Telescope Command Reference
Manual
2 Celestial Coordinates
2.1 SPECIFICATION OF COORDINATES
This section describes the format to be used when
specifying equatorial polar coordinates of astronomical objects for:
(i) the entry of coordinates for the telescope
TRACK/COORDINATE command;
(ii) the format of individual records in a telescope
Coordinate File;
(iii) general usage throughout Mt Stromlo and Siding
Spring Observatories.
Observers are strongly advised to use this format in
their own programs wherever equatorial polar coordinates (right ascension
and declination) are needed in the interests of uniformity and convenience.
To this end the two parsing routines TEL_Parse_Coordinate_String and TEL_Parse_Equinox_String
are available in the TELLIB library; see the MSSSO Telescopes User Programming
Manual .
Equatorial coordinates should always
be thought of as an ordered triplet- a d e
where e specifies
the particular equatorial system (equator and equinox). The first two quantities
are meaningless without the third which must either be present or default
to some appropriate coordinate system specification. The order of the three
quantities differs here from some other standards and telescope projects,
but this is necessary in order that unnecessary information can be ommitted
simply without the need for place-holders in the record or command line.
The use of double quotation marks (") to delimit the name field has been
dictated by the need to provide flexibility in the length of object names
and to overcome the clumsy command-line handling strategy imposed on us
by the VMS operating system.
The data required for complete specification of a
celestial position are listed below; in a particular instance many of them
can be omitted in which case they default to sensible values as discussed.
| Object name |
|
| R.A. |
a |
| Declination |
d |
| Coordinate system specification |
FK4, FK5 or Apparent Place. |
| Equinox |
(the epoch of the mean equator and equinox) |
| Epoch |
(time zero for the proper motion correction) |
| Proper motion in RA |
ma |
| Proper motion in Declination |
md |
| Parallax |
p |
| Radial Velocity |
Vr |
A coordinate string in general takes the format:
["object name"]
ad
[equinox [(epoch)]] [mamd
[p [Vr]]]
[!comment]
where the square brackets indicate optional parameters
in the usual fashion.
2.1.1 Object Name
The object name is an optional string of printable
characters and, if present, it must be enclosed in double quotation
marks. The case (upper or lower) of alphabetic characters is preserved
and the name may contain embedded spaces or tabs . It may occur anywhere
in the coordinate string and is excised from the string before the string
is parsed for the other fields or subfields; it is the only field which
does not have a preassigned position.
The object name field may have any length provided
the length of the entire coordinate string does not exceed the system parameter
TEL_K_Coord_Length
(132 characters).
"^^NGC 2516^offset^by
2 arcsec"
"^^+163, -67.3$^"
"SN1987A 'if
it is still there' <TAB> 29/4/87"
are all valid object names.
The system stores and handles an object name exactly
as it was supplied by the observer. However, when trying to match object
names in a file with a name supplied in a TRACK command, all spaces and
tabs are ignored, and the case of alphabetic characters is ignored.
2.1.2 Right Ascension
The right ascension field comprises one, two or
three subfields specifying the hours, minutes and seconds of time for the
coordinate.
The minutes and/or seconds subfield(s) may be omitted
by decimalizing the preceding field; the last subfield is defined as the
one with the decimal point or the third one encountered.
One or more spaces or tabs or a comma in conjunction
with spaces and tabs are assumed to separate the subfields. Thus the following
strings specify valid R.A. coordinates:
13 46 52
13 46 52.20
13 46.87
13.781167
13, 46.87,
The following are not valid R.A. fields:
13 47
13
Right Ascension is a mandatory
parameter.
2.1.3 Declination
The declination field comprises one, two or three
subfields specifying the degrees, arcminutes and arcseconds for the coordinate.
Normally three subfields are expected but the field may be prematurely
terminated by decimalizing the arcminutes or degrees field (just as for
the R.A. field).
