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Kanzelhöhe H-α Imaging 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 the diffraction limit for the telescope of 1.65 arcsec at 656 nm. According to
Nyquists 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 2 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.
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Scheme of the H-α imaging system.
- CCD1: Position for a second camera which can be set to a cadence of 4 frames/sec
- CCD2: main camera
- M1: stowable mirror for eyepiece
- Sp: Beamsplitter for CCD1 and CCD2
- Lyot Hα:Hα filte
- IF: inclined H-α interference filter with a FWHM of 10 nm
- F1, F2:Field lenses
- AL: Objective lense (1650/100 mm) coated with gold
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