Each co-ordinate Frame has several attributes that determine its properties. This section lists the most important of these. See SUN/210 for a complete description of the properties of Frames and their attributes. The application WCSATTRIB can be used to examine attribute values associated with the current Frame of an NDF, and change the values of those that are not read-only.
Note that the Digits value acts only as a means of determining a default value for the Format attribute. Its effects are overridden if a Format string is set explicitly for an axis.
The default Digits value for a Frame is 7
. If a value fewer than 1 is supplied, then 1
is used instead.
If the Frame is actually a SkyFrame (e.g. describes celestial longitude and latitude), then the Digits value specifies the total number of digits in the formatted axis value–-that is, the sum of the hours (or degrees), minutes and seconds digits.
Type:
m51
. This results in axis
values being formatted with four digits of precision when they are displayed by any
application, so long as no value has been set for the Format attribute. The Domain attribute also controls how Frames align with each other. If the Domain value in a Frame is set, then only Frames with the same Domain value can be aligned with it.
Some Frames are given standard Domain values when they are created (e.g. GRID, FRACTION, PIXEL, AXIS, SKY, SPECTRUM, CURPIC, NDC, BASEPIC, GRAPHICS). Frames created by the user (for instance, using WCSADD) can have any Domain value, but the standard Domain names should be avoided unless the standard meanings are appropriate for the Frame being created.
Type:
m51
is the PIXEL Frame, then this command takes a
‘snap-shot’ of the PIXEL Frame and stores it as a new Frame with Domain OLDPIXEL in the
WCS component. Subsequent changes to the PIXEL Frame (for instance, produced by
applications that rotate, or move the contents of the NDF) will not effect the OLDPIXEL
Frame, which will thus provide a ‘frozen’ record of the original PIXEL Frame. All Domain values are converted to uppercase and white space is removed before use.
"
observed"
sideband, and the other sideband
as the "
image"
sideband.
The value is accessed as a position in the spectral system specified by the System
attribute, but is stored internally as topocentric frequency. Thus, if the System
attribute of the DSBSpecFrame is set to "
VRAD"
, the Unit attribute set to "
m/s"
and
the StdOfRest attribute set to "
LSRK"
, then values for the DSBCentre attribute should
be supplied as radio velocity in units of "
m/s"
relative to the kinematic LSR
(alternative units may be used by appending a suitable units string to the end of the
value). This value is then converted to topocentric frequency and stored. If (say) the
Unit attribute is subsequently changed to "
km/s"
before retrieving the current
value of the DSBCentre attribute, the stored topocentric frequency will be
converted back to LSRK radio velocity, this time in units of "
km/s"
, before being
returned.
The default value for this attribute is 30 GHz.
The Epoch value is important in cases where the co-ordinate system changes with time. For instance, when considering celestial co-ordinate systems, possible reasons for change include: (i) changing aberration of light caused by the observer’s velocity (e.g. due to the Earth’s motion around the Sun), (ii) changing gravitational deflection by the Sun due to changes in the observer’s position with time, (iii) fictitious motion due to rotation of non-inertial co-ordinate systems (e.g. the old FK4 system), and (iv) proper motion of the source itself (although this last effect is not handled by the SkyFrame class because it affects individual sources rather than the co-ordinate system as a whole).
The Epoch attribute is stored as a Modified Julian Date, and is not usually changed.
Care must be taken to distinguish the Epoch value, which relates to motion (or apparent motion) of the source, from the superficially similar Equinox value. The latter is used to qualify a co-ordinate system which is itself in motion in a (notionally) predictable way as a result of being referred to a slowly moving reference plane (e.g. the equator).
See the description of the System attribute for details of which qualifying attributes apply to each celestial co-ordinate system.
Besselian Epoch: Expressed in decimal years, with or without decimal places ("B1950"
or
"B1976.13"
for example).
Julian Epoch: Expressed in decimal years, with or without decimal places ("J2000"
or
"J2100.9"
for example).
Year: Decimal years, with or without decimal places ("1996.8"
for example). Such values
are interpreted as a Besselian epoch (see above) if less than 1984.0 and as a Julian
epoch otherwise.
Julian Date: With or without decimal places ("JD 2454321.9"
for example).
Modified Julian Date: With or without decimal places ("MJD 54321.4"
for example).
Gregorian Calendar Date: With the month expressed either as an integer or a 3-character
abbreviation, and with optional decimal places to represent a fraction of a day
("1996-10-2"
or "1996-Oct-2.6"
for example). If no fractional part of a day is given,
the time refers to the start of the day (zero hours).
