Cryostat Disassembly

The Berger-Lahr motor drive racks must be depowered before the motor cables are unplugged, otherwise they will suffer damage.

As the cryogenic stepper motors are revealed they should be protected by placing some protective plastic film over the yellow label to prevent dirt getting into the bearing.

Many of these procedures require use of cranes and some manual lifting.   It is recommended that at least one person trained in slinging techniques is present throughout.

2.1   Unbolt and remove the section of the calibration unit which covers the cryostat window. This section contains the virtually irreplaceable polarimetry optics and so it should be treated extremely carefully. They should be removed from the unit and stored safely as soon as possible.

2.2   Protect the cryostat window from water vapour damage by mounting the window cover plate with its six mounting screws.

2.3   Remove the flux collector assembly followed by the optics housing and store in a suitable safe, dry place.

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3.1   Mechanism Check

Using CCS dm screens do a force datum on the mechanisms in turn.   One person should observe the mechanism itself for motion, noise etc., whilst the other should observe the dm screen for datum switch operations etc.

Before driving the motors remove all perspex films used to cover the motor shafts.

Open the (nitrogen gas filled) cryostat to atmosphere, preferably using a controllable rotary valve rather than the gate valve on the turbo pump.   

  Be sure to check that all sensors have been stable at or above ambient temperature for several hours, to ensure that condensation will not form in the opened cryostat.

Temperature Sensor Check

Indicate temperature reading either directly from scanners or from CCS

4   Removing the Front (Imager) Cover (1 hour)

4.1   Rotate the cryostat until the interface between the centre section and the front section is horizontal.   Pull out the two alignment dowels at the bottom corners which align the front cover and hence calibration unit to the rest of the cryostat.

4.2   Because access to bolts along the side nearest the manipulator is restricted, these should be removed first by rotating the cryostat such that the front face is vertical.

4.3   Now rotate the cryostat until the front face is horizontal.   And attach the four corner eye bolts and using two slings, one along each side, attach to the overhead crane.   Remove the remaining bolts retaining the front cover.

4.4  Lift off the front cover (mass of front cover = 220kg) and lower it onto its flat storage plate for later evacuation and storage. Note that the unused set of fore-optics could be stored on the flat cover so you may wish to leave the front section suspended until you have removed them.

4.5   Check the front section MLI blanket for damage, undo the tape and velcro pads which secure it to the MLI on the centre section of the cold optics, and remove for storage , ideally under vacuum.

Figure 4.4 The front section multi-layer insulation.

Radiation shield cover

4.6   Remove the sensor plate hiding the Temperature sensor on the Front Radiation shield and disconnect. The loose connector and lead can be taped to the focus translation stage for safety.   Replace the plate on the front for safe keeping.   With the front face vertical, undo the sixteen screws on periphery of shield, the three shorter screws at the copper thermal wick and lastly the eight quick release catches to remove the front section.

Store the screws and associated belleville washers in the box provided, keeping the shorter wick screws separate.

Place the perspex cover over the UKIRT fore-optics by reusing some of the screws holding on the front shiny plate.

This procedure should only be undertaken by competent personnel who have had previous instruction from a member of the Michelle electronics team.

It is not necessary to remove the whole of the 4K arm in order to remove the array housing.

First check that the instrument is earthed through a low impedance safety earth connection (see installing the detector box and array)

Remove the section of radiation shield in front of the 4K arm.

Undo the screws for the four clock and bias connectors, there are two screws per connector, remember to slacken a screw on either side of each connector alternately.

Remove the earth tags connected to the circular copper cold block, disconnect the temperature sensor connector and remove the right angle copper block which is attached to the centre of the circular copper block at the end of a wick from the 4K arm, replace the single M6 screw and invar bush in the circular copper block for safe keeping.

Carefully move the cables, wires and wicks now hanging from the end of the 4K arm to the side, remove the baffle plate (three M3 socket screws) from along the gap between the detector box radiation shield and the front optical bench before undoing the eight M3 button head screws to remove the main detector box radiation shield.  
Do not undo the four M3 socket cap screws.

