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Telescope Tracking

Corrector/cartridge installation/removal procedures and interlocks

Sloan Digital Sky Survey Telescope Technical Note 19970730

Walter A. Siegmund

Contents

Introduction

Light is transported from 640 galaxies and other targets on the focal surface of the Sloan Digital Sky Survey (SDSS) 2.5-m telescope to the two SDSS spectrographs by optical fibers, one for each target. A system of nine spectrographic cartridges, quick exchange mechanisms, controls and procedures allows the spectra of a different set of targets to be obtained about once an hour.

Each spectrographic cartridge supports and protects the optical fibers, slit-heads and plug-plate. One end of each fiber is terminated in a steel plug. These plugs are plugged into holes drilled in a ø795 mm aluminum plug-plate that is deformed to match the telescope focal surface. The other ends of the fibers terminate on one of two fiber slit plates. Each fiber slit plate, in turn is part of the fiber slit-head.

Cartridges plug into a socket on the back of the telescope. As part of the mating process, the pair of associated slit-heads plug into receptacles in the spectrographs. A computer controlled instrument lift is used to install and remove cartridges. The same computer controls latches that preload the cartridge against its kinematic mounts on the telescope and the slit heads against their kinematic mounts in the spectrographs.

The day after a successful night of spectrographic observations, each used plug-plate is unplugged and removed from its cartridge. Then, a new plug-plate is installed in the cartridge and plugged leaving the cartridge ready for use the following night.

During periods of excellent image quality and sky clarity, the cartridge that is on the telescope is removed along with the spectrographic corrector, a lens located just in front of the plug-plate. This allows the SDSS camera to be installed using the same mechanisms and computer controls that is used for cartridge interchange.

The 2.5-m telescope enclosure is very close fitting and rolls off the telescope for observations. The telescope is stowed pointed at the horizon and cannot be moved significantly inside the enclosure. All instrument exchange operations are performed with the telescope pointed at the zenith with the enclosure rolled off the telescope.

The SDSS camera and the spectrographic corrector are very expensive and fragile. Major damage to either would have a very large adverse impact on the survey project. Consequently, a motion control processor (MCP) controls and sequences the exchange and an electronic interlock system (ILK) monitors the interchange of these components. The function of the latter system is to interrupt the operation and alert the observers through the MCP if trouble is detected.

Two individuals, called observers, are expected to operate the 2.5-m telescope and monitor the quality of the data produced. They are responsible for changing instruments, i.e., spectrographic cartridges, SDSS camera, etc., during the night. Prior to an instrument exchange, the cool observer (so-called since the outside temperature is generally below the room temperature and to contrast with the warm observer who remains in the warm control room) goes to the telescope to verify that everything is ready for the exchange. The warm observer remains in the control room and completes the last observations prior to the exchange. During the exchange process the two cool observers have available two-way voice communication, e.g., a hands-free intercom. Also, the warm observer can view the activities of the cool observer on a video monitor, although the low level illumination does not provide high quality images.

Upon completion of the last observations, the warm observer starts the desired instrument exchange script and passes responsibility for the exchange to the cool observer. The cool observer performs the manual operations and monitors automatic operations.

After the instrument exchange is completed, responsibility for the telescope passes back to the warm observer who begins the sequence leading to the next observations. The cool observer finishes any exchange tasks remaining and does preliminary preparation for the next anticipated exchange, as appropriate, before returning to the control room to assist with the observations.

This report provides a narrative that can be used to develop the functional specifications for that portion of the motion control processor (MCP) and the interlock (ILK) system that controls and monitors cartridge exchange and spectrographic corrector installation and removal. Also, it is a draft of the procedures that will be used for these processes.

Installing/removing the spectrographic corrector

The spectrographic corrector can only be installed or removed with the telescope enclosure rolled off the telescope and with the telescope vertical. When the spectrographic corrector is off the telescope, it is stored in a sealed box (the cat house) mounted near the middle of the south wall of the telescope enclosure. It is stored on edge with the sky side facing the wall. The box contains an air dryer, is purged with dry N2 or is heated to prevent condensation on the corrector. The corrector can only be put into or taken out of the cat house when the telescope enclosure is rolled off the telescope due to the proximity of the wind baffle when the enclosure is over the telescope.

