Connected groups of pixels are accepted as objects if they have more than a minimum number of pixels. Such groups may be rejected if they contact the edges of the data array.
Threshold estimation is performed using either a percentage data point (i.e. the value for which this percentage of pixels have a lower value) or by using a standard deviation and background value determined by fitting a gaussian to the data histogram.
If the logging system has been initialised using CCDSETUP, then the value specified there will be
used. Otherwise, the default is
TERMINAL – Send output to the terminal only
LOGFILE – Send output to the logfile only (see the LOGFILE parameter)
BOTH – Send output to both the terminal and the logfile
NEITHER – Produce no output at all
If the logging system has been initialised using CCDSETUP then the value specified there will be
used. Otherwise, the default is
These may be specified as list of comma separated names, using indirection if required, OR, as a single
modification element (of the input image names). The simplest modification element is the asterisk
which means call each of the output lists the same name as the corresponding input images.
signifies that all the images in the current directory should be used and the output lists should have the same names.
Other types of modification can also occur, such as:
which means call the position lists the same as the input images but put
" after the
names. Replacement of a specified string with another in the output file names can also be
this replaces the string
" in any of the output names.
If wildcarded names for the input images are used then is it recommended that wildcards are also used for the position list names (the order of input names is not guaranteed).
The output files contain a integer index for each image feature followed by the X and Y centroid (formed using all the intensity information) and finally the mean intensity of pixels in the group. [.DAT]
The algorithm used for calculating the values of percentiles for threshold determination should give good results even in the presence of pixel values which lie very far away from the bulk of the data. However, the sampling of the histogram used to estimate the mode and standard deviation may be poor in the presence of extreme outliers. If there are extreme outliers therefore, the percentile method (USEPER set to TRUE) of determining the threshold should be used.
The histogram used by FINDOBJ when USEPER is FALSE is formed by (if necessary) re-binning until the BINFRAC criterion is met, it is expected that this will always result in a well sampled histogram. The background value is the mode of this histogram and is not refined during the gaussian fitting. The gaussian fitting just estimates the standard deviation of the background and uses a fixed peak value and position (the mode of the histogram) and iterates rejecting bins whose counts fall below 20 percent of the peak value, stopping when either 3 iterations have been performed or the standard deviation does not change by more than one bin width in data values.
FINDOBJ is optimised to determine a reliable detection threshold and is not concerned with the accurate determination of the background value on a frame (as it performs no photometric measurements). For this reason the histogram which it uses to determine the background value is made in such a way that it is usually very well sampled (probably oversampled, for most other purposes). FINDOBJ should not be used in a manner for which it is not suited without understanding how if differs from other more specialized routines.
NDF extension items.
On exit the CURRENT_LIST items in the CCDPACK extensions (.MORE.CCDPACK) of the input NDFs are set to the names of the appropriate output lists. These items will be used by other CCDPACK position list processing routines to automatically access the lists.
Output position list format.
CCDPACK format - Position lists in CCDPACK are formatted files whose first three columns are interpreted as the following.
Column 1: an integer identifier
Column 2: the X position
Column 3: the Y position
The column one value must be an integer and is used to identify positions which may have different locations but are to be considered as the same point. Comments may be included in the file using the characters # and !. Columns may be separated by the use of commas or spaces.
In all cases the coordinates in position lists are pixel coordinates.
"value is the value assigned on the last run of the application. If the application has not been run then the
"defaults, as shown in the parameter help, apply. The exceptions to this rule are:
THRESH – dynamic value
Retaining parameter values has the advantage of allowing you to define the default behaviour of the application but does mean that additional care needs to be taken when re-using the application after a break of sometime. The intrinsic default behaviour of the application may be restored by using the RESET keyword on the command line.
Certain parameters (LOGTO and LOGFILE) have global values. These global values will always take precedence, except when an assignment is made on the command line. Global values may be set and reset using the CCDSETUP and CCDCLEAR commands.
This routine correctly processes the DATA and QUALITY components of an NDF data structure. Bad pixels and all non-complex numeric data types can be handled.