1) Imager: The imaging camera
consists of the main body containing the CCDs and their dewars, and the saddle
bags which contain the power supplies and the 10 L transfer dewars. The saddle
always stays with the camera during normal operations, and is only removed
when the camera is serviced.
2) Spectrograph: The spectrograph
bodies are permanently mounted to the rotator. When we talk about changing
instruments, we are talking about the cartridges which take the 640 fibers,
holding one end of each in position on the focal plane in a plug plate, and
route them to one of two slitheads which send the light into the spectrographs
themselves. The cartridges require a corrector lens to be installed above the
plug plate which is not present when the imager is installed.
a) Spectrographic Corrector
3) Engineering Camera: A modified
spectrographic cartridge without slitheads and containing a CCD camera of the
same type used in the spectroscopic guider. It is used to check optical
collimation and to acquire pointing model data.
Instrument Lift: This is a hydraulic
lift located at the center of the fork of the telescope. When the telescope is
at the zenith and the rotator is in the proper position and an instrument has
been rolled into position above the lift, the lift will move the instrument
between its cart and its mount position on the rotator. The lift is controlled
via a hand paddle near the lift.
Latches: A series of pneumatic
latches hold the instruments and accessories on the rotator. They are
controlled via a panel on the outside of the mirror cell of the telescope (
OSS ). There are three sets of latches. The first are the three primary
instrument latches. They hold the instrument against the kinematic mounts on
the rotator. The second are the corrector latches. In spectroscopy mode, they
hold the spectroscopic corrector lens in place. In imaging mode they serve as
a backup set of latches holding the camera. Each of set of latches are spaced
120 degrees apart around the Cassegrain hole in the rotator. The two sets are
offset by 60 degrees. Thus there is a latch every 60 degrees around the
rotator, and they alternate designation between primary and corrector latch. A
third set of latches is used for the imager only. They are called Saddle
latches and hold the camera saddle bags.
Cartridge Cart: The cart is used to
transport cartridges between the cartridge rack and the instrument lift. It
can hold two cartridges at once. This is for the situation when changing from
one cartridge to another. Currently the corrector is also transported using
the cartridge cart. The corrector rides on top of a cartridge.
Imager Operations Cart: The cart is
used move the imager between the dog house and the instrument lift, and rolls
on a set of permanently installed rails.
Dog House: This enclosure for the
imager is located on the rotating portion of the floor which surround the
telescope out to a radius of about six feet. The dog house is mounted "behind"
the telescope in line with the fork. That is in the same plane that the
altitude axis moves in, but on the opposite side of the fork from the
Cartridge Rack: This is a nine bay
hydraulic rack which is accessed through a garage door in the Support
Building. There are three levels which can each hold up to three cartridges.
The rack stops in one of three positions so that each level can be accessed by
the manipulator to remove or replace cartridges.
The manipulator is used to transfer cartridges between the rack
and the cart. It picks up the cartridge by a couple of handles mounted on the
outside of the cartridge body, above its center of gravity. When retrieving a
cartridge it raises one slightly off the rack, is then swung out so that the
cartridge is suspended above the cart. The manipulator then lowers the
cartridge onto the cart. This is all controlled by a deadman switch in the
manipulator handle. It is up to the operator to guide the cartridge and
initiate raising and lowering.
The instrument change system is fully functional. It is
capable of mounting, dismounting, and storing all of the instruments. It is
not however, completely finished. Currently instruments are mounted and
dismounted in manual mode. By the summer there will be an automatic mode in
which the Motion Control Processor ( MCP ) will monitor and restrict actions
through the interlocks system. This will be the normal mode of operation
during instrument change. When implemented it will increase operational
efficiency, and definitely increase the safety of the instrument change
The spectrographic corrector handling and storage will change
significantly over the next couple of months. Currently the corrector is
transported, installed, and removed from the telescope on top of a cartridge.
It is stored in the cartridge rack, and has a special fixture to allow it to
be handled with the manipulator. This is a time consuming and somewhat nerve
racking process. The plan is to construct a special cart which is used to
install and remove the corrector. When not in use, the corrector will be
stored in its cart in an enclosure mounted to the north side of the 2.5m
There a couple of maintenance issues which should be raised
here. First, this is a complicated system and requires several hardware and
software subsystems to work in concert. Thus modifications to any of the
systems involved must be considered very carefully before changes are made.
The software is handled through a version control system called CVS. Hardware
modifications are made through the Change Control Board. These facilities must
be used and combined with extensive testing to prevent interruptions to
operations due to unforeseen consequences of changes.
The state of the telescope and
instrument change system is conveyed to the interlock logic through a large
number of switches and sensors. It is a vital maintenance issue that these
switches and sensors remain in proper adjustment and working order, so that
incidents of false or misleading signals sent to the system are minimized.
There isn't a much more frustrating experience than being kept from doing
something because the interlock system is being fed incorrect or misleading
information ( the only thing worse is having the system act improperly when
given correct input ).