A sign character + or - should preceed the degrees
subfield and is applied to the coordinate as a whole. The + sign is not
mandatory for coordinates of northern declinations but its use is strongly
recommended because it makes coordinate lists more easily readable. Thus
the following strings are valid specifications for a (southern) declination:
-0 26 21.6
-0 26.36
-0.439333
-0 26 22
The following are not valid Declination fields:
-0 26
-1
-^0 26 21.6
(space
between sign and degrees)
Declination is a mandatory
parameter.
2.1.4 Equinox
The equinox parameter defines two items of data:
(i) the coordinate system: FK4 mean place, FK5 mean
place, or apparent place;
(ii) the epoch of the equinox and equator (for FK4
or FK5).
It takes one of the following forms:
[B]yyyy[.y..]
[J]yyyy[.y..]
A[PPARENT]
Examples of valid equinox specifications are:
| FK4: |
B1950 |
B1950.0 |
1950 |
1921.31 |
B1988 |
| FK5: |
J2000 |
J2000.0 |
2007 |
2008.217 |
J1920 |
| Apparent: |
A |
AP |
APP |
APPAR |
etc |
If the leading B or J character does not appear the
coordinate system defaults to FK5 for epochs 1984.0 and later, and to FK4
for epochs before 1984.0
Apparent coordinates are interpreted as the geocentric
position of the object with respect to the true equinox and equator of
date.
If the equinox parameter is omitted both the equinox
and epoch parameters (see below) default to the value of the configuration
variable Default_File_Equinox which can be set by the observer using the
CONFIGURE command; see §1.3.
2.1.5 Epoch
In some circumstances it is necessary to distinguish
between the epoch defining the equinox and equator of the mean coordinate
system and the epoch which is used as the zero point for proper motion
calculations. If the two are not the same the epoch is appended to the
equinox parameter in parentheses thus:
equinox(epoch)
for example J2000.0(1985.31)or
B1950(1975)
A leading B or J character is permitted
in the epoch string e.g. J2000(J1987.1)
but the difference in the resultant epoch time value is negligible for
most purposes.
Where the epoch parameter is omitted it defaults
to the equinox value.
2.1.6 Proper Motions
The proper motion fields ma
and md
are optional but, if supplied, must both be present. Each may be any valid
numeric (real or integer) string and are interpreted thus:
| ma |
proper motion in R.A. (da/dt)
in seconds of time per century. |
| md |
proper motion in Declination (dd/dt)
in arcseconds per century. |
The century referred to is the Besselian century
(taken to be the length of the tropical year at B1900.0) of 36524.2198781
days for FK4 positions, and the Julian century of 36525 days for FK5 positions.
Proper motions are only relevant for the mean place
FK4 and FK5 systems. If the proper motions are omitted they default to
zero for the FK5 system and to a small fictitious proper motion for an
FK4 system. This means that in both cases the object is assumed to have
zero space motion with respect to an inertial frame.
The proper motion fields must be present if the parallax
and radial velocity fields (which follow them) are specified.
2.1.7 Parallax
The annual parallax ? is specified as a real or
integer value in units of arcseconds.
Parallax is optional but must be present if the radial
velocity (which follows it) is specified.
2.1.8 Radial Velocity
The radial velocity Vr is specified as a real or
integer value in units of km/second (positive for velocities outward).
Radial velocity is an optional parameter.
2.1.9 Comment Field
The exclamation mark character ! can be used to
include information of a comment nature in the coordinate record. It must
be placed after all of the coordinate fields which are used by the telescope
control system and signifies that the remainder of the record is comment.
The system makes no use of the comment field.
2.1.10 Field Delimiters
The fields of a coordinate specification may be
separated by the following means in any combination:
(i) one or more spaces;
(ii) one or more tab characters;
(iii) any combination of spaces and tab characters;
(iv) a single comma;
(v) a single comma combined in any order with one
or more spaces and/or tabs.
The above applies equally well to the subfields
of RA and declination. It is not necessary to adhere to the one form of
delimiter throughout a coordinate record; for example a common form might
be to have commas separating the fields but spaces between the hours, minutes,
seconds of RA and degrees, arcminutes, arcseconds of declination.