Gregorian Date and Time: Any calendar date (as above) but with a fraction of a day
expressed as hours, minutes and seconds ("1996-Oct-2 12:13:56.985"
for example).
When enquiring Epoch values, the format used is the “Year” format described under “Input Formats”. This is a value in decimal years, which will be a Besselian epoch if less than 1984.0, and a Julian epoch otherwise.
Both of these planes are in motion and their positions are difficult to specify precisely. In practice, therefore, a model ecliptic and/or equator are used instead. These, together with the point on the sky that defines the co-ordinate origin (the intersection of the two planes termed the ‘mean equinox’) move with time according to some models that remove the more-rapid fluctuations. The SkyFrame class supports both the old FK4 and the current FK5 models.
The position of a fixed source expressed in any of these co-ordinate systems will appear to change with time due to movement of the co-ordinate system itself (rather than motion of the source). Such co-ordinate systems must therefore be qualified by a moment in time (the ‘epoch of the mean equinox’ or ‘equinox’ for short) which allows the position of the model co-ordinate system on the sky to be determined. This is the rôle of the Equinox attribute.
The Equinox attribute is stored as a Modified Julian Date, but when setting or getting its value you may use the same formats as for the Epoch attribute (q.v.).
Type:
Care must be taken to distinguish the Equinox value, which relates to the definition of a time-dependent co-ordinate system (based on solar-system reference planes which are in motion), from the superficially similar Epoch value. The latter is used to qualify co-ordinate systems where the positions of sources change with time (or appear to do so) for a variety of other reasons, such as aberration of light caused by the observer’s motion, etc.
See the description of the System attribute for details of which qualifying attributes apply to each celestial co-ordinate system.
If no Format value is set for a Frame axis, a default value is supplied instead. This is based on the value of the Digits, or Digits(axis) attribute and is chosen so that it displays the requested number of digits of precision.
The interpretation of this string depends on whether or not the Frame is a SkyFrame. If
it is not, the string is interpreted as a format-specification string to be passed to
the C “printf
” function (e.g. "%1.7G"
) in order to format a single co-ordinate value
(supplied as a double-precision number).
For SkyFrames, the syntax and default value of the Format string is re-defined to allow the formatting of sexagesimal values as appropriate for the particular celestial co-ordinate system being represented. The syntax of SkyFrame Format strings is described (below) in the “SkyFrame Formats” section.
Type:
"+"
:
Indicates that a plus sign should be prefixed to positive values. By default, no plus
sign is used.
"z"
: Indicates that leading zeros should be prefixed to the value so that the first
field is of constant width, as would be required in a fixed-width table (leading zeros
are always prefixed to any fields that follow). By default, no leading zeros are
added.
"i"
: Use the standard ISO field separator (a colon) between fields. This is the default
behaviour.
"b"
: Use a blank to separate fields.
"l"
: Use a letter ("h"
/"d"
, "m"
or "s"
as appropriate) to separate fields.
"g"
: This is the same as "l"
, except that the separator characters are displayed as
small superscripts when drawn on a graphical device.
"d"
: Include a degrees field. Expressing the angle purely in degrees is also the
default if none of "h"
, "m"
, "s"
or "t"
are given.
"h"
: Express the angle as a time and include an hours field (where 24 hours correspond
to 360 degrees). Expressing the angle purely in hours is also the default if "t"
is
given without either "m"
or "s"
.
"m"
: Include a minutes field. By default this is not included.
"s"
: Include a seconds field. By default this is not included. This request is ignored
if "d"
or "h"
is given, unless a minutes field is also included.
"t"
: Express the angle as a time (where 24 hours correspond to 360 degrees). This
option is ignored if either "d"
or "h"
is given and is intended for use where the value
is to be expressed purely in minutes and/or seconds of time (with no hours field). If
"t"
is given without "d"
, "h"
, "m"
or "s"
being present, then it is equivalent to
"h"
.
"."
: Indicates that decimal places are to be given for the final field in the formatted
string (whichever field this is). The "."
should be followed immediately by an unsigned
integer which gives the number of decimal places required. By default, no decimal
places are given.
