antenna recommissioning after metrology system upgradeparrar/tran/msu/comm2011.pdf · { 21-10044a...

Atacama PathfinderEXperiment

Metrology Upgrade

APEX-APX-PRR-0000

Revision: 2.3

Release: October 12, 2012

Category: 4

Author: F. Montenegro

Antenna Recommissioning afterMetrology System Upgrade

F. Montenegro & Rodrigo Parra

(on behalf of the APEX Science Operations Group)and

G. Wieching (APEX), A. Lundgren (ALMA),A. Belloche, R. Gunsten, D. Muders, (MPIfR)

Keywords: Metrology, Upgrade, Commissioning, APEX

Author Signature: F. Montenegro Date: October 12, 2012

Approved by: NN

Institute: NNN

Signature: NN

Date: Month Date, Year

Released by: NN

Institute: NNN

Signature: NN

Date: Month Date, Year

APEX Antenna Recommissioning after Metrology System Upgrade

Change Record

Revision Date Author Section/ RemarksPage affected

0 2011-03-26 F. Montenegro All Initial Version1.0 2012-09-14 F. Montenegro All Major revision2.0 2012-10-04 F. Montenegro 4.2 Incl. pointing section by R. Parra2.1 2012-10-04 F. Schuller All Reorganization, minor corrections2.2 2012-10-08 F. Montenegro All Including some comments by GWI2.3 2012-10-10 R. Parra All Finalizing Comments and tidy up.

Create Date: October 12, 2012 Contact author: F. Montenegro


Contents

1 Reference documents 4

2 Introduction 5

3 Metrology system 53.1 Previous tiltmeters setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

3.1.1 Internal calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63.1.2 Recalibration with pointing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

3.2 Linear Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

4 Metrology System Upgrade (MSU) 104.1 Control system upgrade . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104.2 Linear Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104.3 ACU, PTC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114.4 Tiltmeters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114.5 Drive Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114.6 Other Post-upgrade important changes . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

5 Recommissioning 135.1 Tracking Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

5.1.1 Design of the performed tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145.1.2 Tests before MSU upgrade (19-26 Jan 2011) . . . . . . . . . . . . . . . . . . . . 145.1.3 Tests after MSU upgrade (16-24 Feb 2011) . . . . . . . . . . . . . . . . . . . . 165.1.4 Tests after first adjustment of ACU tracking parameters (01-09 Mar 2011) . . . . 205.1.5 Tests after second adjustment of the parameters set (09-25 Mar 2011) . . . . . . 205.1.6 Tests 1.5 years after MSU (21 Sep 2012) . . . . . . . . . . . . . . . . . . . . . . 215.1.7 Conclusions from dedicated tests . . . . . . . . . . . . . . . . . . . . . . . . . . 215.1.8 Tracking monitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

5.2 Pointing Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 245.2.1 Linear Sensors: Night versus day comparison . . . . . . . . . . . . . . . . . . . . 245.2.2 Linear sensors signature removal . . . . . . . . . . . . . . . . . . . . . . . . . . 255.2.3 Tiltmeters: Bias Thermal Compensation . . . . . . . . . . . . . . . . . . . . . . 265.2.4 Deviation of gravity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 275.2.5 Tiltmeter measurements: Clockwise vs counterclockwise . . . . . . . . . . . . . 295.2.6 Pointing monitoring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

5.3 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

6 Summary and conclusion 34



1 Reference documents

– VERTEX document OM1001135-32712 Control System User Manual - Part 8 - Technical Descriptionof Pointing Error Compensations

– APEX-APX-PRR-0001 APEX Tracking performance (September 09, 2004)

– APEX-APX-PRR-0008 Optical Pointing Status April 2005

– APEX-APX-TRE-0023 Improving APEX using tiltmeters

– APEX-APX-TRE-0036 APEX Pointing - Status report (September 2010)

– TN-1010057-31470 Vertex Technical note Error Compensation (May 2011)

– 21-10044A Upgrade of Antenna Control Unit and Metrology System. Technical Description

– SATR-1010057-32100 ACU Site Acceptance Test. Antenna Control Unit

– SATR-1010057-32700 SR-MMS Site Acceptance Test. Metrology System / Subreflector Hexapod



2 Introduction

The APEX telescope design was based on a prototype of the ALMA antennas. The experience gainedduring various years of operations of that prototype version has been valuable to improve the designof the current ALMA production antennas developed by VERTEX. Therefore, these implement severalimprovements with respect to the APEX design, in particular regarding the metrology system, aimed atoptimizing the pointing accuracy on sky.

The metrology system installed at APEX in 2004 has never been implemented to perform accuratepointing corrections on real time. This has led to the development of various operational procedureswhich use only partially the metrology system.

However, it is clear the necessity to exploit the system in its full capabilities to minimize the pointingoffsets due to thermal deformations of the antenna structure and changes in the antenna tilt. Thisis especially important when performing high-frequency observations, for which the beam becomescomparable in size to the pointing corrections provided by metrology sensors. This full implementation isnow possible at APEX after the metrology upgrade since the system was previously not usable.

A major upgrade of the metrology system took place at APEX in February 2011. The two maincomponents of this new metrology system are linear sensors and tiltmeters. Their way of working isbriefly described below, in sections 3.1 and 3.2, respectively. Apart from those devices, there is thehardware/software which collects the data from them and computes real time pointing corrections to beapplied on top of the current pointing model.

Contemporaneously with the metrology system upgrade, some other changes have been done at APEX,like the ones in the drive system, which affect the tracking performance, as well as upgrades on the APEXcontrol system and related software. They are described in Section 4.

3 Metrology system

3.1 Previous tiltmeters setup

The telescope tilt, or deviation of the azimuth axis with respect to the vertical direction, can be quantifiedusing two parameters, AN, AW. These are, respectively, the projections (in arcsec) of this deviation in thedirections North-South and East-West.

In order to determine these parameters, the embedded metrology system includes three biaxial tiltmetersinstalled on the telescope mount (Manufacturer: Applied Geomechanics; models: 711-2 and 716-2). Twoon each arm of the yoke (T2 and T3) and one (T1) over the azimuth bearing (see Figure 3.1). Thesetiltmeters are assumed to be sufficiently well aligned with the geometrical/geodetical setup of the mountto be used as fiducial references to improve the pointing accuracy.

Each of these tiltmeters are biaxial and measure (in their reference system) the inclination tilt x andtilt y. Depending on their location, these reference systems relate to the yoke coordinate system as shownin Figure 3.2.

So far, due to the partial implementation of the metrology system, periodic offline fits to tiltmeter mea-surements conducted while swinging the telescope around the clock have been used (measure tilt script)in an attempt to estimate the present value of the AN,AW vector. However, this technique is not sensitiveto changes occurring between such measurements, e.g. due to thermal deformation of the structure, whichcan in turn produce substantial pointing errors. The overall effect (assumed to be purely geometrical) ofthe AN,AW vector on the pointing corrections vector field is given by:

[∆A,∆E] '[− sin(A) tan(E) − cos(A)− cos(A) tan(E) + sin(A)

] [AN

AW

](3.1)



Y

Az=0

Yoke seen from above

X X X

YY

T2 T1 T3

Figure 3.1: Location and orientation of the three tiltmeter units. Note the anti-parallel orientation of T2and T3 which translates in the phase difference on the traces shown in following sections.

where A and E are respectively the pointing azimuth and elevation. The devastating effect of the tan(E)term, especially at high elevations, makes that an intra-hour variation of a few asec in AN,AW can produceinvisible pointing errors as large as a high frequency beam, consequently rendering the observations ofweak targets hopeless.

