5.4.1. Telescope Pointing Software Libraries#

The TCS pointing and tracking control will be built around a core set of software libraries: TCSpk [Wall12a], SLALIB [Wall12b], and tpk [Terr06], which are used on many telescopes currently in operation, e.g., Magellan, Keck, GTC, VLT, Gemini, and LBT. The software libraries provide the following functionalities:

  • SLALIB [Wall12b] – A library of over 200 functions in ANSI C for positional astronomy, SLALIB comprises functions and numerical methods that calculate spherical trigonometry, vectors and matrices, and coordinate and time transformations required to achieve milliarcsecond pointing accuracy to celestial targets. Key examples of positional astronomy computations and transformations performed by SLALIB include:

    • Celestial – catalog coordinates such as FK4, FK5, galactic, geocentric coordinates, ICRS, proper motion, target proper motion, parallax, and radial velocity.

    • Terrestrial – atmospheric refraction, diurnal aberrations, precession and nutation, mean place to apparent and observed place, focal plane astrometry.

    • Time – time systems (UTC, GST, LAST, etc.), calendar, epochs, ephemerides

    SLALIB supports both IAU 2000/2006 precession-nutation and Earth rotation models in both classical (equinox-based) and Conventional International Origin forms. Details of the celestial coordinate systems used by the TCS and the transformation flow going from user specified celestial coordinates to observed directions are discussed below.

  • TCSpk [Wall12a] – A library of roughly 70 functions in ANSI C makes up the TCS Pointing Kernel (TCSpk), around which the TCS is designed. The TCSpk library provides modular functions that can be assembled in flexible ways to implement telescope pointing patterns and strategies, and that place few constraints on the design and potential capabilities of the TCS. TCSpk interfaces with SLALIB for positional astronomy calculations so that the complexities of transforming coordinates are transparent to the designer. TCSpk also interfaces with TPOINT for calculating pointing corrections.

  • tpk [Terr06] – The Telescope Pointing Kernel (tpk) is a C++ library of classes that is layered on top of TCSpk and SLALIB to create virtual telescopes (see below) for the mount, rotators, guiders, instruments, etc., and to produce encoder angle demands that point the telescope, and that position rotator and guiders based on inputs from the virtual telescopes.

  • TPOINT [Wall94] – Software for analyzing telescope pointing, TPOINT helps to assess and measure misalignments and flexures in the telescope and instruments. Data produced by TCSpk / tpk interface directly with TPOINT for analysis, while collimation and flexure errors measured by TPOINT can directly feed back into TCSpk / tpk to reduce pointing errors.