Finds the centroids of star-like features in an NDF
Four methods are available for obtaining the initial positions, selected using Parameter MODE:
from the parameter system (see Parameter INIT);
Using a graphics cursor to indicate the feature in a previously displayed data array (see Parameter DEVICE);
from a specified positions list (see Parameter INCAT); or
from a simple text file containing a list of co-ordinates (see Parameter COIN).
In the first two modes the application loops, asking for new feature co-ordinates until it is told to quit or encounters an error.
The results may optionally be written to an output positions list which can be used to pass the positions on to another application (see Parameter OUTCAT), or to a log file geared more towards human readers, including details of the input parameters (see Parameter LOGFILE).
The uncertainty in the centroid positions may be estimated if variance values are available within the supplied NDF (see Parameter CERROR).
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. [!]
"1996.8"
for example). Such values are interpreted as a Besselian epoch if less than
1984.0 and as a Julian epoch otherwise. TRUE
, errors in the
centroided position will be calculated. The input NDF must contain a VARIANCE component in order
to compute errors. [FALSE]
"File"
. Each line should contain the formatted axis values for a single position, in the
current Frame of the NDF. Axis values can be separated by spaces, tabs or commas. The
file may contain comment lines with the first character #
or !
. TRUE
, a detailed description of the co-ordinate Frame in which the centroided
positions will be reported is displayed before the positions themselves. [
current value]
"Cursor"
. [
Current graphics device]
TRUE
, then the supplied
guesses for the centroid positions will be included in the screen and log file output, together
with the accurate positions. [
current value]
"Catalogue"
. ":"
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 (!
).
[!]
"Cursor"
. 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
values.
"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.
[
current value]
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, 2
gives
a cross, 3
gives an asterisk, 7
gives a triangle. The value must be larger than or equal to
−31.
[
current value]
[9]
[9.0]
"Interface"
— positions are obtained using Parameter INIT.
"Cursor"
— positions are obtained using the graphics cursor of the device specified by Parameter
DEVICE.
"Catalogue"
— positions are obtained from a positions list using Parameter INCAT.
"File"
— positions are obtained from a text file using Parameter COIN.
[
current value]
[]
[100]
!
) is supplied, no output catalogue is produced. See also
Parameter CATFRAME. [!]
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:
<name>=<value>
where <name>
is the name of a plotting attribute, and
<value>
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). [
current value]
TRUE
, if array features are
positive above the background. [TRUE]
[9]
!
) 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. [!]
[0.05]
centroids.log
. All positions are supplied and reported in the current co-ordinate Frame of the NDF. A description of
the co-ordinate Frame being used is given if Parameter DESCRIBE is set to a TRUE
value. Application
WCSFRAME can be used to change the current co-ordinate Frame of the NDF before running this
application if required.
In Cursor or Interface mode, only the first 200 supplied positions will be stored in the output catalogue. Any further positions will be displayed on the screen but not stored in the output catalogue.
The centroid positions are not displayed on the screen when the message filter environment variable
MSG_FILTER is set to QUIET
. The creation of output parameters and files is unaffected by
MSG_FILTER.
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 achieved.
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.
The processing of bad pixels and all non-complex numeric data types is supported.