Gaia Data Release 1 Documentation release 0
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Table 2.4: Fortran format of GEPC3.0. Name Format Unit description Ob j name A23 —– unique coordinate based identifier RA h I2 hour Right Ascension Hour RA m I2 minute Right Ascension Minute RA s F7.4 second Right Ascension Second err RA F6.4 arcsec Right Ascension Error DEC d I3 degrees Declination Degrees DEC m I2 arcmin Declination Minute DEC m F6.3 arcsec Declination Second err DEC F5.3 arcsec Declination Error pm RA F8.2 mas yr −1 proper motion in RA err pm RA F6.2 mas yr −1 error of proper motion in RA, currently not used pm RA F8.2 mas yr −1 proper motion in DEC err pm RA F6.2 mas yr −1 error of proper motion in DEC, currently not used U cal F6.3 mag calibrated U magnitude err U F6.3 mag error of U magnitude B cal F6.3 mag calibrated B magnitude err B F6.3 mag error of B magnitude V cal F6.3 mag calibrated V magnitude err V F6.3 mag error of V magnitude R cal F6.3 mag calibrated R magnitude err R F6.3 mag error of R magnitude I cal F6.3 mag calibrated I magnitude err I F6.3 mag error of I magnitude z cal F6.3 mag calibrated z magnitude err z F6.3 mag error of z magnitude G F6.3 mag Gaia G magnitude G RVS F6.3 mag Gaia RVS magnitude G BP F6.3 mag Gaia BP magnitude G RP F6.3 mag Gaia RP magnitude S tellarity F4.2 —– Stellarity index, 1 = star; 0 = galaxy Notes. This table shows the formats and the columns in which the data is stored. 88 Since the start of the nominal mission, the first and third of these items are re-checked whenever the scanning law touches the ecliptic poles — which is quite regularly about once per month for 1–2 scans per pole and field of view each — while the second item is now being covered by a daily comparison of bright-star measurements with independent Tycho-2 magnitudes. All this is part of the daily First-Look data processing. 2.2.5 The Attitude Star Catalogue (ASC) Author(s): Ricky Smart The Attitude Star Catalogue (ASC) was commissioned by the Gaia Data Processing and Analysis Consortium (DPAC) in 2006 to allow a first reconstruction of the attitude of Gaia. Eventually it will be replaced by a catalogue constructed from the Gaia observations but for at least the first two years a precompiled ground based catalogue was needed. The ASC entries were required to be of a high astrometric precision, isolated from other bright G > 13.7 objects, and, brighter than the 2D window threshold of the Gaia instrument. The first version delivered to the DPAC in September 2013 was simply a subset of the Initial Gaia Source List (IGSL) described in Smart & Nicastro (2014) identified by the parameter toggleASC =1. Early commissioning usage and an examination of the ASC subset revealed a number of repeat entries for the same object and entries that did not meet the isolation requirements. Since the reliability of the ASC was fundamental to the Gaia mission a new re-compilation was requested in January 2014. The new separate ASC was delivered to DPAC in April 2014 and is available from the IGSL website. 2.2.5.1 Construction The source catalogues used and their order of inclusion were: • Hipparcos (Perryman et al. 1997): The photometry from the original Hipparcos Catalogue and the astrometric parameters from the update by van Leeuwen (2007b) when published. Initially all entries were included regardless of the known errors, e.g. also for entries that are considered erroneous. Since inclusion in the ASC requires an estimate of the G-mag the unreal entries were excluded as part of the cleaning phase. • Tycho-2 (Høg et al. 2000): This catalogue forms the backbone of all the major ground based cata- logues currently available. It was made from a combination of the Tycho star mapper observations on the Hipparcos satellite (Høg et al. 1997), the Astrographic Catalogue and 143 other ground-based catalogues. • Sky2000 (Myers et al. 2001): The SKYMAP Star Catalogue System is a list of all stars with either measured Johnson blue or visual magnitudes brighter than 9.0. The version used here had 299167 entries of which 212 were not in the combined Hipparcos + Tycho-2 catalogues. Sky2000 provides positions at 2000, proper motions and a blue and visual magnitude. We assumed the positions to have an error of 100 mas, the proper motions an error of 10 mas yr −1 , and an error of 0.6 in the ASC magnitudes derived from the Sky2000 values. • UCAC4 (Zacharias et al. 2013): The USNO CCD Astrograph Catalogue version 4 is the most pre- cise all-sky astrometric catalogue in the range V =10–16 currently available. There are no original standard magnitudes in this catalogue. • GSC2.3 (Lasker et al. 2008): The Second Guide Star Catalogue version 2.3 forms the bulk of the