Comment calculer une image maître du flat-field avec
ISIS
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The
flat-field image represents the response of the detector surface at illumination
when the input optical flux is white (without spectral
lines). This image is
particularly
easy to obtain with the spectrograph LISA. The latter
(option) incorporate a retractable filament lamp dedicated to this function
(tungsten lamp). The exposure
time is characteristic of a
dozen seconds. Tip: If possible adopt always the same exposure time during shooting calibration images. On the right, acquisition of an image sequence of 13 flat-field from Audela software. The exposure time is 10 seconds (typical value with LISA and the internal tungsten lamp). Adjust the exposure time to cover much of the dynamics of the camera to the peak intensity flat-field image, while taking care not to saturate. Tip: The worst thing that can happen during the acquisition a flat-field image is to saturate all or part of the image. Take a small margin on the exposure time for this never happens! Another tip: Do not hesitate to acquire a large number individual tunsten images. We will add later to get a flat-field with a high signal to noise ratio. A value of 10 to 15 images is a good choice. The
name of the sequence of images acquired consists of the name of the
star being observed and the suffix "_tung" for mean that
the flat-field is associated with this item. Similarly, for images of the neon lamp we will name such as generic "hd103287_neon-". Remember that you may need to perform several "Tungsten" and "Neon" in the night. You will recognize them at the count, which is facilitated by the strategy described above. |
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From "Master images" tab, calculate a master image flat-field images from individual hd103287_tung-1, hd103287_tung-2, ..., hd103287_tung-13. The result is the image flat_hd103287.fit in the
working directory. The
adoption of this name helps to
remember that the flat in question was
made following observation of the
star HD103287. Do not select the "Vertical gain correction" (usefull only for peculiar situations, i.e. observation of very faint objects). |
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