2.1.11 Auxiliary Data Records
When used as the first character of a record, the
sharp character # indicates to the telescope control system that that particular
record does not contain coordinate data. Any number of such Auxiliary Data
records may be included in telescope coordinate files. They are not used
in any way by the telescope control system (just like comment fields) but
in the case of the 2.3 metre telescope they are made available to observer
or external programs in the following manner: Any Auxiliary Data records
immediately following a coordinate record are assumed to be associated
with that object; the first four such records for the current object are
stored in a user-accessible area of the telescope database and may be parsed
or interpreted in any way desired.
2.1.12 Continuation of a Command Line
Although not part of this Coordinate Specification,
it should be noted here that the minus sign is used to indicate continuation
of a DCL or TELESCOPE command onto subsequent lines. The VAX Command Language
Interpreter interprets a minus (-) as a continuation indicator only if
it is the last character in the line; it will not misinterpret negative
declinations or proper motions, nor will minus signs in object names affect
the correct decoding of the coordinate string.
Note, however, that a minus sign inside a quoted
string intended as an object name cannot function as a continuation character.
An object name which is very long should simply be allowed to wrap on the
screen; alternatively terminate the coordinate values with a continuation
(-) and start the object name on a fresh line.
For example if one tried to enter the command lines:
TEL$
TRACK/COORD "This supernova SN1987A is a partic- <RET>
_TEL$ ularly interesting object" 5 23.7 - <RET>
_TEL$ -69 46.0 1987.4 <RET>
a DCL error would be signalled at the first line;
but the following command lines:
TEL$
TRACK/COORD 5 23.7- <RET>
_TEL$ -69 46.0 1987.4 - <RET>
_TEL$ "This supernova SN1987A is a particularly
interesting object" <RET>
will be correctly decoded to:
| object name |
This supernova SN1987A is a particularly interesting
object |
| RA |
5 23 42.0 |
| Dec |
-69 46 00 |
| coord system |
FK5 |
| equinox |
J1987.4 |
| epoch |
J1987.4 |
| ma |
0 |
| md |
0 |
| p |
0 |
| Vr |
0 |
2.2 COORDINATE TRANSFORMATIONS
The MSSSO Telescope control systems are designed
to handle coordinates in equatorial polar form with considerable flexibility
and accuracy. Other forms of celestial coordinate input such as galactic
latitude and longitude or geocentric or heliocentric rectangular coordinates
have not been implemented because of their reduced usefullness. The various
coordinate transformations involved in controlling the telescope are described
below and reference should be made to Figure 2.1 or 2.2 Celestial Coordinate
Transformations (for your telescope).
2.2.1 Equatorial Coordinate Systems
Equatorial coordinate systems differ in their equator
and equinox which establish the reference plane and a particular fiducial
point on it. The FK4 system of a particular Bessellian epoch is denoted
by the letter B before the epoch year thus B1950.0, B1975, B1981.3 etc.
The new FK5 system and epochs referred to the Julian century of 36525 days
have been in use since the beginning of 1984 and are denoted by the letter
J before the epoch year e.g. J2000.0, J1975, J1988.6 etc. If you omit the
B or J prefix the system will assume a sensible default (see §2.1.4);
however it is desirable to use this convention because it is a universally
accepted means of avoiding any confusion of which epoch or coordinate system
is meant. For more information on this subject see The Supplement to The
Astronomical Almanac 1984 -‘The Improved IAU System of Astronomical
Constants, Time Scales and Reference Frame’.
As well as mean place coordinate systems discussed
above, it is possible to use a geocentric coordinate system referred to
the true equinox and equator of date. This Geocentric Apparent system is
specified by using the word APPARENT (or any abbreviation of it) in lieu
of the normal equinox specification. Please ignore
the semantic anomaly-you supply the word APPARENT when the system expects
a mean equinox and equator specification; we did not wish to unnecessarily
complicate the data entry requirements.