Refit an ESD lead to the circular copper cold block.

To remove the detector box cover loosen all fourteen M3 slotted screws, Then remove the eight connector screws first, followed by the six screws holding the cover to the detector box, It will be necessary to temporarily disconnect the ESD lead in order to remove this cover.

Fit shorting plugs to the four connector positions before finally removing the ESD lead connection to the circular cold block.

Undo the four M3 socket cap screws at the corners of the inner gold array housing and carefully remove it from the detector housing. Note the positions and arrangement of the copper washers, G10 spacers and mylar insulators these should be replaced in the detector housing along with the screws for safe keeping.

Cover over the square hole into the array housing and transfer it to an ESD dissipative bag then store it away safely, preferably in a clean room environment or if one is not available then in a protected dry atmosphere.

5.3  Disconnect the array cables at the four connectors (2 jackscrews each) at the back of the detector box, the connector to the array temperature sensor and heater and the small black connector to the DT470 temperature sensor. Immediately attach four 1 million Ohm grounding paths, running from the detector focus and translation mechanism to locations as listed below, and as shown in Figure 5.3.

1. To the top of the Joule-Thomson cooler, which should in turn be connected to the telescope ground.

2. To a conducting mat placed on which the person installing the detector box should stand.

3. To a wrist strap on the installer.

4. To the grounding stud on the gold detector box or (as shown in the Figure) to the base of the detector cold finger.

   

   Figure 5.3 Grounding the detector box when the detector cables are not connected. (Cooling arm has been removed).

5.4 Unscrew the single bolt with its Invar washer which holds the copper block to the back surface of the array, and remove the 25K shield by undoing its 3 screws.

5.5 Undo the screws which hold the array connectors to the inside of the array housing cover, and then remove the array housing cover by undoing its 6 screws. (see Figure 5.3).

5.6 Remove the gold detector box by undoing its four screws and their insulating washers, and transfer it to a grounded antistatic tray or bag for immediate transfer to a clean and electrically safe area. The aperture in the gold box over the array should be covered.

Figure 5.6 The gold detector box ready for removal and storage (should be grounded!).

6   Removing the 4K arm (30 minutes)

   This Procedure should only be undertaken by competent personnel who have had previous instruction from a member of the Michelle electronics team.

   First check that the instrument is earthed through a low impedance safety earth connection (See installing the detector box and array)

Connect an ESD lead from a conducting mat to the copper block on the corner of the detector box. Ensure that the detector box assembly is properly grounded through the cryostat and that the person who will be removing the 4K arm is wearing a connected wrist strap, gloves and will be standing on the mat.

Undo the button head screws from end section of radiation shield and remove both components, replace the screws in copper arm and secure the two pieces of radiation shield together for safe keeping.

Remove the front and side sections of the box surrounding the gold 4K plate at bottom of JT radiation shield by undoing the socket head screws from along the bottom and back edges.

Remove last three pieces of the radiation shield around the copper arm and the curved pieces from around the gold 4K plate.      Note that one of these pieces is fixed by acorn nuts to studs on the plate and the other is only guided by screws at the rear left between the JT radiation shield and the main optical bench.

All the pieces of the 4K arm radiation shield should now have been removed, the curved shaped bracket attached to the bottom of the JT radiation shield need only be removed if you intend removing the JT system from the cryostat.

Undo the screws and disconnect the four clock and bias connectors, there are two screws per connector, remember to slacken a screw on either side of each connector alternately.

6.   Remove the front and right hand side covers (Al sheet) from the J-T cooler. (Note that one of the covers is only guided by the screws at the rear left - IRB). Then remove all shields from the copper arm.

   First Ensure that the 4K arm and its radiation shield have been removed.

Remove the curved shape bracket from the bottom of the JT radiation shield.

Remove the two halves of split ring and the 0-ring through front of cryostat.

Ensure that any pressure is released from the JT system then remove the Helium pressure relief bends and fit protection caps.

Rotate cryostat until JT system is horizontal.

Position the JT handling frame on the lifting trolley then line up the frame with the six hexagonal stand offs on the JT plate then secure in place using nuts.

