Background
The DEIMOS gratings must be able to clamp up at all DEIMOS
rotator position angles, a task made difficult by the varying
gravity and subsequent flexure of the grating select mechanism.
To achieve the accurate positioning required for each grating
slider's cone to insert properly into the "pin", the system
employs a two-stage alignment process:
- Coarse move. First, the slider is moved into
approximate position based on the absolute encoder value by
slewing to the position given by the GRATTABn
keyword, then it makes a slow move until a flag on the stage
interrupts an optical switch.
- Fine move. With the exact position now determined,
the select stage makes a relative move of the number of
encoder steps given by the keyword GRATOFFn. This
should put the stage into position where it can clamp up.
Tuning the GRATOFFn keywords is a labrious process
that is made much easier through scripts provided by Bob Kibrick
of UCO/Lick. This document describes the tuning procedure and
documents instances of tuning.
Procedure
Follow these steps to determine the appropriate values of
GRATOFFn:
- Obtain summit permission to rotate DEIMOS.
- Open a terminal as any DEIMOS account at any of the DEIMOS
Linux hosts and launch script:
optimal_offset_driver 2 3 4
Execution time for 3 sliders is around 6h30m.
- The following two files will be saved
on /s/sdata1004/grating_offsets:
optimal_offset_driver.csh.2020oct22.clampup
optimal_offset_driver.csh.2020oct22.results
- Extract results by running
the PlotDeimosGratingOffset script as follows:
PlotDeimosGratingOffset /s/sdata1004/grating_offsets/ optimal_offset_driver.csh.2020oct22.clampup 45000 58000 60000 AUTO AUTO AUTO AUTO
- Review plot saved on /s/sdata1004/grating_offsets
using the command:
eog optimal_offset_driver_2020oct22.png
- Update the stowed GRATOFFn values included in the
deimosplus.GRATEOFF keyword with the new values from
the plot.
- Check the old stowed values:
-
gshow -s deimosplus grateoff
- Write the new values to deimosplus.GRATEOFF:
-
modify -s deimosplus
GRATEOFF="deimot.GRATOFF1 0 deimot.GRATOFF2 44000
deimot.GRATOFF3 56500 deimot.GRATOFF4 56500"
-
gshow -s deimosplus grateoff
- Test the new values are properly loaded:
-
modify -s deimosplus godefault=1
-
gshow -s deimot gratoff1 gratoff2 gratoff3 gratoff4
- Optional: Have Lick staff enter new values of
GRATOFFn into Galil code and burn into controller
ROM.
Typical grating offset adjustment plot
