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Sloan Digital Sky Survey
Review of Observing Systems and Survey Operations

Instrument Change System Overview
Dan Long
April 11, 2000

Instrument Change System

The instrument change system for the SDSS 2.5m telescope consists of the equipment required to remove and install one of two types of instrumentation on the rotator of the 2.5m telescope. Also included are the storage areas for the instruments and the equipment for handling and transporting the instruments between the storage areas and the telescope.


Instruments 1 and 2 are the science instruments for the survey. The engineering camera is diagnostic tool for engineering and maintenance tasks. The instruments mount to the rotator of the telescope at the Cassegrain position via a system of latches. Each instrument is conveyed from its storage area to the telescope via a cart. The instrument lift, located in the telescope fork, is then used to raise or lower the instrument between the cart and the rotator.


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
b) Spectrographic Cartridge

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 secondary truss.

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.

Cartridge Manipulator: 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.

Current Status

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 process.

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 enclosure.

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 ).

Review of Observing Systems and Survey Operations
Apache Point Observatory
April 25-27, 2000

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