McMath-Pierce FTS beam splitter change


Close off the vacuum pumps to Babar; (closed is up). Then open the air release (A/R). Remove the B port kilroy mount. Open one of the back tank windows to be sure the tank is at atmosphere. Close the air release valve. Hook up the crane and lift the tank. Use the 4-by-4's on the floor to block up the tank top. Switch on the electronics racks before bringing the castings up on oil.

Remove the movable input aperture from the front of the FTS and replace it with the fixed aperture (found in the detector cabinet). Set up the laser so that it's shining in through the front, centering the beam on the input aperture and the collimator mirror. Refer to the chart at the end of this document to determine which windows, mirrors, catseyes, and catseyes position. If the beamsplitter change requires exchange of the mirrors, swap out the mirrors one by one, before the catseyes. Realign the interferometer after each mirror change, using target circles on cards set up at the output ports. Use each mirror's individual adjustment knobs to regain the bullseye, or its centering, on the targets. Check the mirrors for smudges, etc., as they are exchanged. Clean them, if necessary, with alcohol and blow-dry with N2 or Ar.

Remove the lead blocks, noting their positions, from the top of the the south catseye. Carefully position the new catseye, making certain that it has seated correctly in its detents. Take care to avoid knocking the retro-mirrors at the east end of the catseye carrage. Note that offsetting the catseyes involves moving the catseye toward the center of itUs travel. It can be useful (particularly when working alone) to put a "WLF rod" in the track before swapping out each catseye. Replace the lead blocks and realign the catseye by recentering the "bullseye" using the south 8-inch flat.

Swap the north catseye in a similar manner to the south. Note that removal of the south catseye requires removal of the sum channel beamsplitter and "periscope setup" at the east end of the catseye. It may be convenient to move the thermometer. Install the new catseye and its lead blocks, and recenter the "bullseye" with the north 8-inch flat. Replace the sum channel beamsplitter and periscope setup. In order to realign the sum beamsplitter, first get back to a "rough" alignment by using the non-lighted hand paddle to close the gap between the two laser dots which should appear toward the top of the fume hood along the east wall of the FTS room. Plug hand paddle into "SUM 6V" receptacle (at FTS control panel), supplying at least 6V with the power supply. When the 2 spots appear on top of one another there should be enough SUM voltage on the meter to finish the alignment by maximizing the voltage.

After the necessary mirror and/or catseye exchanges have been completed, the main beamsplitter can be exchanged. It is useful to position one of the rolling carts alongside Babar to do the beamsplitter swap. Have the allen head set and the pair of slotted-end rods (found in the optics cabinet) within reach. Climb up into tank. Remove the lead weights taking care about the rubber viton damping pads. Remove the top plate (watch that pad!) and remove the screw attaching the bridge. Remove the bridge. Remove the masking from the beamsplitter and unplug the beamsplitter electrical power (pushing on the bracket while pulling on the plug). Use the two slotted-end rods to guide the beamsplitter during removal; carefully place rods through holes at either side of the base of the beamsplitter and be sure they seat firmly before lifting the BS. Be sure to control the connectors on the beamsplitters to avoid the possibility of a swinging plug damaging any optics. The UV beamsplitter requires the removal of the brass counterweight on the north side of the beamsplitter mount. Lift out the BS, carefully install the new one and reconnect the plug immediately. Remove the guide rods slowly, as they can pop suddenly from their moorings. Try "unscrewing" the rods. Reinstall the brass counterweight on the beamsplitter mount if it was removed.

Return the FTS to rough alignment as follows:

  1. Place cards at the "B" camera and "A" output positions.
  2. Adjust north 8-inch flat to get a bullseye. If the bullseye is too far off to see, first move the 8" flat to get the 2 laser spots on the BS to overlap.
  3. Move both carriages to opposite end of travel, maintaining carriage positions with with the long WLF rods, and recheck bullseye.
  4. Adjust collimator to half of the necessary correction for regaining bullseye.
  5. Return to shutdown (remove WLF rods) and obtain bullseye with the north 8-inch flat once again.
  6. Repeat steps 3-5 till bullseyes are roughly the same at both ends of Travel.

