INT Prime-Focus Autoguider:
External Interfaces -
TCS Interface

Gareth Jones

Issue 1.2; 24th August 1995

Royal Greenwich Observatory,
Madingley Road,
Cambridge CB3 0HJ

Telephone (01223) 374000
Fax (01223) 374700

1 Introduction

1.1 Purpose

This document describes the format of the packet to be provided by the autoguider to the TCS computer.

This document should be read by those involved in interfacing the autoguider and TCS computer.

1.2 Acronyms and abbreviations

INT Isaac Newton Telescope

INTPFA Isaac Newton Telescope Prime-Focus Autoguider

TCS Telescope Control System

1.3 References

[1]. "INT Prime-Focus Autoguider: User requirements"
Guy Rixon, Gareth Jones
RGO document INT-PF-1

[2]. "INT Prime-Focus Autoguider: Software requirements"
Gareth Jones
RGO document INT-PF-6

2 TCS interface specification

2.1 User requirements

Requirement from R.Laing; 21/04/95

The autoguider sub-system shall provide position estimates accurate to 2 decimal places to the TCS, in order to prevent the introduction of rounding errors. The values are measured in CCD pixels.

Requirement from R.Laing; 24/05/95

The autoguider sub-system shall provide position information to an accuracy of better than 0.1 arcsec, preferably 0.05 arcsec, with a nominal rate of 1 position error calculation per second, for normal guiding, and a maximum rate of 10 position error calculations per second, for tracking analysis.

Requirement from WHT design; 12/04/95

The autoguider shall provide information on the quality of the position estimate to the TCS.

2.2 Software requirements

Requirement : The system shall communicate with the TCS computer using an RS-232 compatible connection.

Requirement : The data transferred to the TCS will include the x and y guide position as coded values and a value indicating the time to the next packet.

Requirement : The ordering of the TCS packet shall be, x guide position followed by y guide position followed by time value.

Requirement : The system shall provide TCS correction values accurate to 2 decimal places, and to 5 significant digits.

Requirement : The system shall provide time/code values in fixed point format with 2 decimal places and 4 significant digits; the value is signed.

Requirement : The system shall provide all values in the TCS packet in ASCII (8-bit) text format.

2.3 TCS protocol

2.3.1 Design decisions

The Baud rate for the autoguider/TCS RS-232 connection shall be 9600 baud, although the software will be written to make this value programmable for values of 1200 baud, for example: this is to circumvent any problems that may occur in the reliability of a higher speed RS-232 link.

To meet the quality requirements, the time/code value will be used in the following manner:

Positive value Time until next packet

Zero Autoguider is terminating guide loop - this is the last packet

Negative value The packet contains suspect data (for example, when the image
is such that an accurate guide estimate cannot be made -
analogous to the guiding suspended mode on the WHT

The position estimates are referenced to the readout corner of the CCD and are measured in pixels.

The autoguider/TCS communication is assumed to to have been disrupted, or have failed, if twice the stated time until the next packet is exceeded (as suggested by Guy Rixon and Robert Laing 21/08/95).

2.3.2 Packet protocol

The following diagram illustrates the packet protocol to be used between the autoguider and the TCS computer.

Figure 1. Illustration of the TCS packet Packet definition

packet ::= xGuidePosition SPACE yGuidePosition SPACE code CR

xGuidePosition ::= {s0000p00 ..... s9999p99} Representing the position
accurate to 0.01 pixels

yGuidePosition ::= {s0000p00 ..... s9999p99} Representing the position
accurate to 0.01 pixels

code ::= time | terminating | suspended

time ::= {00000p01 ..... 99999p99} Representing time until next packet accurate
to 10 msec. Note that although values up to
99999.99 can be represented, only values up
to 9999.99 are valid - in order to be
symmetric with the suspended time values.

terminating ::= 00000p00 | -0000p00 Informs the TCS that the autoguider is
terminating the guide loop

suspended ::= {-0000p01 ..... -9999p99} Informs the TCS that the reliability of the x
and y guide data in this packet cannot be
guarantied. The time until the next packet is
still represented (see definition of time)

s ::= 0 | - Represents the sign of the value

p ::= ASCII code ('.') Represents the decimal point

SPACE ::= ASCII code (' '), 0x20 The field separator

CR ::= ASCII code 0x0d The end-of-packet marker Units

xGuidePosition Measured in pixels from the readout corner of the CCD

yGuidePosition Measured in pixels from the readout corner of the CCD

code Time measured in seconds

2.4 Comparison with current WHT system

The current FORTH based WHT autoguider has a packet structure where the position estimates are provided accurate to 4 significant figures (i.e. 1 decimal place) and the time/code is 4 significant figures with 2 decimal places (i.e. accurate to 10 msec). No SPACE delimiters are provided and the position of the decimal point is implied. The packet is CR terminated and the units of the values are as described in this document; the time coding scheme is also identical.

2.5 Autoguider self-test requirements

The autoguider software provides functions for power-on self-test (POST) and for several internal test modes. Some of these modes rely on the RS-232 connection to the TCS computer being placed in loop-back mode. However, due to the design of the RS-232 driver device on the MVME147 MPU card, the output is still driven in loop-back mode.

The system will use a simple ASCII text string, terminated by CR in order to perform these tests. The test packet will be constructed such that it is of identical length to the 27 character TCS packet. Its content may contain numeric values, but the first character of the packet is guarantied to be non-numeric and the string will contain no spaces.

Since the TCS packet will be read by a FORTRAN program in the TCS computer (in the first instance), the space delimited TCS packet described above can easily be decoded. The test string described here will cause the parsing of the string to fail if an attempt is made to extract the x, y guide position.

The maximum rate of test packets will not exceed the 10 packet/second rate limit imposed by the specifications on the normal TCS packets.

2.6 Error correction

The TCS packet will contain no error-correction information since a bit-error in the ASCII text string will have a 95% chance of converting a valid character to an invalid one (based on the fact that each character can have 256 values, only 14 of which are valid in the TCS packet).

Although parity could be enabled on the RS-232 connection, the system will assume that packets are transmitted with no parity. This decision can be reviewed at a later date should data corruption become a problem.

Appendix A. Document history

Issue 1.1; 22/08/95 Original document.

Issue 1.2; 24/08/95 Update as a result of meeting between P. Bunclark, B. Gentles, M. Fisher and G. Jones.