The spectrographic corrector is transported between the telescope and the cat house using the cartridge cart. It is carried atop any of the spectrographic cartridges. The corrector and its housing weighs about 490 N (110 lbs) and a cartridge weighs about 1300 N (300 lbs).

Removing the cartridge/corrector assembly

Normally, removal of the cartridge/corrector assembly will followed immediately by installation of the SDSS camera or stowing of the telescope. The corrector is always removed with a cartridge, i.e., the cartridge/corrector assembly. If a cartridge is not present, one must be installed.

  1. The cartridge cart (assumed empty) is moved to the cartridge holding area. This is the area on the grating adjacent to the SDSS enclosure observing floor. In this area, the cool observer and cartridge cart are clear of the telescope and wind baffle.
  2. The cool observer waits for the telescope to slew to the cartridge exchange orientation.
  3. The telescope finishes taking data. The warm observer verifies that the data are (at least) nominally satisfactory and issues the command to proceed.
  4. The telescope slews to the instrument exchange orientation.
  5. The MCP engages the telescope axis brakes when the telescope axes have stopped. It turns off the telescope axis servos.
  6. The MCP verifies the the telescope axes are at the correct angles for instrument exchange. It turns on a light to indicate that instrument exchange may proceed.
  7. The cool observer pushes one of the stop buttons. In this state, the telescope and wind baffle axes are prevented from moving until the cool observer releases all buttons.
  8. The cool observer turns on low level illumination lights.
  9. The cart is moved under the telescope and positioned at the proper mechanical detent.

The warm observer starts the program in the MCP for lowering the cartridge/corrector assembly on the cartridge cart. The MCP software routine for lowering the cartridge/corrector assembly is enabled only if

  1. The cartridge/corrector assembly is on the telescope.
  2. All the cartridge and corrector latches are fully latched.
  3. The corrector protection bolts are fully engaged.
  4. The telescope altitude is 90° and the azimuth is 120° east of south, i.e., the instrument exchange angles.
  5. The instrument rotator is at 0°, the instrument exchange angle.
  6. One or more stop buttons are depressed. The telescope and wind baffle axes drive power is off and the telescope and wind baffle altitude brakes are engaged.
  7. The cartridge cart is identified and an empty position is in the proper position on the instrument lift.

The cool observer pushes the deadman lift up switch. The lift begins. The cart floor engagement switch indicates engagement. The cartridge present switch continues to indicate that no cartridge is present. The lifting force is monitored to make sure the envelope in force/height space established for the bare cart floor is not exceeded. Lifting stops immediately when the envelope is exceeded or the lift force limit switch trips. The cool observer releases the deadman lift up switch. During the lift, the MCP rebalances the telescope for no corrector/cartridge assembly.

The MCP enables the latch controls only if

  1. the instrument lift height and force is within tolerance. (If the instrument lift height and force is within tolerance, the cartridge/corrector assembly is supported by the instrument lift.)
  2. The lift force limit switch detects the force limit.

The cool observer manually disengages the two corrector protection bolts. The cool observer unlatches the 3 corrector latches and 3 cartridge latches. The cool observer and MCP must check after an appropriate interval (a few seconds) that they are in FACT fully unlatched.

The MCP software routine for lowering the lift is enabled only if

  1. The corrector latches are fully unlatched.
  2. The cartridge latches are fully unlatched.
  3. The spectrograph slit-head latches are unlatched.
  4. The corrector protection bolts are fully disengaged.
  5. The slit-head door is fully open.

The cool observer performs a visual inspection to insure that it is safe to lower the instrument lift. The cool observer pushes the deadman lift down switch. The MCP lowers the cartridge/corrector assembly. The lowering force is monitored to make sure the envelope in force/height space established for cartridge/corrector assembly lowering is not exceeded. Lowering stops immediately when the envelope is exceeded.

The cool observer releases the deadman lift down switch. The MCP closes the spectrograph slit-head doors only if

  1. The cartridge present switch indicates no cartridge present.
  2. The cart floor engagement switch indicates no engagement.
  3. The lift is fully down.

The cool observer is informed that it is OK to roll the cartridge cart, with the, to the cartridge holding area. The cool observer does so. If the cartridge/corrector assembly has been removed so that the SDSS camera can be installed, then that procedure begins.

