Maintenance of the Link to Hipparcos Technical

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Introduction densification frame 1 reference optical HCRF. the discuss and We for programs these HCRF. inappropriate between is the effort link significant often the of Thus, frame of densification maintenance the stars. the the faint sky, and of the into programs distribution on channeled denser (HCRF) distributed been a Frame sparsely requiring has Reference are programs Celestial which astrometric Hipparcos and stars the astronomical bright since of Unfortunately, comprised is satellite. astrometric Hipparcos the Abstract. USA D.C., Washington (USNO), Observatory Naval U.S. Hipparcos Zacharias N. to link the of Maintenance h pia aeeghatoercrfrnefaei urnl ae nosrain aeby made observations on based currently is frame reference astrometric wavelength optical The noeve n h ttso hs rjcsi ie eo.Mc is time. Much this at below. issues given frame is reference projects on results these without of progress status in critical work the 3) itself. and link and, overview frame wavelengths, An reference (IR) optical to 1) infrared radio to the extension categories: on frame 2) 3 check wavelengths, Hipparcos optical into several the at fall frame sense, of These reference broad the a of ongoing. in densification are issue programs maintenance Lowell observational system the Hipparcos at the presented For were 2005). and Zacharias, results 2004 & of First (Zacharias end meeting by progress. astrometry completed in were are Observations the ongoing reductions test link. The externally ICRF to epoch. potential to 2005 the HCRF the has at program axis link extragalactic and per the USNO error about HCRF in standard approaching error estimated time, the with mas possible attempt between increases 3 The systems first link 2 2004). the al., a original between et alignment the mission (Bobylev ICRF upon Hipparcos extragalactic improve successful the to the made after was years future. near 10 the the Over in have evaluation projects assess an some externally such However, to allow Tycho-2. ready to and yet potential the HCRF not deriving pos- between are are before link here errors the discussed possible systematic projects as end, The faint good the sible. as at Particularly HCRF which sub-mas catalogs, motions. the the proper epoch onto derived on early put been ground-based, be have been many hand to have of other have believed combination the is data on a motions which from Tycho proper system, the Tycho-2 depend same The and the here level. Catalogue on catalogs Hipparcos put densification the been of successful accuracy how The mission on space 1997). Hipparcos/Tycho Hip- the (ESA central on data the based are near Positions (1991.25) limits). epoch aspects. its parcos 2 (within involves HCRF actually the Tycho-2 The on Catalogue is directly. Tycho-2 which stars the 2000), reference on as al., rely used et projects be (Høg densification to current sparse all too Hipparcos and Almost below, bright mentioned too as Reference are cases Celestial stars many Hipparcos in the However, defines (HCRF). work. stars Frame densification Hipparcos and link of frame subset reference This astrometric Manchester, for used Assembly, C,G,O,V,X) be (flags to multiplicity General are known without (24th stars Hipparcos only B1.2 2000) resolution IAU Following Technical IERS Note 73 No. 34 74 No. 34 . UCAC 2.1 Densification 2 IERS Note Technical utos h vr10 ae 45T)aebiglae oRI disk RAID 2006. to before loaded ready being be are achieve not TB) will to re- (4.5 UCAC3 re-reduced photometric however, tapes and arrays; be 1200 astrometric over will on The data improve ductions. raw and pixel completeness higher the in a UCAC3 48 IAU over final 2003 the contains the at For UCAC2 released 2004). was The al., and et sky (Zacharias data. the re- Sydney of SPM the PMM 86% covers on and as stars, performed NPM well million plates the as AGK2 of machine, the measuring measures utiliz- of StarScan provided re-measures on Washington are recent based USNO of motions are part proper positions a improved UCAC2 ing and De- the catalog mid UCAC1 Tycho-2 to to up the Contrary observations includes 2002. (UCAC2) cember release data astrograph. Peak second December the The Kitt in at last performed observing was The parallel program with link 2004. extragalactic 2004 May the in for Arizona. run completed in 0.9m were (NOFS) CTIO observations Station from Flagstaff sky Observatory relocated All Naval was the astrograph to the (Chile) 2001, September/October 1998). (Monet, In A2 and 1998), USNO (R al., the stars et stars (Urban bright faint data (AC2000) of Catalogue motions Astrographic Catalogue mainly proper Tycho original UCAC1 the on (HCRF). rely positions System (Tycho-1). positions Hipparcos was containing UCAC1 the (UCAC1) the on with 2000), stars of release range, million al., Reductions 27 mag first et of 14 motions The (Zacharias proper to 2000 and 10 R=16. March about about in the of published in survey magnitude stars all-sky limiting for This a positions Catalog). mas the Astrograph 20 is gives CCD frame reference (USNO optical the project of UCAC densification the for program major A USNO-B. the to to attributed UCAC be the can for which example of an have most shows merely differences, 1 positions Figure USNO-B catalog NOMAD. into original copied NOMAD The between been into differences exist. systematic merged do limitations, catalogs catalogs their all individual within Although ICRF the catalog. on NOMAD notare compiled NOMAD. but into a copied NOMAD not are catalogs is in UCAC, original given photometry the Tycho-2, from are and columns Hipparcos, astrometry Cross-references all including 2MASS. best” catalogs ”the and various star USNO-B, each from For picked is 2004). (Zacha- al. Dataset Astrometric et Merged rias Observatory Naval the are the is 2005 given, NOMAD beyond is years range sigma Release 1 a magnitude. are the When estimates. errors on mainly positional cat- coordinate. depend each quoted per errors in The errors general stars (M). external = but millions of G estimated program number in and given The astrometric astrometric, is = all-sky astrometry. alog = on S A emphasize wavelengths. only, is: no optical sky survey, ”type” the at in column are area below) the selected programs. (see of densification project meaning ground-based 2MASS The the the of except overview date. All an launch gives a is to 1 There close Table mission programs. space astrometric densification (Zacharias, no (Röser 1998) ground-based currently about space-based reviews and IAU the 1998) in given is overview An aneac fteln oHipparcos to link the of Maintenance ≤ 12 .Zacharias N. m utilize ) N. Zacharias IERS Maintenance of the link to Hipparcos Technical

