MODS IMCS Collimator TTF Zero Point Calibration Procedure

Overview

The MODS Image Motion Compensation System (IMCS) nulls image motion on the CCD detectors due to a combination of gravity- and thermal-induced flexure, stochastic strain relief in the structure ("ticks-and-pops"), and rotator "print through" into the structure (the dominant term is gravity-induced flexure). It works by using a closed-loop control system that nulls image motion by steering the red- and blue-channel collimator mirrors in tip/tilt to keep an IR laser spot launched just below the focal plane centered on germanium quad-cell detectors mounted in each camera just above the science CCDs. The IR laser beam shares the same light path as the science beams, which means that IMCS correction is only active when the shutter is open. The MODS data-taking system takes care of coordinating acquisition of science images and keeping the optics aligned with the IMCS system.

The collimator mirrors are each mounted on three tip/tilt/focus (TTF) actuators that control their orientation. The IMCS software measures the motion of the IR laser spot on the quad cell and computes correction signals sent to the TTF actuators to steer the mirror while leaving the mirror focus position (Z-piston) and mirror surface vertex fixed relative to the instrument slit plane. In this way the tip/tip compensation for "flexure" does not introduce any (additional) optical aberrations.

For each instrument configuration (e.g., dual grating, red imaging, blue prism, etc.), we need to measure a set of TTF zero points for the collimator mirror. When the telescope is pointed and the instrument configured (e.g., for dual grating spectroscopy), the collimator mirrors TTF actuators are set to these zero-point values so that when the IMCS loop is started, it starts from a known reference configuration. We measure these at elevation 60 deg and the reference rotator position angle so that for any particular region we can point LBT to on the sky, weshould be close to these zero points. The TTF zero points are stored in the bconfig.tab and rconfig.tab files in the configuration folders on the instrument server computer (e.g., host "mods1").

In general, TTF zero points only need to be re-measured when changes are made to the internal optical alignment of a MODS instrument, and we generally remeasure them each time the instrument is reinstalled on LBT after being dismounted from the telescope (e.g., for summer shutdown or hopefully-rare off-telescope major maintenance). We expect that it will be a once-a-year activity as part of post-summer restart of the observatory.

This wiki document describes the procedure for measuring the TTF zero points and updating the runtime configuration files used by the MODS Control Panel GUI.

Measure the IMCS TTF Zero Points

The measurement can be made during the daytime with the full enclosure lights on. The telescope needs to be pointed at the following positions for each instrument:

  MODS1 MODS2
Elevation 60 deg 60 deg
Rotator 207 deg 195 deg
The telescope needs to be static (not tracking) during the TTF measurements, so this is best done during the daytime or during a cloudy night.

Login as the LBTO user, start the MODS Control Panel GUI for the instrument being calibrated (MODS1 or MODS2), and make sure that the IMCS agents for the Red and Blue channels for that instrument are running. Wake MODS in the usual way, and make sure the IMCS IR laser is enabled. You should also launch modsDisp so you can watch the images as they are acquired.

Now go to the MODS support script directory
    cd /lbt/lbto/mods/support/Dual/

and execute the imcsTTFs.pro script using execBinoMODS to collect data for both MODS1 and MODS2 simultaneously (or =execMODS --mods1 for MODS1 or execMODS --mods2 for MODS2):
    execBinoMODS imcsTTFs.pro

This script puts the instrument into each of the 9 major configurations (dual/red/blue imaging/grating/prism) and locks the IMCS for each mode. It then takes a 1Kx1K snapshot image in the channel(s) being calibrated. A total of 12 images will be taken, 6 in each channel: dual-mode imaging, grating, and prism, then direct channel-only mode imaging, grating, and prism configurations.

The script takes ~15 minutes to execute. The measurement procedure is the same for both MODS instruments.

The TTF zero-point values are stored in the image FITS headers as the COLTTFA, COLTTFB, and COLTTFC keywords. To get a quick summary, copy the FITS images from /newdata/ into a working directory with only the FITS images just acquired and type
    gethead dichname gratname colttfa colttfb colttfc *.fits

To see just the settings for a single mode, e.g., red-channel dual-mode on MODS1, pipe output from gethead to grep like this:
   gethead dichmode gratname colttfa colttfb colttfc mods1r*.fits | grep Dual

whereas the blue-channel direct ("blue-only") mode TTFs would be found by piping gethead output to grep like this:
   gethead dichmode gratname colttfa colttfb colttfc mods1b*.fits | grep Blue

and so forth.

Update the MODS Configuration Files

The TTF zero points are stored in the bconfig.tab and rconfig.tab files for each MODS. These files are on the instrument server for each instrument under the mods account.

For example, for MODS1, ssh into the mods1 computer as user mods and then cd into the Config/MODS1 directory :

   cd ~/Config/MODS1/

For MODS2, ssh into the mods2 computer and then cd into the Config/MODS2 directory.

There should be copies of the current bconfig and rconfig files in the 00IMCS/ folder, with names like rconfig_20111102.tab, which contain copies of the config files for past measurements.

The config files are mostly comments, the relevant parts are at the bottom. For example, the bottom of the rconfig.tab file will look like this, with two blocks of "presets", direct mode (here "red-only" as this is the red-channel config file), and the dual mode:
   # Measured: 2011 Nov 01, El=60deg, rotAngle=207 (PA=0 at Meridian) 
   # Measured By: R. Pogge, OSU           
   #
   # Direct-Mode Presets
   #
   Imaging  Flat    Any    1200 15944 14768 12663 3Kx3K  1 1 0 0 Slow
   Grating  G670L   GG495  1750 15993 14830 12560 Full   1 1 0 0 Slow
   Prism    P700L   GG495  1550 15943 14855 12588 4Kx3K  1 1 0 0 Slow
   Acquire  Flat    r_sdss 1250 15944 14768 12663 3Kx3K  2 2 0 0 Fast
   #
   # Dual-Mode Presets
   #
   DImaging Flat    Any    1200 15885 14812 12688 3Kx3K  1 1 0 0 Slow
   DGrating G670L   GG495  1750 15932 14858 12594 Full   1 1 0 0 Slow
   DPrism   P700L   GG495  1550 15892 14888 12605 4Kx3K  1 1 0 0 Slow
   DAcquire Flat    r_sdss 1250 15885 14812 12688 3Kx3K  2 2 0 0 Fast

The TTF values are in columns 5 thru 7 for TTFA thru TTFC, respectively, highlighted above (see the comments inside these files for details on all the different configuration parameters and their units in each column). Note that while the Acquire and DAcquire presets are no longer used and may be omitted from future iterations, for now just use the TTF zero points for the Imaging and DImaging modes, respectively.

Replace the values of the TTFs zero points with the values extracted from the calibration image FITS headers acquired above. Please leave the rest of the configuration lines unchanged.

Also remember to "sign" and date your changes along with any other information you think might be relevant. There are two places to note this, in the bottom block and at the top of the files.

Once you've finished, make copies in the 00IMCS folder:
    cp bconfig.tab 00IMCS/bconfig_ccyymmdd.tab
    cp rconfig.tab 00IMCS/rconfig_ccyymmdd.tab

where ccyymmdd is the date you made the measurements. Then cd into the 00IMCS folder, and update the 00README.txt file with a summary of when you remeasured the TTF zero points. This file constitutes the historical record of the measurements.

Finally, once you've updated the bconfig.tab and rconfig.tab files, restart the MODS GUI, and test to make sure the IMCS zero points are entered correctly.

-- %USERSIG{RichardPogge - 2015-11-22}%
Topic revision: r6 - 20 Dec 2023, OlgaKuhn
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