOVER3.SCH
- M1 Cover-M3 Baffle
- Vertical Position Interconnections 18-06-90CS-E-1518
Shutter Control M3-1 Unit
- Arm A Control 1 03-02-91CS-E-1715
- Arm B Control 2 03-02-91CS-E-1715
- List of Components3
- PCBCS-P-1715
M3-1 Unit Mounting Plate 18-06-90CS-E-1562
- Cabling Overview 26-06-90CABOVER1.SCH
M3-2 Unit Mounting Plate 18-06-90CS-E-1527
- Cabling Overview 26-06-90CABOVER2.SCH
M3 Cabling Overview 05-09-90EXTOVER.SCH
Interconnections M3-1, M3-2 Units
- Cables C1 and C4 05-09-88CS-E-1563
- Cables C2 and C3 18-06-90CS-E-1564
Cable Analog Interface 25-04-89CS-E-1566

10.6.2 Data Sheets

Inland Power Servo Amplifier DCM 70/14 Operating Instructions 11
ESO 32-bit Incremental Encoder Counter Card
Heidenhain Incremental Encoder RON 7052
Heidenhain Evaluation and Interpolation Electronics EXE 702B 2
Saunders-Roe Mercury Tilt Switch - Part TS81
Accustar Ratiometric Inclinometer5