The combination of high resolution, large collecting area, and sensitivity to high energy X-rays will make it possible for AXAF to study faint sources, sometimes strongly absorbed, even in crowded fields. These unprecedented capabilities challenge us to maximize the quantity and quality of observations by achieving the best possible pointing and guiding of the spacecraft, and enabling the construction of excellent images and accurate aspect reconstruction. This article is an update on work underway to achieve a star catalog best suited to this purpose for the AXAF mission.
As is normal for an X-ray mission, pointing and guiding are done with a co-aligned optical imaging system (see SIN, newsletter #3). The AXAF Aspect Camera Assembly (ACA), a 10.4cm, f/9.5 R-C optical reflector with two swappable 1024² CCDs, measures the image positions of up to 8 selected target stars in its ( 1.4 x 1.4 deg² ) field of view. The AXAF on-board computer uses gyro attitude data and ACA stellar image centroids for real-time pointing control and attitude determination, using positions and magnitudes from the AXAF Guide and Aspect Star Catalog.
The absolute celestial pointing accuracy of the spacecraft is better than 30arcsec (99% of the time, in any 1 sec interval). Post-facto aspect determination is required for observations over 100 sec to compensate for the apparent motion of the X-ray image on the focal plane of the science instruments. The aspect solution as a function of time provides an RMS image diameter smaller than 0.5 arcsec within the central 5 arcmin field-of-view (increasing to 10 arcsec at a radius of 20 arcmin). Dithered or even tracking observation modes will be available in special circumstances that also require accurate pointing and aspect determinations as a function of time.
The AXAF Guide/Aspect Star Catalog (AGASC) will be used for
pointing control and attitude determination and for aspect
reconstruction of timed X-ray events in order to
form 0.5 arcsec X-ray images
with absolute positions good to <1arcsec.
When finished, AGASC should contain enough stars to select with the
Aspect Camera a minimum of 5 stars suitable for an astrometric
solution, 95% of the time, anywhere on the sky. This requires a star
density of about 5 deg 
to a magnitude limit in the instrumental
`ACA' system of 
. Since the ACA bandpass is wide (peaking
near 7000Å), predicted `ACA mags' ( 
)
are currently determined from
published magnitudes
and colors when available. After launch, actual ACA
mags from the AXAF Aspect Camera may be stored and/or used for
recalibration of the (V) relation as a function of color.
To select the best set of guide/aspect stars for each planned
observation, we seek a complete catalog with both magnitudes and
colors. Given the transformation between V and , the
reddest expected stars have ( 
)=3.6. Including an error
budget of 0.7 mag in colors and transformations, the AGASC should
ideally be as complete as possible, with colors, to 
.
Accurate star positions (either of recent epoch or including proper motions) are required to meet the top level AXAF requirement that X-ray photons can be assigned absolute sky coordinates accurate to better than 1 arcsec. Proper motions are needed to attain this. Parallax data are also necessary, since parallaxes are not random, and in many cases will exceed position errors.
No all-sky catalog currently exists to provide colors for all objects
to 
, let alone positions, proper motions and parallaxes.
Therefore, to amass as much of this data as possible, AGASC1.2 will
incorporate the best measurements (based on lowest quoted error) for
photometry, positions, proper motions, and parallax for all stars from
1) the Hubble Guide Star Catalog (GSC1.1), 2) the Positions and Proper
Motions Catalog (PPM), and 3) the Tycho Output Catalog (TYC).
1) The Hubble Space Telescope Guide Star Catalog (GSC1.1), constructed to support the operational need of the Hubble Space Telescope for off-axis guide stars, contains 18,819,291 objects in the magnitude range 7 - 16, of which more than 15 million are classified as stars. GSC photometry is given in the natural systems defined by the individual plates in the GSC collection (generally J or V), and photometric uncertainties are typically 0.3 mag.
2) The Positions and Proper Motions (PPM) Catalog provides
magnitudes, stellar spectral types, and proper motions for stars down
to 
, for about 400,000 stars.
Colors are still needed for the majority of stars in AGASC1.1, since merge with the PPM provided colors (from spectral types) for only the brightest 2% of GSC1.1 objects. Also, PPM-derived colors are probably very uncertain, since they are interpolated from listed Spectral Types, with no reddening information.
3) The Tycho Output Catalog (TYC) is a product of the Tycho star
mappers on the Hipparcos satellite (Hög et al. 1995, A&A, 304,
150). TYC provides reliable colors for 1,058,332 stars, nearly complete to
In the current error budget, using the highly accurate Tycho star
positions (vs. the GSC1.1) can improve the absolute aspect by
30 - 50%. Astrometric errors for Tycho stars ( 
arcsec), are of the same order as parallaxes expected for many
bright ( 
) stars typically selected by the AXAF Star Selection
Algorithm (SSA), so parallax measurements in TYC will also be included in
AGASC1.2.
The combination of these three catalogs provides all the most critical
star data over the entire celestial sphere to a depth of .
AGASC1.2 was released at the end of August 1997.
In simulations, the SSA picks stars 95% of the time having V<10.1. A few percent of stars are both faint and red enough that although they may not have a color provided by the PPM or TYC catalogs, they may be bright enough in the ACA bandpass to spoil the ACA stellar image centroids of nearby stars used for guiding. These unrecognized potential `spoiler stars' provide the motivation for further expansion of AGASC1.2 with stellar color data, when reliable data becomes available. News of further developments of the AXAF star catalog will be posted in future newsletter articles, and on the Mission Planning page of the AXAF Science Center WWW site.
Paul Green