photometry and defines the red and blue magnitudes (B J and R F ) as this is the sky survey with the 89 largest coverage on a precise homogeneous photometric system. The only variation with the public version is that we removed the multiple entries discussed in section 4.2 of Lasker et al. (2008). This was done by insisting that only one entry from any objects with position di fferences of less than 10 mas were kept selecting Tycho-2 or Sky2000 over other entries. • PPMXL (Roeser et al. 2010): The Positions and Proper Motions ‘Extra Large’ Catalogue,produced from a combination of the USNO–B (Monet et al. 2003) and the Two Micron Sky Survey point source catalogue (Epchtein et al. 1999). This catalogue was included to provide magnitudes for those entries that did not have them in the previous catalogues. In addition any objects in the Washington Double Star catalogue (Mason et al. 2010) or the Tycho Double Star catalogue (Fabricius et al. 2002) were indicated as probable members of a binary system. The first version of the ASC was a subset of the IGSL and consequently was derived using the procedure in Smart & Nicastro (2014). In summary we produced a master list of objects starting with the large faint catalogues, pro- gressively adding other catalogues and increasing the master list as entries from new catalogues were unmatched. The catalogues of bright objects were then matched to a large master list which resulted in mismatches of the bright objects to noise or faint objects near the true bright objects. Also it was found that the large Schmidt catalogues in the overlap region between plates often had many multiple entries of the same objects, this can be seen in the sky plot of the PPMXL. These multiple entries, if they were bright enough, were included as ASC sources. The first on-ground attitude reconstruction of Gaia is described in detail in Gaia DR1 papers. The goal of this attitude reconstruction is to provide the attitude with an accuracy of 50 mas for the first year when the ASC will be the primary source of reference objects. Later in the mission it is planned to replace this catalogue with one produced by Gaia with an expected accuracy of 5 mas. This reconstruction requires at least one 2D measurement per second and per field of view which equates into a minimum density of 75 stars per square degree. To be automatically assigned a 2D window the star must have a G < 13 but this does not provide enough calibration sources especially near the galactic poles. The compromise was to provide a list of faint calibration stars to a G =13.4 which sets the limiting magnitude of the ASC. Note that, originally the limit of the ASC was set to G =14.0 however a change of the procedure allowed a relaxation of that requirement to G=13.4. The crossmatching radius in the first on-ground attitude reconstruction will be between 20–30 , the precise value to be optimized during the commissioning phase. Hence we conservatively require that all ASC sources are isolated at the level of 40 . This would potentially allow up to 8000 ASC entries per square degree and not violate the isolation criteria. Following this consideration, in the original subset of the IGSL that constituted the ASC, we just lowered the magnitude limit to reduce the number of stars to less than 1000. We then assumed the isolation criteria would always be met when there were this many objects per square degree. However, because of the multiple entries, uncatalogued binary systems and general non uniform distribution it was found that the isolation requirement of the ASC subset was violated. To address the isolation and duplicate entry issues the ASC was reproduced from scratch using the catalogues listed above in the order given. The production of the ASC starts with all objects in the Hipparcos catalogue as a master list, the other catalogues are input and matched to this master list with a matching radius of 5 . All entries from the input catalogue not matched are included as new master list objects. If more than one entry from the input catalogue matches the master list only the closest is considered matched and a new entry is generated for the others. In this way the master list grows with each included catalogue. Since the first catalogues are composed of bright objects they are sparse and the chances of a mismatch between the input catalogues and the master list was reduced. The confusion at the bright end of the master list was in this way minimized. When the large, dense Schmidt plate based catalogues are included there is still the possibility that non-real entries are matched to bright objects and the 90 Yüklə 5,01 Kb. Dostları ilə paylaş:
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