Otherwise, it is likely that the telescope will be stowed and the building closed. The cool observer performs a visual inspection for obstructions. If everything is satisfactory, the cool observer releases all stop buttons. This allows the telescope and wind baffle to move under computer control and should only be done after the cool observer and cart are clear of the SDSS enclosure observing floor. The cool observer and the MCP notify the warm observer that the exchange is complete.

The warm observer starts the sequence of commands to install the SDSS camera or to stow the telescope. If the command to stow the telescope is issued, the MCP turns off the exchange light. The MCP releases the telescope brakes and enables the telescope drive servos only if

  1. The telescope is in balance, i.e., the counterweight positions are correct.
  2. The MCP receives an INIT command from the TCC.
  3. The cart is not present.

Transporting the corrector to the cat house

To move the corrector to the cat house, the following procedure is defined.

  1. The corrector cover is installed to protect the corrector.
  2. The cartridge/corrector assembly is moved to the cat house using the cartridge cart.
  3. The cartridge cart is moved to the cat house and is pushed against locating surfaces.
  4. The cat house is opened and the corrector lift mechanism is attached to the corrector.
  5. The corrector is transferred to the cat house.
  6. The cool observer removes the plug-plate cover from its storage location and installs it on the cartridge.

Transporting the corrector to the instrument lift.

The corrector installation process begins with a cartridge on the cartridge cart and located outside the cartridge storage area. Normally this will be the first cartridge to be used for spectrographic observations. It is likely that the engineering camera cartridge can be used to move the corrector. If so, this cartridge may be not be stored in the cartridge storage area and the procedure for transferring it to the cartridge cart may be different.

To move the corrector to the instrument lift, the following procedure is defined.

  1. The cartridge cart is moved to the cat house and is pushed against locating surfaces.
  2. The cool observer removes the plug-plate cover and places it in its storage location.
  3. The corrector is transferred to the cartridge.
  4. The cat house is closed.
  5. The cartridge cart is moved to the cartridge holding area. This is the area on the grating adjacent to the SDSS enclosure observing floor. In this area, the cool observer and cartridge cart are clear of the telescope and wind baffle.
  6. The cool observer waits for the telescope to slew to the cartridge exchange orientation.
  7. The warm observer issues the command to proceed with the installation.
  8. The telescope slews to the instrument exchange orientation.
  9. The MCP engages the telescope axis brakes when the telescope axes have stopped. It turns off the telescope axis servos.
  10. The MCP verifies the the telescope axes are at the correct angles for instrument exchange. It turns on the exchange light to indicate that instrument exchange may proceed.
  11. The cool observer pushes one of the stop buttons. In this state, the telescope and wind baffle axes are prevented from moving until the cool observer releases all buttons.
  12. The cartridge/corrector assembly is moved on its cart under the telescope and positioned at the proper mechanical detent.

Installing the cartridge/corrector assembly

The corrector is always installed with a cartridge, i.e., the cartridge/corrector assembly. The warm observer starts the program in the MCP for lifting the cartridge/corrector assembly. The MCP software routine for lifting the cartridge/corrector assembly is enabled only if

  1. The telescope is bare--the SDSS camera, another cartridge or the engineering camera are not already installed.
  2. The telescope altitude is 90° and the azimuth is 120° east of south, i.e., the instrument exchange angles.
  3. The instrument rotator is at 0°, the instrument exchange angle.
  4. One or more stop buttons are depressed. The telescope and wind baffle axes drive power is off and the telescope and wind baffle altitude brakes are engaged.
  5. The cartridge/corrector assembly is on the cartridge cart. However, it does not need to distinguish between a cartridge and the cartridge/corrector assembly prior to lifting since the weight difference is evident once lifting begins.
  6. The cartridge cart is positively identified with a cartridge properly positioned with respect to the lift.
  7. The cartridge and corrector latches are fully unlatched.
  8. The two manually installed corrector protection bolts are fully disengaged.
  9. The spectrograph slit-head latches are unlatched.

The cool observer removes and stows the corrector cover. The cool observer pushes the deadman lift up switch. The MCP opens the slit-head doors. Once the slit-head doors are fully open, the lift begins. The cart floor engagement switch indicates engagement. The cartridge present switch indicates that a cartridge is present. The lifting force is monitored to make sure the envelope in force/height space established for the cartridge/corrector assembly is not exceeded. Lifting stops immediately when the envelope is exceeded or the lift force limit switch trips. The cool observer releases the deadman lift up switch.