Despite these limitations, the systematic usage of this script has allowed us to build a database of tiltmeasurements, keeping track of significant changes in the antenna tilt on weekly, monthly and yearlytimescales (see document APEX-APX-TRE-0036 for details).

As will be explained in Section 4.4, the new approach will consist of using only the tiltmeter T1 locatedover the azimuth bearing to estimate the pointing corrections.

3.1.1 Internal calibration

The tiltmeters readouts are temperature dependent. The manufacturer provides a calibration recipein order to convert the raw voltage readouts into microradians. These equations are supposed to beimplemented into the ACU, as shown in Figure 3.3 (this figure was extracted from Vertex DocumentOM1001135-32712, Control System User Manual - Part 8 - Technical Description of Pointing ErrorCompensations).

To the resulting corrections one has to add offsets to both axis x and y. These offsets compensate foralignment errors of the tiltmeters. Taking this into consideration, the final equation governing the internaltiltmeter calibration is:

TiltACU = 10T (sfcal +KsTcal −Kz0 −KztT + Tiltoff ) (3.2)

where the relevant constants applicable to the tiltmeters installed in 2005 are given in the in-plantcalibration data sheets in the mentioned manual, and summarized in Figure 3.4. These constants shouldalso be stored in the ACU to implement such calibration in an automatic fashion



Figure 3.2: Relationship between the TM coordinate systems and the yoke coordinate system.

Figure 3.3: Equations governing the internal calibration of the tiltmeters as a function of temperature

3.1.2 Recalibration with pointing

On top of the manufacturer calibration, there is an additional calibration needed to make the AN, AW

coefficients measured by the tiltmeters match those derived while fitting a pointing model to a set of ob-served stars. This second calibration has been in the past implemented in the script measure tilt.apecs

and it contains a scaling factor to the tilt derived by the ACU plus a term depending on the temperature:

Tiltx = Ix × TiltACU,x + pxT (3.3)

Tilty = Iy × TiltACU,y + pyT (3.4)

The way to determine the px, py constants is to collect enough tilt measurements (i.e. using themeasure tilt.apecs script) covering a sufficiently high range of temperatures. The scaling factors Ix,Iy can be calculated comparing, for a series of pointing runs, the tilt as obtained from equation 3.2 withthat derived by the pointing model fit.

The coefficients derived in the last recalibrations, which were in use just before the metrology upgrade(February 2011), are listed in Table 1. There, values are provided for each tiltmeter and for each axis.



Figure 3.4: Calibration constants for the three tiltmeters

3.2 Linear Sensors

Four high precision linear sensors are mounted in the yoke arms. They are mounted pairwise one eachside of the elevation tube, and measure the movement with respect to the vertical CFRP (carbon fiberreinforced plastic) struts. Their positions are shown in Figure 3.5. The linear sensors installed at theconstruction phase of APEX and mounted till Feb 2011 were Linear Gap Displacement Transducers(LGDTs) manufactured by the company BEI. They measured distances to the target surface by emittinglight with an IR LED and then measuring the reflected emission. They had a 1%-linearity region of 3,75mm and the analog output was 0−10 V.

Any difference in the readings of the two sensors at right and left side of each encoder indicates that theazimuth (steel) structure is tilted compared to the temperature stable CFRP encoder mount, resulting ina pure elevation error.

The distance between the two linear sensors in one arm, d, is 1440 mm, while the distance D betweenone sensor on one arm and the corresponding sensor on the other arm is 5700 mm. The linear sensors aresensitive only to a tilt of the Yoke arms relative to the Yoke base, and they are not able to measure anytilt of the ground. The elevation offsets are calculated averaging the tilt measured by the linear sensorsat both sides, as follows:

∆L =(L1B − L1A) + (L2B − L2A)

2(3.5)



Tiltmeter Ix Iy px (asec/degC) py (asec/degC)T1 1.0480443 1.0256347 0.0248090 0.0961730T2 0.9530092 1.0052551 0.0172317 0.0027851T3 0.9804320 1.0137433 −0.0737580 0.1244756

Table 1: Additional calibration for the tiltmeters (implemented in the old measure tilt script), in useuntil February 2011.

Figure 3.5: Location of the four linear sensors.

And the resulting error in elevations is given by:

∆ELS = tan−1(

∆L

1440mm

)radians (3.6)

At APEX the old set of linear sensors could never be properly calibrated. On one hand, the devicesthemselves had some readout problems that prevented having good and repeatable measurements. Inaddition, the algorithms implemented in the old PTC which would compute the pointing offset correctionswere not accurate either. Despite this, various attempts were done in the past in order to measure theeffect of the linear sensor corrections in the pointing accuracy. As an example, this was tested in 2005(see document APEX-APX-PRR-0008) and only a very slight improvement in the overall rms of thefitted pointing model was found (it improved by 0.2 arcsec with respect to LS not enabled) but there wasno apparent improvement in the rms in elevation.



4 Metrology System Upgrade (MSU)

Within the 2011 summer shutdown period, several days (from Jan 21st to Feb 17th) were scheduledto execute the metrology system and other related upgrades. For this purpose, personnel from VER-TEX (K. Willmeroth and H. Becker) were working at the antenna in the different upgrades with thesupport/help/local supervision by G. Wieching (APEX) and D. Muders (MPI). In this temporal lapseseveral important modifications/replacements were done, which are briefly described in this section. Fora more detailed overview of the planned modifications, see VERTEX document 21-10044#A.

After this major intervention there has been an extensive period (about 1.5 years) for testing, debuggingand calibrating the new metrology and drive systems. A list of open items remained unsolved justafter the visit by VERTEX, including issues with the initialization process of the sub-reflector hexapod,excessive noise of the tiltmeter installed on the azimuth bearing or the incomplete execution of theantenna self-test.

To address some of these open items, a second visit by VERTEX (L. Stenvers) was scheduled on May23rd 2011 who replaced the tiltmeter by a new unit, and a third visit during the shutdown period in 2012(Jan 27th to Feb 6th) by T. Jurges and M. Funke who addressed some of the issues with the hexapodinitialization, tracking issues and antenna self-test.More recently (winter 2012), several hardware replacements were done, regarding servo amplifiers andtheir cabling to telescope drives, which significantly improved tracking performance.

4.1 Control system upgrade

Before any hardware replacement, a series of software upgrades were done on 2011 January 21st regardingcontrol system:

– Servers operative system: From Scientific Linux v5.2 to v5.5

– ALMA common Software (ACS) from v8.0 to v9.0

– New VxWorks kernel with network improvements

– APEX control system (APECS) from v2.2 to v2.3

These changes were performed in all servers both in Sequitor and Chajnantor by D. Muders in remotefrom Bonn, supported by M. Cardenas and J. P. Araneda at APEX. After the replacements, a few dayswere spent to test the new software, observing modes, pointing runs, etc.