2.2.2 File Coordinates
It is important to understand the distinction between
the FILE COORDINATES which are the observer's original source of coordinate
information and the CONTROL COORDINATES (BASE or TRACKING COORDINATES)
which are the coordinates in which the virtual telescope is being controlled,
in which all observer jog, offset, scan and rate control is executed, and
which are displayed in the double-height heading on top of the display
VDU(s). The observer has independent control over both coordinate systems.
The FILE coordinate system can be an FK4 catalogue mean place, an FK5 catalogue
mean place or Geocentric Apparent and likewise the CONTROL coordinates
in the following combinations:
file coordinate system tracking coordinate
system
| FK4 |
FK4 |
| FK5 |
FK5 |
| FK4 |
FK5 |
| FK5 |
FK4 |
| APPARENT |
APPARENT |
Note the restricted manner in which apparent coordinates
are handled-if the file coordinates are an apparent place then the displayed
tracking coordinates must be apparent too. In this case the display variables
J2000_RA and J2000_Dec may be useful; these give the tracking position
of the telescope in the J2000.0 FK5 mean coordinate system at the current
epoch i.e. date.
Coordinate data entered directly at the control VDU
using the TRACK/COORDINATE command is also referred to as 'File Coordinates'
and is handled similarly to data read from a Coordinate File.
2.2.3 Base Coordinates
The BASE COORDINATES represent the geocentric position
of the selected object at date (i.e. with space motion and parallax corrected
for the current epoch) in the coordinate system specified by the configuration
variable Tracking_Equinox; see §2.2.4 below. They are calculated once
only from the File Coordinate information whenever a new object is tracked
or when the command TRACK/SOURCE is issued.
2.2.4 Tracking Coordinates
The TRACKING COORDINATES (at the top of the observer’s
display) represent the instantaneous position of the telescope optic axis
(currently selected aperture) in the coordinate system specified by the
configuration variable Tracking_Equinox. On the 2.3 metre telescope they
are obtained from the Base Coordinates above by adding the accumulated
offsets due to jog, offset, scan and rate generation; this process takes
place continuously at the control loop frequency of 20 Hz. On the 74 inch
and 50 inch telescopes, they are calculated from the axis position encoder
readings.
The observer may change the Tracking Equinox at any
time (even whilst tracking) with the command:
CONFIGURE TRACKING_EQUINOX equinox_spec
There may be a momentary ‘glitch’ in the position
of the star whilst the tracking calculations are re-initialized but the
final position on the sky will remain unchanged, even though its displayed
coordinates will have changed and the route by which it is calculated may
have changed radically.
2.2.5 The Distinction between Equinox and
Epoch
As discussed in §2.2.1, the equinox parameter
specifies the coordinate system whereas epoch denotes the absolute time
for which the object's position is specified i.e. time ‘zero’ for the space
motion calculation. For a position obtained from a catalogue the equinox
and epoch are usually the same but in some cases, such as radio source
positions where no proper motion information is determinable, they may
be the position at 1975.0 (epoch=1975.0) referred to the 1950.0 coordinate
system (equinox=B1950.0) or similar combination.
2.2.6 E-terms (FK4)
The elliptic motion of the earth in its orbit is
responsible for the small components (less than 0.35 arcseconds) of annual
aberration called elliptic aberration or e-terms. These terms are approximately
constant for a particular star and before 1976 the published mean place
was augmented by these elliptic terms and the conventional correction for
annual aberration included only terms for circular motion. Thus all published
FK4 catalogue positions are affected by elliptic aberration.
Star positions in the FK5/J2000 system do not include
the elliptic aberration terms and the correction for annual aberration
now employs the total velocity of the earth with respect to the barycentre.
The telescope system takes this difference into account
rigorously by removing the e-terms from FK4 coordinates before attempting
to correct for space motion or precession to a different epoch. It must
be noted that, when the Tracking Equinox is set to a FK4 system, the appropriate
e-terms are not added back in. Thus any FK4 tracking coordinates read from
the display are "true" mean places unaffected by aberration of any kind
and may differ from the equivalent catalogue mean place by an error not
exceeding 0.343 arcseconds. Only the displayed FK4 coordinates are affected;
the eventual conversion of FK4 positions to the J2000 system and thence
apparent place is accurate to a few milliarcseconds.