All of the above format specifiers are case-insensitive. If several characters make
conflicting requests (e.g. if both "i"
and "b"
appear), then the character occurring
last takes precedence, except that "d"
and "h"
always override "t"
.
my_data
, so that axis values are
formatted as floating-point values using a minimum field width of ten characters, and
displaying five significant figures. An exponent is used if necessary. ngc5128
, so that axis values are
formatted as separate degrees, minutes and seconds field, separated by blanks. The
seconds field has two decimal places. This assumes the current co-ordinate Frame
in the NDF is a celestial co-ordinate Frame (i.e. a SkyFrame). When specifying this attribute by name, it should be subscripted with the number of the Frame axis to which it applies.
"
centre frequency"
is the topocentric
frequency in Hz corresponding to the current value of the DSBCentre attribute.
The value of the IF attribute may be positive or negative: a positive value
results in the LO frequency being above the central frequency, whilst a negative
aattIF value results in the LO frequency being below the central frequency. The
sign of the IF attribute value determines the default value for the SideBand
attribute.
When setting a new value for this attribute, the units in which the frequency value is
supplied may be indicated by appending a suitable string to the end of the formatted
value. If the units are not specified, then the supplied value is assumed to be in
units of GHz. For instance, the following strings all result in an IF of 4 GHz being
used: "
4.0"
, "
4.0 GHz"
, "
4.0E9 Hz"
, etc.
When getting the value of this attribute, the returned value is always in units of GHz. The default value for this attribute is 4 GHz.
The value is calculated by first transforming the rest frequency (given by the RestFreq
attribute) from the standard of rest of the source (given by the SourceVel and
SourceVRF attributes) to the standard of rest of the observer (i.e. the topocentric
standard of rest). The resulting topocentric frequency is assumed to be in the same
sideband as the value given for the DSBCentre attribute (the "
observed"
sideband), and
is transformed to the other sideband (the "
image"
sideband). The new frequency is
converted back to the standard of rest of the source, and the resulting value is
returned as the attribute value, in units of GHz.
If a Label value has not been set for a Frame axis, then a suitable default is supplied, depending on whether or not the Frame is a SkyFrame.
The default for simple Frames is the string "Axis <
, where
n
>"<
is
>1
, 2
, etc. for each successive axis.
The default labels for specialised Frames (SkyFrames, SpecFrames, etc.) depend on the
particular co-ordinate system represented by the Frame (e.g. "Right ascension"
,
"Galactic latitude"
, "Frequency"
, "Wavelength in air"
, etc.).
Type:
my_data
, to the string "IRAS
data (marked in white)"
. Axis labels are intended purely for interpretation by human readers and not by software.
When specifying this attribute by name, it should be subscripted with the number of the Frame axis to which it applies.
my_data
. The value is stored internally in radians, but is converted to and from a degrees
string for access. Some example input formats are: "22:19:23.2"
, "22 19 23.2"
,
"22:19.387"
, "22.32311"
, "N22.32311"
, "-45.6"
, "S45.6"
. As indicated, the sign of the
latitude can optionally be indicated using characters "N"
and "S"
in place of the usual
"+"
and
"-"
. When converting the stored value to a string, the format "
[s]dd:mm:ss.s"
is used,
when "[s]"
is "N"
or "S"
.
The value is stored internally in radians, but is converted to and from a degrees string
for access. Some example input formats are: "155:19:23.2"
, "155 19 23.2"
, "155:19.387"
,
"155.32311"
, "E155.32311"
, "-204.67689"
, "W204.67689"
. As indicated, the sign of the
longitude can optionally be indicated using characters "E"
and "W"
in place of the usual
"+"
and
"-"
. When converting the stored value to a string, the format "
[s]ddd:mm:ss.s"
is used,
when "[s]"
is "E"
or "W"
and the numerical value is chosen to be less than 180 degrees.
The RefRA and RefDec attributes are stored internally in radians, but are converted to
and from a string for access. The format "
hh:mm:ss.ss"
is used for RefRA, and
"
dd:mm:ss.s"
is used for RefDec.
RefRA has a default value of zero.
When setting a new value for this attribute, the new value can be supplied either
directly as a frequency, or indirectly as a wavelength or energy, in which case the
supplied value is converted to a frequency before being stored. The nature of the
supplied value is indicated by appending text to the end of the numerical value
indicating the units in which the value is supplied. If the units are not specified,
then the supplied value is assumed to be a frequency in units of GHz. If the supplied
unit is a unit of frequency, the supplied value is assumed to be a frequency in the
given units. If the supplied unit is a unit of length, the supplied value is assumed to
be a (vacuum) wavelength. If the supplied unit is a unit of energy, the supplied value
is assumed to be an energy. For instance, the following strings all result in a
rest frequency of around 1.4E14 Hz being used: "1.4E5"
, "1.4E14 Hz"
, "1.4E14
s∗∗-1"
,
"1.4E5 GHz"
, "2.14E-6 m"
, "21400 Angstrom"
, "9.28E-20 J"
, "9.28E-13 erg"
, "0.58 eV"
,
etc.
