The following statistics are displayed:
The total number of pixels within the aperture
The number of good pixels within the aperture
The total data sum within the aperture
The standard deviation on the total data sum (that is, the square root of the sum of the individual pixel variances)
The mean pixel value within the aperture
The standard deviation on the mean pixel value (that is, the standard deviation on the total data sum divided by the number of values)
The standard deviation of the pixel values within the aperture
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.
!) 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.
":"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.
"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.
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
A generic option where
"SPEC" requests the spectral axis,
"TIME" selects the time axis,
"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.
TRUEvalue 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.
outline.dat. The file contains an ARD description of the required aperture. The results are written to the log file
The statistics are not displayed on the screen when the message filter environment variable
MSG_FILTER is set to
QUIET. The creation of output parameters and the log file is unaffected by