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TPOINT a telescope pointing analysis system
Version 18.20, for Unix 10th December 2011

Published by:

Tpoint





Software

www.tpsoft.demon.co.uk

TPOINT is a trade mark.


Information in this document is sub ject to change without notice. c Copyright 2011 P. T. Wallace. All Rights Reserved. Reproduction, adaptation, or translation without prior written permission is prohibited, except as allowed under the copyright laws.


CONTENTS

iii

Contents
Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 INTRODUCTION 1.1 Installing TPOINT on your Unix 1.1.1 Standard distribution . . 1.1.2 Full distribution . . . . . 1.2 System overview . . . . . . . . 1.3 How TPOINT is used . . . . . . 1.4 The TPOINT philosophy . . . . 1.5 Running TPOINT . . . . . . . 1.6 An example TPOINT session . 1.7 Syntax . . . . . . . . . . . . . . 2 MODELING 2.1 The pointing terms . . . 2.2 Sizes of coefficients . . . 2.3 Saving models . . . . . . 2.4 RMS and PSD estimates v v 1 1 1 2 3 3 4 5 7 11 12 12 17 17 20

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3 GRAPHICS 21 3.1 Using the G. . . commands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 3.2 The E9 and A9 plots . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 4 POINTING DATA 4.1 Sky coverage and mapping order . . . 4.1.1 German equatorials . . . . . . 4.2 Making the observations . . . . . . . 4.2.1 Altazimuths and rotators . . . 4.3 Reading and editing the observations 4.4 Different sorts of coordinates . . . . . 4.5 INDAT data formats . . . . . . . . . 4.5.1 Caption record . . . . . . . . 4.5.2 Option records . . . . . . . . 4.5.3 Run parameters record . . . . 4.5.4 Observation records . . . . . . 4.5.5 Subset records . . . . . . . . . 4.6 Non-standard data formats . . . . . . 4.7 Star catalogs . . . . . . . . . . . . . 4.8 Coordinate ranges, and "beyond the p 4.9 Sample data . . . . . . . . . . . . . . 4.10 Simulated pointing tests . . . . . . . 30 30 30 33 33 35 36 37 38 38 38 39 41 42 42 43 44 46

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iv 5 ADVANCED TECHNIQUES 5.1 Building models . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2 Exposing unmodeled effects . . . . . . . . . . . . . . . . . . . . 5.3 Physical versus empirical . . . . . . . . . . . . . . . . . . . . . . 5.4 Command scripts . . . . . . . . . . . . . . . . . . . . . . . . . . 5.5 The UNFIT command . . . . . . . . . . . . . . . . . . . . . . . 5.6 Reducing multiple data sets . . . . . . . . . . . . . . . . . . . . 5.7 Combining data with UNFIT and OUTDAT . . . . . . . . . . . 5.8 Using data subsets . . . . . . . . . . . . . . . . . . . . . . . . . 5.8.1 Controlling subset membership ­ INDAT and SUBSET . 5.8.2 Model terms for data subsets ­ USE, LOSE, FIX, CHAIN 5.9 Order of terms . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.10 Rigorous fitting . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.11 The generalized gimbal mount . . . . . . . . . . . . . . . . . . . 5.12 Off-axis pointing origin . . . . . . . . . . . . . . . . . . . . . . . 5.13 Batch mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 HOW TO ALIGN YOUR POLAR AXIS

CONTENTS 48 48 49 50 50 51 52 52 53 54 54 55 56 57 57 59 60

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7 REFERENCE SECTION 63 7.1 Commands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 7.2 Pointing terms: names and sign conventions . . . . . . . . . . . . . . . . . . . 102 8 THE INITIALIZATION FILES 138 8.1 The TPOINT initialization file . . . . . . . . . . . . . . . . . . . . . . . . . . . 138 8.2 The TPG (TPOINT graphics) initialization file . . . . . . . . . . . . . . . . . 140 9 COMMAND SUMMARY 143


v

Preface
TPOINT is a software system for analysing telescope pointing. It is a descendant of systems that have been in use at ma jor observatories for many years, including optical, IR, mm and radio telescopes. Its origins go back to the author's work on the Anglo-Australian 3.9 m telescope in the 1970s. The version to be described in this manual runs on Unix platforms; other versions exist which run on Microsoft Windows PCs.

