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How to take and analyze Twoprobe data

Description

Twoprobe is a program for measuring telescope collimation using two Shack-Hartmann (S‑H) probes simultaneously; one actively corrects the telescope at the center of the field, while the other passively measures the Zernike terms around the edge of the field.

Setup

Normally, probe 2 is the active S‑H (the same as for routine observing) and probe 1 is the passive S‑H. On IMACS, probe 3 is always the active S‑H, and probe 2 is always the passive S‑H.

Configure the twoprobe S-H computer

The passive S‑H is run from a remote computer located in the Clay equipment room which can be configured for use with either Baade or Clay. To reconfigure this computer, type reconfig-oc2 in a terminal window on the operator’s computer. It will either respond:

ssh: connect to host magXoc2 port 22: No route to host

in which case, the computer is already correctly configured, or:

Connection to magXoc2 closed by remote host.

in which case, the computer is being reconfigured. This will take a minute or two; you can check whether it is ready by typing "ping $MAGOC2".

Set up the telescope and guiders

  • Slew the telescope to a dense field of stars, such as an open cluster or an area of the Milky Way. The field should be chosen so that the airmass does not change much over the course of the observations; a field in the southeastern sky at 50-60 degrees of elevation often works well.

  • Place a star for use with the active S‑H at the center of the field. Make sure there is a good selection of appropriate S‑H stars spaced around the edges of the field.

  • Turn the rotator to a position near one of its limits (±165 degrees).

  • On the TCS PC, make sure the set values for the secondary mirror y, x, h, and v axes are set to the proper values for the port using the commands hset, vset, etc. (If no values have been determined for the port, set the values to zero.)

  • Except on IMACS (where the active probe cannot move): On the Guider PC, type ‘XYn 0 0’ (where n is the guider number for the active probe) to move the probe to the center of the field.

  • On the GCAM PC for the active probe, type ‘mm 3’ to set the move mode to move the telescope, not the probe. Center the star in the active probe aperture, start the S‑H (by pressing F3 and F10), and allow it to run while the rest of the setup is completed.

  • On the GCAM PC for the passive probe, type ‘mm 2’ to set the move mode to move the probe, not the telescope.

  • In GMAP, select a star for the passive probe in the half of the field nearest the probe’s home position (left half for probe 1, right half for probe 2), and as far from the center of the field as possible. (On IMACS, try to stay near the center of the probe area if you have any choice at all.)

Set up the active S‑H software

  • Wait until all active S‑H Zernike terms except TILT (Z01 and Z02) are less than ±0.100; this indicates that the active S‑H has found the proper correction for the telescope mirrors. If the TILT Zernike terms (Z01 and Z02) are larger than ±10.0 at any point, recenter the star in the aperture.

  • On the GCAM PC for the active probe, stop the S‑H (by pressing F1 and F9).

  • In the active S‑H IRAF window:

    • Exit hartmannloop by typing CTRL-C.

    • Type twoprobeactive, and then type hartmannloop.

Set up the passive S‑H software

  • In a terminal window on the operator’s computer (Alpaca or Vicuna):

    • Type copytemplates to synchronize the templates with the remote computer.

    • Type activeoptics-oc2. This will open IRAF and SAOImage windows for the passive S‑H.

  • In the passive S‑H IRAF window, type twoprobepassive, and then type hartmannloop.

  • On the GCAM PC for the passive probe:

    • Type exit to exit the guider camera software.

    • Type ‘COPY MAGxOC2n.INI GCAM.INI’ (where x is the telescope number [1=Baade, 2=Clay] and n is the guider number).

    • Restart the PC by typing CTRL-ALT-DEL.

  • On the GCAM PC for the passive probe, start the S‑H (by pressing F3 and F10).

  • On the operator’s computer, make sure the S‑H image is received and processed properly in the passive S‑H windows.

Taking a data set

Overview

The data set will start with the rotator near a limit (±165 degrees) and then rotate in 45 degree increments, ending when the rotator reaches the other limit. There will be a total of 8 positions. The instructions above should leave the telescope already set up for the first position.

Before starting

  • Note down the set and ima values for the secondary mirror y, x, h, v, and z axes.

  • On the GCAM PC, stop the S‑H for both probes (by pressing F1 and F9).

Procedure for each rotator position

  1. In a terminal window on the operator’s computer:

    1. Type ‘twopgcamrec filename’ to start the S‑H on both probes and begin recording data. (The filename should contain an instrument name and a data set number (e.g. IMACS_1), and will not change during a data set. The script will automatically add the .passive and .active file extensions.)

    2. Wait for the S‑H systems to take three images and both IRAF windows to display “waiting for new image”.

    3. Type twopgcamstop to stop the S‑H systems and data recording. Don’t worry about stopping as soon as possible; there is plenty of time (30 seconds) to stop the recording after the last image is finished. It is better to record extra data than stop recording too soon.

  2. On the TCS computer, type dr 45 (or -45) to turn the rotator 45 degrees. Do not use the MGTCS control panel to turn the rotator. (If the rotator is near the limit of travel, the data set is complete. Stop.)

  3. On the GCAM PC for the active probe, align the star at the center of the field with the aperture. Do not start the active S‑H yet.

  4. In GMAP, select a new star for the passive probe, as close as possible to the same position each time (unless there is already one in the camera field).

  5. On the GCAM PC for the passive probe, align the star with the aperture.

  6. Return to step 1 to record the next data point.

After finishing

  • Note down the new ima values for the secondary mirror y, x, h, v, and z axes. The values for the h and v axes should not have changed.

  • On the GCAM PC for the active probe, restart the S‑H (by pressing F1 and F9) to keep the mirror tuned while the data is analyzed. The passive probe can be used to guide the telescope.

Analyzing data and using the result

For the time being, the telescope scientist or other designated expert will perform the analysis of the Twoprobe data and provide new hset and vset values for the secondary mirror, and tilt or voltage values for the tertiary mirror.

Cleanup

  • On the GCAM PC for the passive probe:

    • Exit the guider camera software on the passive probe guider camera PC.

    • Type COPY GCaaaaan.INI GCAM.INI (where aaaaa is the first 5 letters of the name of the operator’s computer, e.g. ALPAC or VICUN, and n is the guider number).

    • Restart the PC by typing CTRL-ALT-DEL.

  • In both IRAF windows, exit hartmannloop by typing CTRL-C, and then type twoprobeoff.

  • Close the passive IRAF and SAOImage windows.

  • In a terminal window on the operator’s workstation, type copytwoprobe to copy the passive data files from the remote computer.

  • Record the final secondary and tertiary mirror values in the nightly report.

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