Prototyping GMT Telescope Metrology on LBT
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Prototyping the GMT Telescope Metrology System on LBT Presented by Andrew Rakich Optical Designer

Summary Description of Laser Truss Metrology System Etalon Absolute Multiline Technology Prototyping on LBT Results to date Future work

The GMT Telescope Metrology System  In 2016 GMTO adopted the idea of a “Telescope Metrology System” (TMS) into the baseline plan. ▪ Seen to address a gap between the expected accuracy of openloop modelling of optics positions given thermal and gravity deflections, and “good” starting points for the Acquisition, Guiding and Wavefront Sensing system (AGWS). ▪ The system allows the measurement and control of all mirror segments, aligning them to the Gregorian Rotator axis, with errors at the micron level, and with measurement cadences of several seconds. ▪ Of several 3-D metrology...

Etalon Absolute Metrology Technology (EAMT)  A commercial absolute distance measuring interferometer system was selected for the laser truss. ▪ Up to 800 independent metrology channels. ▪ Measurement uncertainty (in air) 0.5 µm/m ▪ Maximum measurement frequency > 500 kHz (can be used as vibrometer with much higher accuracy for high frequencies) ▪ Measurement length > 30 m ▪ Simple measurement channel consisting only of telecom fiber, collimator and triple reflector (no electrical systems on telescope) ▪ Almost unlimited fiber length possible (several kilometers) ▪ Eye safe infrared...

Prototyping on LBT  In 2017 GMTO proposed to prototype the metrology system on LBT. ▪ Gives LBTO the opportunity to trial new metrology technology without large capital expenditure. ▪ Opportunity for GMTO to gain first-hand and long-term experience with Laser Truss metrology on a working telescope. ▪ LBT 22.5 m optical baseline and 8.4 m diameter borosilicate mirrors give the closest match to GMT of any working large telescope today. ▪ Conveniently located, not so far from Pasadena, and with a common partner institution in Steward Observatory. ▪ GMTO technical staff already very familiar...

Prototyping Phases  The prototyping effort is divided into three phases: ▪ Phase 1: Deploy laser truss on the prime focus cameras, measuring each primary mirror with respect to the corresponding corrector. Test hardware, develop software. Integrate with the Telescope Control System (TCS). ▪ Phase 2: Deploy Laser Truss for the Gregorian Telescope. Measure M1, M2 and M3 relative to an instrument rotator. Integrate with TCS. ▪ Phase 3: Control both Gregorian Telescopes and measure between right and left telescopes for interferometric baseline control. ▪ At this stage Phase 1 is basically...

Andrew Rakich Prototyping the GMT Telescope Metrology System on LBT

Closed dome measurements: Swing Arm Repeatability 15 Andrew Rakich Prototyping the GMT Telescope Metrology System on LBT

Closed dome measurements: Thermal Drift Andrew Rakich Prototyping the GMT Telescope Metrology System on LBT

Andrew Rakich Closed dome measurements: Elevation 0.1 Prototyping the GMT Telescope Metrology System on LBT

Closed dome measurements: Signal strength over the elevation range Andrew Rakich Prototyping the GMT Telescope Metrology System on LBT

“Passive” Observing Andrew Rakich Prototyping the GMT Telescope Metrology System on LBT

Software The EAMT system has its own software that fully controls the unit. This software can be accessed by other programs on the observatory network via a TCP/IP interface. All of the control, measurement and analysis functions of the native EAMT software can be called by another program over TCP/IP. For our prototyping effort a Python script has been written that commands the EAMT to take a measurement, requests the resultant data on differential motion of the target retroreflectors, and does the necessary mathematics to produce a vector of M1 mirror position commands ݑ, Űݑ, ưݑ, ǰݑ Űݑ ,...

Software  The change in relative position of the mirror and corrector at a given TMS measurement can be decomposed into “good” and “bad” changes. The good changes are the various commanded mirror position changes that have been applied since the original TMS reference measurement was made.  These must to be taken into account before applying corrections to the mirror position. Essentially the “goalposts” move and this must be taken into account.  The PERL script interface to the TCS subtracts the following known offsets from the raw measurement and applies the difference to a mirror...

Active Control  Very limited use of the system for full active control has so far been made, as the prime focus correctors are only in use ~ 10% of the time and further engineering time, which is heavily subscribed, is required to deliver a system ready for routine observing.  The following result shows the TMS maintaining alignment during a transiting exo-planet observation that tracked a defocused star through Zenith for 40 minutes.  There was no noticeable change in the PSF during the observation, a result that improved on a similar observation without the TMS. Andrew Rakich...

Active Control  When the variables of telescope and software conspired to work during our limited time in active control of the telescope, the results have been promising.  After collimating the telescope at a high elevation, and locking optics positions with the TMS, the telescope can be slewed to low elevations with no noticeable misalignment of the optics.  With the latest modifications to the control software, further engineering time is required to qualify the system for routine use during science operations. Andrew Rakich Prototyping the GMT Telescope Metrology System on LBT

Lessons learned to date. ▪ Hardware robust and reliable. ▪ TCP/IP communication and control works well. ▪ Collimator alignment maintenance is not an issue. ▪ 1” collimators give superior results to ½” collimators. ▪ Good signal maintained over collimation model range of LBT. ▪ There are no measurable stray light issues for science instruments utilizing CCD detectors. ▪ Achievable accuracies appear to be excellent and as expected. ▪ Range of temperature from +20C to -10C handled without problems from operability point of view. ▪ Temperature measurement in situ is important. ▪ System...