The following procedures describe the calibration of LWS opto-mechanical assemblies.

Summary

• General Information
• Setup
• Aperture and Grating Mechanism Calibration Procedure
• Filter Mechanism Calibration Procedure
• Sector Wheel Calibration Procedure
• ZnSe Window Flipper Calibration Procedure

General Information

The LWS instrument is required to be fully operational and cryogenically cooled to operating temperatures prior to proceeding with the calibration procedure. A calibration is neccessary  prior to an observing run and/or following a dewar cool down from ambient temperatures. The purpose of the calibration is to map stepper motor counts to the corresponding physical position of named motor configurations. For example, the aperture name open corresponds to an apraw (raw number of motor counts ) value of approximately 50,000 motor counts.  The apraw value can vary by several thousand counts from run to run, presumably due to thermally induced changes in the opto-mechanical nature of the instrument.  These procedures describe how to adjust the mechanism using raw motor counts and then update the configuration files that map the named locations to raw motor counts. The calibration files are stored on keokea under the following names:

• ~lws/v3-0/cfg/lws_aperture.config
• ~lws/v3-0/cfg/lws_grating.config
• ~lws/v3-0/cfg/lws_filter.config
• ~lws/v3-0/cfg/lws_secwheel.config
• /kroot/kss/lws/install/motor_defaults_script

The calibration files are owned by the lws account and other users (besides root) should not have write privileges. There are other calibration files in these directories that don't directly apply to this calibration procedure but the interested reader is encouraged to review these files as they may be useful towards other performance issues of the instrument.

Before editing any configuration file, please backup the original file with a date suffix for the purpose of easily reverting to the previous configuration. For example:

        cp lws_aperture.config lws_aperture.config.2002jul18

Setup

1. Set up data acquisition
• Control detector temperature to 8.5 K (turn det heater on)
• Turn level shifter power on (det power on)
• Set data acquisition mode to video
• Confirm live image data is updating in quickview via the current frame count in buffer control widget.

Note: Since the instrument configuration is unknown, the background level in QuickView may only a few percent or saturated at 100% or somewhere in between.
• Set image display to "raw-dc frame" in the quickview buffer control widget
2. Home the sector wheel
3. Set sector wheel to mirror

The sector wheel reflects the cold dewar back onto itself to avoid saturation during afternoon setup procedures. Quickview should now indicate a non saturated background (expect in the unlikely event that a broadband spectroscopy filter is installed and the instrument is in imaging mode)
4. Home the grating wheel

NOTE for all motors: The home sequence can be monitored by watching the "ask_motor" commands in the coyote tip window and by watching the quickview image display. The homing sequence is painfully slow so have patience. Occasionally homing will fail to complete as evidenced by by the tip window being stuck in a constant ask_motor message scroll. In this case issue the command

        tellmotor K

This command halts any and all motor moves. Upon a home failure go ahead and select the motor to named position and retry the home. The second home sequence will often succeed. If the home refuses to complete and the tellmotor command is issued, its likely that the position is close to the proper home position and the calibration procedure can continue, although it will only be valid until the next motor controller power cycle.
5. Select Grating Mirror position.
6. Home the Aperture Wheel.
7. Select Aperture Open.
8. Home the Filter Wheel.
9. Select Filter 11.7.
10. Select ZnSe Window In.

The ZnSe window will cause the background to be around 40 to 50% when imaging the sector wheel It is always safer to work with the ZnSe window in to protect the very hygroscopic KBr dewar window.
11. Initialization complete

LWS should now be in imaging mode at 11.7 µm. In the image display you should notice a mostly illuminated field of view (un-differenced raw DC frame / no accumulation) with several bad or hot pixels. If the image shows signs of vignetting near the edges of the detector, it is likely that the calibration is required. Even if there is no vignetting, the calibration procedure should be completed if the instrument is being used for the first time since a thermal cycle.

Aperture and Grating Calibration

1. Verify that aperture wheel is selected to OPEN
2. In a keokea window, issue the command

          show -s lws apraw

   and note the reported value.
3. Adjust the aperture wheel up and down (as appears on the QuickView) using the command

          modify -s lws apraw

   until the mid point value is determined for the unvignetted field of view in the vertical direction.
4. Edit the file ~lws/v3-0/cfg/lws_aperture.config to specify the new value for the open position.
5. Verify that grating is selected to MIRROR.
6. Issue the command

          show -s lws graraw

   on keokea and note value.
7. Adjust grating left and right (as appears on the quickview) with

          modify -s lws graraw

   until the mid-point value is determined for the unvignetted field of view in the horizontal direction.
8. Edit the file ~lws/v3-0/cfg/lws_grating.config to specify the new value for the mirror position.
9. Set aperture to 3 pixel slit
10. Adjust the aperture using the command

           modify -s lws apraw

    until the best value for the vertical position of the slit is determined. Note that measuring the slit top and bottom centroids with the slit align widget may be useful for this process.
11. Edit the file ~lws/v3-0/cfg/lws_aperture.config to specify the new value for the 3 pixel position.
12. Select Aperture to 6 pixel slit
13. Adjust aperture with

           modify -s lws apraw

    until the best value for the vertical position of the slit is determined.
14. Edit the file ~/v3-0/cfg/lws_aperture.config to specify the new value for the 6 pixel position.
15. Select Grating to LRES zeroth order with the command

           modify -s lws graname = Lres0th

16. Adjust the grating with

           modify -s lws graraw

    until the best value for the slit centered position (x = 64 on the array) is determined. Note that the flux will be much lower compared to the mirror and a histogram equalized stretch may help with the slit display.
17. Edit the file ~lws/v3-0/cfg/lws_grating.config to specify the new value for the Lres0th position.
18. Edit the file /kroot/kss/lws/install/motor_defaults_script to specify the new value for the ZPOSLOW. ZPOSLOW is the same graraw as the Lres0th position but this keyword is used by the software that computes the grating position for a user specified central wavelength for spectroscopy.
19. Set grating to HRES zeroth order with the command

