J. van Harmelen, 17-NOV-1988, V3.1

J. van Harmelen, 1993, V4.0 View Command added

J. van Harmelen, 25 October 1996, V4.1 Configure Command added, general review.

J. van Harmelen, 7 July 1999 V4.2 Other Telescope Commands added

                Autoguide Command added, Status Command added

J. van Harmelen, 12 July 2000 V4.3 Added “Halt” command for small telescope control systems

 

 

 

SPECIFICATION OF A CONNECTION BETWEEN TELESCOPE AND INSTRUMENT COMPUTERS

          (ETS_LINK)

 

 

 

 

1. SCOPE

This document relates to the connection of stand alone instrument control computers to telescope control computers for the purpose of conveying telescope coordinate and time information. Other information and control commands have been added to the list of commands.

It does not concern instrument control by VAX computers, either by the telescope computer itself, or by other networked VAX's, as user programs have access to telescope data and can control the telescope by the methods described in the 2.3m Telescope Observer's Manual (Chapter 12).

 

The use of a dedicated serial line for this purpose should be regarded as a simple and convenient solution for the desired data exchange. At a later time network connection will considered.

2. HARDWARE

The Telescope and Instrument computers are to be connected by R5232 ports, operating at 9600 bps, no parity, one stop bit.

 

On a VAX this will be a general purpose terminal port, on a PDP11/24 port SLU2:, on other PDPll's a comparable port with the same address as SLU2:, on other computers a convenient serial port.

3. DATA EXCHANGE PROTOCOL

All data exchange will be in ASCII format.

 

The Instrument computer is the master of the datalink: it requests information when needed, by sending a command to the Telescope computer, which will then reply with the requested data, or with an error message.

 

The Telescope computer is available for exchange of information at any time while the telescope system is running. When the telescope system is not running, there will be no reply from the telescope computer to any Instrument computer commands. The instrument computer software shall implement a time-out to cope with this condition.

4. COMMAND AND DATA FORMATS

The command and reply repertoire described below is thought to cover the requirements of most existing instruments which are controlled by their own computer systems, but may need future extension. The CONFIGURE and VIEW commands are two such extensions.

 

The VAX Telescope computers use DCL parse routines and will generate the error message "UNRECOGNISED COMMAND" if the commands can not be recognised. Other types of telescope control computers shall mimic the behaviour of a VAX/VMS machine as much as possible. All systems implement as a minimum the TELESCOPE, COORDINATES, TIME and VIEW commands. Commands which control the telescope are only implemented on a needs basis.

 

A command or qualifier can be abbreviated to the minimum number of characters necessary to uniquely identify it, but not to less than two characters, and has to be terminated with a carriage return <CR>. No more than four characters shall be required to uniquely identify a command or qualifier. The command strings are not case sensitive.

 

All replies are terminated with a <CR><LF> combination. All alphabetic information in the reply messages will be in capitals.

 

The commands TELESCOPE, COORDINATES, TIME, VIEW and CONFIGURE are available together with qualifiers to indicate which coordinates or time are required, and whether the reply values will be in 'string' or 'real' format.

 

4.1 CONFIGURE

The command:          CONFIGURE  configuration_variable  value

 

shall be accepted by the telescope system as if it were issued on the command line at the telescope control terminal, or the equivalent information entered in the configuration dialog in menu driven systems. Refer to the Telescope Manuals for further details.

 

A <CR><LF> reply indicates successful completion of the command, any error is signalled with the reply:                   UNRECOGNISED COMMAND.

 

A special case is the control of the Dome_Control configuration variable, which on successful completion reports the previous value:

 

          DOME_CONTROL WAS MANUAL  or  DOME_CONTROL WAS AUTOMATIC

 

This addition to the command repertoire introduced in version 4.1 has only been implemented on the 74” and 50” telescope systems. Other systems will be revised as need for this command arises.

