Beginning of the night: Focus, center and absorb pointing offsets¶

Overview¶

This procedure must be followed by the observers before the beginning of dark time, prior to starting science or engineering activities. It will focus the telescope, center a target and absorb the pointing offsets.

Prerequisites¶

The telescope is open and ready for on-sky operations: it has been vented (if applicable), mirror cover and dome shutter are open, AOS corrections are enabled.
The sky brightness is low enough that the WEP (wavefront estimate pipeline) can converge successfully. This condition is normally achieved around 10-15 minutes before the end of nautical twilight or dark time.

Post-Condition¶

Telescope is focused and pointing offsets have been absorbed. The system is ready to start scheduler-driven observations or engineering tasks.

Procedure Steps¶

Confirm sky background is dark enough.

As a general rule, a good time to start executing the following SAL scripts is 10-15 minutes before the end of nautical twilight.

Launch a WEP sequence on an 8th magnitude star to use the wavefront estimation pipeline to measure wavefront error and perform mirror alignment and focus.

From the LOVE ATQueue panel, under AVAILABLE SCRIPTS on the left side, find and load the script auxtel/latiss_wep_align.py by clicking on the blue icon. It is included under External scripts.

In the configuring Script: latiss_wep_align window, the script configuration is introduced under CONFIG. Traditionally, we perform the first WEP of the night towards the south and at mid-elevations on an 8-mag star. With the configuration below, the script will query SIMBAD for a target around the provided az and el coordinates and with the specified maximum brightness mag_limit.

find_target:
    az: 180
    el: 60
    mag_limit: 8.0

Click on Add and the script will be added to the end of the script queue.

../../../_images/WEP_AuxTel.png — Screenshot of LOVE launching a WEP.¶

Command the telescope to measure and correct for any nightly pointing zero-point offsets using the SAL script auxtel/correct_pointing.py.

The steps the script perform in the background are:

Reset the pointing and hexapod x and y offsets.
Acquire target. How to select the source is summarized below.
Find the offset between the brightest source in the field and the boresight.
Offset the telescope to place the source at the center of the detector.
The telescope will absorb the measured pointing offsets.

Nominally, the same target the telescope is tracking and focused on will be used to correct the pointing. However, this functionality is not yet implemented and is included in DM-37841 <https://jira.lsstcorp.org/browse/DM-37841>

For now, correct the zero-point offsets in the same area of the sky as the WEP was done earlier. With the same WEP configuration, add the external script auxtel/correct_pointing.py to the ATQueue.

az: 180
el: 60
mag_limit: 8.0

The script will then find and use a target around the given coordinates az and el and magnitude limit mag_limit.

../../../_images/CorrectPointing_AuxTel_sameCoordinatesasCWFS.png — LOVE launching the script to correct the pointing using a source at around `az` 180 and `el` 60 and magnitude limit `mag_limit` 8.0.¶

If the configuration cell is left empty, the script will automatically find a target around az 90 deg and el 60 deg, with a mag_limit of 8.0.

Contact Personnel¶

This procedure was last modified on Feb 16, 2023.

This procedure was written by TDB. The following are contributors: none.

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