The MCP enables the latch controls only if

  1. The instrument lift height and force is within tolerance.
  2. The lift force limit switch detects the force limit.
  3. The instrument ID switches indicate registration with the kinematic mounts and proper identification.

The cool observer latches the 3 corrector latches and 3 cartridge latches. The cool observer and MCP must check after an appropriate interval (a few seconds) that they are in FACT fully latched. The cool observer disables the latches. The MCP latches the slit-head latches. The cool observer manually engages the corrector protection bolts.

The MCP software routine for lowering the lift is enabled only if:

  1. The corrector and cartridge latches are fully latched.
  2. The corrector protection bolts are fully engaged.

If the slit-head latches fail to latch, an error message is displayed. However, the lift is not disabled. The cool observer performs a visual inspection to insure that it is safe to lower the instrument lift. The cool observer pushes the deadman lift down switch. The MCP lowers the bare cart floor (leaving the cartridge/corrector assembly installed). The lowering force is monitored to make sure the envelope in force/height space established for bare cart floor lowering is not exceeded. While lowering, the MCP rebalances the telescope for the corrector/cartridge assembly.

Lowering stops immediately when the envelope is exceeded. The cool observer releases the deadman lift down switch. The cool observer is informed that it is OK to roll the cartridge cart to the cartridge holding area only if

  1. The cartridge present switch indicates no cartridge present.
  2. The cart floor engagement switch indicates no engagement.
  3. The lift is fully down.

The cool observer does so and performs a visual inspection. If everything is satisfactory, the cool observer turns off the low level illumination and releases all stop buttons. This allows the telescope and wind baffle to move under computer control and should only be done after the cool observer and cart are clear of the SDSS enclosure observing floor. The cool observer and the MCP notify the warm observer that the exchange is complete.

The warm observer starts the sequence of commands that begin the next observation. The MCP turns off the exchange light. The MCP releases the telescope brakes and enables the telescope drive servos only if

  1. The telescope is in balance, i.e., the counterweight positions are correct.
  2. The MCP receives an INIT command from the TCC.
  3. The cart is not present.

Exchanging spectrographic cartridges

Spectrographic cartridges can only be installed or removed with the telescope enclosure rolled off the telescope and with the telescope vertical. Spectrographic cartridges are stored in the cartridge storage area of the SDSS support building. About sunset, the inside door of the cartridge storage area is closed and the outside door is opened. Also, the cartridge storage area exhaust fan is turned on. This allows the cartridges to equilibrate to the outside temperature.

The cartridge cart has places for two cartridges. During the brief period after a used cartridge has been removed and before a new cartridge is installed both positions are occupied. However, most of the time, the cart carries only one cartridge. The position nearest the cart handle is normally used for the "new" cartridge, i.e, the next unused cartridge to be installed on the telescope. The other position is normally used for the "used" cartridge, i.e., the most recently removed used cartridge that is to be moved to the cartridge storage area. However, this convention will not be interlocked or rigidly enforced.

During evening twilight, the cartridge cart is moved from its storage location to the cartridge storage area, and the first cartridge of the night is transferred to the new cartridge position on the cart. The wind-forced ventilation of the cartridge on its cart will enhance equilibration (as long as the cartridge is radiatively isolated from the sky by low emissivity coatings).

Cartridge exchange is a significant part of the time required for spectrographic observations. Consequently, it is desirable that the time spent changing cartridges be minimized. Also, it should be noted that the cartridges are robust and inexpensive, at least by comparison to the SDSS camera and the spectrographic corrector. As a result, it is desirable to emphasize time efficiency more and protection less as compared with the handling processes for the SDSS camera or spectrographic corrector. However, these processes should be as similar as possible unless efficiency dictates otherwise, to minimize errors.

Moving the new cartridge to the instrument lift

The cartridge exchange process begins with the new cartridge on the cartridge cart and located outside the cartridge storage area.

