HOW TO DO WITHOUT A SHUTTER
The readout time in a low noise CCD camera is relatively long.
In case of the Audine camera the time needed for digitising the image
is 5 seconds in binning 2x2 and 15 seconds in binning 1x1 mode. During this time
the CCD is still exposed to light, the objects in the field produce a parasitic artefact manifesting itself as trails.
This phenomenon is called smearing.
The traditional solution consists of preventing the light from reaching the CCD during reading, by mounting an electromechanical shutter, controlled by the camera electronics. The following images show an alternative that doesn't use a shutter at all.
In this case treating the images afterwards with a compensating routine is sufficient to remove the smearing effect (the DECONVFLAT
command in Pisco or QMiPS32). This procedure is extremely effective in the vast majority of cases (it only fails locally in those parts of the image where a saturated star is in the field).
It is economical (important in the Audine philosophy) and totally eliminates mechanical trouble
(sometimes a shutter is a fragile element).
Demonstration of the effectiveness of removing the smearing effect afterwards.
The image on the left is a composite of 5 exposures of 2 minutes of the neighbourhood
of galaxy NGC 677 (190 mm telescope). The individual images were acquired without
shutter in binning 1x1 mode. In this photonegative representation the smearing
effect manifests itself as dark trails coming from the brightest stars. The
white column is a defect of the CCD. On the right, the individual images are
treated with the DECONVFLAT command and composited afterwards. For erasing the
vertical defect of the CCD we make use of the information in the adjacent columns.
The smearing effect has disappeared almost completely and the resolution of
the image stays intact.
