FAST: Hardware updates

FAST CCD

John Geary and Steve Amato mounted and tested a new CCD (FAST3, a UA STA520A 2688x512 chip) in a dewar in Cambridge. The CCD and dewar arrived on the mountain on 14 October 2005. We found problems with the chip that required the dewar to travel back to Cambridge. As of April 2006, the chip showed unexplained behavior that resulted in lower sensitivity than for FAST2. Tests showed the problem was in the software. Ted debugged the CCD controls, with the happy result that the CCD is in good shape as of June 2006.

As of the end of June 2006, FAST3 (S/N 4377), has replaced FAST2. It requires no UV-flooding, a significant advantage for Ridge operations.

FAST optics

Warren Brown had replacement optics for FAST re-coated during July 2003. He replaced and aligned the internal optics during August 2003 shutdown. He also replaced a sol-gel recoated corrector. The result was a marked increase in throughput (see the plot here), especially at blue wavelengths. Throughput measurements were made with standard stars and a 5" slit. The increase in blue throughput has revealed that we have significant second-order contamination in FAST spectra. The plot shown here shows the spectrum of the blue standard BD+28 without the WG360 order-blocking filter (3700 A) divided by the spectrum through the filter (thanks to Tom Matheson). If you are concerned with spectrophotometry, you may wish to use the WG360 filter for your observations.

We periodically observe standards with the 5" aperture to monitor the throughput of the telescope and FAST. See the plots here and here for Warren's most recent analysis, with FAST3 starting July 2006. Re-aluminizing the primary in August 2005 returned the total system throughput close to what we measured in September 2003, immediately after the FAST refurbishment. Dusting off the internal optics of FAST (carefully blowing and vacuuming) in early November 2005 yielded a nearly 10% throughput improvement. By April 2006, we were back to the levels preceding the re-aluminization in 2005. Dusting off the internal optics of FAST is now on a monthly schedule, as for the corrector.

FAST alignment and corrector cleaning procedures
08/22/2003 Warren Brown

Equipment for FAST Alignment:
Optical bench, mounting blocks, tie-down bars, long threaded rods.
Laser, 24" post, post holder, tip/tilt stage, translation stage.
FAST Power supply, control box, laptop with serial interface and FAST.BAS.
Allen set, crescent wrench, needlenose pliers, small phillips and standard screwdrivers, tape, scissors.

FAST Alignment procedure:

• Move FAST to optical bench.
  The optical bench in August 2003 was located in the old Gamma-ray control shed, now the HAT shed. I called Karen to get a forklift operator. We used the forklift to pick up FAST in its rack and carry it down to the shed. We then used the forklift to pick up FAST from its aluminum handles, lift it out of the rack, and set the spectrograph on a piece of wood + foam on the doorstep of the shed. After removing the handles, we tipped the flange through the doorway and onto the optical bench. Then we lifted and carried the opposite end up onto the bench.

• Mount FAST on optical bench.
  Use the 3 aluminum mounting blocks as shown. The two short ones go near the flange, with the 1/4-20 bolt holes facing away from the instrument. The tall aluminum block is placed on the far end. The blocks should only touch the metal rim, not the sidewalls, of the spectrograph. Finally, use the long threaded rods and the aluminum tie-down bars to firmly hold FAST onto the blocks.

• Connect the control box etc.
  I used the FAST control box to manually open the shutter. The calibration lamps are too dim to be used in daylight alignment. The laptop is needed to command the COMP mirror out of the optical path, and to move the collimator mirror to a nominal focus position.

• Align the laser.
  I used the laser from the 60-inch alignment tool. The laser should shine through the center of the slit, reflect off the center of the collimator mirror, reflect off the center of the grating, and hit the center of the fold mirror. I used the pinhole mounted in the spare slit mask holder to define the center of the slit. The old collimator mirror has an inscribed center mark. The aluminum disk with the plastic tip can be used to inscribe a circular mark on the collimator if needed. The grating only needs to be centered approximately, but should be tilted so the beam falls in the center of the fold mirror. The plastic insert with the white dot (and white plastic screw behind it) can be placed in the fold mirror hole to mark the center of the fold mirror.

• Put in grating surrogate.
  Remove one of the gratings (I chose the 1200 line, since it is used the least) and replace it with the grating surrogate. The center mirror on the grating surrogate was bonded into place while the whole assembly was lying face-down on a granite table. Thus the mirror can be assumed to be co-planar with the aluminum grating surrogate.

• Verify optic spacings.
  I used the alignment rods to verify the spacings between the optics. The brass rods are to be used in conjunction with the inside micrometer + delrin tip and the 24.000" aluminum rod in the Hecto spectrograph room at the MMT. The nominal spacings (from Zemax) are summarized on the attached page.