The Beginning of the End

Pull out the FTS Control Unit drawer. Turn unit off in the drawer with the toggle switch inside left of the drawer and pull out the leftmost card(A2) and replace it with the "old" Input card. Look up the typical gain in the FTS Optics Log for the beamsplitter going in and install the appropriate gain resistors. Power-up the drawer. Adjust the diode screws and feed mirror alignment on the beamsplitter mount, to achieve a maximum DIFF output voltage. Compare this with past DIFF voltages for the BS being used listed in the Optics Log. Check the DIFF voltage at the middle and far end of travel as well as in shut down. Replace the beamsplitter weights, damping pads and baffles.

Set up the "cheap" diode at the output and a white light source at the input. un the lamp at about 6 amps. Connect the power cable, A port signal cable and pre-amp diode plug into input plug, and adjust the detector gain to get arount 3 volts output on the A+B Sum meter. Switch to "Si" setting on the A & B Analog filter inputs. If the switch is in the "InSb" possition, the meter will deflect in the negative direction.

Boot XX (See the FTS User's Guide) and load the FTS software. From the Main Menu go to the Utilities Menu and sellect the Beam Splitter Menu. Cursor down until the BS being installed is high-lighted and hit return. Back in the Main Menu sellect "WLF" - Automatic" and it should find the centeral fringe. If the central fringe wasn't close to the center of the screen, replace the value in the Beam Splitter Menu with a value closer to proper number.

Turn the dither control (Drake Shake) on. Center the maximum modulation point in the O-scope's diplay with the "GO+" and/or "GO-" buttons on the FTS Command Unit. Turn frequency all the way down and use the hand paddle with red lights at 18 volts to adjust the BSUs tip ("fluf" the BS).

The end of the End

In order to ensure that everything in the tank is as it should be before the tank top is lowered, the following checklist is provided:

  1. Thermometer positioning
  2. Collimator inspection flat positioning
  3. North 8-in. flat gear collars tight
  4. Beamsplitter:
  5. Difference amplitude modulation
  6. Difference voltage drift (near - far)
  7. Lead weights positioned
  8. 2nd input (looking at Curly)
  9. Table balance
  10. O-ring properly seated

Now is the time to install new windows, when necessary. Remove the A-port window. Remove, clean, and reinstall the O-ring. The new window retaining ring should be "finger-tight to snug" only. Then remove the B port kilroy, and install the new window the same as at A port. Replace the input port window, using a slotted flat-head screwdriver to remove the retaining ring screws.

Take one last look to see that everything in out of the tank that should be and the the O-ring is seated properly. Lower the FTS top. Realign the Laser at the input onto the collimator to correct for the wedge of the input window. Center the bullseye in the red laser circle at the output with the unlighted hand paddle plugged in to the 8" Flat plug and with the power supply set to at least 12 volts. Move the catseyes to the far end of their travel with the "WLF" - "manual" command. Move the catseye 1.8e6 fringes (0.9e6 for offset catseyes). Move the bullseye 1/2 way back (all the way for offset catseyes) to the center of the laser spot at the output. Repeat these steps until the bullseye is centered at both ends of travel.

Center the laser spot in the center of the A-port output by tilting the A-port camera mirror. Then do the same for the B-port using the B-port camera mirror. Close all the vacuum ports.

Turn off the Servo motor. Wait for the catseyes to settle and the two square waves have settled on the O-scope diplay. Power down the oil pump. Once the oil pressure reaches zero, the other electronics racks can be turned off.

The instrument should be off oil at least 30 minutes before initiating the pumpdown procedure. To begin pumpdown, turn on the two pumps and flip the "Rough" toggle switch down (on the FTS Control Panel).

              Beamsplitter   Windows    Mirrors    Catseyes   Offset
                  UV          CaF2        Al         Al         No
                  Vis         CaF2        Ag         Ag         No
                  CaF2        CaF2        Ag         Ag         No
                  KCl         KCl         Ag         Ag         Yes

Return to the FTS Notes book.