CFHT operating manual - Homepage Usask

More documents

Recommendations

Info

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; 1 of 2 08/07/04 11:33 PM
ESPaDOnS: observing procedures http://webast.ast.obs-mip.fr/magnetisme/espadons_new/procedures.html series of check exposures (with polarised Q=1 or U=1 illumination and given waveplate configurations) to verify that the polarimetric analysis is behaving as expected; additional series of flat fields in case the scientic program involves observing very bright stars at extremely high signal to noise ratios. 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 2 of 2 08/07/04 11:33 PM
Page 1 and 2:
CFHT Observatory Manual - cover htt
Page 3 and 4:
CFHT Observatory Manual - Table of
Page 5 and 6:
CFHT Observatory Manual - Intro (Se
Page 7 and 8:
CFHT Observatory Manual - Site (Sec
Page 9 and 10:
Page 11 and 12:
Page 13 and 14:
Page 15 and 16:
CFHT Observatory Manual - Observato
Page 17 and 18:
Page 19 and 20:
Page 21 and 22:
Page 23 and 24:
Page 25 and 26:
CFHT_ObservatoryManual_Floor1 http:
Page 27 and 28:
CFHT_ObservatoryManual_Floor2 http:
Page 29 and 30: CFHT_ObservatoryManual_Floor4 http:
Page 31 and 32: CFHT Observatory Manual - Observato
Page 59 and 60: CFHT’s Web page for ESPaDOnS http
Page 61 and 62: Home Page of ESPaDOnS project http:
Page 63 and 64: ESPaDOnS: instrument details and co
Page 65 and 66: ESPaDOnS: spectral domain and resol
Page 67 and 68: ESPaDOnS: spectral response and glo
Page 69 and 70: ESPaDOnS: thermal response and spec
Page 71 and 72: ESPaDOnS: performances of Fresnel r
Page 73 and 74: ESPaDOnS: ccd readout modes and cha
Page 75 and 76: ESPaDOnS: control software and user
Page 77 and 78: ESPaDOnS: viewing, guiding and expo
Page 79: ESPaDOnS: temperature and pressure
Page 83 and 84: ESPaDOnS: data reduction routines h
Page 85 and 86: ESPaDOnS: instrument status http://
Page 87 and 88: ESPaDOnS: description of critical i
Page 89 and 90: ESPaDOnS: examples of frames http:/
Page 91 and 92: ESPaDOnS: examples of spectra http:
Page 93 and 94: ESPaDOnS: picture gallery http://we
Page 99 and 100: ESPaDOnS: project team and budget h
Page 101: ObservatoryManual - Maps http://www
show all

CFHT operating manual - Homepage Usask

Create successful ePaper yourself

Delete template?

Save as template?