  1. About two minutes prior to the end of the previous spectrographic observation, the cartridge cart is moved to the cartridge holding area. This is the area on the grating adjacent to the SDSS enclosure observing floor. In this area, the cool observer and cartridge cart are clear of the telescope and wind baffle.
  2. The cool observer removes the plug-plate cover and the slit head covers and places them in their storage locations.
  3. The cool observer waits for the telescope to slew to the cartridge exchange orientation.
  4. The telescope finishes taking data. The warm observer verifies that the data are at least nominally satisfactory and issues the command to proceed.
  5. The telescope slews to the instrument exchange orientation.
  6. The MCP engages the telescope axis brakes when the telescope axes have stopped. It turns off the telescope axis servos.
  7. The MCP verifies the the telescope axes are at the correct angles for instrument exchange. It turns on the exchange light to indicate that instrument exchange may proceed.
  8. The cool observer pushes one of the stop buttons. In this state, the telescope and wind baffle axes are prevented from moving until the cool observer releases all buttons.
  9. The cool observer turns on low level illumination lights.
  10. With the telescope secured, the cool observer moves the cartridge cart under the telescope and positions the used cartridge position over the instrument lift at the proper mechanical detent.

Removing a used cartridge

The cool observer starts the program in the MCP for removing a used cartridge. Normally removal of a used cartridge would be followed immediately by installation of a new cartridge. However, it should be possible to execute removal independently of installation. The bare cart floor can be then be lifted only if

  1. The telescope has the spectrographic corrector installed.
  2. It must have a cartridge or engineering camera installed. This interlock is necessary since the SDSS camera could be damaged by attempting to remove the SDSS camera with the cartridge cart.
  3. The telescope altitude is 90° and the azimuth is 120° east of south, i.e., the instrument exchange angles.
  4. The instrument rotator is at 0°, the instrument exchange angle.
  5. One or more stop buttons are depressed. The telescope and wind baffle axes drive power is off and the telescope and wind baffle altitude brakes are engaged.
  6. The cartridge cart is positively identified and properly positioned with respect to the lift.
  7. The cartridge and corrector latches are all fully latched.
  8. The corrector protection bolts are fully engaged.

The cool observer pushes the deadman lift up switch. The lift begins. The cart floor engagement switch indicates engagement. The cartridge present switch continues to indicate that no cartridge is present. The lifting force is monitored to make sure the envelope in force/height space established for the bare cart floor is not exceeded. Lifting stops immediately when the envelope is exceeded or the lift force limit switch trips. (A simpler approach would likely suffice for lifting cartridges but it is suggested that it may be simpler to implement the same algorithm that is used for the camera.) The cool observer releases the deadman lift up switch. During the lift, the MCP rebalances the telescope for the corrector with no cartridge.

The MCP software routine for disengaging the latches is enabled only if

  1. The instrument lift height and force is within tolerance.
  2. The lift force limit switch detects the force limit.
  3. The spectrograph slit-head latches are unlatched.

If the instrument lift height and force is within tolerance, the cartridge is supported by the lift. The spectrograph slit-head latches are unlatched. The cool observer unlatches the 3 cartridge latches. The cool observer and MCP must check after an appropriate interval (a few seconds) that they are in FACT fully unlatched.

The MCP software routine for lowering the lift is enabled only if:

  1. The spectrograph slit-head latches are unlatched.
  2. The cartridge latches are fully unlatched.
  3. The corrector latches are fully latched.
  4. The corrector protection bolts are fully engaged.
  5. The slit-head door is fully open.

The cool observer starts the program to lower the cartridge. The cool observer pushes the deadman lift down switch. Lowering begins. The lowering force is monitored to make sure the envelope in force/height space established for cartridge lowering is not exceeded.

Lowering stops immediately when the envelope is exceeded. The cool observer releases the deadman lift down switch. The MCP closes the spectrograph slit-head doors only if

  1. The cartridge present switch indicates no cartridge present.
  2. The cart floor engagement switch indicates no engagement.
  3. The lift is fully down.

The cool observer is informed that it is OK to roll the cartridge cart so the new cartridge position is located properly with respect to the instrument lift. The cool observer rolls the cartridge cart under the telescope and positions the new cartridge over the instrument lift at the proper mechanical detent.