Acknowledgements
The author is pleased to acknowledge all those who have given advice and encouragement during TPOINT's long history, especially Peter Gillingham, Joe Wampler, Don Morton, Steve Lee, Robert Laing, Ken Elliott, Hilton Lewis, Bob Kibrick, Fang Yanling, Russ Owen, Steve, Dan, Matt and Tom Bisque, Krister Wirenstrand, Wayne Rosing, Jeff Mangum and David Terrett.


vi


1

1

INTRODUCTION

TPOINT is a software system for analysing telescope pointing. It is used by observatories around the world: AAT, UKST, UKIRT, LPO, WIYN, ARC, Keck, JCMT, WHT, INT, NTT, VLT, Gemini, SOAR, GBT, LBT and many other ma jor telescopes use TPOINT routinely to maintain their pointing models and to monitor performance. Using TPOINT you can assess the potential for accurate pointing in your telescope and measure misalignments and flexures. The information TPOINT generates can be used in telescope control systems (TCSs) to improve setting accuracy and to enable good coordinates to be logged at any time. In addition, misalignments can be addressed mechanically, for example through adjustments to the polar axis. Using the sophisticated pointing models that can be developed with TPOINT, the best large telescopes deliver pointing accuracy below the 2 arcsecond level and some approach 1 arcsecond, roughly the diameter of the larger satellites of Jupiter. The best amateur telescopes can easily beat 1 arcminute, placing targets such as faint galaxies reliably in the center of even a high-power eyepiece.

1.1
1.1.1

Installing TPOINT on your Unix system
Standard distribution

The standard TPOINT package includes a makefile and all the files necessary to build an executable system, for a specific Unix. The only pre-requisites are the Unix operating system itself and the Tcl/Tk package, which is used for graphics. The package is in the form of separate files, either on a single ISO 9660 format CD-ROM or downloaded as a tar file. Installation is straightforward, and the makefile can easily be adapted should the default installation procedure not be ideal. This is how to install TPOINT: 1. If CD-ROM, mount it and either cd to it or copy the contents to a directory on the hard disk and cd there. If a downloaded tar file, extract its contents into a directory on the hard disk and cd there. 2. The documentation is supplied on a .pdf file. Copy it to an appropriate place on your system. 3. Review the makefile for compatibility with local conventions. By default, files will be placed in the directories $HOME/bin and $HOME/etc/tpoint, creating these directories if they do not already exist. If these destinations are not suitable, either edit the makefile, or alternatively when executing the makefile use make INSTALL_DIR= to override the $HOME default. 4. Invoke make.


2

1 INTRODUCTION 5. Make sure the script tpoint and the executable tpt are in your path. (The makefile puts them in $HOME/bin by default.) 6. Should de-installation become necessary, type make deinstall.

To run TPOINT, cd to the directory you wish to work from and type tpoint. If the program fails to start, or if error messages appear during the start-up phase, or if the graphics commands fail to work, the things to check first are (a) paths, (b) the location and contents of the tpoint.ini file, (c) the location and contents of the tpg.ini file and (d) the existence, location and version of the Tcl/Tk software. Details of how to use special versions of tpoint.ini and/or tpg.ini are given in Sections 8 and 8, near the end.

