Figure 1: Spectral sensitivity of the Kodak KAI4021 CCD, taken from the manufacturer's product description. The blue, green and red line denote the positions of the CaII K, Continuum (whitelight) and Hα filters.

The image acquisition application is written in C++ and makes use of the Common Vision Blox library. It is running under Windows and grabs continuously frames from the camera. Each frame is evaluated in a user defined rectangle (area-of-interest AOI) with regard to mean pixel value and standard deviation. The mean is used to control the exposure time and keeping the brightness level of the images fairly constant, the standard deviation is a measure for the blurring which is the main factor of the seeing at exposure times of some milliseconds which freeze the image motion component. The image with the best seeing of a consecutive number of frames is then written onto harddisk, the standard format is FITS, JPEG copies are optional. The whole procedure can be repeated after a user defined interval for automatic acquisition of time series. A block diagram and further details of the software can be found in Otruba, W.: 2005, Hvar Observatory Bulletin 29, 279.

Figure 2: A screen-shot of the Ca4M-image acquisition application. In the center of the window the live image from the camera can be seen. The white box is indicating the AOI (area-of-interest) which is used for the calculation of the image parameters. The parameters are displayed in real-time in the right part of the main window. At the left hand side there are the control buttons for taking dark current frames, snapping images right on user requests, activating the recording of time series and taking overexposed images for observing prominences. Time intervals, number of frames used for frame selection and AOI-coordinates or a fixed exposure time can be specified via pull-down menus and dialog boxes.