Installing a new cartridge

The cool observer starts the program in the MCP for installing a new cartridge. It should be possible to execute this procedure independent of the procedure to remove a used cartridge. The operation can begin only if

  1. The telescope has the spectrographic corrector installed.
  2. It must NOT have a cartridge, the engineering camera, or the SDSS camera installed.
  3. The corrector latches are fully latched. The cartridge latches are fully unlatched.
  4. The corrector protection bolts are fully engaged.
  5. The telescope altitude is 90° and the azimuth is 120° east of south. This implies that the telescope enclosure is rolled off the telescope.
  6. The instrument rotator is at 0°, the instrument exchange angle.
  7. One or more stop buttons are depressed. The telescope and wind baffle axes drive power is off and the telescope and wind baffle altitude brakes are engaged.
  8. The cartridge cart is positively identified and properly positioned with respect to the lift.
  9. The spectrograph slit-head latches are unlatched.

The cool observer pushes the deadman lift up switch. The MCP opens the slit-head doors. Once the slit-head doors are fully open, the lift begins. The cart floor engagement switch indicates engagement. The cartridge present switch indicates that a cartridge is present. The lifting force is monitored to make sure the envelope in force/height space established for the cartridge is not exceeded. Lifting stops immediately when the envelope is exceeded or the lift force limit switch trips. The cool observer releases the deadman lift up switch.

The MCP enables the latch controls only if

  1. The instrument lift height and force is within tolerance.
  2. The lift force limit switch detects the force limit.
  3. The instrument ID switches indicate registration with the kinematic mounts and proper identification.

The cool observer latches the 3 cartridge latches. The cool observer and MCP must check after an appropriate interval (a few seconds) that they are in FACT fully latched. The MCP latches the slit-head latches.

The MCP software routine for lowering the lift is enabled only if

  1. The corrector and cartridge latches are fully latched.
  2. The corrector protection bolts are fully engaged.

If the slit-head latches fail to latch, an error message is displayed. However, the lift is not disabled. The cool observer pushes the deadman lift down switch. The MCP lowers the bare cart floor (leaving the cartridge installed). The lowering force is monitored to make sure the envelope in force/height space established for bare cart floor lowering is not exceeded. While lowering, the MCP rebalances the telescope for the corrector/cartridge assembly.

Lowering stops immediately when the envelope is exceeded. The cool observer releases the deadman lift down switch. The cool observer is informed that it is OK to roll the cartridge cart to the cartridge holding area only if

  1. The cartridge present switch indicates no cartridge present.
  2. The cart floor engagement switch indicates no engagement.
  3. The lift is fully down.

The cool observer rolls the cartridge cart to the cartridge holding area and performs a visual inspection. If everything is satisfactory, the cool observer turns off the low level illumination and releases all stop buttons. This allows the telescope and wind baffle to move under computer control and should only be done after the cool observer and cart are clear of the SDSS enclosure observing floor. The cool observer and the MCP notify the warm observer that the exchange is complete.

The warm observer starts the sequence of commands that begin the next observation. The MCP turns off the exchange light. The MCP releases the telescope brakes and enables the telescope drive servos only if

  1. The telescope is in balance, i.e., the counterweight positions are correct.
  2. The MCP receives an INIT command from the TCC.
  3. The cart is not present.

The cool observer removes the plug-plate cover and the slit head covers from their storage locations and installs them on the used cartridge. The cart is moved from the cartridge holding area to the cartridge storage area. The used cartridge is moved to a storage shelf and a new cartridge is moved to the new position on the cart. The cool observer secures the cartridge cart and returns to the control room to assist with the observations.

Engineering camera

The engineering camera is similar to a cartridge. Handling procedures and programs are analogous to those for a cartridge. However, it has no slit-heads. It is treated similar to a cartridge by the interlock controller and the MCP. Adjustments must be made because its weight is about 1200 N (270 lbs). Also, since it has no slit-heads, the spectrograph slit-head doors are left closed and the slit-head latches are left unlatched when it is installed.