2.2.7 Transformation between FK4 and FK5
The rigorous conversion of positions from the FK4
system to the FK5 system requires:
(i) removal of the e-terms
(ii) precession within the FK4 system to B1950.0
(iii) transformation from B1950.0 to J2000.0 using
a 6x6 matrix to account for the relative motion between the two frames
and its effect on space motion, and finally
(iv) precession to the required FK5 equinox and equator.
The transformation from B1950.0 to J2000.0 and (from
J2000.0 to B1950.0) is given in "recipe" form in section B of the Astronomical
Almanac and discussed in detail in:
Standish E. M., (1982) Conversion
of Positions and Proper Motions from B1950.0 to the IAU System at J2000.0
Astronomy and Astrophysics 115, 20-22, 1982.
Aoki S., Sôma H., Kinoshita H.,
Inoue K. (1983) Conversion matrix of epoch B1950.0 FK4-based positions
of stars to epoch J2000.0 positions in accordance with the new IAU resolutions
Astronomy and Astrophysics 128, 263-267, 1983.
These transformations are implemented for the convenience
of observers as TELLIB library routines TEL_B1950_to_J2000 and TEL_J2000_to_B1950.
The telescope control system does not call these routines whilst tracking
because of time constraints but instead employs simplified versions of
the transformations. Nevertheless, the pointing errors so caused are only
a few hundreths of an arcsecond.
2.3 COORDINATE FILES
Coordinate Files are text files which contain the
celestial coordinates of objects to be observed and are an important aid
to observational efficiency. They have the default file type ".COORD" and
can be edited, listed or printed just like any other text file.
A Coordinate File is selected for use by the command:
TRACK/FILE=coordfilespec
and the file specification can, as usual, be abbreviated
to just the filename if the file is of type ".COORD" and resides in the
current default directory MSO_USER:[yourname.OBSERVE].
The system responds with a message showing the full
file specification of the Coordinate File selected.
Two system files may be of interest to observers.
The file TEL_DEFAULT:CATALOGUE_J2000 contains 438
stars whose positions and proper motions are known to be accurate for use
in pointing calibration tests. They are evenly distributed on the sky and
are all about 4th magnitude.
The current position of the telescope may be saved
using one of the commands:
TRACK/SAVE=TRACKING objectname
TRACK/SAVE=BASE objectname
These cause the Tracking Coordinates (current position
of the telescope) or the Base Coordinates (the coordinates of the currently
selected object) to be appended as a new record to the currently selected
Coordinate File; see the description of the TRACK command in §1.3.
2.3.1 Format of Coordinate Files
Coordinate Files consist of coordinate records,
auxiliary data records, and comment records as described below.
Any number of Coordinate records may be present.
Each contains the equatorial polar coordinates of one object and must comply
with the format described in §2.1 above. An absolute minimum specification
is the RA and Declination of the object; if no equinox is specified the
current value of the configuration variable Default_File_Equinox is used.
When all fields are present the format appears thus:
"object name" ad
equinox(epoch) mamdp
Vr !comment
Note that the object name may appear any where in
the record but must be enclosed in double quotation marks. See §2.1
for an itemized description of the format.
2.3.2 Comment Records
An entire record may be used for the purpose of
comment or annotation by using an exclamation mark (!) as the first character.
Any number of these lines may occur in any position in the file and they
are completely ignored by the system.
2.3.3 Auxiliary Data Records
Auxiliary Data records can be used to include in
a Coordinate File information for an instrument or user program. Auxiliary
Data records are identified by a sharp character (#) in the first character
position, are up to 80 characters in length and there may be any number
in the file. They are not used in any way by the telescope control system
but are made available to observer or an external program in the following
manner: Any Auxiliary Data records immediately following a Coordinate record
are assumed to be associated with that object; the first four such records
for the current object are stored within the telescope system so that user
programs which call TELLIB routines may access them. See the MSSSO Telescopes
User Programming Manual for more details.