During the shutdown period in 2012 additional major changes were done on the control system, includingreplacement of servers by more powerful machines (control3 and observer3 replacing, respectively, control2and apexdev2), and the upgrade to ACS 10.1 and APECS 2.5.

4.2 Linear Sensors

Four new displacement sensors have been installed on the yoke replacing the old models. The new modelsare manufactured by SOLARTRON and the are Linear Variable Differential Transformers (LVDT) withspring-push technology. Their operational range is ±1 mm and the signal output ±10 V. The analog todigital converter has been kept the same as in the old configuration.

This new set of linear sensors have an accuracy of ∼ 2µm and a resolution < 0.1µm. They are used inthe ALMA production antennas and are well characterized.

After the installation, Various calibrations and adjustments were performed by VERTEX in all fourdevices. Figure 4.1 show the correlation between Yoke temperature and linear sensor displacement.

DispLS = 0.035T mm (4.1)



Figure 4.1: Calibration of the Linear Sensor displacements as a function of yoke temperature.

4.3 ACU, PTC

The Antenna Control Unit (ACU) has been replaced in February 2011. The new unit (8200) replaces theold model (8100) and is now similar to the ones used onboard ALMA production antennas. The new ACU8200 contains two CPUs based on Pentium M processors and the Graphical Unit Interface (GUI) has beenimproved with respect to the older setup. In addition, it is now possible to do remote connections in orderto command the antenna without being present at the instrument container. This is of great help in emer-gency cases (failures close to Sun avoidance, telescope stuck during night time close to hardware limits, etc).

The Pointing Computer (PTC) has been modified so it contains a similar CPU as the one in the newACU. From the functional point of view, PTC will remain a mere data handler and interface to existinghardware, since many of its past functions will be handled by the new ACU. No GUI will be necessaryanymore for this computer and no special spares are needed for its CPU, since the one for the ACU canalso be used, if needed. The CAN interface and power supply were unchanged.

4.4 Tiltmeters

The original plan was to keep the same set of tiltmeters in their original locations, but only use the oneinstalled above the azimuth bearing for real-time corrections, in the same way this is done in the ALMAproduction antennas. The only foreseen change was to provide a different set of calibration parametersinto the ACU.

However, it was found at a later stage that this particular tiltmeter showed quite noisy readoutsmaking impossible to generate accurate pointing corrections. For this reason, a new device was in-stalled/aligned/calibrated by VERTEX on 2011 May 23rd, replacing the old one. It was found laterthat this device experienced from freezing issues at low temperatures, and for this reason a new unitwas installed in 2012 Feb 19th with a different electrolyte. The new and recalibrated tiltmeter hasan improved noise floor relative to the old one (at least by factor of 10, see Figure 4.2). However, theelectrolite within this unit was freezing at low temperatures so it was replaced by yet another one in midFebruary 2012 (The same procedure was conducted at ALMA).

4.5 Drive Control

The main significant improvements implemented in 2011 affecting the drive behavior were:

– There are now different parameter sets for the tracking submodes

– Additional parameters have been included to allow for better optimization



Figure 4.2: Tiltmeter raw readouts for 10 minutes. The antenna was stopped during the measurements.The red curve corresponds to the old tiltmeter and it was recorded on 2011-04-23, while the blue curvefrom the new tiltmeter was recorded on 2011-05-24, just after its installation.

– Improved profiler algorithm for transitions, turnarounds and steps

– Elevation servo-amplifier has been tuned with a “one-parameter-set”

The single parameter setup has cured the memory losses of the azimuth servo amplifiers in the past. Theidea is that this can also be possible in elevation. These memory losses were causing strong oscillationsin elevation, extensively reported at APEX in the last few years.

In 2012 new servo amplifiers have been installed in the drive cabinet by new models (series Lenze 9400)with the line filters mounted underneath. A full set of amplifiers (4) is nowadays in use. Improvementswere done in the connectors and cables (both control and power cables) linking drives with servo amplifiers.

4.6 Other Post-upgrade important changes

It is important to note that after several weeks of operation with the new metrology system, it was foundin an holography session that the sub-reflector was heavily damaged having it lost a considerable fractionof its aluminum layer. This forced a replacement by a spare unit on 2011 May 29th. This change hasa strong impact on the pointing models created after this replacement, since various pointing constantshave changed and that means that older historical pointing data cannot be used anymore in the creationof new pointing models.



5 Recommissioning

The commissioning of the new system has been done in several steps after the various upgrades. At ainitial stage (from 2011 January 26th to February 11th), VERTEX personnel installed and tested thefunctionality of the new system. G. Wieching (APEX) and D. Muders (MPI) were there to supervisethe results of the tests. The verification process was done following a series of pre-designed acceptancetests with the scope of checking the performance of interlocks, drive system, ACU, metrology, hexapod,monitoring & Control, and integration with ABM. The detailed description of these tests and the obtainedresults are available in documents SATR-1010057-32700 SR-MMS and SATR-1010057-32100 ACU.

After the antenna was accepted, VERTEX gave a training presentation to the APEX staff present atthe base on 2011 Feb 16th. In this presentation the performed changes were outlined, and the majoroperational differences with respect to the previous system were highlighted.

In the last 1.5 years, a strong effort has been done by the APEX Sciops group in order to understand thefunctionality of the upgraded metrology system and characterize its performance. The final scope is tomake optimal use of the upgraded metrology system to improve as much as possible the pointing accuracyon-sky. In addition, several procedures have been established to quantify the tracking performance duringnormal operations.

5.1 Tracking Performance

The general idea of this chapter is to get a status of the tracking performance before the MSU, andcompare with the performance after the upgrade. According to VERTEX, the improvements on the ACUAlgorithms, control loop parameters and tuning of the servo amplifiers will result in a better trackingperformance (see details in Section 4.5).

At the beginning, the procedure consisted of a series of dedicated tests in order to check the trackingperformance as a function of position on the sky, etc. During the last 12 months, a strong effort has beendone to substitute these dedicated tests by a more efficient procedure. Scripts have been created in orderto quantify the tracking efficiency on regular scans (science, pointing, calibration, etc), and a databasehas been put in place to collect historical data. This tracking monitoring program is currently being donein a completely automatic fashion, and allows us to have:

– A more homogeneous dataset to compare tracking deviations computed in the same way.

– Much better temporal resolution, since we collect data virtually at every time.

– Much quicker notice of tracking degradation, which allows to take the appropriate actions.

– Possibilities to correlate bad performances with technical modifications or other environmental effects(temperature, wind speed or wind direction).

Coming back to the dedicated tests, different blocks were scheduled at different stages:

- Before the software upgrade (ACS + APECS). 19-22 Jan 2011

- After the software upgrade but before the MSU. 22-26 Jan 2011

- After the MSU. 16-24 Feb 2011

- After fine-tuning the parameter set for the servo amplifiers

- About 1.5 years from the upgrade, after several servo replacements and recabling.

We will first focus on the comparison of these test at the different stages, and only at the end say somepreliminary results about the tracking monitoring plan.