When getting the value of this attribute, the returned value is always a frequency in units of GHz.
"
usb"
(for upper sideband), "
lsb"
(for lower sideband),
or "
lo"
(for offset from the local oscillator frequency). When setting a new
value, any of the strings "
lsb"
, "
usb"
, "
observed"
, "
image"
or "
lo"
may be
supplied (case insensitive). The "
observed"
sideband is which ever sideband
(upper or lower) contains the central spectral position given by attribute
DSBCentre, and the "
image"
sideband is the other sideband. It is the sign of the
IF attribute which determines if the observed sideband is the upper or lower
sideband. The default value for SideBand is the observed sideband. This attribute (together with SourceVRF, RefRA, and RefDec) defines the ‘Source’ standard of rest (see attribute StdOfRest). This is a rest frame that is moving towards the position given by RefRA and RefDec at a velocity given by SourceVel (in km/s). When setting a value for SourceVel using WCSATTRIB, the velocity should be supplied in the rest frame specified by the SourceVRF attribute. Likewise, when getting the value of SourceVel, it will be returned in the rest frame specified by the SourceVRF attribute.
The default value is zero.
Type:
If the value of SourceVRF is changed, the value stored for SourceVel will be converted from the old to the new rest frame.
The values that can be supplied are the same as for the StdOfRest attribute (except
that SourceVRF cannot be set to "Source"
). The default value is "Helio"
.
The default StdOfRest value is "Helio"
.
"Topocentric"
, "Topocent"
or "Topo"
: The observers rest-frame (assumed to be on the
surface of the earth). Spectra recorded in this standard of rest suffer a Doppler shift
which varies over the course of a day because of the rotation of the observer around
the axis of the earth. This standard of rest must be qualified using the ObsLat,
ObsLon, Epoch, RefRA, and RefDec attributes.
"Geocentric"
, "Geocentr"
or "Geo"
: The rest-frame of the earth centre. Spectra recorded
in this standard of rest suffer a Doppler shift which varies over the course of a year
because of the rotation of the earth around the Sun. This standard of rest must be
qualified using the Epoch, RefRA, and RefDec attributes.
"Barycentric"
, "Barycent"
or "Bary"
: The rest-frame of the solar-system barycentre.
Spectra recorded in this standard of rest suffer a Doppler shift which depends both on
the velocity of the Sun through the Local Standard of Rest, and on the movement of the
planets through the solar system. This standard of rest must be qualified using the
Epoch, RefRA, and RefDec attributes.
"Heliocentric"
, "Heliocen"
or "Helio"
: The rest-frame of the Sun. Spectra recorded in
this standard of rest suffer a Doppler shift which depends on the velocity of the Sun
through the Local Standard of Rest. This standard of rest must be qualified using the
RefRA and RefDec attributes.
"LSR"
, "LSRK"
: The rest-frame of the kinematical Local Standard of Rest. Spectra
recorded in this standard of rest suffer a Doppler shift which depends on the velocity
of the kinematical Local Standard of Rest through the galaxy. This standard of rest
must be qualified using the RefRA and RefDec attributes.
"LSRD"
: The rest-frame of the dynamical Local Standard of Rest. Spectra recorded in
this standard of rest suffer a Doppler shift which depends on the velocity of the
dynamical Local Standard of Rest through the galaxy. This standard of rest must be
qualified using the RefRA and RefDec attributes.
"Galactic"
, "Galactoc"
or "Gal"
: The rest-frame of the galactic centre. Spectra
recorded in this standard of rest suffer a Doppler shift which depends on the velocity
of the galactic centre through the local group. This standard of rest must be qualified
using the RefRA and RefDec attributes.
"Local_group"
, "Localgrp"
or "LG"
: The rest-frame of the local group. This standard of
rest must be qualified using the RefRA and RefDec attributes.
"Source"
, or "src"
: The rest-frame of the source. This standard of rest must be
qualified using the RefRA, RefDec, and SourceVel attributes.
Where more than one alternative System value is shown above, the first of these will be returned when an enquiry is made.
"RA"
and "Dec"
(for right ascension and
declination).
If a Symbol value has not been set for a Frame axis, then a suitable default is supplied.