Undo the nine M5 screws with Invar spacers, G10 washers and G10 bushes from cryostat plate fitting the alignment pins provided sequentially as the screws are removed.

Once JT system has been retracted, slide the frame back on the trolley and secure with clamps.

Mechanism Check

Figure 8. Removing the Front Optical Bench.

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8.6 Cover the exposed field rotator optics and the opening to the spectrometer camera with the perspex covers provided.  (Spectrometer cover needs to be taped in place.)

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9.1   Following removal of the Front Optical Bench a perspex cover should have been fitted to the front of the slit and field rotator. If not find and fit.   Disconnect and remove the two motor cables.   Disconnect the two status cables at the Radiation Shield end only and tape to the base of the assembly.   Disconnect the thermal wicks at the motor end, replacing the screws in the motor housing for storage.   Screws for the wicks should be replaced in the motor housings for storage.

9.2   Remove the tape at the rear of assembly used for light sealing.  Rotate the instrument until the Slit/Field Rotator assembly is well supported, as shown in Figure 9.2. Undo the three bolts which secure the assembly to the MOB and remove it, taking care not to damage the cabling on the Radiation Shield, in particular where the field rotator motor passes the bracket on the radiation shield.

Figure 9.2 The orientation of the field rotator and slit wheel assembly for removal.

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10   Removing the Closed Cycle Cooler (20 minutes)

10.1 Undo the 12 screws holding the two copper coloured flat plates which connect the copper wicking between the adapter plate at the end of the second stage of the cooler and the main optical bench, (as shown in Figure 10.1). Remove the wick.

Figure 10.1 The closed cycle cooler second stage flexible wicking. The position of the temperature sensor on the adapter plate(now copper) is shown inset.

Figure 10.4 Unbolting the CTI1020 cold head from the vacuum vessel.

Figure 10.5 Removing the CTI1020 cold head from the vacuum vessel. Note that the copper wicking has been replaced by a solid copper sleeve.

11.1 Position the cryostat with the grating drum at the top and the interface between the grating and the centre section horizontal. Remove the bolts connecting the grating drum vacuum cover to the centre section.

   Note that as with the front cover, the screws nearest the manipulator may need to be removed first with the interface surface vertical to ease access.   

Install eyebolts in the grating drum cover, lift it off (mass = 100kg), and lower it onto a table. (see Figure 11.1).

Figure 11.1 Removing the grating drum vacuum cover

• Ensure all motor drive cards have been depowered before disconnecting motor cables.
• Leave the wavelength drive motor wick attached at the motor end to help in drum installation.
• The wick attached to the motor which rotates the drum is tricky to access
             (see Figure 11.4).

Figure 11.4 The thermal wicks on the grating drive motors. The hand is pointing to the wavelength drive wick at the left (note the grey snubber bar on the radiation shield). The right hand picture shows the difficult to access grating exchange motor wick.

11.5 Fit five grey coloured bar shaped snubbers to the centre section radiation shield using the radiation shield screws to finger tightness. (they act as guides when withdrawing the drum). Each snubber is numbered as is its location on the radiation shield.

11.6   Fit two lifting shackles (the third is not required) to the drum on the side nearest to the motors, undo the four captive bolts holding it to the centre section using the special tools provided (two Allen key ended bars 57cm and 67cm long) and lift it out of the centre section as shown in Figure 10.6 (drum mass 135 kg).

Figure 11.6 Removing the grating drum.

11.7   As soon as the drum is raised far enough mount the protective perspex cover over the open face of the centre section to protect the spectrometer collimator and camera optics.   Four short screws are needed.

11.8   Unplug and remove the three grating exchange motor drive and datum sensor cables from the centre section.

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Figure 12.1. Removing the support collars on the nitrogen fill tubes.

Figure 12.2 Removing the collet clamps from the MOB nitrogen fill tubes (Joule-Thomson cooler removed).

Figure 12.3 Undoing the bolts holding the MOB onto the truss.

Figure 12.4 Lifting the MOB from the vacuum vessel, with the fill tubes being guided through the radiation shield.