5.1.1 Design of the performed tests

Several tests were done moving the antenna to a given position and then tracking this position with thecommand on() for a given number of seconds. Different integration times were chosen, and differentpositions were explored in order to test different tracking velocities in both axes:

– Transiting sources (North) at high/medium/low elevation

– Transiting sources (South) at high/medium/low elevation

– Rising sources

– Setting sources

Most test were done using the standard elevation mode el aux mode(0), which means using both the A−and B−cabin drives. This is the standard situation for normal operations. However, some other trackingtests were done using el aux mode(1) (using only the A−cabin drive) and el aux mode(2) (using onlythe B−cabin drive), to check their performance. These modes are useful whenever there are critical issuesaffecting one of the drives, so it is important to have at least a basic characterization of their performances.

When the data were taken, the strategy was to quantitatively evaluate the performance of the trackingfollowing the same methodology as in document APEX-APX-PRR-0001 (APEX tracking performance,September 09, 2004): The following quantities were calculated for the whole duration of the scan:

– ∆Az = Actual Az − Commanded Az

– ∆El = Actual El − Commanded El

Then for the whole scan duration, the average and RMS of these quantities has been computed. Thecommanded and actual positions of the antenna are stored with a resolution of 48 ms inside theMONITOR table written by the FitsWriter. This information has been used to calculate their differencealong the duration of the scan. The results are shown in the tables below. According to VERTEXspecifications, tracking should be better than 0.6” RMS. Plots have also been generated showing thetelescope trajectory for every scan, as well as the quantities defined above and the tracking velocities inAz and El (see Figure 5.1 as an example).

5.1.2 Tests before MSU upgrade (19-26 Jan 2011)

Many tracking tests were done on Jan 19th, before the ACS + APECS upgrades. The idea was tocharacterize the tracking just before any upgrade was performed, to have a recent reference status. InTable 2 the list of scans taken in this period is shown.

The azimuth tracking is mostly according to specifications. When using both drives, only for three scanswe see RMS ∆Az > 0.6”. They are 3489 and 3463 with RMS just above specs (smaller than 0.7”) andscan 3494 with RMS a bit worse (0.84”). The RMS is higher in these scans because there is some kindof smooth variation in the azimuth velocity with time (see Figure 5.1). These variations were possiblyrelated to the implementation of the tracking loop into the ACU.

We see in Table 2 that about 1 of every 5 scans show a large RMS in elevation and the reason is thepresence of some strong ”jumps” in the (actual-commanded) signal, which seem to be stronger whilethe source is rising. In Figure 5.3 one can see the higher RMS on scans for which sources are onthe East (rising sources). There are some examples of transiting sources for which these jumps canbe seen only before the transit (e.g. scan 3465, 3988). These jumps are, in this case, likely beingintroduced by the B-cabin drive. This is evident from scan 3454 (see Figure 5.2) for which only thismotor is being used. In addition, scans for which the A-cabin drive was only used do not show these jumps.

The jumps like those shown in Figure 5.2 were likely produced because of the servo amplifiers sufferingfrom ”memory losses”, i.e., loosing their parameter set. Every time this has happened it has been



Scan aux time Move Az0 El0 Mean RMSAz El Az El

3456 0 600 Transit N 6 78.6 0.028 0.012 0.431 0.5243463 0 600 Transit N 4 78.72 0.028 −0.002 0.408 0.3023465 0 300 Transit N 10 85 0.073 0.012 0.368 0.5933468 0 300 Transit N 0.5 20 0.006 0.003 0.381 0.1473469 0 300 Transit N 1 57 0.009 0.003 0.631 0.2353987 0 1800 Transit N −16 82.1 0.036 −0.008 0.346 0.4163470 0 300 Transit S 179 84.7 −0.019 −0.003 0.297 0.2163473 0 300 Transit S 179 27 −0.002 0.001 0.091 0.0233474 0 300 Transit S 179 45 −0.004 0.002 0.297 0.1323478 0 300 Transit S 180 25.7 −0.001 0.005 0.083 0.0183988 0 1800 Transit S −185 81.5 −0.018 −0.005 0.422 0.3213471 0 300 Rising 178 19 0.004 0.014 0.082 0.6603472 0 300 Rising 178 19 0.005 −0.002 0.097 0.4973477 0 300 Rising 72.8 74.2 0.005 0.082 0.307 1.3403495 0 300 Rising 37 85 0.077 0.020 0.305 0.5513643 0 1000 Rising 18.5 48 0.008 −0.004 0.638 0.5473645 0 1500 Rising 89 14 0.002 0.008 0.294 0.9053677 0 1000 Rising 114 5.8 0.003 0.018 0.332 1.0053992 0 1800 Rising 93 68.5 0.004 0.016 0.271 1.0553494 0 300 Setting −71.2 82 −0.003 −0.007 0.840 0.457

3480 1 300 Transit S 180 25.7 −0.004 0.000 0.074 0.0374011 1 1800 Setting 78 −93.3 −0.004 −0.003 0.196 0.462

4015 1 1800 Rising 105.2 71 0.000 −0.006 0.336 0.6793454 2 600 Transit N 3 78.7 0.029 0.268 0.535 2.119

Table 2: Tracking results before the ACS+APECS upgrade. The second column shows the auxiliary modeused in elevation. In bold face appear those RMS values out of specs, higher than 0.6”.

Figure 5.1: Tracking plot of scan 3494, taken on 2011-01-19. There are smooth variations in azimuth veloc-ity with time which translate into higher positionalRMS.

Figure 5.2: Tracking plot of scan 3454, taken on 2011-01-19 in el aux mode(2). Strong oscillations were in-troduced by B-cabin drive.

necessary to reload the parameter set again in order to recover normal operations of the servos. One ofthis losses occurred on 2011-01-22 and the parameters were reloaded again on 2011-01-23. After this,a few more scans were taken and the strong oscillations in elevation were gone. The results from the



tracking tests are shown in Table 3.


4294 0 600 Rising 16 78.3 0.027 −0.007 0.525 0.2104299 0 600 Rising −181 174 0.006 0.000 0.109 0.1764301 0 600 Rising 72 70 0.010 −0.007 0.790 0.2534407 0 1800 Rising 70 70 0.010 −0.006 0.779 0.243

4296 1 600 Setting −28 77.4 0.014 −0.005 0.288 0.6804420 1 1800 Rising 70 70 0.009 −0.007 0.829 0.670

4433 2 1800 Rising 70 70 0.009 −0.006 0.830 0.8754295 2 600 Setting −2 78.7 0.026 −0.008 0.508 0.851

Table 3: Summary of the tracking results after the ACS+APECS upgrade and after reloading the param-eter set in the elevation servo amplifier.

The average RMS in elevation of the 4 scans with el aux mode(0) is 0.220”. In azimuth, we see still thepresence of some ”waves” which make the corresponding RMS higher. Examples of this are scans 4301(see Figure 5.4 ), 4407, 4420, 4433. As found later, these waves were resonances due to a non appropriateselection of the parameters for the servos. In addition, scans 4295 and 4296, which use auxiliary modesin elevation, get slightly out of specifications as well.

Summarizing, before the MSU frequent memory losses from the servo amplifiers were present. They led toservo amplifiers producing oscillations of different magnitudes in elevation, which degraded the trackingperformance out of specifications.