Notes on instrument exchange

  1. The instrument lift can be programmed to move at four speeds. It will move slowly for the exchange of fragile and expensive instruments and while the instrument is near either the kinematic mounts on the back of the telescope or the cart on the floor.
  2. The instrument lift incorporates load cells that monitor the force exerted by the lift on the instrument. When the instrument is supported by the floor, this force is zero. But,when the instrument is supported by the lift, this will generally be the weight of the instrument (since acceleration is small) and when the instrument is mated to the kinematic mount, the force will be increased by the mating force. In addition, the height of the lift is monitored.
  3. One way to make use of this information is to define an envelope on a graph of force v. height. If this envelope is exceeded, e.g., due to an obstruction, lifting or lowering is terminated. Upon termination, the height and instrument id switches (if relevant) are checked to distinguish between normal and abnormal termination.
  4. The instrument latches are described in detail in "SDSS Camera Interface and Interlocks", J. Gunn, 7 Sept 1996, message 24 in the 2.5-m Telescope Controls archive. The instrument switches are described in "Interlocks for the 2.5-meter SDSS Telescope", John Anderson, Jr., 4 Nov 1996.
  5. The cat house transfer mechanism is a manually operated lift that allows precision transfer of the corrector between the cartridge and the cat house. The corrector is rotated 90 degrees during transfer so that the sky side of the corrector faces the wall. This reduces the enclosure floor area required.
  6. As an alternative to the cat house, it has been proposed that a special cart be built for the corrector and that a storage shed be added near the support building for the corrector on its special cart, the cartridge cart, and a cartridge or the engineering camera. The SDSS camera transfer cart may be stored in this shed also. Clearly, the corrector exchange procedure must be modified if this approach is adopted. A cartridge can be stored on the cartridge cart if it is convenient to do so.
  7. If no additional storage is available, the engineering camera cartridge will normally be stored in the lower level of the telescope enclosure. To use the engineering camera cartridge, it is lifted to the observing level using the 1 ton hoist and placed on the cartridge cart. During telescope testing, however, it will likely be stored in the cartridge storage area and the ninth cartridge will be stored elsewhere.
  8. It is necessary to locate the cartridge carts accurately before lifts are begun so that the conical pins on the underside of the lifting plate will find their sockets. The acceptance range is TBD.
  9. The deadman lift up switch or the deadman lift down switch must be depressed for the lift to operate. The cool observer releases the switch at the end of any lift or lowering operation or if anything abnormal occurs. Releasing the switch stops the lift. Both switches are enabled whenever the interlock logic indicates that the corresponding motion is permitted. Normally, this allows the cool observer to reverse the direction of motion if a problem occurs.
  10. The instrument, corrector and saddle latches are controlled by pushbuttons in the Instrument Latch Control Box on the front of the telescope. They are only enabled by the MCP when it is deemed safe to allow the cool observer operate the latches. They are disabled once latch operation is complete, i.e., within a few seconds.
  11. The MCP must wait for the telescope axes to stop moving before engaging the axis brakes. However, it is rare in servo systems that the measured velocity is exactly zero. It may be better to use the servo velocity command as an indication that the servo system is halted.
  12. During an exchange, the MCP controls the process. Commands from the TCC are not executed and error messages are generated. The MCP has control whenever the exchange light is on.
  13. In an emergency, the telescope can stowed most easily if it is either balanced or bottom heavy. Consequently, telescope rebalance commands are issued after installing an instrument and before removing an instrument. This insures that the telescope is either balanced or bottom heavy.
  14. The MCP always attempts to rebalance the telescope during instrument exchange. However, instrument exchange is not interlocked with telescope balance. This allows cartridge exchange without waiting for rebalance to complete and does not link moving the counterweights to the operation of the latches and instrument lift. Installation of a new cartridge interrupts a previous used cartridge removal rebalance process, if it is not complete. The MCP immediately begins rebalancing the telescope for the installed cartridge.
  15. The MCP generates an error message if a rebalance command is not successful. Also, the MCP verifies that counterweight positions are correct for the configuration of the telescope prior to releasing the telescope brakes and enabling the axis servos.

Tolerances for the instrument kinematic mount

The kinematic mounts and latches on the instrument rotator have a ± 6.3 mm acceptance range in both x and y. Consequently, the instrument must be located properly with respect to the instrument lift, its kinematic mounting surfaces must be mounted correctly and the instrument lift must not wobble significantly, even in the presence of wind or a load that is out of balance. Also, the telescope altitude and rotator axes must be at the correct angles for instrument exchange.