 

4.2 COORDINATES

 If the telescope is tracking, this command causes the telescope computer to reply with the message:

 

          "object name"  ra  dec  equinoxstring

 

The object name is only included if available, and is always in capitals and enclosed in double quotation marks.

 

The format of RA and DEC variables depends on the format qualifiers and is described in detail under the /REAL and /STRING headers.

 

The equinox string contains seven or eight characters: the FK4/FK5 identifier "B" or "J", followed by the epoch as "yyyy.y" (seven characters), or the string "APPARENT" (eight characters) when   apparent place is selected for the telescope display.

 

If the telescope is not tracking, an error message is generated:

          TELESCOPE NOT TRACKING

 

If the coordinate data are inaccessible or suspect, the reply is:

         DATA ACCESS ERROR

 

/BASE

The requested coordinates are Base Coordinates (refer to 2.3m Telescope Observer's Manual for definition).  (Not available on telescopes which do not support the TRACK command)

 

/FILE

The requested coordinates are File Coordinates (refer to 2.3m Telescope Observer's Manual for definition). (Not available on telescopes which do not support the TRACK command)

 

/TRACK (default)

The requested coordinates are the current telescope Tracking Coordinates (refer to 2.3m Telescope Observer's Manual for definition).

 

/REAL

RA and Dec are supplied in ASCII representation of their floating point values:

                0 < RA  < 2*PI,                  format: r.rrrrrr

          -PI/2 < Dec < PI/2,               format: sd.dddddd (s=sign)

 

The sign is only included if it is negative.

 

/STRING (default)

RA and Dec are supplied in string format:

 

          RA:     hh mm ss.d          Dec:    sdd mm ss (first s=sign)

 

If the information is inaccessible or suspect, the reply is:

          DATA ACCESS ERROR

 

4.3 TIME

This command causes the telescope computer to reply with the message:

 

          mjd  sidereal_time  selected_time  selected_date

 

"mjd" is the Modified Julian Date (defined as JD - 2400000.5) in the format: ddddd.dddddd

"sidereal_time" is Local Apparent Sidereal Time

"selected_time" is either Universal Time or Civil Time as specified by the qualifiers /UT and /CT.

      

The format of "sidereal_time" and "selected_time" variables depends on the format qualifiers and is described in detail under the /REAL and /STRING headers.

 

"selected_date" is either UT Date or Civil Date as specified by the qualifiers /UT and /CT.

The "selected_date" is returned in string format:

 

          dd-mmm-yyyy

 

If the time information is inaccessible or suspect, the reply is:

          DATA ACCESS ERROR

 

/CT

The "selected time" variable is returned as Civil Time: Australian Eastern Standard Time or Australian Eastern Daylightsaving Time as applicable (UT + 10, resp UT + 11 hours).

 

/UT (default)

The "selected time" variable is returned as Universal Time.

 

/REAL

selected_time and sidereal time are supplied in ASCII representation of their floating point values:

 

           0 < time < 2*PI format: t.tttttt

 

/STRING (default)

selected time, which is UT or Civil Time depending on the /UT and /CT qualifiers, and sidereal time are returned in string format:

 

          hh:mm:ss.s     in the range 0 to 24 hours.

 

4.4 TELESCOPE

 This command causes the telescope computer to reply with an identification message:

 

              tel_id_string  astr_latitude  astr_east_longitude  height_AMSL

 

"tel_id_string" is the site and telescope identifier, a fixed length string of fifteen characters left justified, in the format:                 SSO 2.3METRE or MSO 74INCH etc.

 

"astr_latitude" and " astr_east_longitude " are the astronomical latitude and east longitude in decimal degrees in the format:

 

              sdd.ddddd and ddd.ddddd (s=sign)

 

"height_AMSL" is the integer value of the height above sea level of the telescope in metres: hhhhh

 

4.5 VIEW

The VIEW command permits an observer to see the value of any telescope display variable

(even if it is not currently featured on one of the display screens).