5.1.3 Tests after MSU upgrade (16-24 Feb 2011)

We performed similar tests after the metrology system upgrade (new parameter set and new control loopin the ACU). Thirty-two scans were taken using the different auxiliary modes for elevation tracking. InTable 4 the results of these tests are summarized.

The first conclusion is that the tracking performance in elevation has proven to be well within specs(using normal operation mode el aux mode(0)). None of the 15 scans taken showed an RMS higher than

Figure 5.3: RMS of the elevation tracking as a function of the azimuth position of the source. The higherRMS values correspond to rising sources (on East).



Figure 5.4: Tracking plot for scan 4301 taken on 2011-03-23 after reloading the parameter set in theelevation servo amplifier. Some ”waves” can be seen in azimuth which make the RMS being out ofspecifications.

Figure 5.5: A temporal zoom on the ∆Az signal shows oscillations which result in a higher RMS. Takenfrom scan 4955.




4913 0 600 Setting −14 80 0.030 −0.018 1.717 0.2704914 0 600 Transiting N 4 80 0.054 −0.018 0.417 0.1904953 0 600 Setting 181 45 −0.001 0.001 0.386 0.1594955 0 600 Rising 7 45 0.003 −0.007 0.745 0.1734956 0 1200 Rising 3 45 0.004 −0.004 0.412 0.1504960 0 600 Setting −90 20 0.002 0.001 0.234 0.3714962 0 600 Rising −183 40 −0.001 −0.002 0.222 0.1774964 0 600 Transiting N 5 84 0.098 −0.024 0.997 0.1794966 0 600 Setting −85 18 0.002 0.000 0.219 0.3764969 0 600 Rising 90 10 0.002 0.002 0.217 0.3595046 0 600 Transiting N 9 75 0.012 −0.011 0.717 0.2035048 0 600 Transiting N 5 75 0.013 −0.011 0.319 0.1605050 0 600 Rising 5 45 0.004 −0.007 0.269 0.1845054 0 600 Rising 5 15 0.003 −0.005 0.291 0.1505056 0 600 Transiting N 5 80 0.057 −0.016 0.306 0.1835683 0 600 Rising 90 24 0.002 −0.004 0.216 0.332

5686 1 600 Rising 90 24 0.002 −0.003 0.224 1.3775689 1 600 Rising 90 60 0.003 −0.012 0.223 1.5325692 1 600 Transiting N 9 84 0.102 −0.021 1.256 1.1965695 1 600 Transiting N 3.2 45 0.004 −0.004 0.253 0.8205698 1 600 Rising 177.8 34.8 −0.001 −0.006 0.132 0.8755701 1 600 Transiting S 177 79.9 −0.009 −0.009 0.818 0.9675704 1 600 Setting −90.2 71 −0.002 -0.037 0.195 1.9715707 1 600 Setting −90.2 71 0.002 0.000 0.208 1.858

5710 2 600 Rising 90 23.5 0.002 −0.007 0.231 1.7065713 2 600 Rising 90 60.5 0.003 −0.012 0.244 1.9005716 2 600 Rising −2 84 0.076 −0.033 0.935 1.6675719 2 600 Setting −0.5 45 0.005 −0.003 0.298 0.9785722 2 600 Setting 187 83.9 −0.019 −0.031 1.221 1.5775725 2 600 Setting 180 35 −0.001 0.001 0.138 0.4115728 2 600 Setting −90 70 −0.002 −0.015 0.192 2.2505731 2 600 Setting −90 30 0.001 0.002 0.205 1.981

Table 4: Summary of the tracking results after the MSU.



Figure 5.6: FFT of the quantities ∆Az and ∆El. There are strong peaks at 2.5 Hz which produce theripples in the azimuth tracking that can be seen e.g. in Figure 5.5.

the 0.6”, being the average 0.226”, a similar value as found before the upgrade (after the removal of theoscillations in elevation).

The auxiliary modes (using only 1 drive) are clearly beyond the specifications for normal operations. Thisis fine, since always mode 0 should be used. However, switching to only one of the drives for whatever rea-son (e.g. in case the other one is damaged) results in a substantial degradation of the tracking performance.

In azimuth, there were a few scans that showed RMS values higher than the specifications. The reasonwas that for these scans the tracking loop was entering in some kind of resonance that created theseoscillations (see Figure 5.5 as an example) with a frequency of 2.5 Hz (period of 400 ms). When theseoscillations occurred, the RMS in azimuth increased, as happened in scans 4913, 4955, 4964, 5046. Figure5.6 shows the frequency decomposition (FFT) of scans 4913 and 4955, showing a strong peak at 2.5 Hz.A peak at the same frequency (not so intense) is seen in elevation, but this is probably the oscillation inazimuth as seen by the elevation encoder.

These oscillations seemed to affect only part of the scan (as in the case of scans 4913 and 5046), butsometimes also the whole scan of 600 seconds (e.g., scans 4955, 4964). This behavior happened in 4 outof 16 (25%) of the scans taken in this period (16-24 Feb). When using auxiliary modes (in elevation)the behavior of the azimuth tracking was still the same (appear in 25% of all the scans taken in bothel aux mode(1) (2 out of 8) and el aux mode(2) (2 out of 8)).

Why the tracking loop was entering this resonance was still to be understood. After asking VERTEXabout this issue, they reported that this could happen when the telescope is moving at a certain azimuthspeed. They stated that:

“The reason for this behavior is that the tracking velocity at this very time [when oscillations start]crossed the threshold of 0.02 deg/s (higher velocity before the transition, i.e. when the tracking error wassmaller). Below this threshold the Az axis is operated at a higher gain in order to improve the trackingaccuracy. While this feature helps at low velocities - as the measurements made during SAT have shown- it obviously does no more help at az velocities of 0.02 deg/s - quite the opposite, the tracking becomesworse as your plots show .”



5.1.4 Tests after first adjustment of ACU tracking parameters (01-09 Mar 2011)

VERTEX recommended to change the Kpp p Factor Velocity Slow Azimuth parameter from 0.02 to0.01 deg/s in order to get rid of the observed oscillations in azimuth.These adjustments were done onMarch 1st, and subsequent tracking tests were done. Results are shown in Table 5.


6197 0 300 Setting 180 25 −0.003 0.001 0.034 0.1206198 0 300 Setting 182 30 −0.005 −0.001 0.073 0.1506199 0 300 Rising 177 45 −0.003 −0.009 0.403 0.2016200 0 300 Setting 180 55 −0.002 −0.002 0.520 0.2016201 0 300 Transiting S 180 65 −0.006 −0.004 1.062 0.1856202 0 300 Transiting S 180 75 −0.017 −0.007 0.388 0.1776269 0 1200 Rising 21 85 0.000 −0.004 0.307 0.3916282 0 600 Transiting N 5 85 0.106 −0.033 0.822 0.2836285 0 600 Transiting N 2 56 0.006 −0.005 0.662 0.3166287 0 1200 Transiting N 1 32 0.004 −0.003 0.484 0.2866285 0 600 Transiting N 2 56 0.006 −0.005 0.662 0.3166289 0 600 Transiting S −180 57 −0.001 0.001 0.716 0.2786307 0 600 Setting −90 41 0.001 0.001 0.247 0.3546308 0 600 Setting −95 70 −0.002 −0.004 0.096 0.3386309 0 600 Setting −69 78 0.000 −0.012 0.648 0.3006310 0 600 Rising −220 42 0.001 −0.005 0.124 0.302

Table 5: Summary of the tracking results after the first fine-tuning of the servo parameters.

