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# kpolar: regridding to polar co-ordinates

This tool allows you to regrid an image using a rectangular to polar transformation. It allows you to see the azimuthal variation of intensity around a given centre. This can be useful for looking at the (optical) structure of galaxies, or doing close inspection of a dirty beam (quite useful when doing radio astronomy).

To use this programme, all you need is an image file, which you load with the Files button. Once your image is loaded, you need to define an ellipse:

- Press the right mouse button to define the centre, then drag the mouse while keeping the button pressed, to define the size and shape of the ellipse
- If you press and release the 'r' key (still keeping the mouse button down) this will toggle on `rotation mode' so that now when you drag the mouse the ellipse will rotate. Press 'r' again to turn off rotation mode so you can continue changing the size and shape of the ellipse
- Once you are happy with your ellipse, release the mouse button and the polar image will be computed and displayed in the Polar Window. The vertical axis is azimuth (angle) and the horizontal axis is radius

Please note: this is an experimental programme that (at present) does this transformation in a simpleminded way. It works by binning the input image in radius and azimuth directions using a set of concentric elliptical annuli. Each annulus has a width of one pixel. Some points that you may wish to consider when using this task are listed below:

- The programme does simple nearest-neighbour interpolation to produce the output intensities; the absolute flux scale will probably not be perfectly preserved
- No 'world' (i.e. angle in degrees, radius in arcseconds) co-ordinate system is attached to the image. No allowance is made for images which have different scales on their x and y axes
- The centre position of the ellipses is not fitted in any way, it is exactly where you place it

Feedback on improvements to this task (including code!) is welcome.

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