FAST: Hardware updates


Created 09/03/03 by EF
Updated 01/26/12 by EF


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:



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