After these test we continued having noisy trackings in azimuth. However, we identified that the problemwas only in those scans for which the tracking speed was around 0.01 deg/s. That was basically the sameproblem as we had before the adjustment of Kpp p but now at a different velocity. We reported theseissues again to VERTEX.

5.1.5 Tests after second adjustment of the parameters set (09-25 Mar 2011)

To the light of the previous results, VERTEX recommended a subsequent change in Kpp p Factor

Velocity Slow Azimuth from 0.01 to 0.004 deg/s. This change was implemented on March, 9th. Somemore tracking tests were taken to verify that this was finally the solution to make these oscillationsdisappear (see Table 6).

With these new settings, none of the tracking tests shows any RMS higher than 0.6” neither in azimuthnor elevation. Therefore we did not continue with any further modifications and these tracking parameterswere used for normal operations from that time on.


7502 0 600 Setting −78 48 0.001 −0.002 0.262 0.4277503 0 600 Transiting N 5 80 0.058 −0.017 0.389 0.2137504 0 600 Setting −78 48 0.004 −0.001 0.237 0.1797505 0 600 Setting −70 22 0.003 0.000 0.320 0.4039131 0 600 Transiting N 1 45 0.013 −0.001 0.455 0.1739132 0 600 Rising 20 88 0.002 −0.004 0.218 0.3459133 0 600 Rising 65 30 0.002 −0.007 0.275 0.3579134 0 600 Transiting N 0 71 0.029 −0.007 0.332 0.165

Table 6: Summary of the tracking results after the second fine-tuning of the servo parameters.



5.1.6 Tests 1.5 years after MSU (21 Sep 2012)

To have a more recent status of the tracking performance, similar tests were scheduled during technicaltime in Sep 2012. It is important to consider that some months prior to these tests, other modificationswere done in the antenna drive system. Firstly, a full set of amplifiers were replaced by new ones.Secondly, a detailed inspection was done of the cables connecting these amplifiers to the drives themselves.Actually, this revision pointed out a few problems affecting the tracking performance:

– Very noisy feedback resolver signal measured in the control cables.

– Drive power cables were in bad shape.

These problems were mostly due to loose cable connections and sparkling contacts. As a consequence,some work was done in order to replace the resolver cables and connectors, and if necessary the powerconnectors. Additionally, control cables affecting B-cabin were replaced by new ones.

Results of the tracking tests after these modifications are summarized in Table 7.

As we can see, antenna tracking performs within specs in elevation, being RMS well below 0.6” in100 per cent of the scans. In azimuth, some of the smooth variations introduced by the control loopimplementation are present in some scans. Therefore, there is a slight increase of the RMS above 0.6” inazimuth, but this affects to < 10 per cent of the scans.

5.1.7 Conclusions from dedicated tests

The changes made by VERTEX in the drive control system (see section 4.5) in combination with variousexchanges of the servo amplifiers, the improvements in connectors and recabling have turned into asignificant improvement of the tracking performance: the strong oscillations in elevation that the antenna


77882 0 300 −93.3 15.1 0.002 −0.002 0.205 0.43077883 0 300 −96.9 34.9 0.001 −0.003 0.151 0.37077884 0 300 −100.4 54.4 −0.001 −0.002 0.081 0.40877885 0 300 −87.0 30.9 0.003 0.000 0.204 0.40177886 0 300 −118.8 32.6 0.001 −0.000 0.076 0.39977887 0 300 −74.3 61.7 0.003 −0.005 0.236 0.34477888 0 300 −7.5 55.9 0.018 −0.006 0.587 0.22477889 0 300 6.4 33.1 0.006 −0.004 0.416 0.25577890 0 300 79.1 52.4 0.004 −0.010 0.257 0.39777891 0 300 79.8 17.8 0.001 −0.004 0.233 0.37377892 0 300 155.9 62.6 −0.007 −0.011 0.425 0.30777893 0 300 140.0 36.1 −0.000 −0.007 0.075 0.32777894 0 300 50.6 32.8 0.001 −0.007 0.228 0.33877895 0 300 21.9 19.6 0.001 −0.004 0.214 0.31377896 0 300 −3.7 73.2 0.038 −0.010 0.248 0.23177897 0 300 −164.4 73.6 −0.030 −0.013 0.333 0.28577898 0 300 −192.3 65.1 −0.013 −0.012 0.722 0.23977899 0 300 −176.0 53.0 −0.003 −0.001 0.217 0.22477900 0 300 −154.2 11.9 0.000 −0.002 0.105 0.28977901 0 300 −188.1 18.1 0.002 −0.003 0.060 0.24877904 0 300 192.9 38.3 −0.003 −0.001 0.166 0.25977905 0 300 64.3 73.1 0.022 −0.020 0.749 0.36077906 0 300 29.7 66.4 0.026 −0.014 0.366 0.30677907 0 300 −30.8 39.6 0.011 −0.001 0.488 0.325

Table 7: Summary of the tracking results 1.5 years after the MSU, servo amplifier replacements, andrecabling.



was suffering relatively often during the previous years are now much less frequent. However, the ultimateorigin of these problems is presently not completely understood.

Smaller 2.5-Hz oscillations were identified in the azimuth tracking, which turned out to happen atcertain tracking speeds. After reporting this feature to VERTEX they fine-tuned the parameters of thecontrol loop but these oscillations are still affecting a fraction of the scans (less than 10 per cent). Theseoscillations occur when there is a gain change in the tracking loop at certain tracking speeds. It is possiblethat the current value of the relevant parameter is the best that can be achieved. One should considerthat modifications in these control-loop parameters might have side effects difficult to predict.

The final result is that tracking is within specifications in elevation, i.e., RMS of the deviations are nothigher than 0.6”. This is true when using the normal mode for antenna operation (i.e., el aux mode(0)

in elevation). Using other auxiliary modes which forces the use of a single pair of drives (either the onescloser to the A− or to the B−cabin) degrades substantially (by a factor 5-6, on average) the RMS of thetracking. In Azimuth, tracking is within specs in 90 percent of the cases due to the causes exposed above.

The average RMS for the elevation tracking is now 0.32”, approximately 0.18” less than the valuebefore the MSU (when these strong oscillations were present in some of the scans). This average has beencomputed considering tracking positions all over the sky and therefore at different tracking speeds. Forazimuth tracking, the average RMS is 0.28”, a bit smaller than the value before the MSU (it was 0.35”).

5.1.8 Tracking monitor

Since May 2012 a continuous monitoring of the tracking performance is being done. The mainpurpose is to judge the tracking quality to do critical observations (e.g, at high frequency or when us-ing photometric mode for bolometers), but also to quickly identify misbehaviors in order to try to fix them.

Currently, every scan for which the telescope is tracking the sky for a considerable number of seconds isreduced on the spot by a script (Tracking Tool). Results on the RMS in both axes together with otherrelevant quantities (wind speed or wind direction, etc) are stored in a database. Plots similar to Figure 5.1are generated for all these scans, and are available for inspection. A web page shows the RMS of trackingin both axes from the last 24 hours, so observers can judge in advance when tracking is starting to degrade.

