Laser Tracker Alignment System#
Abstract
This technical note provides an overview of the Laser Tracker Alignment System. It details the system’s operational framework, software interfaces, and guidelines to operate it.
Introduction#
At the Vera C. Rubin Observatory, the Laser Tracker plays a crucial role in the precision alignment of the observatory’s optical elements. Positioned at the center of the M1M3 cell, it utilizes a laser to accurately measure the positions of Spherically Mounted Retroreflectors (SMRs) distributed around the Camera, M2, and M1M3.
Operational Overview#
The operation of the Laser Tracker is facilitated through a Commandable SAL Component (CSC), which interfaces with the custom T2SA software installed on the laser tracker computer. The T2SA software on its turn interfaces with Spatial Analyzer, the proprietary software developed by the laser tracker’s manufacturer. Remote desktop access to the laser tracker computer is available for operational control and monitoring, with access details provided within the “lasetracker” Slack channel. This setup ensures a seamless operational flow and allows for efficient management and troubleshooting of the system.
Computing Target offsets#
In our previous approach to computing offsets on M2 and the Camera, we employed a method where we would measure the target (we will choose “Camera” for this explanation) points and update the reference FrameCAM by creating the Frame_CAM_meas. Notably, FrameCAM was centered at the origin of M1M3. Due to this configuration, Frame_CAM_meas was also centered at the origin of M1M3, leading to observed displacements in the frame caused by rotation in the hexapod, since the hexapod point of rotation is at the Camera instead of at that origin. These rotational displacements were artifacts that would not have been apparent if FrameCAM had been centered directly at the center of the CAM target.
Indeed, take the example of 0.15deg rotation about x in the hexapod. This rotation would cause the FrameCAM at the center of M1M3 to move,
In the following figure, we can observe the artifcats seen when moving the hexapod in dRY.
To avoid these artifacts, we need to adopt a new method. This method is detailed in one of the initial schmeatic drawings the vendor made, as seen below. Initially, we measure M1M3 and establish this measurement as the base or working frame by setting the tracker at the center of M1M3. All subsequent measurements are then made with respect to this M1M3 frame. Then, we proceed to measure M2 and the camera. The measurement frame for M2 (Frame_M2_meas) is compared to the default FrameM2, but crucially, this time the frame is centered at the actual location of the M2 target center. This adjustment allows us to correct for any real displacements without the interference of assumed distances between M1M3 and the target. It ensures that the default distance between M1M3 and the target is not erroneously generating displacement artifacts.
Currently we execute measurements with this commands:
await self.model.measure_target("M1M3")
await self.model.measure_target(target)
target_frame_name = self.get_target_name(target)
reference_frame_name = self.get_target_name("M1M3")
target_offset = await self.model.get_target_offset(
target=target_frame_name, reference_pointgroup=reference_frame_name
)
We propose to change the method to the following:
await self.model.measure_target("M1M3")
await self.model.measure_target(target)
target_frame_name = self.get_target_name(target)
reference_frame_name = self.get_default_target_name(target)
target_offset = await self.model.get_target_offset(
target=target_frame_name, reference_pointgroup=reference_frame_name
)
Note that this also involves changing the default Z coordinate of FrameCAM and FrameM2 to their nominal Z position with respect to M1M3.
It is important to understand that the new method is doing two things:
When measuring the target M1M3, it is setting the reference frame to the center of M1M3. This means any measurements after will be in that frame of reference.
We measure the offset between the default target offset, which is the optical optimal position of the target w.r.t. to M1M3, and the measured target offset. This is the actual offset of the target w.r.t. to perfect optical alignment.
Regular Operations#
The Laser Tracker system is routinely operated at the beginning of each night’s observations to align all optical elements within the observatory. This operation is supported by a series of specialized scripts designed to facilitate the alignment process:
maintel/lasertracker/measure.py: This script measures a target and calculates the offset of that target (either M2 or Camera) with respect to M1M3.maintel/lasertracker/align.py: With inputs for a target and a tolerance level, this script measures the chosen target and iteratively realigns the telescope using the measured offsets until the specified tolerance is achieved.
Special Procedures#
In addition to routine alignments, specific procedures are in place to ensure comprehensive system readiness and operational integrity:
BLOCK-246: Tests that all targets are measurable by the laser tracker. It goes through all the targets and ensures no errors are found when measuring them.
BLOCK-197: Tests the sequential misalignment and realignment of all optical elements, testing and confirming the system’s capability to accurately realign after intentional displacement.
System Maintenance#
The laser tracker computer should not be powered down using the maintel/lasertracker/shut_down.py script.
Restarting the system requires physical presence at the observatory for manual reboot, a procedure designed
to minimize downtime and ensure continuous operational readiness.
Note
For detailed instructions on accessing and operating the Laser Tracker Alignment System, please refer to the “lasetracker” Slack channel, where updates and access credentials are regularly provided.