The default Symbol value supplied for simple Frames is the string
“<Domain
><n
>”,
where <n
>
is 1
, 2
, etc. for successive axes, and
<Domain>
is the value of the Frame’s Domain attribute (truncated if necessary so that the final
string does not exceed 15 characters). If no Domain value has been set, "x"
is used as the
<Domain>
value in constructing this default string.
Specialised Frames (SkyFrame, SpecFrame,etc.) re-define the default Symbol value to be appropriate for the particular co-ordinate system being represented.
Type:
my_data
, to the string "AR"
. When specifying this attribute by name, it should be subscripted with the number of the Frame axis to which it applies.
"Cartesian"
, and may not be altered. "FK4"
: The old FK4 (barycentric) equatorial co-ordinate system, which should be
qualified by an Equinox value. The underlying model on which this is based is
non-inertial and rotates slowly with time, so for accurate work FK4 co-ordinate systems
should also be qualified by an Epoch value.
"FK4-NO-E"
or "FK4_NO_E"
: The old FK4 (barycentric) equatorial system but without the
E-terms of aberration (e.g. some radio catalogues). This co-ordinate system should
also be qualified by both an Equinox and an Epoch value.
"FK5"
or "EQUATORIAL"
: The modern FK5 (barycentric) equatorial co-ordinate system. This
should be qualified by an Equinox value.
"GAPPT"
, "GEOCENTRIC"
or "APPARENT"
: The geocentric apparent equatorial co-ordinate
system, which gives the apparent positions of sources relative to the true
plane of the Earth’s equator and the equinox (the co-ordinate origin) at a time
specified by the qualifying Epoch value. (Note that no Equinox is needed to qualify
this co-ordinate system because no model ‘mean equinox’ is involved.) These
co-ordinates give the apparent right ascension and declination of a source for a
specified date of observation, and therefore form an approximate basis for
pointing a telescope. Note, however, that they are applicable to a fictitious
observer at the Earth’s centre, and therefore ignore such effects as atmospheric
refraction and the (normally much smaller) aberration of light due to the rotational
velocity of the Earth’s surface. Geocentric apparent co-ordinates are derived from
the standard FK5 (J2000.0) barycentric co-ordinates by taking account of the
gravitational deflection of light by the Sun (usually small), the aberration of
light caused by the motion of the Earth’s centre with respect to the barycentre
(larger), and the precession and nutation of the Earth’s spin axis (normally larger
still).
"ECLIPTIC"
: Ecliptic co-ordinates (IAU 1980), referred to the ecliptic and mean equinox
specified by the qualifying Equinox value.
"GALACTIC"
: Galactic co-ordinates (IAU 1958).
"SUPERGALACTIC"
: De Vaucouleurs Supergalactic co-ordinates.
"UNKNOWN"
: Any other general spherical co-ordinate system. No Mapping can be
created between a pair of SkyFrames if either of the SkyFrames has System set to
"Unknown"
.
Currently, the default System value is "FK5"
.
"WAVE"
–-wavelength):
"FREQ"
: Frequency (Hz)
"ENER"
or "ENERGY"
: Energy (J)
"WAVN"
or "WAVENUM"
: Wave-number (1/m)
"WAVE"
or "WAVELEN"
: Vacuum wavelength (m)
"AWAV"
or "AIRWAVE"
: Wave-length in air (m)
"VRAD"
or "VRADIO"
: Radio velocity (m/s)
"VOPT"
or "VOPTICAL"
: Optical velocity (m/s)
"ZOPT"
or "REDSHIFT"
: Redshift (dimensionless)
"BETA"
: Beta factor (dimensionless)
"VELO"
or "VREL"
: Relativistic velocity (m/s)
The default value for the Unit attribute for each system is shown in parentheses. Note, changes to the Unit attribute for a SpecFrame will result in the Mapping from pixel to spectral co-ordinates being modified in order to reflect the change in units.
TimeFrame
"
MJD"
):
"
MJD"
: Modified Julian Date (d)
"
JD"
: Julian Date (d)
"
JEPOCH"
: Julian epoch (yr)
"
BEPOCH"
: Besselian (yr)
The default value for the Unit attribute for each system is shown in parentheses.
Strictly, these systems should not allow changes to be made to the units. For instance,
the usual definition of "
MJD"
and "
JD"
include the statement that the values will be in
units of days. However, AST does allow the use of other units with all the above
supported systems (except BEPOCH), on the understanding that conversion to the
"
correct"
units involves nothing more than a simple scaling (1 yr = 365.25 d, 1 d = 24
h, 1 h = 60 min, 1 min = 60 s). Besselian epoch values are defined in terms of
tropical years of 365.2422 days, rather than the usual Julian year of 365.25
days. Therefore, to avoid any confusion, the Unit attribute is automatically
cleared to "
yr"
when a System value of BEPOCH System is selected, and an error is
reported if any attempt is subsequently made to change the Unit attribute.
