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Pointing and Tracking ATST Control System: Pointing and Tracking ATST/PTW/001 P.T.Wallace Ref: ATST/PTW/001 CCLRC/RAL ATST Pointing and Tracking Issue: 1 Date: July 6, 2006 Page: 2 CHANGE RECORD Issue Date Reason for change 1 July 6, 2006 New document Ref: ATST/PTW/001 CCLRC/RAL ATST Pointing and Tracking Issue: 1 Date: July 6, 2006 Page: 3 Contents 1 INTRODUCTION ...............................................................................................................4 2 THE INTERFACE TO THE TELESCOPE CONTROL SYSTEM ...............................5 2.1 POINTING AT SOLAR TARGETS .......................................................................................5 2.2 TCSPK ARCHITECTURE..................................................................................................5 2.3 TPOINT........................................................................................................................6 2.4 SLALIB........................................................................................................................6 2.5 POINTING-KERNEL FEATURES........................................................................................7 2.6 ARCHITECTURAL CONSIDERATIONS...............................................................................8 2.7 DIFFERENTIAL ROTATION..............................................................................................9 2.8 ZENITH BLIND SPOT.....................................................................................................10 3 COORDINATE SYSTEMS ..............................................................................................15 3.1 HELIOGRAPHIC (HG)...................................................................................................16 3.2 STONYHURST ..............................................................................................................16 3.3 HELIOPROJECTIVE (HP) ..............................................................................................16 3.4 HELIOCENTRIC (HC) ...................................................................................................17 3.5 TOPOCENTRIC APPARENT PLACE (AP).........................................................................17 3.6 GEOCENTRIC APPARENT PLACE ...................................................................................18 4 SOLAR EPHEMERIDES .................................................................................................19 4.1 HELIOCENTRIC EARTH EPHEMERIS ..............................................................................19 4.2 SOLAR PHYSICAL EPHEMERIS ......................................................................................19 5 THE SOLLIB LIBRARY OF C FUNCTIONS...............................................................21 5.1 THE SOLLIB LIBRARY...............................................................................................21 5.2 STRATEGIES FOR USING SOLLIB IN A TCS APPLICATION ...........................................22 5.3 TRANSFORMATION PATHS ...........................................................................................24 5.4 THE DATA CONTEXT ....................................................................................................25 5.5 COMPONENT ALGORITHMS ..........................................................................................26 5.6 PROGRAMMER’S REFERENCE.......................................................................................33 6 BIBLIOGRAPHY..............................................................................................................53 Ref: ATST/PTW/001 CCLRC/RAL ATST Pointing and Tracking Issue: 1 Date: July 6, 2006 Page: 4 1 Introduction The pointing problem for ATST has two aspects: 1. Given the coordinates of a solar surface or coronal feature (the “target”) we want to control the telescope mount and movable optics so that the image of the feature appears in the right place in the focal plane. 2. Given the coordinates to which the mount and movable optics have been set, we want to know the solar coordinates imaged at a given place in the focal plane. These two capabilities are complementary, linked by the same transformation chain and differing only in the direction in which the chain is traversed. The first one is about pointing and tracking; the second addresses displayed coordinates and World Coordinate Systems (WCS). The extreme ends of the chain, so far as this report is concerned, are (i) the heliographic longitude and latitude (L,B)1 and (ii) the demanded coordinates sent to the telescope control system (TCS). There are a number of intermediate coordinate systems, some of which have uses in their own right. Section 2 of the report deals with the TCS pointing kernel, a relatively self- contained component from the internal complexities of which the solar observer can be insulated. Section 3 reviews the choice of supported coordinate systems, the dominating influence on the observer’s perception of the ATST pointing controls. Section 4 discusses methods for generating the required solar ephemerides, both orbital and physical. Section 5 of the report describes a library of C functions, SOLLIB, that performs all the required transformations. 1 The symbols L and B mean the heliographic longitude and latitude respectively. It should be noted that not only are other symbols in use (λ and Φ for longitude, Ψ and Θ for latitude, in various pairings) but solar observers generally use the opposite order: latitude then longitude. Unfortunately, the latter practice is at odds with the SLALIB/TCSpk convention, which is the normal mathematical one: longitude first. Here, for software consistency, we adopt the longitude then latitude order; the ATST user interface may be different. Beware! Ref: ATST/PTW/001 CCLRC/RAL ATST Pointing and Tracking Issue: 1 Date: July 6, 2006 Page: 5 2 The Interface to the Telescope Control System Because of the need to calibrate the pointing of ATST using stars, and to reduce development costs, it is proposed to use a proprietary telescope pointing kernel (TCSpk). This software is principally intended to track celestial targets (stars etc.), and so solar observing will be treated as an extension, as when tracking any solar-system target. 2.1 Pointing at solar targets The interface to the pointing kernel will be topocentric apparent place, continuously recalculated in order to generate the requisite non-sidereal tracking rates. The starting point for this calculation will usually be heliographic coordinates (L,B), most often for photospheric targets, but sometimes (x,y,z) heliocentric coordinates, suitable for coronal features, and on occasion helioprojective coordinates (x,y), referring to places on the solar disk. The pointing calculation will be quite rigorous, taking properly into account planetary aberration (i.e. light time) and diurnal parallax. Light deflection (i.e. the Sun’s gravitational lens effect) can be neglected, as for solar features it is always below 4 mas, though it could be up to 1.75″ for a coronal feature distant from the Sun and seen against the limb.2 Working in apparent (α,δ) has the disadvantage that the coordinates of the main object of interest, namely the Sun, are continuously changing. The motion in latitude could be reduced were the kernel to support ecliptic coordinates, and the longitude drift could be reduced by working in terms of the mean Sun. However, they would never quite go away – diurnal, monthly, synodic and annual terms would always be present – making such strategies less advantageous. Moreover, the rates in (α,δ) change so smoothly that keeping the solar coordinates and their rates of change up-to- date to the required accuracy is straightforward. 2.2 TCSpk architecture TCSpk is a suite of about 60 ANSI C functions, around which a telescope control system can be developed. It has been used on a number of projects including SOAR and LBT. The TCSpk functions implement the astrometric pointing kernel part of a TCS in a rigorous, general and modular way, 2 As with other ephemeris minutiae, consistency with other solar observers will often be a more important consideration than rigor. Ref: ATST/PTW/001 CCLRC/RAL ATST Pointing and Tracking Issue: 1 Date: July 6, 2006 Page: 6 insulating the TCS designer from many intricacies. The algorithms used by TCSpk are described in Wallace (2002). Because TCSpk does a specialized job and is merely a part of a complete TCS, it is designed to place as few constraints on the TCS developer as possible. There is no preferred operating system or user-interface style for example, and even the real-time requirements can be met in a variety of ways. This flexibility will make TCSpk easy to integrate within an ATST control system design. For example, TCSpk lends itself to use in a C++ design, providing the mathematical algorithms without dictating the way they are grouped and interconnected; experience with the SOAR and LBT control systems has provided many insights into the best ways of dealing with the object-oriented design issues. TCSpk uses the SLALIB library for all its positional-astronomy transformations and a subset of the TPOINT pointing-analysis software (see below) for calculating pointing corrections. Both are de facto standards, reducing maintenance concerns. 2.3 TPOINT Pointing analysis will be via the proprietary TPOINT system. This is a straightforward command-driven system that can be used interactively or run automatically using scripts. The rigorous TCSpk transformations
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