Setting Up for Double-Pass
- Place mirrors (with compensator) in position. Compensator-mirror
is on the south side. The mirror alone is on the north side. Use
the medium size lead bricks (about 1/4 brick) to hold the mirror
bases down. Be sure the brick do not block the beams coming from
- The mirrors should be positioned to allow the HeNe reference beam
to pass near the flat edge of each mirror. The reference beam should
be unobscured at both ends of the catseye travel. Be sure the oil
guards will not bump the mirrors when the catseyes travel to the
- The orientation of the compensator is set be aligning the mirror base
(not the base plate) parallel with the granite block edge.
- Send the expanded/collimated HeNe laser through the 8 mm entrance
aperture and center it on the collimator. Adjust the south mirror
to superimpose the return beam at the center of the collimator.
Check that the beams are superimposed at both ends of of catseye
travel. The return beam cam the "chopped" by flicking a finger in
front of the lollipop.
- Place the camera mirror (89" focal length) at the east-most position
at B-input. Adjust the lollipop to place the HeNe spot at the center
of the camera mirror.
- Mask the reference beam at the lollipop.
- Looking at the output spot, adjust the north mirror to superimpose
the output beams. A bull's eye and central fringe should be centered
on the spot (check at both end and center of catseye travel).
- Close the tank (with window installed). Mount a Kilroy at B-input.
Open the ports.
- Check the position and super-position of the beams at the collimator.
Adjust the south mirror, if necessary.
- Adjust the camera mirror to center the output mean at the Kilroy.
- Check the super-position of the beams at they're output. Adjust the
north mirror for maximum modulation (~85%, narrow band). Check the
central fringe at both ends of travel.
- Check the line-shape with acetylene (C2H2,10cm, 200microns, 700-850cm-1)
before pumping down.
- Pump down "slowly". Check the modulation and line-shape.
- In Double-pass mode, some parameter combinations do not work:
- M=3D 470 S=3D 2500 Does not work
- M=3D 235 S=3D 2500 Does work
- M=3D 470 S=3D 2400 Does work