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Table 1. Important, recent catalogs for astrometry. Name of mag. bandpass number pos.err. type release comment catalog range (approx.) of stars (mas) year ACR-2 10–17.5 R 1.2 M 26– 60 S 1999 equator. areas M2000 – 17 V 2.3 M 35–100 S 2001 +11 to +18 decl. CMC12 9 – 17 r’ 6.3 M 36–113 S 2002 −3 to +3 decl. CMC13 9 – 17 r’ many 35–100 S 2003 +3 to +30 decl. UCAC1 8 – 16 579-643 27 M 25– 70 (A) 2000 −90 to −15 decl. UCAC2 8 – 16 ... 48 M 20– 70 (A) 2003 −90 to +40 decl. UCAC3 8 – 16 ... 70 M 20– 70 A 2006 all sky GSC-2.2 12 – 20 B,V 456 M 300 G 2002 posit., photom. USNO-A2 12 – 20 ... 526 M 200 G 2001 posit., photom. USNO-B 12 – 20 ... 1000 M 200 G 2002 incl. proper motions SDSS 15 – 23 u,b,v,r,z many 50–150 G 2003 about 5000 sq.deg 2MASS J,K,H 471 M 60–100 G 2003 infrared survey NOMAD -1 – 20 B,V,R,J,H,K 1100 M 10–200 A 2005 merged data set

Fig. 1. UCAC2 minus USNO-B systematic position diﬀerences. Examples are shown as a function of magnitude and right ascension.

