Using the output Catalogue

By default, the output catalogue created by FINDCLUMPS and EXTRACTCLUMPS will be a standard FITS binary table, with the following columns:

Peak1

: The first co-ordinate at the centre of the pixel with the highest data value in the clump.

Peak2

: The second co-ordinate at the centre of the pixel with the highest data value in the clump.

Peak3

: The third co-ordinate at the centre of the pixel with the highest data value in the clump.

Cen1

: The first co-ordinate of the clump centroid.

Cen2

: The second co-ordinate of the clump centroid.

Cen3

: The third co-ordinate of the clump centroid.

Size1

: The size of the clump along the first axis, in pixels.

Size2

: The size of the clump along the second axis, in pixels.

Size3

: The size of the clump along the third axis, in pixels.

Sum

: The total data sum in the clump.

Peak

: The peak value in the clump.

Volume

: The total number of pixels falling within the clump (a volume for 3D data and an area for 2D data).

The co-ordinate system used depends on the value supplied for the WCSPAR parameter. If WCSPAR is set to TRUE, then current WCS co-ordinate system in the input NDF will be used. If WCSPAR is left at its default value of FALSE, then the input NDF's PIXEL co-ordinate system is used. The centroid position is the weighted mean of the pixel co-ordinate values at the centre of all the pixels in the clump, with the pixel values being used as the weights (the final pixel position will be converted to a WCS position for storage in the catalogue if WCSPAR is TRUE):

$$\begin{eqnarray*} Cen1 & = & \frac{\sum_{k}X_{k}.D_{k}}{\sum_{k}D_{k}} \\ Cen... ...D_{k}} \\ Cen3 & = & \frac{\sum_{k}Z_{k}.D_{k}}{\sum_{k}D_{k}} \end{eqnarray*}$$

The clump sizes are the standard deviation of the pixel co-ordinate values about the centroid position, weighted by the pixel values. For a Gaussian profile, this size value is equal to the standard deviation of the Gaussian. These sizes are then corrected to remove the effect of the instrumental smoothing specified by the FwhmBeam and VeloRes configuration parameters (assuming the DECONV parameter is set to TRUE):

$$\begin{eqnarray*} Size1 & = & \sqrt{\frac{\sum_{k}D_{k}.(X_{k}-Cen1)^{2}}{\sum_... ...rac{\sum_{k}D_{k}.(Z_{k}-Cen3)^{2}}{\sum_{k}D_{k}} - b_{z}^{2} } \end{eqnarray*}$$

where $b_{x}$, $b_{y}$ and $b_{z}$ are the beam widths implied by configuration parameters FwhmBeam and VeloRes. Clumps are excluded from the returned list if the clump size before correction is smaller than the specified beam width. The final widths are converted to WCS units for storage in the catalogue if WCSPAR is TRUE.

If DECONV is TRUE, the peak value in the clump is also corrected to take account of the smoothing produced by the instrumental beam. Such smoothing will result in the observed peak value being less than the real peak value, by an amount that increases as the clump area gets smaller. The correction factor assumes that the clumps has a Gaussian profile and is determined by the requirement that the total data sum within the corrected clump equals the total data sum within the uncorrected clump:

$$\begin{eqnarray*} Peak & = & D_{max}.sqrt{ (Size1'.Size2'.Size3')/(Size1.Size2.Size3) } \end{eqnarray*}$$

where the primed sizes refer to the clump sizes before the correction for the instrumental beam width.

The recommended way to explore and examine the output catalogue is to use the Starlink catalogue browser, TOPCAT (see http://www.starlink.ac.uk/topcat/).