• Replace Collimator.
  The collimator mirror is accessed from the bottom plate of FAST. Four bolts need to be unscrewed, and the (partial) ring holding the mirror carefully pulled out on its pins (top and bottom). The collimator mirror is aligned by shimming the three pads. Note that two radial pads are hard points, and one radial pad is spring loaded. It is important that the mirror be pushed firmly against the hard points to obtain repeatable positioning. The collimator mirror should be centered to within +-1 mm, and aligned to < 0.2 deg side-side and < 0.5 deg up-down.

• Replace the Schmidt Corrector. The Schmidt Corrector is basically a UBK7 window with a slight bump in the middle of one side. The corrector should be oriented with the bump pointing towards the fold mirror. The tolerances are very forgiving: +-5 mm centration and +- 5 deg tip/tilt. I chose to leave the corrector mount as-is during the August 2003 alignment, since the ~3mm de-center I observed was within spec.

• Replace the Fold Mirror.
  Tilt the grating surrogate off-axis so that the laser reflects back to the surrogate and its position can be marked. The final alignment is verified by centering the beam on the Fold Mirror (using the white dot insert) and making sure the laser reflects to the center of the Camera Mirror using the mirror insert.
  The fold mirror is shimmed on three pads. I found I could simply set the mirror in place to do a rough alignment before putting the clamps in for the final alignment.
  The fold mirror is accessed from the dewar flange. The three pinned, spring-loaded clamps are difficult to access. The two clamps on the left-hand side (looking at the back of the mirror) have the hard radial pads. Special care needs to be taken with the two loose springs that spring-load the bare metal clamp on the right-hand side; when inserting that clamp I kept the springs from falling out by using small pieces of tape to hold them to the clamp, and then needlenose pliers to carefully remove the tape when I was done.

• Replace the Camera Mirror.
  The Camera Mirror should be aligned so that the Grating -> Fold -> Camera retro-reflection lines up on itself. The three clamps are easily accessed from the main side panel, and the camera mirror is shimmed on its three pads.

• Glyptol the bolts.
  To keep the bolts from loosening, dab a touch of epoxy on them. Same for the bolts on the grating mounts.

• Pack-up and move FAST.
  Wipe down the surfaces and vacuum the inside of FAST. Blow accumulated dust off of the optics. Reverse set-up procedure and move FAST back to dome.

• Put dewar on and take COMPS and FLATS.
  I was unable to do this in August 2003, but the final check is to take a set of focus COMPS and some FLATS to see how everything works.

1.5m Corrector Replacement / Cleaning Procedure:

This procedure is easiest when the primary mirror is disassembled for re-aluminization, and the corrector lens barrel is easily accessible. However, at other times, leave the mirror covers closed, tilt to az 60d, el 45d for easy access, remove the stove pipe and then the 4 outer screws that hold the barrel to the cell (on the top flange, not the recessed screws). Then, carefully lift the barrel by the handles, making note of the alignment white marks.

Note: The top corrector lens is a convex meniscus lens that sticks up over 0.6 inch, so it is all too easy to accidentally bump its surface. Be careful, and wear latex gloves when working with the lenses.

Carefully screw two 6-32 threaded rods or 2" bolts into the convex meniscus lens ring. Use the 6-32 bolts to unscrew the entire corrector lens assembly from the barrel. The plastic retainer ring should unscrew fairly easily. Once you have removed the ring, use the same 6-32 bolts screwed onto the corrector lens assembly to lift it up, out of the barrel. Reverse for re-assembly.
The corrector lens assembly consists of three stainless steel rings. The lenses are bonded into the top two rings; the bottom ring is a spacer. Three 4-40 bolts hold the three rings together. In addition, the rings holding the lenses are pinned.

The new corrector lenses with the Spectrum Thin Film dielectric AR coating (harder than sol-gel) can be cleaned as follows:
1. Blow off the surfaces (i.e. with dry N2).
2. Wash surfaces with reagent-grade isopropyl alcohol and lens tissue. NOTE: Do not squirt and let liquid run off. Instead, DAUB very lightly and slowly with wadded-up Kim-wipe tissues. Take your time!
Squirting isopropyl alcohol on lens 1 as it was held vertically (as per instructions from Spectrum Thin Films) resulted in small spots on the coating, as shown here.

The lesson is: Do not allow water to stay on the lenses!

Picture gallery:

FAST alignment
FAST alignment (on bench I)
FAST alignment (on bench II)
FAST tolerances
FAST New Cam
FAST New Coll
FAST New Fold
FAST New Old Coll