            modify -s lws graname = Hres0th

20. Adjust the grating with

           modify -s lws graraw

    until the best value for the slit centered position (x = 64 on the array) is determined. Note that the flux will be much lower compared to the mirror and a histogram equalized stretch may help with the slit display.
21. Edit the file ~lws/v3-0/cfg/lws_grating.config to specify the new value for the Hres0th position.
22. Edit the file /kroot/kss/lws/install/motor_defaults_script to specify the new value for the ZPOSHIGH. ZPOSLOW is the same graraw as the Lres0th position but this keyword is used by the software that computes the grating position for a user specified central wavelength for spectroscopy.
23. Kill the XLWS and XPOSE-LWS GUIs.
24. Restart the motor daemon software with the command

           restart_motord

Note that if the ZPOS values did not change as a result of the calibration and the the motor_defaults_script was not edited that the motor daemon does not need to be restarted.
25. Verify the new values for ZPOSLOW and ZPOSHIGH by showing the keywords.
26. Restart LWS GUIs.

Filter Wheel Calibration

There are two methods for calibrating the filter wheel, one that can be done in the afternoon and one that requires the image of a bright star at night. The night sky calibration is preferred for fine tuning the filter since it is more accurate. However, it is rare that the filter wheel drifts out of calibration and the afternoon calibration is acceptable for checking the position accuracy after a thermal cycle.

Afternoon Filter Wheel Calibration

1. Set the instrument configuration to imaging mode:
   • aperture = OPEN
   • filter = 11.7 µm
   • grating = MIRROR
   • sector wheel = MIRROR
2. Set the data acquisition to video, quickview to dynamic. Confirm that the image is updating with a background level around 50%.
3. Record the background level displayed in quickview
4. Issue the following command on keokea:

          show -s lws filraw

   and note the value.
5. Increase filraw by steps of 2000 with the command

          modify -s filraw = nnnnnn

   until the background level drops by 20%. For example:

          m filraw = 322000
          m filraw = 324000

6. Restore the filter the original position recorded in the step above.
7. Decrease filraw by steps of 2000 with the command "

          modify -s filraw = nnnnnn

   until the background level drops by 20%. For example

       m filraw = 318000
       m filraw = 316000

8. The optimum filraw position is where the best throughput occurs which is the mid-point between two points where the background (counts) drops significantly below optimum. If this position is significantly different, ±4000 motor steps, than the original filraw recorded above, then the night sky filter wheel calibration procedure is recommended.

Night Sky Filter Wheel Calibration

1. Configure LWS for L-band imaging mode:
   • grating = MIRROR
   • aperture = OPEN
   • filter = L
   • window = OUT or IN
   • sector = HOME or OPEN
2. Acquire V = 1 or brighter star (suggestion: IRTF standard). No guiding is required.
3. Start chopping with 20" throw and center star on LWS.
4. Apply +175 nm PMFM and verify that individual segments are discernible and all are within the LWS field of view. Adjust PMFM as needed. If seeing is poor, more PMFM can be used but offsetting the telescope will be required to check the outer segments.
5. Verify alignment of LWS filter wheel.:
   • Display current filter position via command

            show -s lws filraw

     Note the returned value (should be 320000)
   • Change filraw in steps of 2000 until segments on one side of PMFM image (tel pupil) become vignetted; e.g.,
     • m filraw = 322000
     • m filraw = 324000
     then check the other side:
     • m filraw = 318000
     • m filraw = 316000
   • verify value of filraw that corresponds to best filter alignment and note value.
6. Adjust values for mapping FILNAME to FILRAW in the file ~lws/v3-0/cfg/lws_filter.config. Each filraw value will need to change and should be equally spaced at 37,500 motor counts apart
7. Restart XLWS GUI

Sector Wheel Calibration

The sector wheel is used for calibrations and afternoon checkout. The unit is external to the dewar and it is rare that it needs calibrated. It is typically only needed after disassembly of the unit.

1. Have a summit technician monitor the sector wheel.
2. Home the sector wheel by selecting HOME/OPEN in the XLWS GUI. Have technician confirm proper homing of the wheel and that the wheel is position in an open positions in front of the dewar window.
3. Select Sector MIRROR position and have the technician confirm the wheel is positioned such that a polished aluminum sector paddle is positioned in front of the dewar window.
4. Select Sector "DARK" position and have the technician confirm the wheel is positioned such that a dark ector paddle is positioned in front of the dewar window.

Note: need instructions here on what to do if wheel is out of position. What command are used to rotate the wheel? Which configuration files must be updated?

ZnSe Window Flipper Calibration

The ZnSe window is used to protect the hygroscopic KBr dewar window from moisture. It is controlled by a stepper motor that flips it out of the way when the conditions are dry enough that the KBr window can be exposed to the local environment. The control mechanism operates with limit switch feedback and of the limit switches need tweaking.

1. Have a summit technician monitor the window flipper and confirm that the window is in
2. Check the window keyword:

          show -s lws windowin

   and confirm that keyword reports true.
3. Set window to OUT on the XLWS GUI and have the technician confirm the window is out.
4. Select window to IN on the XLWS GUI and have the technician confirm the window is in.
5. Check the window keyword:

         show -s lws windowin

   and confirm that keyword reports true.
6. If the window is not seating properly and the limit switch is not setting, have the technician adjust the mechanism and repeat the steps above unit proper operation is confirmed.

Note: need more explicit instructions on what the technician must do to "adjust the mechanism."

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Last modified: Mon Jul 22 16:03:05 HST 2002