Figure 5.7 shows a plot of tracking RMS as a function of time since this monitor has been in place. Theupper panel shows that the azimuth tracking accuracy has not changed much within these months. Thereis clearly a tail in the distribution of scans for which the tracking is slightly out of specs, in agreementwith the previous results found in the dedicated tests. In elevation we see a complex history, whichreflects the various problems found and interventions done in the drive system in the last half year. It ispromising that after the last interventions (last 20 days in the plot) one can see a notorious improvement,with much less outliers above the specifications threshold.



Figure 5.7: Historical plot of tracking RMS in both axes. Different auxiliary modes are shown withdifferent colors. The shadowed regions indicate VERTEX specifications.



5.2 Pointing Performance

In this section, we summarize the different tests done in order to characterize the metrology system, andto quantify the accuracy of the pointing to date.

5.2.1 Linear Sensors: Night versus day comparison

The linear sensors compensation is much more prominent during daytime due to the increased thermalvariations caused by the Sun. As can be seen in Figure 5.8, the X and Y corrections while tracking exactlythe same region of the sky during night and day (blue and red respectively) can differ as much as 3”.Moreover, the Y axis exhibits larger excursions around the mean correction during the day.

Figure 5.8: Linear Sensor Compensation while tracking on the same portion of the sky during night (blue)and day (red) time.

In order to assess the effect of such compensations, LABOCA pointing models were created using dedicatedobservations conducted during the night and its subsequent day under excellent weather conditions. Thissmall time separation between both runs ensures a similar tilt of the antenna. Observations took placeon Aug 13th, 2012 and more than 30 sources were observed in each run, providing a very good sky coverage.

The derived models are summarized in Table 8 and their predicted residuals displayed in Figure 5.10.Both models are about the same (except perhaps for the HESA parameter) in terms of their pointingparameters and rms of their residuals. Interestingly, the value of IE in both models is the same (withinthe errors) regardless of the significant difference between the red and blue traces in the bottom panelof Figure 5.8 suggesting that the Linear Sensors are indeed correcting real deformations of the mountrendering them invisible to the control system. It is also worth noting that the fitted values for AN, AW arequite similar in both models, confirming the fact that the antenna tilt did not change, and therefore ourcomparison is basically probing the effect of the linear sensors alone.

A similar experiment was conducted using SHFI band 2 on 2012 October 4th. Two pointing runs withabout 30 sources each were conducted, using line pointing in CO(3-2), during the night and during the



Table 8: LABOCA Night and Day pointing models.The terms with the “=” prefix are not fitted andremain fixed at the values in the master model.

Night Day

IA +1662.8405 1.70510 +1664.6804 1.66386

IE +600.5496 1.16087 +600.3355 1.20593

=NPAE -20.0000 -20.0000

CA +50.8117 1.16899 +49.1618 1.19509

AN +51.8205 0.47647 +51.8247 0.39311

AW -33.3283 0.71767 -33.5473 0.66334

HECE -77.9567 1.68516 -76.8421 1.69673

HASA -3.9994 0.81907 -3.6212

=HESE -7.2937 -7.2937

HESA -0.7728 0.86303 -0.4782 0.86262

=HASA2 -3.8546 -3.8546

=HACA2 -1.5593 -1.5593

=HESA2 -1.3085 -1.3085

=HECA2 -2.6841 -2.6841

=HECA3 +0.6093 +0.6093

=HACA3 +1.9979 +1.9979

=HSCA2 -1.4503 -1.4503

Table 9: Night and Day RMS Horizontal (dS) and Zenith Distance (dZ) residuals.

Night Day RatiodS dZ dS dZ dS dZ

LABOCA 2012-08-12 1.40 1.74 1.84 1.53 1.31 0.88HET345 2012-10-04 1.21 1.53 1.85 1.76 1.53 1.15

next morning. The results of both experiments are reported in Table 9. In both cases (LABOCA andHET345), there is a slight increase of the RMS in Az during the day compared to the night results, andhardly any change (within the uncertainties) in elevation. This is a clear indication that the linearsensors compensation is working properly.

Table 10: On sky RMS with LABOCA before and after implementing a new model

dS dZOn-sky Before 1.51 1.93

TPoint fit result 1.02 1.26On-sky After 2.02 1.12

Ratio After / TPoint 1.98 0.89

Additional tests were conducted with LABOCA on 2012 October 4th, but this time the idea was tocompare the pointing residuals in dedicated pointing runs before and after computing and uploading anew model. This new model was derived from the data collected in the first run. Both pointing runstook place during the night; each run consisted in observing more than 30 sources, with good S/N. Themeasured RMS on-sky are shown in Table 10. The performance on sky is still significantly worsethan the theoretical value provided by TPoint, especially for azimuth. This inconsistency be-tween model and reality may be (partly) explained by the tiltmeter biases being 1–2′′ arcsec away from zero.

5.2.2 Linear sensors signature removal

When rotating in azimuth, the linear sensors produce a signature tracing the triangular structure of thesupport cone because the CFRO support structure which the sensors are attached to is not 100% stiff.Such compensation is spurious in nature and must be removed. This is done by substracting the simpleharmonic model given by the following formula (see VERTEX TN-1010057-31470 Technical Note on ErrorCompensations):



A2 sin(2Az − φ2) +A3 sin(3Az − φ3) (5.1)

Analyzing pointing data (see Figure 5.9), it was found that the parameters loaded in the ACU wereuncalibrated default values. A new set of parameters (see Table 11) was calculated following the standardALMA procedure recommended by VERTEX. These newly derived values were loaded into the ACUin May 2012 and since then, they are the values in place. Although the irreducible residuals decreasedsignificantly, there is still a small hint of it as can be seen in the residuals of subsequent optical pointingruns. Perhaps is normal?

Table 11: Linear Sensors signature model parametersA2 φ2 A3 φ3 C

[asec] [deg] [asec] [deg] [asec]Default Values -0.7500 45.0000 -0.3000 8.0000

Fit to Science Data -0.7205 135.5905 0.2964 -3.9763 0.0213Fit to Swing (current) -0.7239 135.8769 -0.3894 -6.4114 0.0177

Figure 5.9: Horizontal Residuals versus (wrapped) azimuth. Clockwise from top-left: LABOCA pointingmodel on-Sky Horizontal offsets, Optical Run on-sky X-offsets multiplied by cosine elevation, Optical RunLinear Sensor horizontal compensations and VERTEX Linear Sensors horizontal correction formula usingthe parameters currently loaded in the ACU.

5.2.3 Tiltmeters: Bias Thermal Compensation

The Tiltmeter bias is modeled to change linearly with temperature (see details in Section 3.1.1) . TheACU allows the implementation of an open loop piecewise linear compensation whose parameters needto be calibrated using field data. We have been systematically populating the temperature domain andrecalibrating the compensation constants.

The current calibration was implemented on June 2012 and covers the range of temperatures between−3 and 8 deg Celsius, which accounts for basically 80% of the expected temperature range along the



Figure 5.10: LABOCA predicted night and day (left and right respectively) pointing residuals.

year. In Figure 5.11 one can see the variations of the bias with time in the last months. For the coldertemperatures, the tiltmeter Y-axis began to misbehave (see Figure 5.12). Nevertheless, the mean bias isrelatively small compared to the scatter and we are already chasing second order effects.