1.1.2

Full distribution

The full TPOINT distribution includes source code, not only for TPOINT itself but also for the SLALIB library that it uses. Each of these is supplied in source form as one or more tar files, on any appropriate media, for example CD-ROM. As for the executable system, the installation procedure is straightforward and not excessively automated: 1. Create separate directories for the various components: tpoint slalib TPOINT (library and freestanding application) Positional-astronomy library

2. As appropriate, and using conventional techniques, decompress and unpack the contents of the tar file(s) to the directories just created. 3. It is usually necessary to edit the makefiles to suit local directory structures and compiler options. Each makefile contains instructions on what to do. In most cases the changes are all near the beginning of the file. Note that make's environment variables (for example INSTALL_DIR and the compiler options) can be specified on the command line, removing the need to modify the makefiles. 4. For SLALIB, cd to the appropriate directory and invoke make. If you are not using the gcc compiler, refer to the comments near the start of each makefile to see how to invoke make to use your choice of compiler. Then execute a rehash. 5. Go to the directory containing the TPOINT package and invoke make to create both the library and the executable system. (The same advice on running make as in the previous step apply here.)


1.2 System overview

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6. Make sure the script tpoint and the executable tpt are in your path. (The makefile puts them in $HOME/bin by default.) 7. Should de-installation become necessary, type make deinstall. To run TPOINT, cd to the directory you wish to work from and type tpoint. If the program fails to start, or if error messages appear during the start-up phase, or if the graphics commands fail to work, the things to check first are (a) paths, (b) the location and contents of the tpoint.ini file, (c) the location and contents of the tpg.ini file and (d) the existence, location and version of the Tcl/Tk software. Details of how to use special versions of tpoint.ini and/or tpg.ini are given in Sections 8 and 8, near the end.

1.2

System overview

TPOINT is a straightforward, monolithic program which accepts commands from the keyboard. There is a simple but effective "command script" capability, and a log file can be produced, recording what operations were carried out and with what result. Pointing observations are input from ordinary ASCII files (as produced by a text editor) in flexible formats. Graphics output goes to a separate X window; TPOINT can be run on a non-X system but the graphics commands will not work. Some small star catalogs are provided, and some sample data. There is a straightforward text-based hierarchical help system.

1.3

How TPOINT is used

This is how to use TPOINT to investigate the pointing performance of your telescope: 1. Decide which place in the focal plane you want the pointing to refer to. Depending on the size of the telescope and the arrangements for viewing the field, this may be crosswires in an eyepiece, a particular pixel of a CCD, or an autoguider probe. 2. Point your telescope at a sequence of stars of known coordinates (so that the star image is on the crosswires etc.), and record the telescope readouts in a file. 3. Run TPOINT and invoke the INDAT command in order to read in the file of observations. 4. Invoke TPOINT's USE command to specify a likely pointing model for the telescope concerned. The extensive repertoire of TPOINT terms includes zero points, misalignments, flexures and so on as well as general-purpose harmonics and polynomials. There are standard models for both equatorial and altazimuth mounts which make it very easy to get started.


4

1 INTRODUCTION 5. TPOINT's FIT command automatically calculates the optimum size for each term in the model, such that the overall model is the best possible fit to the observations. 6. Using TPOINT's graphics commands, the remaining pointing errors can be inspected. 7. If obvious non-random errors remain, additional terms can be added to the pointing model ­ and removed as necessary ­ and the fit repeated.

The final result estimates how well the telescope is capable of pointing, and what corrections need to be made to achieve this result. The coefficients which TPOINT produces (i.e. the numbers specifying the size of each pointing term) can be fed into the telescope's computer control system to improve the pointing. On small telescopes where there is no computer, they can be used to prepare tables of corrections for various parts of the sky. Another use of TPOINT is to identify opportunities for mechanical adjustment, for example enabling the polar axis of an equatorial mount to be set up accurately even on telescopes which cannot for some reason see the pole.

