Here, we shall briefly discuss the AO control loop, as shown in Figure 2.26. The AO control loop begins with a single wavefront sensor image. For each of the 97 subapertures in each image, the total intensity of the laser spot is calculated. If the intensity is above a certain threshold, the centroid of the Shack-Hartmann spot within the subaperture (i.e. x and y slope of the local wavefront) is calculated, else the subaperture is flagged as having low light and the centroid position is not calculated. The measured slopes are linearized through a look-up table which accounts for the response of the quad-cell. The fiducial centroid positions (or slope offsets) are subtracted from the measured values, and the final values are arranged into a slope vector with 97×2 = 194 values. The measured slopes are multiplied with a reconstructor matrix, which creates a least-squares estimate of the wavefront shape as projected on the 120 deformable mirror actuators and the laser uplink tip-tilt estimate. The estimate of the wavefront error is applied to the deformable mirror through an integral control law with a small leak term that allows the deformable mirror to gracefully tend to a flat position if no slopes can be measured (in low light conditions).
The AO control loop has been demonstrated to run as fast as 1.5 kHz. However, the best operating rate to balance our error sources is 1.2 kHz, which is now the standard rate.
At 1.2 kHz, the wavefront sensor integrates 8 − 9 pulses from the laser beacon (operating at 10 kHz) before the frame is read out and processed. As we will discuss in Chapter3, this bandwidth is sufficient to account for most of the higher order wavefront errors.
• Priming of Cooling Lines: The laser chiller and the heat exchanger are placed in the P60 data room (about 20 to 25 feet below the laser head) and the hoses connecting the laser to the chiller and the electronics rack to the heat exchanger are disconnected at each run. When reconnected, any residual air bubbles occurring in the system must be flushed using a priming pump on each segment of the plumbing. Without this precaution, the bubbles migrate upward and settle at the laser head, preventing cooling of the laser diodes.
• Checking Internal Alignment: At the beginning of each run, the internal align- ment (registration) of the deformable mirror, the field stop, and the wavefront sensor are checked. This ensures that the reconstruction from measured slopes to deformable mirror commands is accurate.
With the deformable mirror at a flat position and using the internal UV source with a large (1 mm diameter) fiber, the alignment of the field stop is checked. The edges of the fiber spots should not overflow the corresponding subaperture boundaries into the neighboring subapertures. The size and position of the field stop is adjusted till the image spot size and position is satisfactory. Then the 1 mm diameter fiber is replaced with a 0.1 mm fiber and again UV fiber source is finely adjusted till the tip- tilt measurements on the wavefront sensor are zeroed (with live feedback from custom LabViewsoftware).
To check the registration of the deformable mirror with the wavefront sensor, a pattern is applied to the deformable mirror that pulls in the four central and four corner actuators on the deformable mirror over an overall flat shape. These create small slope changes on the wavefront sensor that are checked for symmetry. The symmetrical locations imply that the actuators are registered at the corners of the subapertures.
The wavefront sensor lens L1 and L2 are adjusted till the pattern is correctly aligned on the wavefront sensor.
• Renewing Slope Offset Files: After checking the alignment, the wavefront sensor slope fiducial (‘zero’) measurements are recorded as the slope offset files. These are subtracted from the measured slope values to estimate the actual slope variations.
The slope linearization file7 is set to use the linearization table for a flat-topped circular spot expected from the fiber instead of the Gaussian profile expected from an on-sky source. Using the flat shape on the deformable mirror, with the scalar gain of the AO system set to zero and the leak constant of the control law set to unity (such that no correction is applied to the deformable mirror), the AO system is run and slope telemetry is recorded. A MATLAB script converts the average slopes measured in the telemetry to appropriately formatted slope offset files that are used during the observing run.
7The relation of the spot centroid measured in the subaperture to the actual movement of the spot is non-linear and dependent on the spot shape, pixel size etc. We use pre-calculated look-up tables to linearize the centroid measurement. Two separate tables are used for the different profiles expected from the fiber source and the on-sky laser spot.
