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Kanzelhöhe CaIIK Imaging System


Camera System

The experience gained during several years of Hα observations showed that mastering of not always perfect observing and seeing conditions might be even more important than maximum resolution. Key features for the selection of the camera model were

  • presence of an electronic shutter, as mechanical shutters had problems with the huge number of exposures on high cadence time series
  • high frame rate for use of frame selection
  • software controllable exposure times to adapt for changing opacity of the atmosphere
  • reasonable high resolution (spatial and intensity)
  • digital read-out and progressive scanning which simplifies the data acquisition

The selected JAI Pulnix TM--4200GE features a Kodak CCD chip with microlenses and a built-in electronic shutter, the maximum of the spectral sensitivity is close to the observed band. The edge length of 7.4 μm of the 2kx2k square pixels corresponds to 1.04 arcsec/pix which is not far away from the diffraction limit for the telescope of 0.9 arcsec at 393 nm. According to Nyquist's sampling theorem the image is even more undersampled but it is good compromise between mostly prevailing seeing conditions and camera technology. The shutter can be controlled by the length of an external pulse, typical exposure/integration times are in the range of 15 ms and the maximum frame rate is about 7 frames/s. The output is digitized to 12 bit, the lower level as well as the gain can be set according to the incident light level to exploit the full dynamic range but avoid non-linearity due to saturation. The dual tap CCD has the advantage of higher frame rates but the drawback of the need of a careful tuning of the individual gains and lower levels of the A/D-converter for the two taps to achieve equal dense half-images. Details of the camera setup procedures and parameters can be found in the » KPDC - Data Processing Procedures section.

A GigaBit interface transfers the data to an Intel based 1.8 GHz industrial 19" PC and allows also the control of the camera by the image acquisition software.

Figure 1: The front part of the patrol instrument, with the various objective lens of the telescopes. The new CaIIK system was mounted into the former koronograph light path.

Figure 2: Spectral sensitivity of the Kodak KAI4021 CCD, taken from the manufacturer's product description.

Figure 3: Layout of the Kanzelhöhe Observatory Ca II K Telescope. The chosen lenses L1 and L2 are achromats reducing also non-chromatic aberrations. The effective focal length of the observed system is about 1500 mm which leads to a mean solar image size of about 13.9 mm in diameter. In front of the Ca filter there are two shortpass filters for absorbing (SPF1) and reflecting (SPF2) the wavelengths above approx. 480 nm. The whole system is integrated into the main KSO patrol telescope. For focusing the lens L2 can be shifted back and forth manually with a belt drive..

The image acquisition application is written in C++ and makes use of the Common Vision Blox library. It is running under Windows XP 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 3: 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 black 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.