1.4

The TPOINT philosophy

Many observatories and telescope users have their own ways of fitting models to pointing observations, and in a numerical sense TPOINT may not produce results that are different or better. However, TPOINT is unusual (a) in that the telescope model can be changed during the interactive session and (b) in the variety of its graphical displays. These two features allow rapid exploration of the pointing possibilities of any given telescope and time after time leads to a better model than the one formerly used and to new insights into the telescope's behavior. TPOINT's ability to analyse data from multiple observing sessions, even where zero points etc. have changed, is another unique advantage. The general approach taken by TPOINT is that as far as possible the telescope model should describe real effects (geometrical misalignments, well understood flexures, etc.), and empirical functions should be used only to mop up any remaining systematic errors. There is a school of thought which advocates using empirical functions (for example polynomials or spherical harmonics) for the whole job. However, the TPOINT approach has some advantages: · Simple geometrical misalignments ­ for example a miscentered instrument on a mount which is located at a Nasmyth focus but is not coincident with the elevation axis ­ might require very complicated empirical functions but are simple to deal with analytically. · Direct manipulation of certain geometrical terms while the telescope is in operation can be very useful. An important application of this technique is where a star is switched from one instrument aperture to another simply by changing the collimation parameters. Another example is where the polar axis of a wide field telescope is routinely raised and


1.5 Running TPOINT

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lowered as a function of declination, to minimize the field rotation effects of differential refraction; the polar axis elevation parameter in the pointing model can simply be changed by the same amount and accurate pointing is maintained. · A realistic model of a telescope frequently exposes mechanical deficiencies which can then be diagnosed and cured, or at least understood. Time- and temperature-dependent effects can be pinpointed and remedies sought. · A realistic model is likely to require fewer terms, and the number of stars observed in a pointing test can be correspondingly smaller. · Physically-based models are less likely to misbehave when extrapolating outside the area covered by the available test data. In any case, the models available with TPOINT include an extensive range of empirical terms, giving the TPOINT user the best of both worlds. A good example of this is the AAT model, the terms of which are built into TPOINT as an example of what may be needed to tame a large equatorial with stringent pointing requirements. Though firmly based on well-understood geometrical misalignments and plausible tube flexure corrections, the AAT model also contains complicated harmonics describing what are believed to be flexures in the horseshoe and center section. These flexure models are encapsulated in two terms called HFX and HFD, which are always left at their nominal sizes and are not fitted when individual pointing tests are reduced. HFX and HFD were determined by empirical fits to a very large sample (well over 1000 stars) formed by superimposing the residuals from tests carried out over several years.

1.5

Running TPOINT

To run the TPOINT system from the Unix shell (typically running in an xterm window): tpoint logfile initfile commands messages The four parameters ­ the log file, the initialization file, the command input device and the message output device ­ are all optional and are normally allowed to default, respectively, to the following files: logfile initfile commands messages /dev/null tpoint.ini stdin stdout


6

1 INTRODUCTION

On termination, the log file, if one has been specified, contains a full record of the session, and includes extra information (e.g. correlations between pointing terms) too voluminous to be displayed on the screen during the run. Each TPOINT command line consists of one or more fields separated by spaces. The first field, of up to 6 characters, is the command name, and specifies the action to be performed. The subsequent fields, if any, are the arguments. Where the arguments are numeric, freeformat decoding is performed, and numbers may be entered in a wide variety of formats. The precise way the arguments are interpreted depends on the command in question. TPOINT deals with all forms of input ­ both commands and the various sorts of data file ­ in a standardized way which allows for comments and blank lines to be used and provides consistent handling of lowercase information. Details of these conventions are given in Section 1.7. To abort a command, type Ctrl+C. The command will terminate cleanly as soon as it can, and control will be returned to the operator ready for the next command. To exit normally from TPOINT, use the END command. A full list of all TPOINT commands, in alphabetical order, is given in Section 7.1. A quick reference list can be found at the end. Sequences of TPOINT commands can be executed from a text file called a procedure library. Standard TPOINT syntax applies to such libraries, which may thus contain blank lines and comments (beginning with ! ) to enhance readability. In using procedures from a library, the following commands are involved: CALL x .x PROC x RETURN calls procedure x from the library same: shorthand for CALL x specifies the start of procedure x returns from and ends a procedure

Of these commands, CALL alone is permissible from the keyboard; PROC and RETURN are used only in library procedures. A command CALL x (or .x ) can optionally be followed by arguments, separated by spaces. The argument strings are then substituted into any macros $1, $2,. . . $n that are encountered during execution of the procedure, where $n is the n th string after the procedure name. The standard procedure library file is automatically loaded when TPOINT is started unless a different one is specified on the command line as described above. After loading the library, TPOINT automatically executes a `CALL INIT'; the INIT procedure supplied in the standard library does nothing. Any procedure library loaded as part of start-up must contain an INIT procedure.


