The f/15 chopping secondary for the 2.1-m telescope was the first servo-controlled
secondary to be driven in both orthogonal axes by independent
drivers under computer control. At the time of its construction in 1985,
two-position chopping with single-element infrared detectors was still
in use, and the two-axis capability permitted the continuous selection
of both position angle and amplitude, up to 90 arcsec. At the relatively
short infrared wavelengths (
The secondary was designed at a focal ratio of f/15 to be compatible with
the infrared instrumentation at that time. This focal ratio is still used
with SQIID, IRMOS, and some visitor instruments. Because of the size
(375 mm diameter)
and weight of the mirror, great care was taken to design resonance frequencies
above the servo response and to isolate the system from coupling to the
telescope structure. The primary step in this effort was the inclusion of
a "counterforce mirror" having the same moment of inertia, driven out of
phase with respect to the secondary by drivers mounted on a common surface.
The net force of the secondary and counterforce mirrors on the rest of the
support structure is thus nearly zero.
To minimize the thermal background seen by the detector, the entire secondary
top ring was designed to minimize the ambient temperature structures in the
optical path. The support and focus mechanism is smaller in diameter than
the secondary and is thus shielded from the detector. The spider supports
were made as thin as possible. Finally, a reflective cone corresponding to
the area shadowed by the central hole in the primary was mounted in the center
of the secondary; this is designed to reflect skylight into this region of
the secondary, as seen from the detector. This cone can be removed
(section 7) for use with optical instrumentation, where the sky is brighter
than the telescope environment.
The f/15 secondary requires setup in two locations, the telescope itself
and in the control room where the driver electronics are located. Figure
1 is a (rather complex) guide to the wiring connections. A full scale version is available for those requiring
additional scrutiny.
Figure 1: Schematic of Chopping Secondary Electronics
The physical installation of the secondary is a significant effort involving
the day crew. The entire f/8 secondary ring must be removed and lowered
to the ground floor with the crane, and the f/15 secondary ring must be
brought up and installed on the top of the telescope. This procedure takes
at least a couple of hours.
Out in the telescope dome, W15 and W16 plug into a connector box on the
NW side of the telescope, near the level of the declination axis. The cables
are strung on a spring-loaded cable which must be connected to an eye bolt
near the declination axis drive, to keep them from fouling as the telescope
is moved about. Ensure that the cables are plugged in securely, as the
screw-type connector provides the only security against their falling out.
The continuation of these cables must be removed from a dummy connector box
at the top of the telescope and installed on the connectors on the outer
secondary ring junction box.
The flatfield lights on the top of the secondary ring must be plugged in,
using the cable also to power the flatfield lights on the f/8 secondary.
Carefully remove the protective clamshell cover from the secondary itself.
If the "Christmas" alignment lights are required for SQIID, install them at
this time when the secondary is easily accessible. The band containing the
four lights should be secured just behind the mirror, not on the
mirror itself.
The control rack contains five modules for drive and control of the f/15
secondary. There is, in addition, a PAR lock-in amplifier used in the past
for single-channel IR photometry, which should be powered off and ignored.
The five f/15 modules are shown schematically in Fig. 1, although they
are arranged differently in the control rack. Although the f/15 is
no longer used for chopping, the control electronics provide the collimation
tilts and hold the secondary mirror in position, so proper operation of these
electronics is required. A description of the modules and their front
panel controls follows, from top to bottom of the rack:
Figure 2: The Focus Control and Chopper Control Modules
This module controls the focus of the secondary mirror. Only manual control
from the front panel or a hand paddle is possible, and the focus servo
is not implemented. The f/15 secondary focus is self-contained, and the
mirror position is not displayed on the VDU or read into image headers.
This module selects the operating mode and reference input. Front panel
potentiometers control the signal/reference attenuation and offset in
each axis in the ANALOG mode. For the sake of completeness, all of the
functions are described below, although only one configuration is presently
used.
NOTE: The only configuration used with the IR imager/spectrometers is
a static mode in which the OFFSET is employed as a collimation control. For
this application, the SOURCE SELECT must be in the EXT ANALOG position, the
OFFSET switches must be on, and the OFFSET LEVEL is used to adjust the tilt
of the secondary in each axis.
Figure 3: The Chopper and Counterforce Servo Modules
This module contains the servo and drive amplifiers for the chopping secondary
mirror. To protect the mirror and drive electronics, the SERVO button
will activate the servo feedback only when the POWER is on. The RESET button
will reset the servo in the event of an overcurrent condition in the motor
driver amplifier. The LED panel meter may be used to display the X and Y
motor power, and the X and Y POSITION MONITOR BNC jacks may be output to an
oscilloscope or digital voltmeter for diagnostic purposes.
This is identical to the Chopper Servo Module, driving the counterforce
mirror out of phase with the secondary to minimize the net inertia.
This contains power supplies for the focus motor and transducers and has
a single power switch.
As noted above, the OFFSET LEVEL potentiometers are used to tilt the secondary
for collimation purposes. In the event of suspected decollimation, we strongly
suggest consulting with cognizant personnel before adjusting these levels.
In any case, record the settings before making any adjustment, so that
the initial conditions can be recovered. If adjustment yields improved collimation,
record the information on the white board in the control room and
through a note in SERVICE so that the information can be recorded in the log.
As long as all the connections are secure and the electronics are operating
properly, not much can go wrong. The most likely failure is a motor overcurrent
condition, which results in tripping one or both of the servo circuit
breakers. In such an event, the green SERVO LED will go out and the red LED
will light. If this occurs in the mirror servo unit, the mirror will "relax"
and the object will most likely move out of the field. Servo failure in
the counterforce only will have no visible effect and will not affect the
operation if one is not chopping.
If an overcurrent occurs as a result of attempting to overdrive the secondary,
the SERVO RESET button should restore proper operation. If the servo
continues to trip, monitor the X and Y motor currents with the panel meter
on the appropriate servo module. Persistent ramping up to an overcurrent
condition suggests a problem with either hardware (such as physical binding
of the mirror motion) or electronics.
If the image appears comatic and/or the pointing is very bad, check that the
motor servos are enabled (green LED is lit). Also check that the SOURCE
SELECT is on EXT ANALOG and that the OFFSET switches are on.
1. Introduction
< 5
m) now supported at Kitt Peak, chopping
is not required, and the secondary is used in fixed mode, with the
dc offsets providing collimation adjustment.
2. Secondary Mirror
3. Installation & Setup
Secondary Installation
Driver Electronics
The control rack resides permanantly in the control room just inside the
door to the dome. Unless it has been removed for any reason, the AC power
and cables W15 and W16 should be plugged into their appropriate
connectors on the secondary control rack. (Note: The cables W13 and W14
shown in Fig. 1 are no longer used, since the CAMAC-driven options, to say
nothing of the control room CAMAC rack itself, are no longer used).
4. Operation
Description of Control Rack
Focus Control Module (Fig. 2)
Chopper Control Module (Fig. 2)
Chopper Servo Module (Fig. 3)
Counterforce Servo Module (Fig. 3)
Power Supply
Operation Checklists
Startup & Operation
Shutdown
There is no need to shut down the secondary at the end of the night, although
it is a good idea to check that the SERVO ON lights are illuminated at the
beginning of a night's observing. If powering down is necessary (at the
end of an observing run), the following sequence is recommended to avoid
overdriving the motors:
Collimation Adjustment
5. Troubleshooting