 

The command:          VIEW variablename [,...]

 

causes the names, values (and units where appropriate) of the specified display variable(s) to be returned. A list of display variable names for your telescope can be found in the chapter 'Telescope Display' in your Telescope Observer 's Manual. The details of the format of the returned data vary with the type of data requested. Rather than trying to detail all possible cases here, it is recommended to obtain the data by typing in the VIEW command at the control terminal of the 2.3m, 74” or 50” telescopes.

 

Examples:

   command:           reply:

 

   TELESCOPE                                          SSO 2.3METRE     -31.27336 149.06119 1149

   TEL                                                MSO 74INCH       -35.32065 149.02433 768

   COORDINATES                                 "B CENT" 14 03 00.3 -60 19 05 51988.5

   COORD/REAL        "BCENT" 3.678281 -1.052749 J1988.5

   COORD                                              12 34 56.7 +06 54 32 B1950.0

   COORD                                              TELESCOPE NOT TRACKING

   TIME                                               47465.711806 05:41:57.1 17:05:00.0 31-OCT-1988

   TIME/CT                                            47465.711806 05:41:57.1 04:05:00.0 1-NOV-1988

   TIME/REAL/CT      47465.711806 1.492038 1.060288 1-NOV-1988

 

4.6 OTHER TELESCOPE COMMANDS (2.3m, 74” and 50”)

Any telescope command can be given and will be executed. The reply for successful commands is just a <CR>, for unsuccessful commands the error message UNRECOGNISED COMMAND plus the body of the VMS error message generated by the control system. On the 2.3m problems with the delivery of error messages have not yet been solved. It is not possible to discern a command failure.

 

4.7 TRACK/COORDINATE, OFFSET[/BASE] and HALT COMMANDS (40”, 30” and 24”)

For those small telescopes which operate under computer control (40”and 24” to be implemented in winter 2000), the following telescope commands are supported: “TRACK/COORDINATE coordinates” (in standard MSSSO format), “OFFSET ra dec”, “OFFSET/BASE ra dec”, and HALT.  The reply for successful commands is just a <CR>, for unsuccessful commands the error message UNRECOGNISED COMMAND plus the body of the VMS style error message generated by the control system.

 

4.8 AUTOGUIDE COMMAND (2.3m only)

The “AUTOGUIDE ew ns” command puts the sky offsets supplied into the autoguider offset fields of the control database. The telescope will react to this on the next control loop.

 

4.9 STATUS COMMAND

The telescope control system will reply to the “STATUS” command with one of the following strings indicating the current telescope status: “OFF”, “FAULT”, “HALTED”, “WAITING”, “SLEWING”, “TRACKING”. External systems controlling the telescope should use this command to determine when to continue with the next phase of their control program.

5. IMPLEMENTATION ON THE TELESCOPE COMPUTERS

The implementation of the instrument connection on the VAX Telescope computers takes the form of a process as part of the telescope system. The designated serial port is allocated to this process. The process reads from the port and waits till a command arrives. DCL parse routines are used to decode the command, the reply string is assembled from information taken from the telescope database, where necessary using the telescope library routines to do data conversions, and then written out to the port.

 

On other telescope computer types the actions of the VAX computers shall be imitated as closely as possible. The communication port used shall be polled frequently enough not to produce any noticeable delay in reply or be interrupt driven.

6. IMPLEMENTATION ON INSTRUMENT COMPUTERS

The implementation of the instrument connection on PDP11 Instrument computers uses an RT11 device handler to communicate with the serial port. The instrument control program will contain a subroutine which writes the required commands to the port and decodes from the received reply string the

information it requires.

 

On other types of instrument computers there is no prescribed method of implementation. The use of reply parsing routines which are character position dependent is to be discouraged. Small differences in replies between types of telescope computers are possible and should be coped with. Examples of these differences are the number of significant digits in scientific notation of variable values returned by the VIEW command and the number of spaces separating fields.