75 76 No. 34 . 2MASS 3.1 observations IR 3 projects densification Other 2.2 IERS Note Technical al .Wrdwd e ik odnicto projects. densification to links web wide World 2. Table NOMAD 2MASS SDSS A,B USNO GSC-2.2 UCAC CMC12,13 M2000 ACR-2 Project www.nofs.navy.mil/nomad www.ipac.caltech.edu/2mass/ www.sdss.org www.nofs.navy.mil/projects/pmm/ www-gsss.stsci.edu/gsc/gsc2/GSC2home.htm ad.usno.navy.mil/ucac dwe/SRF/cmc12.html www.ast.cam.ac.uk/ www.obs.u-bordeaux1.fr/m2a/soubiran/equipe/JF/M2000/m2000.htm /pub/outgoing/calibregions ftp.nofs.navy.mil link saon 0mswt uhsalrsseai znladmagnitude and coordinate (zonal position systematic catalog smaller much per with errors. error dependent) mas average 80 the astrometry around and The (2MASS) is HCRF sources. Survey the point tele- Sky million identical on 400 2 All is over with for 2-micron obtained wavelengths south) been the into (north, has scopes is (J,H,K) frame photometry radio Hipparcos IR or the project. optical extend than to other program major The A in possible. planned is also catalogs future. are position near dependent) various location the as the declination well between as zonal, comparison color on (mainly of detailed function typically sky a magnitude, as the Variations of in level. function mas published 30 a to been as 20 catalogs the errors has ground-based systematic astrometric, Tycho-2 current, mainly The and have now. 2000). UCAC1 available al. are the et (Zacharias data between 2. preliminary high Table only comparison in the and provided A ongoing are to are projects due mentioned projects 2 all Most for about links web of wide factor World a UCAC. accu- of more by density are stars and positions Tycho-2 accuracy SDSS where using the positions, than data, UCAC is rate UCAC preliminary Astrometry using and Tycho-2 pole. When pho- via north 5-band available. HCRF galactic deep the the its to around concluded tied areas (SDSS) in Survey survey Sky tometric Digital Sloan Hipparcos the The to link to stars Tycho use & system. projects Pier these (Stone, All Scanning years 1999). Astrometric few Monet, a Flagstaff since Calibra- available the Astrometric are (FASTT) from the Telescope Transit of obtained zone release (ACR) degree second +18 Regions The to tion 2002). +11 been the al. has for et catalog Observatory (Ducourant M2000 Bordeaux the the ex- project, by similar northern A re-furbished produced A a observed. 2002). with being &Helmer, is mode Irwin tension drift-scan (Evans, in instrument observed circle been zone transit declination has ef- The degree joint Spain. +30 a and to is UK, Denmark, The program, from ACR. CMC13) institutes and of and fort M2000 (CMC12 CMC, Circle the Meridian are Carlsberg projects densification major Other aneac fteln oHipparcos to link the of Maintenance .Zacharias N. − 3 N. Zacharias IERS Maintenance of the link to Hipparcos Technical

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In 2001, pre-release 2MASS data was obtained by USNO from R. Stien- ing (private communication) and a comparison with UCAC positions was made. Overall a very good agreement was found, with largest systematic position differences being 50 mas. The results helped to better under- stand systematic errors in the 2MASS data and lead to improvements in the reduction pipeline. For the final release a significant improvement in the accuracy of 2MASS positions was achieved, with similar random errors in positions as reported on the preliminary data (Zacharias et al., 2005). End of 2002 new, preliminary 2MASS data were received and compared to preliminary UCAC2 positions. Observations for both projects were obtained almost at the same epoch. The 2MASS data is strictly reduced with Tycho-2 data for reference stars, as is UCAC2. Thus for stars not in Tycho-2 (mainly R ≥ 12) the UCAC2–2MASS differences are truly external. Some results are presented in Figures 2 and 3. Systematic errors as a function of magnitude are small except for the critical link area around 9 to 11th magnitude. There are also significant variations as a function of location in the sky and color of the stars (Figure 3). Note, the systematic differences are highlighted in these plots due to binning of a large number of stars. The average precision of a single star is about 20 to 70 mas in UCAC2 and 60 to 100 mas in 2MASS, while the systematic errors are only on the 10 to 20 mas level.