Figure 5.11: TM bias offsets and Temperature versus time. See Figure 5.12 for a different perspective ofthe same dataset.

5.2.4 Deviation of gravity

The on-sky pointing residuals in elevation contained a clear sine-wave of constant amplitude not seen inthe TPoint fits. Deep analysis revealed that the cause of this residuals was that the Deviation of Gravityconstants hard-coded within the control system were wrong1. The elevation offset caused by the telescopetilt is given by:

∆E = −AN cos(A) +AW sin(A) (5.2)

1The origin of the values used so far is obscure and could not be verified.



Figure 5.12: TM bias versus temperature. See Figure 5.11 for a different perspective of the same dataset.

Figure 5.13: Top: Blue: on-Sky elevation residuals. Red: Best harmonic fit (AN,AW ) =(1.14,−3.87) asec. Bottom: On Sky residuals of optical run observed after including the derived DoGcorrection in TILT.PTG.

This formula was fit to optical pointing data shown in Figure 5.13 obtaining (AN,AW ) = (1.14,−3.87)arcsec. These values were written in the TILT.PTG file and subsequently an optical run was ob-served to check the effect. The new on-sky residuals don’t have such a prominent sine-wave anymore



and the on-sky rms dropped from 2.7 to 1.7 asec. However, the residuals still contain some ir-reducible structure which could be attributed to miscalibration of the Linear Sensors (see previous section).

5.2.5 Tiltmeter measurements: Clockwise vs counterclockwise

ClockWise (CW) and Counter ClockWise (CCW) Tiltmeter measurements result in different values asdifferent as 1 asec for both the bias and absolute telescope tilt (see Figures 5.14 and 5.15). Also interestingto notice is the variation of X-bias over the night (Figure 5.15, top), although the temperature did notchange significantly, according to the measurements.Such effect can be explained by a lose azimuth axis reacting differently to CW and CCW twisting forces.For the Y axis, the bias difference between CW and CCW measurements seems to converge to 1 asec asthe settling time increases (Figure 5.15, bottom). This unmodeled hysteresis-like effect could contributeto the higher measured on-sky RMS as compared to the result given by TPoint. However, dedicatedobservations performed on 2012 October 4th (pointing on the same source coming from the left or fromthe right) could not show any measurable signature of hysteresis.



Figure 5.14: TM Hysteresis Measurements. Top: Telescope azimuth describing swinging path and settlingtime used for the measurements. Center: TM Readout. Bottom: Blow-up of the region indicated bythe green rectangle in the center panel.



Figure 5.15: Top: AN, AW, X-bias and Y-bias derived from Tiltmeter measurements, where the swing isdone CW (green dots) or CCW (red dots). The X axis is a running number, which increases with time.Bottom: Difference in AN, AW, X-bias and Y-bias between CW and CCW measurements, as a functionof settling time.



5.2.6 Pointing monitoring

Since May 2012, we are routinely using a tool (developed by Michael Dumke) to monitor quantitativelythe accuracy of our pointing models. This tool computes the average values and standard deviations of theabsolute pointing corrections (i.e. CA and IE) over a given period of time. This is done per instrument.The evolution of these standard deviations is shown for all facility receivers in Figure 5.16. In summary,the pointing accuracy has improved significantly in the last months, in particular for LABOCAand SHFI, with RMS below or of order 2′′ in each axis since August this year. The improvement is morevisible for the RMS in elevation, which is probably related with the less frequent bad-tracking events (seeSect. 5.1.6); interestingly, there seem to be a correlation between Figures 5.16 and 5.7, which shows theperiods of time when the tracking in El was bad.

Figure 5.16: Standard deviations of absolute pointing corrections in azimuth (top) and elevation (bottom)for the facility receivers, plotted against time.

5.3 Conclusions

Although there are still open issues which are currently being investigated, the pointing performance hasimproved significantly relative to the previous year. The on-sky optical pointing residuals are much closerto the predicted values because not modelled systematic effects introduced by the wrong operation of themetrology system have been finally removed. In particular, the unexplainable absolute offset (IA, IE)relative to the models and the unreductible structure of the Zenith Distance residuals versus Azimuthhave been minimized. Before the upgrade, typical on sky residuals were almost a factor of two larger thanwe have now.The VERTEX replacement and fine tuning of critical components of the drive and metrology systemshas been crucial for this achievement. A new level of understanding of the various control loops has beenreached, allowing the refinement of our procedures. Most of the first order effects affecting the pointingperformance are currently under control. We are now trying to nail down smaller second order effects



which are detectable and measurable by the first time.

The topics still under investigation are:

– Linear Sensor Calibration. The parameters for the signature subtraction have to be fine tuned.

– Deviation of Gravity Calibration. Dedicated observations to reach the bottom of this issue needto be conducted.

– Tiltmeter bias thermal compensation calibration. Experiments using the piecewise capabilityof the ACU need to be conducted. Artificially changing the TM temperature is an option that cannot be discarded.

– Effect of focus drifts in pointing at high elevations. This effect has is presently not fullyunderstood. Laser ranger measurements have been conducted on the subreflector but no final con-clusion has yet been achieved. The formulae used to produce the radio pointing data may eventuallyneed revision.

– Spurious azimuth jumps associated to wrapping switch. This is homework for VERTEX

– Effect of bad tracking (oscillations) on metrology and pointing. We have enough data toanalyze and design observations to investigate if bad tracking has any impact on the correctionsprovided by the metrology system.



6 Summary and conclusion

The main results obtained after 1.5 years of debugging, investigating and monitoring are summarizedbelow.

– Tracking accuracy is mostly within specs, specially in elevation. In azimuth it degrades about 0.1-0.2arcsec out of specs for about 10 percent of the scans, which is probably fine. In general, our testssee a slight improvement in the tracking RMS in both axes with respect to before the MSU (undernormal working conditions, i.e., excluding the presence of anomalous oscillations). This improvementcould be due to the modifications of the tracking loop implemented in the ACU by VERTEX.

– The tracking oscillations that used to affect operations relatively often are now much less frequent.The reasons for this improvement are the replacement of the servo amplifiers by newer units, theimplementation of new parameter sets in those, and improvements in the drives connectors andcabling.

– Linear sensors are working properly since they introduce corrections as large as 3 arcsec that ourpointing models cannot detect. Day to night comparisons for the elevation offsets confirm this,showing similar RMS on sky. A recalibration has been done which removes the spurious signaturewhose imprint we could detect in our pointing data.

– A strong effort has been done to calibrate the TM with temperature. In the last year we havepopulated a good percentage of the temperature domain, but we will continue to improve this inthe future. There are still some relatively small excursions of the TM bias (mostly below 2 arcsec)which have to be understood and kept under control.

This last point is still being investigated, as well as a few other issues (calibration of the linear sensors,proper measurement of the deviation of gravity). These may affect the pointing and partly explain whythe measured on-sky RMS is systematically higher than the prevision given by TPoint.


antenna recommissioning after metrology system upgradeparrar/tran/msu/comm2011.pdf · { 21-10044a...

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