Besselian Epoch: Expressed in decimal years, with or without decimal places ("
B1950"
or
"
B1976.13"
for example).
Julian Epoch: Expressed in decimal years, with or without decimal places ("
J2000"
or
"
J2100.9"
for example).
Units: An unqualified decimal value is interpreted as a value in the system specified
by the TimeFrame’s System attribute, in the units given by the TimeFrame’s Unit
attribute. Alternatively, an appropriate unit string can be appended to the end of the
floating point value ("
123.4 d"
for example), in which case the supplied value is
scaled into the units specified by the Unit attribute.
Julian Date: With or without decimal places ("
JD 2454321.9"
for example).
Modified Julian Date: With or without decimal places ("
MJD 54321.4"
for example).
Gregorian Calendar Date: With the month expressed either as an integer or a
three-character abbreviation, and with optional decimal places to represent a fraction
of a day ("
1996-10-2"
or "
1996-Oct-2.6"
for example). If no fractional part of a day is
given, the time refers to the start of the day (zero hours).
Gregorian Date and Time: Any calendar date (as above) but with a fraction of a day
expressed as hours, minutes and seconds ("
1996-Oct-2 12:13:56.985"
for example). The
date and time can be separated by a space or by a "
T"
(as used by ISO8601 format).
"
Time Scales"
section (below).
The default TimeScale value depends on the current System value; if the current
TimeFrame system is "
Besselian epoch"
the default is "
TT"
, otherwise it is "
TAI"
. Note,
if the System attribute is set so that the TimeFrame represents Besselian Epoch, then
an error will be reported if an attempt is made to set the TimeScale to anything other
than TT.
Note, the supported time scales fall into two groups. The first group containing UT1, GMST, LAST and LMST define time in terms of the orientation of the earth. The second group (containing all the remaining time scales) define time in terms of an atomic process. Since the rate of rotation of the earth varies in an unpredictable way, conversion between two timescales in different groups relies on a value being supplied for the Dut1 attribute. This attribute specifies the difference between the UT1 and UTC time scales, in seconds, and defaults to zero. See the documentation for the Dut1 attribute in SUN/210 for further details.
"TAI"
– International Atomic Time
"UTC"
– Coordinated Universal Time
"UT1"
– Universal Time
"GMST"
– Greenwich Mean Sidereal Time
"LAST"
– Local Apparent Sidereal Time
"LMST"
– Local Mean Sidereal Time
"TT"
– Terrestrial Time
"TDB"
– Barycentric Dynamical Time
"TCB"
– Barycentric Coordinate Time
"TCG"
– Geocentric Coordinate Time
"LT"
– Local Time (the offset from UTC is given by attribute LTOffset)
An very informative description of these and other time scales is available at
http://www.ucolick.org/∼sla/leapsecs/timescales.htm
.
"Detector Co-ordinates"
or "Galactic
Co-ordinates"
.
If a Title value has not been set for a Frame, then a suitable default is supplied.
The default supplied by the Frame class is "<n>-d co-ordinate system"
, where <n>
is the
number of Frame axes (Naxes attribute).
Specialised Frames (SkyFrame, SpecFrame, etc.) re-define the default Title value to be appropriate to the particular co-ordinate system being represented.
my_data
, to the string "My own data"
. A Frame’s Title is intended purely for interpretation by human readers and not by software.
Specialised Frames (SkyFrame, SpecFrame, etc.) re-define the default Unit values to be appropriate to the particular co-ordinate system being represented.
For most classes, the Unit attribute is a purely descriptive comment intended for human readers and makes no difference to the operation of the software. However, there are some classes that have active Unit attributes. Changing the Unit attribute for such classes will result in the Mappings within the WCS FrameSet being modified in order to reflect the change in units. By default, only SpecFrames have an active Unit attribute.
In general, the syntax of the Unit attribute should follow the recommendations made in
the FITS standard (see the paper “Representation of world coordinates in FITS” by
Greisen & Calabretta (available at http://www.cv.nrao.edu/fits/documents/wcs/wcs.html
).
When specifying this attribute by name, it should be subscripted with the number of the Frame axis to which it applies (unless the Frame has only one axis).