Fig. 2. Sample comparison between UCAC2 (optical) and 2MASS (IR) pre-release positions. Position diﬀerences for stars in the −30 to 0 degree declination range are shown as a function of UCAC red magnitude. Each dot represents the mean over 4000 stars to highlight systematic diﬀerences.

3.2 Other eﬀorts

At the 1.55m Strand telescope in Flagstaff, test observations were performed with a large format (1k chip) IR detector of selected extragalactic link sources. Currently optical observations are being performed with an even larger field of view (2k detector) and lower noise characteristics. As far as other wavelengths are concerned, radio observations are usu- ally tied directly into the ICRF. X-ray and Gamma-ray observations are being linked to dense optical catalogs like the USNO A and B, thus are typically on the 200 mas level if an optical identification can be made at all.

77 78 No. 34 IERS Note Technical ntergt r hw safnto fclr h aaaesltit elnto oe tpto (top zones declination into split are data The sky. the color. in of area function with variation a clear as UCAC a for shown showing figure are bottom), previous right) the the to on Similar 3. Fig. − MS oiindffrne R ntelf,Dec left, the on (RA differences position 2MASS aneac fteln oHipparcos to link the of Maintenance .Zacharias N. N. Zacharias IERS Maintenance of the link to Hipparcos Technical

Note No. 34

4 Radio–optical extragalactic link program

A comprehensive extragalactic link program is part of the UCAC project. The goal is to eventually provide a link between the ICRS and the UCAC, independent of HCRF (using block adjustment procedures) and then quasi externally assess the HCRF-ICRF link accuracy. As a first step traditionally reduced CCD frames provide optical positions of ICRF counterparts supposedly on the HCRF, using Tycho-2 and UCAC anonymous secondary reference stars in a 2-step procedure. Preliminary results from a subset of the data were published (Assafin et al., 2003; Zacharias & Zacharias, 2005). The Assafin et al. paper is based on optical positions of about 120 ICRF sources which were obtained in 3 observing runs at CTIO and KPNO during 2001, supplemented with some observations by the 1.6-meter LNA (Brazil) of optically faint targets. It uses the UCAC2 general-survey as reference stars. The Zacharias & Zacharias paper is the first to utilize the special USNO astrograph observations performed in parallel with the deep CCD imaging. Advantages of this approach are:

same bandpass of astrograph and deep CCD frames (579–643 nm): minimizing differential refraction effects of the atmosphere simultaneous observations of astrograph and deep field observations to avoid problems with unknown proper motions of anonymous secondary stars used to link the data extra observing at the astrograph provides much higher precision of star positions than regular UCAC survey data alone east and west of pier observations with the astrograph of ICRF fields provide good calibration of systematic errors to enhance the astrometric accuracy beyond the general UCAC survey data

The goal of this project is to improve the accuracy of the HCRF to ICRF link by a factor of 2 beyond what has been obtained initially with the Hipparcos and ground-based eﬀort. This observational program was completed by end of 2004 and a total of about 500 ICRF sources (including extensions 1 and 2) are likely to show measurable optical counterparts on our deep CCD imaging data, covering both the northern and southern hemisphere.

5 Future Developments

The late Christian de Vegt envisioned a next generation, dedicated, astrometric survey telescope (de Vegt, Laux, & Zacharias, 2003; Laux & Zacharias, 2005). The USNO Robotic Astrometric Telescope (URAT) project is based on this design (Zacharias, 2005). The 0.85 m aperture 3.6m focal length planned instrument has a 4 degree field of view. The goal is to obtain positions of stars on the 5 to 10 mas level for the 14 to 18 mag range with a limiting magnitude of about 20. A direct tie to the defining ICRF sources would be possible. Narrow bandpass observations will also allow to access Hipparcos stars. Funds have been secured for the detector development (large-format, monolithic chip), and optical design studies are in the final stages. Space-based projects of relevance for astrometry are the NASA Space Interferometry Mission (SIM, planet quest), the ESA GAIA mission and

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