CATFRAME = LITERAL (Read)
A string determining the
in which positions are to be stored in the output catalogue associated
with Parameter OUTCAT. The string supplied for CATFRAME can be one of the following
A domain name such as SKY, AXIS, PIXEL.
An integer value giving the index of the required Frame.
An IRAS90 Sky Co-ordinate System (SCS) values such as
"EQUAT(J2000)" (see SUN/163).
If a null (
!) value is supplied, the positions will be stored in the current Frame.
The epoch at which the sky positions stored in the output catalogue were
determined. It will only be accessed if an epoch value is needed to qualify the co-ordinate Frame
specified by COLFRAME. If required, it should be given as a decimal years value, with or without
decimal places (
"1996.8" for example). Such values are interpreted as a Besselian epoch if less than
1984.0 and as a Julian epoch otherwise.
CERROR = _LOGICAL (Read)
TRUE, errors in the
centroided position will be calculated. The input NDF must contain a VARIANCE component in order
to compute errors.
COIN = FILENAME (Read)
Name of a text file containing the initial
guesses at the co-ordinates of features to be centroided. Only accessed if Parameter MODE is given the
. Each line should contain the formatted axis values
for a single position, in the
of the NDF. Axis values can be separated by spaces, tabs or commas. The
file may contain comment lines with the first character
DESCRIBE = _LOGICAL
TRUE, a detailed description of the co-ordinate Frame in which the centroided
positions will be reported is displayed before the positions themselves.
DEVICE = DEVICE (Read)
The graphics device which is to be used to give the initial
guesses at the centroid positions. Only accessed if Parameter MODE is given the value
[Current graphics device
GUESS = _LOGICAL (Read)
TRUE, then the supplied
guesses for the centroid positions will be included in the screen and log file output, together
with the accurate positions.
INCAT = FILENAME (Read)
containing a positions list giving the initial guesses at the centroid positions, such as produced
by applications CURSOR, LISTMAKE. Only accessed if Parameter MODE is given the
INIT = LITERAL (Read)
An initial guess at the co-ordinates of the
next feature to be centroided, in the current co-ordinate Frame of the NDF (supplying a
":" will display details of the current co-ordinate Frame). The position should be
supplied as a list of formatted axis values separated by spaces or commas. INIT is only
accessed if parameter MODE is given the value
"Interface". If the initial co-ordinates are
supplied on the command line only one centroid will be found; otherwise the application
will ask for further guesses, which may be terminated by supplying the null value (
LOGFILE = FILENAME (Read)
Name of the text file to log the results. If null, there
will be no logging. Note this is intended for the human reader and is not intended for
passing to other applications.
MARK = LITERAL (Read)
Only accessed if Parameter
MODE is given the value
. It indicates which positions are to be marked on the
screen using the marker type given by Parameter MARKER. It can take any of the following
"Initial": The position of the cursor when the mouse button is pressed is marked.
"Centroid": The corresponding centroid position is marked.
"None": No positions are marked.
MARKER = _INTEGER (Read)
This parameter is only accessed if Parameter MARK
TRUE. It specifies the type of marker with which each cursor position should be marked, and
should be given as an integer PGPLOT marker type. For instance,
0 gives a box,
1 gives a dot,
3 gives an asterisk,
7 gives a triangle. The value must be larger than or equal to
MAXITER = _INTEGER (Read)
Maximum number of iterations to be used in
the search. It must be in the range 1–9. The dynamic default is 3.
Maximum shift in each dimension allowed between the guess and
output positions in pixels. Each must lie in the range 0.0–26.0. If only a single value is
given, then it will be duplicated to all dimensions. The dynamic default is half of SEARCH
in which the initial co-ordinates are to be obtained. The
supplied string can be one of the following values.
"Interface" — positions are obtained using Parameter INIT.
"Cursor" — positions are obtained using the graphics cursor of the device specified by Parameter
"Catalogue" — positions are obtained from a positions list using Parameter INCAT.
"File" — positions are obtained from a text file using Parameter COIN.
The NDF structure containing the data array to be analysed. In cursor mode (see
Parameter MODE), the run-time default is the displayed data, as recorded in the graphics
database. In other modes, there is no run-time default and the user must supply a value.
NSIM = _INTEGER (Read)
The number of simulations or realisations using the variance
information in order to estimate the error in the centroid position. The uncertainty in the centroid
error decreases as one over the square root of NSIM. The range of acceptable values is
OUTCAT = FILENAME (Write)
The output catalogue in which to store the
centroided positions. If a null value (
!) is supplied, no output catalogue is produced. See also
PLOTSTYLE = GROUP (Read)
A group of attribute settings
describing the style to use when drawing the graphics markers specified by Parameter
A comma-separated list of strings should be given in which each string is either
an attribute setting, or the name of a text file preceded by an up-arrow character
Such text files should contain further comma-separated lists which will be read and interpreted in the
same manner. Attribute settings are applied in the order in which they occur within the list, with later
settings overriding any earlier settings given for the same attribute.
Each individual attribute setting should be of the form:
is the name of a plotting attribute, and
is the value to assign to the attribute. Default values will be used for any unspecified attributes. All
attributes will be defaulted if a null value (
!)—the initial default—is supplied. To apply changes of
style to only the current invocation, begin these attributes with a plus sign. A mixture of persistent
and temporary style changes is achieved by listing all the persistent attributes followed by a plus sign
then the list of temporary attributes.
See Section E for a description of the available attributes. Any unrecognised attributes are ignored (no
error is reported).
POSITIVE = _LOGICAL (Read)
TRUE, if array features are
positive above the background.
SEARCH() = _INTEGER (Read)
Size in pixels of the search
box to be used. If only a single value is given, then it will be duplicated to all dimensions so that a
square, cube or hypercube region is searched. Each value must be odd and lie in the range 3–51.
TITLE = LITERAL (Read)
A title to store with the output catalogue specified by Parameter OUTCAT,
and to display before the centroid positions are listed. If a null (
!) value is supplied, the
title is taken from any input catalogue specified by Parameter INCAT, or is a fixed string
including the name of the NDF.
TOLER = _REAL (Read)
Accuracy in pixels required in
centroiding. Iterations will stop when the shift between successive centroid positions is less than
the accuracy. The accuracy must lie in the range 0.0–2.0.
Each centroid position is obtained by projecting the data values
within a search box centred on the supplied position, on to each axis in turn. This forms a set of
profiles for the feature, one for each axis. An estimate of the background at each point in these profiles
is made and subtracted from the profile. This flattens the profile backgrounds, removing any slope in
the data. Once the profiles have been flattened in this way, and estimate of the background noise in
each is made. The centroid of the feature is then found using only the data above the noise
Successive estimates of the centroid position are made by using the previous estimate of the
centroid as the initial position for another estimation. This loop is repeated up to a maximum
number of iterations, though it normally terminates when a desired accuracy has been
The achieved accuracy is affected by noise, and the presence of non-Gaussian or overlapping features,
but typically an accuracy better than 0.1 pixel is readily attainable for stars. The error in the centroid
position may be estimated by a Monte-Carlo method using the data variance to generate realisations
of the data about the feature (see Parameter CERROR). Each realisation is processed identically
to the actual data, and statistics are formed to derive the standard deviations.