APERADD

Integrates pixel values within an aperture of an NDF

Description:

This routine displays statistics for pixels that lie within a specified aperture of an NDF . The aperture can either be circular (specified by parameters CENTRE and DIAM), or arbitrary (specified by Parameter ARDFILE). If the aperture is specified using parameters CENTRE and DIAM, then it must be either one- or two-dimensional.

The following statistics are displayed:

If individual pixel variances are not available within the input NDF (i.e. if it has no VARIANCE  component), then each pixel is assumed to have a constant variance equal to the variance of the pixel values within the aperture. There is an option to weight pixels so that pixels with larger variances are given less weight (see parameter WEIGHT). The statistics are displayed on the screen and written to output parameters. They may also be written to a log file.

A pixel is included if its centre is within the aperture, and is not included otherwise. This simple approach may not be suitable for accurate aperture photometry, especially where the aperture diameter is less than about ten times the pixel size. A specialist photometry package should be used if accuracy, rather than speed, is paramount.

Usage:

aperadd ndf centre diam

Parameters:

ARDFILE = FILENAME (Read)
The name of an ARD file containing a description of the aperture. This allows apertures of almost any shape to be used. If a null (!) value is supplied then the aperture is assumed to be circular with centre and diameter given by parameters CENTRE and DIAM. ARD files can be created either ‘by hand’ using an editor, or using a specialist application such as ARDGEN.

The co-ordinate system in which positions within the ARD file are given should be indicated by including suitable COFRAME or WCS statements within the file (see SUN/183), but will default to pixel co-ordinates in the absence of any such statements. For instance, starting the file with a line containing the text "COFRAME(SKY,System=FK5)" would indicate that positions are specified in RA/DEC (FK5,J2000). The statement "COFRAME(PIXEL)" indicates explicitly that positions are specified in pixel co-ordinates. [!]

CENTRE = LITERAL (Read)
The co-ordinates of the centre of the circular aperture. Only used if Parameter ARDFILE is set to null. The position must be given in the current co-ordinate Frame  of the NDF (supplying a colon ":" 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. See also parameter USEAXIS. The current co-ordinate Frame can be changed using application WCSFRAME.
DIAM = LITERAL (Read)
The diameter of the circular aperture. Only used if parameter ARDFILE is set to null. If the current co-ordinate Frame  of the NDF is a SKY Frame (e.g. RA and DEC), then the value should be supplied as an increment of celestial latitude (e.g. DEC). Thus, "10.2" means 10.2 degrees, "0:30" would mean 30 arcminutes, and "0:0:1" would mean 1 arcsecond. If the current co-ordinate Frame is not a SKY Frame, then the diameter should be specified as an increment along Axis 1 of the current co-ordinate Frame. Thus, if the current Frame is PIXEL, the value should be given simply as a number of pixels.
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. [!]
MASK = NDF (Write)
An output NDF containing the pixel mask used to evaluate the reported statistics. The NDF will contain a positive integer value for pixels that are included in the statistics, and bad values for all other pixels. The pixel bounds of the NDF will be the smallest needed to encompass all used pixels. [!]
MEAN = _DOUBLE (Write)
The mean of the pixel values within the aperture.
NDF = NDF (Read)
The input NDF.
NGOOD = _INTEGER (Write)
The number of good pixels within the aperture.
NUMPIX = _INTEGER (Write)
The total number of pixels within the aperture.
SIGMA = _DOUBLE (Write)
The standard deviation of the pixel values within the aperture.
SIGMEAN = _DOUBLE (Write)
The standard deviation on the mean pixel value. If variances are available this is the RMS value of the standard deviations associated with each included pixel value. If variances are not available, it is the standard deviation of the pixel values divided by the square root of the number of good pixels in the aperture.
SIGTOTAL = _DOUBLE (Write)
The standard deviation on the total data sum. Only created if variances are available this is the RMS value of the standard deviations associated with each included pixel value. If variances are not available, it is the standard deviation of the pixel values divided by the square root of the number of good pixels in the aperture.
TOTAL = _DOUBLE (Write)
The total of the pixel values within the aperture.
USEAXIS = GROUP (Read)
USEAXIS is only accessed if the current co-ordinate Frame of the NDF has too many axes. A group of strings should be supplied specifying the axes which are to be used when specifying the aperture using parameters ARDFILE, CENTRE and DIAM. Each axis can be specified using one of the following options.
  • Its integer index within the current Frame of the input NDF (in the range 1 to the number of axes in the current Frame).

  • Its Symbol  string such as "RA" or "VRAD".

  • A generic option where "SPEC" requests the spectral axis, "TIME" selects the time axis, "SKYLON" and "SKYLAT" picks the sky longitude and latitude axes respectively. Only those axis domains present are available as options.

A list of acceptable values is displayed if an illegal value is supplied. If a null (!) value is supplied, the axes with the same indices as the two used pixel axes within the NDF are used. [!]

WEIGHT = _LOGICAL (Read)
If a TRUE value is supplied, and the input NDF has a VARIANCE component, then pixels with larger variances will be given smaller weight in the statistics. The weight associated with each pixel is proportional to the reciprocal of its variance. The constant of proportionality is chosen so that the mean weight is unity. The pixel value and pixel variance are multiplied by the pixels weight before being used to calculate the statistics. The calculation of the statistics remains unchanged in all other respects. [FALSE]

Examples:

aperadd neb1 "13.5,201.3" 20
This calculates the statistics of the pixels within a circular aperture of NDF neb1. Assuming the current co-ordinate Frame of neb1 is PIXEL, the aperture is centred at pixel co-ordinates (13.5, 201.3) and has a diameter of 20 pixels.
aperadd neb1 "15:23:43.2 -22:23:34.2" "10:0"
This also calculates the statistics of the pixels within a circular aperture of NDF neb1. Assuming the current co-ordinate Frame of neb1 is a SKY Frame describing RA and DEC, the aperture is centred at RA 15:23:43.2 and DEC -22:23:34.2, and has a diameter of 10 arcminutes.
aperadd ndf=neb1 ardfile=outline.dat logfile=obj1
This calculates the statistics of the pixels within an aperture of NDF neb1 described within the file outline.dat. The file contains an ARD description of the required aperture. The results are written to the log file obj1.

Notes:

ASCII-region-definition Descriptors

The ARD file may be created by ARDGEN or written manually. In the latter case consult SUN/183  for full details of the ARD descriptors and syntax; however, much may be learnt from looking at the ARD files created by ARDGEN and the ARDGEN documentation. There is also a in Section 15.1.1.

Related Applications

KAPPA: STATS, MSTATS, ARDGEN, ARDMASK, ARDPLOT, WCSFRAME.

Implementation Status: