### SKEW

Generates a skewness representation of the image

#### Description:

Performs skewness calculations on an input NDF image file. The resulting skewness image/plot is stored to disk.

Two actions have been taken to reduce the influence of bright objects or cosmic rays:

• the user may elect to employ a cut out pixel count value where any pixel found to be above that value is ignored. The cutout value is determined by the user inputting a global mode value, the background count standard deviation (available via HISTPEAK) and the number of standard deviations above sky level at which the cutout should be.

• a local mean value may be used as the mode.

The user is required to enter the size of the sampling area and the pixel size in arc secs. This is used to define the width of pixel template radius employed. It is assumed that pixels are the same size in the x and y directions.

The skewness value assigned to each pixel of the output image is calculated using the values of pixel count found for all the non-bad pixels within the calculated radius. The value obtained is multiplied by 1000 (or a user defined value) to make display easier.

The modal count value used during the calculation is either the global value (defined by the user) or a local value calculated as required.

The resultant value is some measure of the extent to which the pixel count values surrounding a given pixel are not distributed in a Gaussian manner.

A border is present in the final output image which is the same width as the radius of the template used. Pixels within the border have been assigned the value bad.

#### Usage:

SKEW IN OUT MODET WIDTH PSIZE MULT [BACK] [SIGMA] [NSIGMA] [USEALL]

#### Parameters:

The background pixel count value found in the input NDF. Units counts. Only used if MODET = TRUE.
The name of the NDF that is to be examined.
Used to indicate whether a global modal count value is to be used when calculating the skewness values. The alternative is for the application to calculate and use the local mode value. See BACK. Using a local background calculation can be slow.
A multiplying factor applied to each of the results. Default value is 1000.
The number of standard deviations above the sky level count value, where the pixel count cutoff occurs. Only employed if a global pixel count modal value is in use (MODET = TRUE).
##### OUT = _NDF (Write)
The name of the NDF that will be created.
The size of each pixel in arc seconds. If the image contains a SKY co-ordinate frame this value will be determined automatically.
The standard deviation of the back ground count within the input NDF. Should be determined using a routine such as HISTPEAK which ignores outliers. Only employed if a global pixel count modal value is in use (MODET = TRUE). Units counts.
Used to indicate whether a pixel count threshold is to be a applied when calculating the skewness. Only employed if MODET has been set to ensure that a global modal value is in use.
The width of the sampling area/filter to be passed over the image. Units arc seconds.

#### Examples:

skew in=ic3374 out=skewed modet=false width=10. psize=0.5 mult=1000
A skewness image named SKEWED is generated using IC3374 as the source image. The sampling area from which pixels are selected is 10 arc seconds across. The individual pixel size is .5 arc seconds so the area is 20 pixels across. All the skewness values generated for the output image are multiplied by a factor of 1000, and local background values are used throughout.
skew in=jet out=sjet modet=true width=5. mult=1000. back=2010. useall=true
An output image SJET is generated using JET as the source image. The background count is 2010 and the pixel size will be determined from the WCS component of the source image. All the pixels in the image can be used in the calculation. The sampling area width is 5 arc seconds. All the pixels in the image can be used in the calculation.
skew in=sgp27 out=result modet=true width=8. psize=1. mult=1000. back=4505. sigma=23.7 nsigma=10. useall=false
The output image generated is created by assuming a global background count of 4505. with an associated standard deviation of 23.7 counts. All pixels of a count value greater than 4505$+$23.7x10. are excluded from the calculations.

#### Implementation Status:

As the program stands it is useful for looking at an image to with a view to detecting faint objects and flat-fielding faults. It may be easily extended by the user to provide plots showing other statistical quantities such as kurtosis or S/N.