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Axis Arrays   

To define the pixel coordinate system in §[*], we specified the location of each pixel by giving its centre position and width on each axis. Thus, for a given dimension, the pixel centre position C was derived from the corresponding pixel index i according to the formula:

$C(i)=i-\frac{1}{2}$

and its width W was given by:

W(i)=1

An NDF's axis coordinate system extends this idea by allowing each of these centre and width functions to be determined by values stored in a 1-dimensional array. These axis arrays then act as ``look-up tables'' which convert pixel indices into pixel centre coordinates and width values on each axis:


\begin{picture}
(90,80)(-15,-15)

% Pixel array with label above.
\multiput(20,2...
 ...0,-10){\vector(0,1){65}}
\put(-10,58){\makebox(0,0)[b]{\bf Axis 2}}\end{picture}

This allows a wide range of possible coordinate systems to be accommodated. A third axis variance array is also provided as a look-up table to convert pixel indices into variance estimates, which can be used to represent any possible statistical uncertainty in a pixel's centre position.



next up previous
Next: Pixel Positions and Dimensions
Up: THE AXIS COORDINATE SYSTEM
Previous: Axis Coordinates


Starlink User Note 33
R.F. Warren-Smith
11th January 2000
E-mail:rfws@star.rl.ac.uk

Copyright © 2000 Council for the Central Laboratory of the Research Councils