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| Guide to browsing the QSO/Elixir CFHQSIR exposure lists | ||||||||||||
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From the various information located in the image FITS header, the QSO statistics
and the QSO observer and coordinators qualification criteria, a unique ASCII string is
created per image. The principle is to have a single line per acquired exposure with all
the best possible and most complete information to properly define and characterize
that exposure. This represents the core information used by Elixir to select exposures
for processing and validation.
A standard entry for a given exposure is:
The separator is the "|" character and there are a total of 10 fields, some having several sub-fields separated by a the space character " ". The following sections describe each field individually.
The filename: the CFHT odometer number The CFHT uses a unique filename identificator for each exposure. It is a unique incremental 7 digits code. There can't be two exposures with the same odometer number and this is the best identificator for a given exposure.
The observing run: semester and QSO run This field is the granularity element used at CFHT to identify observing runs. The first three letters define the semester (10B for the second semester of the year 2010) and the last three letters define the QSO observing run (Q04 stands for the fourth run of the given semester, with three instruments operating in QSO mode, these index will not be sequential for a given instrument).
The date of observation: Hawaii local time (not UT) at the END of the exposure A string of characters giving the month, the day the local time and the year (year coded only on two digits). Note that this time corresponds to the time the file is created on disk at the end of the exposure.
CFHQSIR Run Identificator and Field Name: CFHTLS Wide tile, epoch and pointing P22 is the program identificator at CFHT for the CFHQSIR Large Program. The CFHTLS Wide tile name (W4.+2-1) is adopted from the traditional CFHTLS convention as observed by MegaCam. The epoch of the CFHQSIR program follows: E1 or E2, and finally the index of the WIRCAM pointing within the tile for the given epoch (see information on the maps for an explanation of the different indexation depending on the epoch).
Field coordinates: right ascension, declination and epoch The fractional seconds on the declination have been removed, but those coordinates are still precise enough to define the pointing within a few arcseconds. Note that these are the telescope control system coordinates for the observed field, not the accurate coordinates of the center of field that could be derived from precise astrometry analysis of the image.
Exposure time: nearest integer value in seconds The program uses exclusively an integration of 75 seconds.
Filter: single letter definition The program uses exclusively the WIRCam Y filter.
Airmass: airmass at the time of the start of the exposure The airmass is coded with two decimals.
Image quality: full image statistics in arcseconds This value is derived from real-time extraction of the image quality at the telescope. The image quality metric is SExtractor's isophotal FWHM.
Sky background: in ADU on the raw data This value is derived from real-time extraction of the image background at the telescope.
The four exposure external information: sky transparency, observing rating, exposure validation, and observing group validation. 1) Sky transparency: SkyProbe provides a robust way to define the absolute value of the sky transparency at the time of the observations (1 point every minute in the V-band only). There can sometimes be a very slight haze that goes undetected. The service observer looks both at the SkyProbe plot (which is found to be reliable in 97% of the cases), the satellite map, and the WIRCam guide stars flux variations to define in absolute if the conditions are or not photometric. The photometric flag is set manually in real-time during the night by the observer for each exposure based on these inputs. The value of the flag can be either "P" for Photometric or "A" for Absorbed. The actual level of absorption can be derived in the V-band from the SkyProbe plot archive, and the MetaData also provide a table of the SkyProbe zero points throughout the whole night (the tabular version of the plot), allowing for a direct determination of the absorption for a given exposure (cloud absorption is grey, but the gap from the V-band to the Y-band is large, some investigations are needed). However the "Observer Rating" and the "Coordinator Rating" flags described below can tell a lot on the level of absorption in the case the "Photometric" flag has been set to the absorbed state. When no data is available, the flag is set to "X". 2) Service Observer rating: This flag ranges from 1 to 5 and is set in real-time during the night by the observer at the telescope. - 1:Taken within, or better than, the conditions defined by the Principal Investigator in terms of seeing, sky background, and sky transparency. - 2:Taken almost within the conditions defined by the Principal Investigator: one of the three parameters (seeing, sky background, sky transparency) is slightly out of boundaries by up to 15%. - 3:Taken outside the conditions defined by the Principal Investigator: one of the parameters (seeing, sky background, sky transparency) is totally out of the accepted boundaries, or two, or all of the parameters are out of bound. - 4:Horrible observing conditions which most likely prevent any science to be done. - 5:A lost cause, nothing to be done with that exposure (can include telescope and camera problems). Note that these three defining parameters are provided in that exposure information line as described above (Image quality, Sky background, Sky Transparency), hence if you see the Service Observer rating flag set to 2, 3 or 4, one can easily conclude the reason why the exposure is considered faulty. When no data is available, the flag is set to "X". 3) QSO Coordinator validation: This flag can be set to "V" for "Validated" or "O" for "Observed". The day following the night when that exposure was obtained, the QSO coordinator checks the service observer rating and other comments to define if that exposure fits within the specifications defined by Principal Investigator. The fact that an exposure is validated does not necessarely mean that its integration time will be accounted for in the time balance allocated to the observing program: this depends on the observing group validation described below. - V: the exposure qualifies for the science goals defined by the Principal Investigator. An exposure can be validated even if the Service Observer rating is equal to 2 since a degradation on of the parameters won't most likely affect the science dramatically. - O: the exposure does not qualify for the science goals defined by the Principal Investigator. The exposure will be processed and distributed anyway but the time used to acquire it is not deducted from the time allocated to the observing program. When no data is available, the flag is set to "X". 4) QSO Coordinator Observing Group validation: This flag can be set to "Q" for "Qualified" or "D" for "Dismissed". CFHQSIR exposures are now taken by sets of at least 14 groups (each group is made of 2 dithered exposure on one WIRCam pointing in a CFHTLS tile). If some exposures within the observing groups are out of specifications *and* will not allow the other validated exposures to be processed (sky subtraction issue due to a varying sky for example) then the entire set of exposures, included the validated ones, will be dismissed in terms of time accounting for that observing program. However, the validated exposures can still be very useful for science, such as the use of epoch separation for CFHQSIR.
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