1.6 An example TPOINT session

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A private library can be loaded at any time during the TPOINT session by using the INPRO command. This supplements whatever has previously been loaded, so there is no need for private libraries to contain copies of the standard procedures ­ though this will do no harm. Indeed, a private library may contain only one procedure if desired, and each time it is reloaded that procedure will replace its previous version. The INIT procedure is not called when INPRO is used from the command line. An INPRO command without arguments restores the original library, without any subsequent additions. By default, procedure commands are not echoed on the screen; they can be made to appear by means of the ECHO ON command. The volume of messages output during the running of complicated procedures can be controlled by means of the MESLEV command. (This command can also be used from the keyboard, though suppressing messages during normal interactive use of TPOINT is not recommended.)

1.6

An example TPOINT session

A painless way to learn how TPOINT works is to reduce some real data, using one of the sample files. Proceed as follows. First start the system: tpoint There will be various announcements, followed by a * prompt. Read in the ukst.dat sample data file, which is from the 1.2 meter UK Schmidt Telescope at Siding Spring Observatory in Australia: INDAT ukst (You may need to specify a full pathname for the ukst.dat file.) The observations will be listed on the screen. Specify the standard geometrical model for an equatorial mount, and then fit the model to the data: USE IH ID NP CH ME MA FIT The values of the pointing coefficients are reported, as well as the root-mean-square (RMS) error on the sky and the population standard deviation (PSD ­ an estimate of what the RMS would be if the number of stars was much larger). There is also information about which observation happens to have the most influence on the model, and whether it looks abnormal.) Plot the residuals (the pointing errors which remain after the TPOINT model has been applied):


8

1 INTRODUCTION

Figure 1: UK Schmidt Telescope, after fitting the standard 6-term model for an equatorial. The central plot is residuals against h, and is characteristic of fork flexure.

CALL E9 (or its shorthand form .E9) The resulting set of graphs is shown in Fig. 1, where the same residuals are displayed in a variety of ways (see Section 3.2). In this particular case, conspicuous systematic effects are present in the central plot, which is dD (declination residual) versus H (hour angle). The term FO (fork flexure, = FO cos h) is a standard one for fork mounted telescopes and produces this characteristic pattern of residuals. Include the term in the model, fit again and plot the residuals: USE FO FIT .E9 See Fig. 2: that the RMS and PSD are much improved, and there are no obvious remaining systematic effects. This simple 7-term model is, in fact, the one used operationally.


1.6 An example TPOINT session

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Figure 2: UK Schmidt Telescope, after adding the fork flexure term FO to the model.

This illustrates the general strategy for modeling a telescope from scratch. Begin by suitably preparing the TPOINT system and inputting the data. Then create a preliminary model consisting of the basic set of geometrical terms, and perform a fit:

INDAT file USE IH ID NP CH MA ME or USE IA IE NPAE CA AW AN FIT

(for an equatorial) (for an altazimuth)

(Procedures for setting up the basic equatorial and altaz models are also provided in the standard library: use CALL EQUAT or CALL ALTAZ.) The fit can be repeated until the solution settles down. Make a note of the population standard deviation, which is an indication of the general quality of the telescope before any modeling of flexure etc. The next step is to display the residuals graphically. A useful way to start is procedure E9 or A9 in the standard library:


