Focussing tool

C2A has a tool to help with focusing. You can get to it through the Image processing window once a star has been selected by tracing a rectangle around it with the mouse then clicking on the focus button on the tool bar.

The focusing principle is very simple:

• The first image in the series must terminate its file name with numerical characters. This image is opened in the C2A image processing window (F11 key). Once this image is open, and after you have adjusted the thresholds, you draw a rectangle around a bright but not saturated star. The PSF (Point Spread Function) will then be presented in the left part of the image processing window. The focusing button will also become active. A click on this button will display the focusing window (see the screen shot below). A thumbnail image of the star is displayed at the top of the window, and just below you will find the FWHM (Full Width at Half Maximum) value for this star. The first point is also placed on the curve plotting the FWHM.

• Then just click on the "Acquisition" button and take pictures while changing gradually the focusing of your telescope. The only constraints are (1) the names of the images must be incremented by one each time (2) the images taken by the instrument must be put in the same folder as the first image was placed. These constraints are usually easy to meet with CCD cameras or digital cameras which number images incrementally and automatically put them in a directory specified by the user.

• Each time a new image arrives in the specified directory, C2A automatically reads the image, extracts the sub-image of the star having the same coordinates as the first image, displays the thumbnail of the star and its associated FWHM and plots a new point on the curve. You can see little by little the curve developing, and you must stop when the curve reaches its lowest point, or go back to the lowest point if you over-shoot. In the example below, the curve oscillates around the optimal focus and slight adjustment is still required to return to the lowest point.

You can specify minimum and maximum values of FWHM that you wish to display on the curve so that you can see better when fine focusing. The “Auto” button lets you establish the limits automatically. When you want to stop focusing, just click the “stop” button or close the window.

In the example below, we have just read the image named "Image_0089.CR2" (it has a raw digital camera format) and are waiting for the next image "Image_0090.CR2". To get information about one of the components of the window, left-click on it in the image below:

All dynamic image formats can be used in focussing.

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