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 the diffraction limit for the telescope of 1.65 arcsec at 656 nm. According to
Nyquist's sampling theorem the image is undersampled but it is good compromise between mostly prevailing
seeing conditions and present camera technology. The shutter can be controlled by the length of an external pulse,
typical exposure/integration times are in the range of 5 ms and the maximum frame rate is about 10 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.
A GigaBit interface transfers the data to an Intel based 3 GHz industrial 19" PC and
allows also the control of the camera by the image acquisition software.
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 1: Spectral sensitivity of the Kodak KAI4021 CCD,
taken from the manufacturer's product description.
Figure 2: Scheme of the H-α imaging system. The old CCD1 is a Pulnix
TM 1010 1 megapixel camera with 10bits. This camera is very fast and now used for guiding the telescope and
for making live-images.
The new Jai Pulnix TM4200GE camera is mounted at position CCD2.
IF ... is an inclined H-α interference filter with a FWHM of 10nm
F1, F2 ... are field lenses
M1,M2 ... are Mirrors
Figure 3: A screen-shot of the image acqusition application.
In the middle of the window the live solar disk image from the cam, in the center the white box indicating the AOI
which is used for the calculation of the image parameters. They are displayed in real-time in the right part of the
main window. Left: the control buttons for taking dark frames, snapping images right on user requests or
for activating the recording of time series. Time intervals, number of frames used for frame selection
and AOI-coordinates or a fixed exposure time can be specified via pull-down menues and dialog boxes.
Additionally with the Button SnapFixExpTimes an image with 5, 20, and 50 ms exposure time is made,
these images are send to the Astronomical Institute of The Academy of Sciences
in Tatranska Lomnica, Slovakia.