187 USING THE TYCHO CATALOGUE FOR AXAF: GUIDING AND ASPECT RECONSTRUCTION FOR HALF-ARCSECOND X-RAY IMAGES P.J. Green, T.A. Aldcroft, M.R. Garcia, P. Slane, J. Vrtilek Smithsonian Astrophysical Observatory High resolution imaging and/or fast timing measure- ABSTRACT ments are enabled by the High Resolution Camera HRC; Kenter 1996. Advances over the high resolu- tion imagers of Einstein and ROSAT include smaller AXAF, the Advanced X-ray Astrophysics Facility will p ore size, larger micro channel plate area, lower back- b e the third satellite in the series of great observato- ground, energy resolution, and charged particle anti- ries in the NASA program, after the Hubble Space coincidence. Telescop e and the Gamma Ray Observatory. At launch in fall 1998, AXAF will carry a high reso- The AXAF CCD Imaging Sp ectrometer ACIS; lution mirror, two imaging detectors, and two sets of Garmire 1997 is a CCD array for simultaneous imag- transmission gratings Holt 1993. Imp ortant AXAF ing and sp ectroscopyE=E =2050 over almost features are: an order of magnitude improvementin the entire AXAF bandpass with high quantum e- spatial resolution, good sensitivity from 0.1{10keV, ciency and low read noise. Pictures of extended ob- and the capability for high sp ectral resolution obser- jects can b e obtained along with sp ectral information vations over most of this range. from each element of the picture. The ACIS-I array comprises 4 CCDs arranged in a square which pro- The Tycho Catalogue from the Hipparcos mission 2 vide a 16 16 arcmin eld. The ACIS-S array has 6 will serve as a primary part of the AXAF Guide CCDs of the same typ e, lined up in a 1 6 con gu- and Asp ect Star Catalog AGASC, particularly as ration. Although ASCA carries a similar CCD array, merged with other star catalogues. Incorp oration of its mirror limits the spatial resolution to 2 arcmin. the Tycho Catalogue into AGASC will provide: 1 accurate p ositions and magnitudes for target acqui- There are two transmission grating sp ectrometers for sition and guiding; 2 the colours necessary for mag- use with either the HRCorACIS detectors. One set nitude transformations to the AXAF Asp ect Camera is optimized for low energies LETG and the other ACA system; 3 high internal astrometric accu- for high energies HETG, providing resolving p owers racy for half-arcsecond p ost-facto image reconstruc- from 100{2000 e.g. Flanagan 1996. tion; and 4 an external astrometric system that will tie AXAF X-ray image astrometry into the b est p osi- tional system exp ected well b eyond the mission life- time and into the next millennium. 2. ASPECT DETERMINATION Key words: space astrometry; X-rays. The AXAF Asp ect Camera Assembly ACA, a 10.4 cm, f/9.5 R-C optical re ector with twoswap- 2 1. INTRODUCTION pable 1024 CCDs, measures the image p ositions of 2 up to 8 selected target stars in its 1:4 1:4 deg eld of view. The AXAF on-b oard computer uses gyro attitude data and ACA stellar image centroids The AXAF X-ray telescop e, after launch in fall for real-time p ointing control and attitude determina- of 1998, will follow an elliptical high-earth orbit tion, using p ositions and magnitudes from the AXAF 140 000 km allowing uninterrupted observing in- Guide and Asp ect Star Catalog. The absolute celes- tervals of more than 48 hours. AXAF's four nested tial p ointing accuracy of the spacecraft is required mirror pairs in Wolter typ e 1 geometry have a fo cal to be < 30 arcsec 99 per cent of the time, in any length of 10 m. The largest mirror's diameter is 1.2 m 1s interval. Post-facto asp ect determination is re- see Figure 1. Small re ection angles and iridium quired for observations over 100 s to comp ensate for coating improve high energy mirror resp onse, while the apparent motion of the X-ray image on the fo cal improved grinding and p olishing techniques yield a plane of the science instruments. The asp ect solu- nal spatial resolution of 0.5 arcsec Van Sp eybro eck tion as a function of time provides for an RMS image 1987. The combination of high resolution, large col- diameter smaller than 0.5 arcsec within the central lecting area, and sensitivity to higher energy X-rays 5 arcmin eld-of-view, increasing to 2 arcsec at a ra- will make it p ossible for AXAF to study faint sources, dius of 20 arcmin. sometimes strongly absorb