10 CALL E9 or CALL A9 (or use the shorthand form .E9 etc.).

1 INTRODUCTION 9 favorite plots for an equatorial 9 favorite plots for an altazimuth

By means of the USE and FIT commands, try out extra terms to reduce any systematic errors. The simple Hooke's-law tube flexure term TF is frequently required; for fork and yoke mounted equatorials try the fork flexure term FO, and for mounts with a declination axis supported at one end only (the English cross-axis and the German mountings are examples) try the declination axis flexure term DAF. When adding new terms, pay attention to the standard deviation of the new coefficient, the effect on other terms, and whether the population standard deviation has been reduced. Remove new terms if they are reported as being indistinguishable from existing ones, or if they lead to the appearance of messages warning that the fit is ill-conditioned. With the exception of the six standard geometrical terms, it is wise to reject any terms which are not at least 2 sigma in size. For each FIT, a table of the correlations between every pair of terms is output to the log file, for inspection after the TPOINT session; values close to unity show terms which are not easily distinguished given the current data. Obvious "outliers" ­ observations much too far out to believe ­ can be temporarily excluded from the fit by using the command MASK n, where n is the observation number. The FIT command itself identifies the most likely candidate, while the OUTL command allows multiple outliers to be identified and MASKed. More detailed advice is given in Section 2. But be sure to try TPOINT's auto-modeling facility: FAUTO In many cases, this gives better results than a purely manual approach, especially as finetuning by hand can always be used as a second step. The current settings of various internal parameters can be displayed as follows: SHOW The list produced by SHOW includes the command names required to change the parameters, and is a useful guide to some of the facilities available within TPOINT. Here is an example:
ADJUST: the model adjusts the telescope to fit the stars. APPEND: the next input file will overwrite the current data list. CAPT: caption plotting is enabled. CLRNG: the plotting zone is cleared before each plot.


1.7 Syntax
ECHO: library procedures execute silently. FITTOL: the ill-conditioning criterion is 0.001. FRAME: a frame will be plotted. MARKH: the marker height is 0.2. MESLEV: messages of all levels are displayed. messages of all levels are logged. PENS: current pens are 18, 5, 17, 12, 13, 2. PLTZ: the plotting zone is from (0.000, 0.000) to (1.000, 1.000). REPLEN: long reports are given in full. TXP: text precision is 2 (stroke). VT: the screen is assumed to be non-vt100. There are 42 observations in the data list, all active; equatorial mount has been specified.

11

1.7

Syntax

All TPOINT commands, procedure library records, star catalog records, model data records, and pointing data records, are sub jected on input to a preliminary vetting and conditioning, as follows: · Comments ­ records which are blank, or which have ! as the first non-space character ­ are logged and displayed if appropriate but are otherwise ignored. · Non-printing characters (TABs for instance) are replaced by single spaces. · Leading blanks are eliminated as appropriate. · Except within string arguments, lowercase characters are converted to uppercase. · Multiple input lines can be concatenated using the backslash continuation character: if an input line ends with a backslash, that backslash and the subsequent newline are ignored. String arguments are groups of characters delimited by pairs of either " or characters to show that they must stay lowercase. When a procedure library file is being input, string argument delimiters are left intact (to allow library commands to have lowercase arguments). For TPOINT commands, string argument delimiters are interpreted by the command routine itself. When star catalog files, pointing data files, and model data files are input, string argument delimiters are removed. File names containing spaces can be used if they are surrounded by quote characters. TPOINT does not support procedure arguments that contain spaces etc.


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2 MODELING

2

MODELING

The pointing model is a sequence of terms selected from an internal repertoire. As well as explicitly formulated terms (geometrical effects for example) there is a generic type covering a wide range of polynomials and harmonics. Terms can be added to the model by means of the USE command. For example, to add the terms for polar axis misalignment: USE ME MA The USE command is also used to re-enable fitting after a FIX command. Terms can be removed from the model by means of the LOSE command. For example, to drop the term PDH2: LOSE PDH2 The value of the coefficient of a single te