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# koords: interactive co-ordinate fitting

This tool allows you to quickly place a co-ordinate system (compute a plate solution) onto an image which does not have one. This is very useful if you have, say, a CCD image which does not have a co-ordinate system on it. All you need is an image of a similar piece of sky which does have a co-ordinate system (such as an image from the Digital Sky Survey). The tool handles flipping and rotation correctly. This tool is much faster to use than the various command-line driven tools currently available (usually less than a minute).

After you start the tool, you will get two windows. The left window should be loaded with the reference image (the image which has a co-ordinate system), by pressing Ref. Images. The right window should be loaded with your target image (the image without the co-ordinate system), by pressing Target Images.

The programme should also work with a photographic negative. The picking algorithm is automatically modified to look for a trough rather than a peak.

Once you have loaded your images, you are now ready to define matching pairs of points (stars). To do this, you must first click (middle) in the left window, near a star, and then click (middle) in the right window near the corresponding star. You then go back to the left window and define another star, and click on it's mate in the right window. You must define at least three pairs of stars.

As you select a star, a green circle is drawn around the fitted centre of the star. If at any time you miss the star, you can just click again (only before going to the other window). If you press Undo Pair, a pair of stars will be removed from the list. As long as some visible part of the star is within the circle cursor (also known as the ``capture region'') when you click (middle), the tool will find the centroid of the star.

You might find it helpful to click the Magnifier button to see the magnifier window. You will see a crosshair which is centred on the current mouse position in the main window. You will also see a green circle which shows the ``capture region'': if any part of the star lies within that circle, when you click (middle) the centroid of the star will be picked. You can adjust the size of the capture region by clicking with the left mouse button in the magnifier window. You will also see a dashed green circle which is slightly larger than the solid green circle. The area between the two circles is called the background annulus. The magnifier will also show small red crosshairs at the exact centroid positions of selected stars.

Once you have selected all your star pairs, you can click on Compute Co-ordinate System and the tool solves for a co-ordinate system. In your command-line window a report of the solution is displayed. If you move your mouse around the right window, you will see the proper world co-ordinates of the pixels. Once you are happy with the co-ordinate solution, you can press the Apply Co-ordinate System button, and then save the image (with the new co-ordinate system) using the Export menu. The coordinate system is written in the header using the standard (CTYPEn, CRVALn, CDELTn, CRPIXn) notation.

The coordinate system is computed under the assumption that the image of the sky has been projected by the telescope optics in a particular way. The projection geometry is assumed to be the Rectangular projection (denoted by the standard projection name ``ARC''), which is used by Schmidt telescopes. This may not be always be the appropriate projection to use, but the errors should only become noticeable for fields larger than 0.5 degree.

# 9.1 Centring Algorithm

The following steps are taken by the picking algorithm:

- find the brightest pixel within the circle cursor (``capture region'')
- proceed from that point by ``walking up the hill'', searching for a neighbouring pixel which is of a greater value. It continues searching until it finds a pixel with no neighbours with a greater value (i.e. a local maximum). This is usually the peak pixel
- find the half-intensity size of the star. To do this it needs an estimate of the background value. The background is computed by finding the median value inside the background annulus
- computes the centroid position by using a two-dimensional weighted mean algorithm. The centroid is computed out to the half-intensity size.

Note that the centring algorithm will not work with heavily clipped images. This is because there is no real peak to work with, rather there is only a plateau. In these cases, the centring algorithm is effectively bypassed and the position you selected will be used for the star position. This allows you to position by eye. Please note that heavily clipped images have lost information, and hence the accuracy of star positions will be limited.

A screen snapshot is available here

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