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Laser Pointing Camera

The LPC is a compact system, including a telescope and a smart CCD camera, that is used to assist a single or multiple Laser Guide Stars (LGS) Adaptive Optics (AO) system to precisely (1") pointing the guide stars to the expected sky position, with a precision that is very difficult or impossible to obtain in open loop. This is achieved by taking images of the pointed area with the LPC and using the on board astrometric software to find the real position of the LGS in the sky, thus allowing to communicate to the laser launch telescopes the ALT-AZ offsets to be applied to put the LGS in the correct position.

 

The LPC allows to have a fast pointing of single or multiple LGS on the AO wave-front sensor (WFS) during the initial acquisition phase of the telescope preset, thus reducing considerably the overheads currently experienced in most LGS-AO systems in operation.

 

By recognizing via astrometric software the field stars as well as the multiple LGS, LPC is insensitive to flexures of the laser launch telescope or of the receiver telescope opto-mechanics.

 

Moreover, LPC gives regularly the photometry and fwhm of the LGS, as well as the scattering of the uplink beams at the height of 10-15km, thus monitoring the presence and evolution of cirrus clouds.

 

Finally, besides eliminating the overhead of the LGS acquisition in the AO WFS during operation, the LPC is a very powerful tool during the calibration of the lasers pointing and WFS wobbling during LGS AO systems commissioning.

LPC main features

  • The LPC is based on a ‘smart’ AST8300 CCD camera with Johnson filters, with on-board computing processor, based on Linux OS, and special customized software

  • The CCD is mounted behind a compact, 15cm aperture Maksutov telescope equipped with an image rotator. It is possible to adopt a different telescope if needed

  • The LPC has a field of view of  ~35x25 arcmin and is able to identify and derive the absolute coordinates of the LGS and the field natural stars, with 100% sky coverage

  • LPC derives the LGS relative position with respect to the main telescope optical axis, with a 3σ error ≤1 arcsec in ALT and AZ and provides the  ΔALT and ΔAZ corrections within ~45 seconds from the user request

  • The LPC can measure the LGS return flux (phot/s/m2), the LGS fwhm in arcsec and the uplink scattering intensity parameter at a certain height above the observatory, in phot/s/sec/arcsec2

  • A 70x45x45cm housing protects the telescope from wind shake and dust and features a motorized cover. The total weight is 35kg. LPC does not have fans or water cooling to remove the heat generated; it uses thermal conduction with the telescope structure, instead.

  • The LPC is a self-contained module, requiring only power supply and a single or dual fiber/LAN Ethernet connection

  • Easy control of operations using ascii strings through TCP/IP Ethernet channel

  • Power 24V @ 1.3A max; @1A typ at 0°C (TEC 70%, deltaT -25°C) regulated temperature

  • Service RS232 UART port, discrete Inputs (Supply_On_Off control; Spare_In), discrete Outputs (Watch_Dog monitor; Laser_Off command; Spare_Out)

  • Complete support service: customization, commissioning and maintenance during operation

Please contact us for documentation and quotations
 

LPC example application: ESO Paranal UT4 telescope 4 LGS unit

LPC has been developed by Astrel Instruments and INAF-OAR (Italian National Institute of Astrophysics - Rome Observatory) for the 4LGSF of the ESO Adaptive Optics Facility, and installed on the UT4 telescope at ESO Paranal Observatory. The LPC commissioning followed the standard ESO path for the installation and verification of new instrumentation and has been timed by the commissioning phases of the main project, the 4LGSF. LPC passed provisional acceptance tests in Garching ESO Headquarter (November 2014) and it was then delivered to Paranal for local acceptance and installation on UT4 during April 2015.

LPC is currently a fully operational instruments on UT4.

LPC commissioning started after the control software integration with the 4LGSF workstation, with its alignment to UT4 optical axis and preliminary astrometry verification of plate scale, focus and sensitivity. During the 6 following nights it was thoroughly tested using the return signal from the first of the four LGS of the new 4LGSF already activated. After two commissioning runs all the design requirements of LPC are fully satisfied. The main results are highlighted in the following list:

  • LPC is functional on the 100% of the sky (full sky coverage), including  empty or crowded star field areas

  • The average time to return the pointing correction to the 4LGSF is about 60 seconds including the exposure time, with the exception of the 47 Tuc field where, due to the paramount number of stars, it takes about 2 minutes

  • End to end budget error for pointing correction is within 1” per axis

  • The average astrometric precision is < 1 arcsec at 3 sigma

  • The photometric precision when exposing for 4 seconds the LGS using a calibrated V-band filter is better than 0.1 mag.

  • The system is operative with Na return flux from 1Mphot/m2/s to 45Mphot/m2/s

  • The passive fan-less heat dump system of the camera Peltier cooler works for a full night without overloading

  • LPC is ready to be operative from a cold start in less than 5 minutes

Images Copyright by ESO

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