The azimuth axis angle is not critical once the instrument is mated to the lift since the lift and the telescope rotate together in azimuth. However, the azimuth axis angle must be correct to align the cartridge tracks between the fixed floor and the rotating circular floor panel. Also, since the cartridge cart is supported both by the fork base and the circular floor panel, misalignment of the telescope azimuth and circular floor panel may prevent proper mating of the cartridge to the instrument lift. See §4.11 HOME POSITION ACCURACY and §4.18 TELESCOPE-TO-WIND BAFFLE ALIGNMENT CLAMPS of "Technical Requirements and Performance Specifications for the 2.5-Meter Telescope" for these tolerances.

Table 1: Instrument kinematic mount position error budget .

Component

1D ± tolerance

Unit

Note

Instrument lift plate

1.5

mm

1

Instrument kinematic mounts

1.5

mm

2

Altitude home position

0.6

mm

3

Rotator home position

0.8

mm

4

Total error

4.4

mm

5
  1. The tolerance of the position of the locating holes on the instrument lift with respect to the instrument rotator kinematic mounts. The instrument lift is at the proper height to mate an instrument with the kinematic mounts. It is loaded with its maximum out of balance load (TBD). The altitude axis and instrument rotator are at their nominal exchange angles.
  2. The tolerance of the instrument kinematic mount and latch features with respect to the locating holes on the instrument lift plate. The instrument is allowed to seat itself on the instrument lift plate prior to the measurement. This specification applies to the design and fabrication of the instrument.
  3. The tolerance of the altitude home position about the nominal home position. It is expressed as a tolerance at the location of the kinematic mounts. See Table 2.
  4. The tolerance of the instrument rotator home position about the nominal home position. It is expressed as a tolerance at the location of the kinematic mounts. See Table 2.

Altitude axis and the instrument rotator enable switches sense whether the altitude and rotator axes are at the correct angle for instrument exchange. They are located near the outer diameter of their respective drive disks. The following Table converts axis home position angular accuracies to position tolerances at the kinematic mounts.

Table 2: Axis home position accuracies. All tolerances are ±.

Axis

Component

Value

Unit

Note

Altitude

Home position accuracy

0.05

deg.

1

Kinematic mount radius

640

mm

2

Kinematic mount position tolerance

0.6

mm

3

Rotator

Home position accuracy

0.1

deg.

4

Kinematic mount radius

440

mm

5

Kinematic mount position tolerance

0.8

mm

6
  1. From §4.11 of "Technical Requirements and Performance Specifications for the 2.5-Meter Telescope".
  2. The distance of the kinematic mounting and latch features from the altitude axis.
  3. The calculated tolerance of the altitude home position at the location of the kinematic mounts.
  4. From §4.11 of "Technical Requirements and Performance Specifications for the 2.5-Meter Telescope".
  5. The distance of the kinematic mounting and latch features from the rotator axis.
  6. The calculated tolerance of the rotator home position at the location of the kinematic mounts.

Faults and abnormal terminations

If a fault occurs during cartridge exchange, the observers should be aware of the following priorities for the protection of equipment from the weather. Generally, these priorities will be satisfied if the telescope can be stowed and the telescope enclosure rolled over the telescope. However, if the instrument lift cannot be lowered completely, it may interfere with stowing the telescope. Also, if the SDSS camera or spectrograph corrector latches cannot be fully latched or unlatched and the protection bolts cannot be engaged, it is not possible to safely retract the instrument lift. In this case, it is better to leave the telescope unprotected from the weather than to risk the SDSS camera or spectrographic corrector. The telescope can be dried and the primary mirror recoated.

  1. SDSS camera
  2. Spectrographic corrector
  3. Telescope and its optical coatings
  4. Engineering camera
  5. Spectrographic cartridge

The design of the MCP must facilitate the efforts of the observers to stow the telescope. Error messages should be developed in cooperation with the observers to insure that they are meaningful and clear. The design should not discourage the observers from interrupting an operation, e.g., by mandating a long delay or a complex procedure before resuming the operation. The design should allow reversal of any operation without defeating any interlocks since this will allow the observer to inspect a suspect mechanism or to clear a fault.

Date created: 07/30/97
            Last modified: 08/14/97
            Copyright © 1997, Walter A. Siegmund
            Walter A. Siegmund
siegmund@astro.washington.edu