### SELFCW

To perform mixed cross-self-correlations on an NDF image file

#### Description:

Performs a mixed cross-self-correlation calculation on an input NDF image file. The resulting correlation image/plot is stored to disk.

The cross-self-correlated image may be used to find faint diffuse objects for a given scale length.

The circular exponential profile template used is of a size that optimises the search for galaxies of the scale length requested by the user.

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 background count value (available via HISTPEAK), the background count standard deviation and the number of standard deviations above sky level at which the cutout should occur.

The user is required to enter a value for the scale length of of the object(s) of interest and also the image pixel size.

The method assumes some sort of symmetry is present in the objects detected but appears to work well on a wide range of image types.

A border is present in the output image which is of the same width as the radius of the template. All pixels within this border are assigned the value bad.

The correlation is optimised by making the template size 1.8x that of the galaxy scale length required. This factor was determined from simulations by Phillipps and Davies at Cardiff.

#### Usage:

SELFCW IN OUT SCALE PSIZE BACK USEALL MULT [SIGMA] [NSIGMA]

#### Parameters:

The modal pixel count value found in the input NDF. Units counts.
The name of the NDF that is to be examined.
The number of standard deviations above the sky level count value at which the pixel count cutoff occurs.
A multiplying factor used to modify the output range.
##### 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 scale length of the galaxies being searched for. Units arc seconds.
The standard deviation of the pixel background count within the input NDF. Should be determined using a routine such as HISTPEAK which ignores outliers.
Used to indicate whether a pixel count threshold is to be a applied when calculating the self-correlation.

#### Examples:

selfcw in=p2 out=scp2 scale=15. psize=0.96 back=1000.2 useall=true
A self-correlation image, optimised for galaxies of a 15 arc second scale length, is generated using image P2 as the input source image and SCP2 as the output image. The pixel size on the image is .96 arc second and the background count value for the source image is 1000.2
selfcw in=lsbg1 out=lsbg2 scale=8. back=444. useall=false sigma=12. nsigma=4.
A self-correlation image, optimised for galaxies of a 8 arc second scale length, is generated using image P2 as the input source image and SCP2 as the output image. The background count value for the source image is 444 and the pixel size in arc seconds will be determined from a SKY frame in the image’s WCS component if possible.

All pixels with a count value greater than 444.$+$12.x4. are excluded from the correlation calculations.

#### Notes:

It is assumed that the x and y axis pixels sizes are the same size.

To establish the statistical significance of a detection, this application should be used in conjunction with MIXUP to allow noise equivalent images to be generated and correlated thereby establishing a 3 sigma limit.