Instructions for complete and accurate characterization of both the polarization efficiency and HWP Zero Angle [written 2007-Mar-02, J. E. Vaillancourt] [minor updates 2007-Mar-28, JEV] [Skip down after step (9) for a simple and fast way to measure just the HWP zero angle] 1) Check the SHARC-II filter wheel: 350 micron = 250, 450 micron = 500 2) Mount the two wire grids, in parallel, on the small aperture plate. a) This is the aperture plate with 5 holes used at the exit side of the Nasmyth tube b) Use a strong tape. Tape to the edges of the smaller side of the grid holders. Do not put tape directly on the solder which bonds the wires onto the holder. 3) Place a piece of nylon, 1/16-inch thick, at the entrance to the Nasmyth tube. a) There is a piece of nylon on the SHARP boxes labeled as being used in the past. If you can't find it, just use another piece of the same thickness. b) You will have to tape the nylon the the Nasmyth tube entrance. 4) Mount a cold load in front of the nylon a) Move the telescope to ZA 35 b) Place a thin piece of plywood horizontally across the bars from the relay optics. If the telescope is at ZA=35 this should be obvious. Hopefully the thin rectangular piece of plywood is still around either on the alidade, sidecab, or lounge. c) Fill the small (styrofoam ?) dewar with LN2. This dewar is also filled with eccosorb. Place it on the plywood platform, centered on the Nasmyth tube. 5) Mount the wire grids, on the aperture plate, at the exit of the Nasmyth tube. a) To place the wires approximately vertically you can site along the top of SHARP box 4 to line up the top of the grid mount parallel to box 4. (This works well for horizontal wires too, but you'll have to make your best guess for anything in between.) b) Make sure the aperture plate is firmly in place, otherwise it may fall out and damage the grids. c) Check the grid angle with the Northwestern electronic level. I usually zero the level on box 4, then hold it flush to the top of th grid mount. Adjust the angle to be within 1 degree of vertical by adjusting the telescope ZA. 6) Take SHARC-II data every 10-20 degrees of the HWP over a range of 90 degrees (I've been doing 10 degree resolution). Use the "Start" button, not the script "Run" button. a) set the FPGA gains to "hi": all_gains hi only if nylon is in place b) set the integration time to 45 - 60 seconds. c) Level Hardware d) set the HWP to your first angle (most likely 50 degrees) - you should see a big change in the level in the twinkle charts e) Have someone hold a hot load (ambient temperature eccosorb) between the cold load dewar and the nylon. There is usually a square of eccosorb mounted to a stick in the sidecab or out on the alidade. f) Level Hardware, yes again g) Remove hot load - you should see the signal in the center 4 pixels of at least one subarray h) Start data file - note the H and V signals in one pair of the center pixels i) Wait for file to finish j) move HWP to next angle k) Repeat steps (e) - (j) until you have gone through 90 degrees l) Be sure to also repeat the first HWP again 7) Repeat steps (5) and (6) with the wire grids horizontal 8) Repeat steps (5) and (6) with the wire grids at 45 degrees. Record the precise angle (i.e. is it 45 deg CW or CCW from vertical). 9) Data Reduction - a) Make a new directory for the current Run b) Get the script "doit1" from sharp@kilauea:Runs/2007Feb/calibs/poleff c) Edit the script "doit1" to run the new data files (in the "foreach..." line) then run the script. d) Make a file with a list of the output *.psig.fits files for a single grid position - for example: ls *.psig.fits > list - you can comment out unwanted files with the "#" character, or just delete them e) In IDL: IDL> .r poleff1 IDL> poleff1, 'list' - This will fit the amplitude, phase, and offset of a sine curve IDL> .r poleff2 IDL> poleff2, 'list' - This will model the offset as a gain difference between H and V ** Here are some simpler and faster schemes for getting just the HWP zero angle: 1) Option 1 - Only steps (1), (2), and (5) above should be necessary (although the grids could be mounted without the aperture) a) Move HWP to best guess for HWP zero (where V signal is smallest) b) Move HWP off by -15 degrees from current position c) Level Hardware d) Move HWP by +10 degrees. You should see a big change in the pixels in the twinkle box. e) Level Hardware f) Move HWP +10 degrees from current position. You should see very little change in the pixels in the twinkle box. g) Move HWP by another +10 degrees. You should now see another big change in the pixels in the twinkle box. - If your observations agree with the noted twinkle-box behavior, this means your estimate of the V-null is good to within 5-10 degrees. 2) Option 2 - Steps (1), (2), (4), and (5) above are necessary, (3) is optional a) Move HWP to best guess for HWP zero (where V signal is smallest) b) Move HWP off by -5 degrees from this position c) Level Hardware with warm load in place d) Note the HWP angle and H & V signals in a corresponding pairs of pixels e) Move HWP by +1 or +2 degrees f) Repeat steps (d) - (e) until you've gone from -5 to +5 degrees g) Do a chi-by-eye to decide if you believe the V-null position - You could probably go from -10 to +10 in bigger steps, depending on the accuracy you want. 3) Option 3 - Steps (1), (2), (4), and (5) above are necessary, (3) is optional a) Set gains to "hi" if using nylon, "lo" otherwise b) Set HWP to starting angle = X c) Level Hardware with warm load in place, then remove warm load. d) Use the IRC Observing Script "SHARP_Standard". Change the default fields to: Time per HWP = 60 seconds Start HWP = X End HWP = X + 100 degrees Step = 10 degrees Chop Throw = 0 e) After script is complete - move back to starting HWP angle f) Level Hardware with warm load in place g) Take one more single HWP file using "Start" button, not "Run" button h) Analyze data in same manner as step (9) above - you should see a low polarization efficiency (~50-60%), but this should not change the HWP zero angle measurement