• Adjusting Laser Focus: Since the laser projector is continuously mounted on the telescope, it is subject to much larger temperature fluctuations and gravity flexing than the Robo-AO Cassegrain optics that are stored in the data room when not mounted on the telescope. Hence, it is important to refocus the laser projector to create a sharp focus at the beacon altitude of 10 km. This procedure should also be repeated if drastic temperature fluctuations (&5◦C) occur during an observing run.
After ensuring that the telescope and the outside air have similar temperatures, the telescope is first focused on a natural star using an automated focusing loop. One may attempt to run the AO system if the laser projector focus is sufficiently close to the optimum value. If the AO system works, a finer estimate of focus can be obtained.
After the telescope is suitably focused, the laser return images are observed on the wavefront sensor camera (read out in the full 80×80 pixel mode). The focus of the laser projector is adjusted using an internal translation stage till the spots are sharpest and their intensity is maximized. If the natural seeing is high (&1.500), it is extremely difficult to optimize the focus since the laser spots can almost fill up the field of view of the wavefront sensor subapertures.
• Telescope Focusing: At the beginning of each observing night, the telescope is focused with an automated focusing routine (written by Kristina Hogstrom). A rea- sonably bright star near the zenith is selected and the telescope focus distance is stepped through, and images are acquired at each focus value. A interpolated focus value that would provide the smallest FWHM is selected.
Through the night, as the AO system is operated, a running average of the focus mode measured by the wavefront sensor is calculated and displayed in the telemetry.
In July 2013, an automated offload of the wavefront sensor focus measurements to the telescope focusing has been implemented.
• Flats and Biases: The Andor EMCCD camera has negligible dark current, but the bias values are not flat over the entire chip. The bias values also vary over different values of the electron-multiplying gains. Biases frames are acquired in each observing mode in the evening before observations.
Flat frames are also acquired in each filter in the evening before the observations. In order to collect sufficient photons on the 0.04300square pixels8 in a reasonable amount of time, Robo-AO uses an industrial halogen floor lamp to illuminate the dome screen over which flats are acquired.
2.4.2 Monitoring
Once the setup and calibration is completed and the target lists are uploaded to the queue scheduler, Robo-AO can operate without human intervention unless a rare unrecoverable error occurs. Robo-AO is intended to be remotely monitored over the internet through a status website.
8As compared to the 0.37800pixels of the GRB camera, the pixels are 70 times smaller in area.
In regular operation, the Robo-AO control software writes a status file for each subsys- tem every second with information about internal workings of the daemons. python and phpscripts continuously convert these status files to images and data tables that are served over the Robo-AO website.
Figure 2.27shows the complete three window setup for monitoring Robo-AO. The left- most panel shows the shape of the deformable mirror (orange circle; left top in the panel), the wavefront sensor image (gray annulus; right top in the panel), and the visible camera image (bottom image in the panel). The deformable mirror image is coded to alert the user with bright colors if the deformable mirror actuators are hitting or are close to hitting the maximum or minimum allowed values. The central panel is anSSHwindow with color-coded logs of the Robo-AO software. The terminal alerts the user with red text and sounds if errors are encountered. The right-most panel is the Robo-AO status webpage. It serves status data from all daemons as data tables. Each value is color-coded to allow the user to check the status at a glance. Green boxes are the normal operating range, yellow is a warning color, and red shows values that are outside operational ranges.
Figure2.27.Robo-AOismonitoredviatelemetryandloggingdatacreatedbythedaemons.Theleft-mostpanelshowstheshapeofthe deformablemirror(orangecircle;lefttopinthepanel),thewavefrontsensorimage(grayannulus;righttopinthepanel), andthevisiblecameraimage(bottomimageinthepanel).Thedeformablemirrorimageiscodedtoalerttheuserwith brightcolorsifthedeformablemirroractuatorsarehittingorareclosetohittingthemaximumorminimumallowedvalues. ThecentralpanelisanSSHwindowwithcolor-codedlogsoftheRobo-AOsoftware.Theterminalalertstheuserwithred textandsoundsiferrorsareencountered.Theright-mostpanelistheRobo-AOstatuswebpage.Itservesstatusdatafrom alldaemonsasdatatables.Eachvalueiscolor-codedtoallowtheusertocheckthestatusataglance.Greenboxesarethe normaloperatingrange,yellowisawarningcolor,andredshowsvaluesthatareoutsideoperationalranges.