CORR

Performs cross-correlations on an image using a galaxy template

Description:

Performs calculations to cross-correlate a circular shaped exponential template with an image. The exponential profile template chosen optimises the chances of identifying faint diffuse galaxies/galaxies of (and near) a scale length defined by the user.

Performs cross-correlation calculations on an input NDF image file. The resulting image/plot is stored in an output NDF.

For each image pixel in turn, all the pixels within a defined radius are identified. The values for each of these in turn have their background values subtracted and the result (F1) multiplied by a factor (F2) generated using an exponential function. The values obtained for all the surrounding image pixels are summed. The total generated is divided by using a normalisation value created by taking the sums of square for F1 and F2, multiplying them together and then taking the square root. This normalised sum is placed in the appropriate pixel of the output image and the program moves on to the next input image pixel to be considered.

The circular elliptical mask used is of a radius 1.8x the scale length requested. Studies undertaken by Phillipps and Davies at Cardiff suggest that this value optimises the detection sensitivity.

The correlation value obtained is multiplied by 1000 (or a user defined value) to make display easier.

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:

CORR 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 image that is to be examined.
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.
OUT = _NDF (Write)
The name of the NDF data 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 to be highlighted in the output image. Units arc seconds.
The standard deviations of the background pixel 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 applied when calculating the correlation.

Examples:

corr in=hh1826 out=correl scale=8. psize=0.3 back=7437. useall=true mult=1000.

Correlates image HH1826 with a mask/template optimised for galaxies of 8 arc seconds scale length. The pixel size on the image is .3 arc second, the background count value 7437 and all the pixels on the image can be used in the calculation. The output image is to be named CORREL.

corr in=forn out=forn4 scale=4. psize=0.22 mult=1000. back=666 useall=false sigma=15 nsigma=3

Correlates image FORN with a mask/template optimised for galaxies of 4 arc seconds scale length. The pixel size is .22 arc seconds and the background count value 666.

Pixels that are brighter than 666$+$15x3 counts are not included in the correlation calculations (USEALL=FALSE). The output image is to be named FORN4.

Notes:

It is assumed that the x and y axis pixels are of 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.