ed, in crowded elds. 188 Figure1. A schematic of the Advanced X-ray Astrophysics Facility AXAF telescope system. Using the gyros and asp ect camera data, the asp ect solution provides an accuracy of < 1 arcsec relativeto reference star lo cations, once p er integration 1, 2, or 4 s. For accurate stellar centroids at the CCD plate scale of 5 arcsec p er pixel, the camera is defo cused, yielding a FWHM of a star image of ab out 9 arcsec. Tolerable centroiding errors for stellar images result when reference star magnitudes in the instrumental `ACA' system are brighter than m =10:2. The AC A ACA bandpass is broad, p eaking near 7000A Fig- ure 2. For the star catalogue, we determine the conversion from common published magnitudes to the AXAF Camera CCD magnitude m as a function of AC A colour by convolving the ACA bandpass with the Bruzual-Persson-Gunn-Stryker stellar sp ectrophoto- metric atlas Gunn & Stryker 1983. The zero- p oint for each lter was established by convolution of the bandpass with a mag=0 sp ectrum of typ e G0V BD+26 3780, normalized to V = 0. The relation go es severely non-linear redder than B V =1:4 see Figure 3. A reasonable t is achieved for the entire colour range as follows: m = V +0:283 0:58B V AC A Figure2. The approximate bandpass of the Aspect Cam- era CCD solid line is plotted over bandpasses of com- 2 4 +0:23B V 0:21B V mon optical lters, B, V, R, and I dashed lines. All these lter transmission curves are normalizedtopeak at unity. 3. REQUIREMENTS FOR THE AXAF STAR CATALOGUE m = 10:2, using the end-of-life eld-of-view of AC A 2 1.79 deg . To attain this goal, we need a catalogue The AXAF Guide/Aspect Star Catalog AGASC, with b oth magnitudes and colours. when nished, should contain enough stars to detect with the Asp ect Camera a minimum of 5 stars suit- `Sp oiler stars' are those whichmay degrade the astro- able for an astrometric solution, 95 per cent of the metric solution obtained from a nearby star used for time, anywhere on the sky. This requires a star den- guiding or asp ect. Red stars fainter than 10.2 mag in 2 sity of 5.11 deg to an instrumental magnitude of their published bandpass, but brighter than 10.2 mag 189 Table 1. A quick reference for the AXAF imaging and spectroscopic capabilities using the HRCorACIS. HRC-I HRC-S units FIELD of VIEW 31 31 7 97 arcmin SPECTRAL RANGE 0:1 10 keV SPATIAL RESOLUTION < 0:5 arcsec TIME RESOLUTION 0.016 s 15 2 1 SENSITIVITY 10 erg cm s for p owerlaw p oint source, t = 300ks, and = 1:4 ACIS-I ACIS-S FIELD of VIEW 17 17 8:5 51 arcmin SPECTRAL RANGE 0:5 8 keV PIXEL SIZE < 0:5 arcsec 14 2 1 SENSITIVITY 10 erg cm s for p owerlaw p oint source, t = 100ks, and = 1:4 4. AGASC1.1 = Hubble + PPM The Hubble Space Telescop e Guide Star Catalog GSC1.1, constructed to supp ort the op erational need of the Hubble Space Telescop e for o -axis guide stars, contains 18 819 291 ob jects in the range 7{ 16 mag, of which more than 15 million are classi ed as stellar. GSC photometry is given in the natural systems de ned by the individual plates in the GSC collection generally J or V, and photometric uncer- tainties are typically 0.3 mag. The Positions and Prop er Motions PPM Cata- logue provides magnitudes, stellar sp ectral typ es, and prop er motions for stars down to V 10 mag, for ab out 400 000 stars. Matching the catalogues, and incorp orating prop er motions, we nd an optimal positional matching tol- erance of 10 arcsec. To that separation, 99.7 per cent of PPM stars are matched. We then p erform a magnitude matching, with a lib eral tolerance of 2 mag. Using the PPM sp ectral typ es, we ob- tain B V colours by interp olation, and express the conversion to V via a single co ecient , where Figure 3. The predicted di erence between the AXAF m = V + B V is used when the PPM mag is GS C Aspect Camera magnitude m and Johnson V mag, AC A visual, or m = B + 1B V is used when GS C as a function of colour Johnson B V . the PPM mag is photographic. The surface density of ob jects matched b etween the 2 GSC1.1 and PPM with m 10:2, is 10.6 deg AC A 2 in the red-sensitive instrumental ACA system are for jbj < 10 , and 4.9 deg for jbj > 80 . This latter a particular danger. Given the transformation be- surface density is still slightly low for AXAF mission tween V and m , the reddest exp ected stars have requirements.