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ESPaDOnS: observing procedures http://webast.ast.obs-mip.fr/magnetisme/espadons_new/procedures.html
ESPaDOnS
observing procedures
Procedures for astronomical observations
Although some observing procedures may depend on the program being carried out, others are essentially dictated by the type
of data being collected. This is the case in particular for spectropolarimetric studies, in which very small amplitude signals
(ranging typically from about 1% of the unpolarised continuum for the largest signals down to about 10ppm for the smallest
ones) are usually being looked for. In this case, it is important to minimise all sorts of spurious signatures that can plague the
data being collected.
Optimally, one would need to record the spectra associated to orthogonal states of a given polarisation both simultaneously (to
avoid mistaking polarisation signatures with temporal variations) and at the same place on the detector (so that pixel to pixel
differences do not affect the results). Since this is obviously impossible, the solution we adopt is to regularly swap the role of
both beams within the instrument by rotating waveplates between exposures. This way, we make sure that both polarisation
states are collected simultaneously (although on different detector regions) within each exposure; we also ensure that the same
region of the ccd detector records both polarisation states (although not simultaneously) to minimise all errors resulting from
flat fielding procedures. This compromise, although not ideal, has the obvious advantage of getting rid of all systematics at first
order. This method is also useful to minimise errors caused by slight waveplates imperfections, and in particular to correct at
first order all crosstalk between circular and linear polarisation states.
In practice, this solution consists in dividing each polarisation exposure in a series of 4 subexposures, each taken in a different
waveplate configuration. Polarisation information is then obtained by processing the complete series of 4 subesposures with the
specific reduction tools, while unpolarised spectra can be derived by individually processing each of the four subexposures.
These observing procedures are implemented in the instrument control software of ESPaDOnS as scripts, chaining
automatically waveplate settings for individual subexposures along with ccd exposure and readout tasks.
Similar procedures can be used for scientific programs interested in measuring very small signals whose origin is not
polarisation but rather temporal variations, such as small spectral variations induced by, eg, atmospheric pulsations, wind
phenomena, activity cycles or extrasolar planets. Although the details of the observing procedure are different, the basic
principles remain the same and aim at minimising all spurious signatures in the collected data. Such procedures are not
implemented yet, but could be added later on specific requests from users.
Calibration sequences
Similarly, it is important to run sequences of calibration exposures to ensure that everything is setup properly for collecting
stellar exposures and reducing them in real time. Such calibration sequences are usually taken once before sunset, and a second
time after sunrise (to keep night time for stellar exposures).
A typical calibration sequence includes at least the following mandatory frames:
one bias frame (null exposure time) to evaluate the magnitude of the ccd readout noise;
one comparison frame (illumination from Th/Ar lamp) to determine the details of the ccd pixel to wavelength
relationship;
a series of ten flat fields (composite illumination from 2 halogen lamps with associated filters) for correcting pixel to
pixel response differences.
Optional (and recommended) calibration exposures to be added to the series are:
one fabry perot exposure to estimate the shape of the slit formed by the image slicer at spectrograph entry with a
better accuracy than with a comparison frame;
one dark frame (no illumination with exposure time similar to that of stellar exposures) to evaluate the amount of
background level in a typical stellar exposure;
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