Parallaxes by CCD's

C. Dahn, US Naval Observatory, Flagstaff

The history of trigonometric stellar parallax determinations using CCD detectors dates back only to 1979 when Dave Monet, then a post-doc at KPNO, applied for and received observing time on the mayall 4-m reflector to investigate the potential of the Fairchild CCD-211 chip for astrometric applications. Dave invited me to collaborate with him and observations commenced in July of 1980. Mounted at prime focus, this chip provided a field of view of only about 1.4x1.9 arcminutes and suitable reference frames were available on only small numbers of targets selected from Luyten's POSS proper motion survey. Results from this investigation have been presented in Monet & Dahn (1983).

In 1983 a program was initiated on the USNO 1.5-m using a TI800x800 chip made available from the HST WF/PC Team and Dave Monet joined the staff in Flagstaff that same year. Results from that program demonstrated that relative parallaxes with formal mean errors in the 0.5 to 1.2 mas range are readily achieved if suitable reference star frames are available (Monet et al. 1992). Unfortunately, good reference frames were often still not found even with the larger field of view (2.7 arcminutes on a side) of this telescope/chip configuration.

Several similar CCD parallax programs were soon initiated at other observatories -- most noteably, the Southern Hemisphere efforts in Australia by Ianna and co-workers (Ianna 1993) and in Chile by Ruiz and co-workers (Ruiz et al. 1990); and also a more limited program at Mt. Wilson carried out by Tinney (1993). Although handicapped by the limited access to general-user telescopes, all three programs have demonstrated the ability to produce high quality parallaxes (formal mean errors in the range 2 - 4 mas).

In 1992 USNO began using a Tek2Kx2K CCD which provided a 11.1x11.1 arcminute field at the 1.5-m telescope. Reductions on observations obtained through April 1995 show that (1) parallaxes with formal mean errors in the range 0.5 - 1.0 mas are routinely being obtained; and (2) no potential target has been found that does not have a suitable reference star frame. The major remaining restriction -- that of magnitude compensation to allow targeting stars brighter than V=13 -- is now being addressed. Tests of a highly uniform neutral density spot providing 9.0 mag of attenuation for the target star and located immediately in front of a Tek2Kx2K CCD are presently in progress and all results thus far are very favorable. This system will permit CCD quality parallax determinations for stars as bright as V=5.

Ianna, P.A. 1993, in "Developments in Astrometry and Their Impact on Astrophysics and Geodynamics," IAU Symp. No. 156, ed. I.I.Mueller and B. Kolaczek, 75.

Monet, D.G. & Dahn, C.C, A.J.,88,1489,1983.

Monet, D.G. et al. 1992, A.J.,103,638.

Ruiz, M.T. et al. 1990, A.J.,100,1270.

Tinney, C.G. 1993, A.J.,105,1169.