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: Optional (and recommended) calibration exposures to be added to the series are: Unless radial velocity information at a precision higher than 50m/s is required, it is not necessary to collect comparison frames throughout the night; using the numerous telluric lines present in the collected stellar frames is usually enough to correct for potential spectral drifts (caused mainly by thermal and pressure fluctuations) across the night with an accuracy of a few tens m/s.

Scripts designed for carrying out automatically such sequences of calibration exposures are already implemented in the instrument control software and can be started with one single command line or with just a few clicks.

© Jean-François Donati, last update May 24 2004