Engineering topics

Van Altena, W.

Publications and source records attributed to Van Altena, W..

4 records

NASA NTRS1993

Periodic low-amplitude variations in the brightness of Proxima Centauri

Using data acquired with Fine Guidance Sensor No. 3 on board the Hubble Space Telescope we find that the brightness of Proxima Centauri varies with an amplitude of 0.01 mag and a period P = 41.6 +/- 0.6 days. We interpret this variation as rotational modulation of star spots or bright regions in the photosphere of Proxima Cen. In a total of 89.36 min of monitoring over 212 days, we detected one major flare and three post-flare brightenings.

Benedict, G. F.↗

NASA NTRS1992

Astrometric performance characteristics of the Hubble Space Telescope fine guidance sensors

Each of the three fine guidance sensors for the Hubble Space Telescope constitutes a sixth science instrument to be used for astrometry. We detail the tests and results used in choosing one of the three sensors to be the prime astrometer. The Astrometry Science Team has chosen Fine Guidance Sensor (FGS) 3. FGS 3 produces position measurements on a star with V = 17 with a peraxis precision of 0.003 arcsec. The interferometer response function should permit double-star astrometry at least down to V = 16 for the central region of FGS 3. In a 10-h test of the stability of POS-mode astrometric measurements made in FGS 3, we found no scale or orientation variations greater than two parts in 100,000. During this same time period, we found no statistically significant systematic guide-star radial separation changes. Spacecraft jitter is found to be the prime determinant of astrometry success or failure.

Benedict, G. F.↗

NASA NTRS1983

Theoretical studies of the effects of grain noise on photographic stellar astrometry and photometry

A method of incorporating the effects of photographic emulsion grain noise into digital image centering algorithms is presented which improves the accuracy of the derived stellar positions and magnitudes. Theoretical formulae are then derived for the limiting error of the center, and the photometric parameters. For IIIa-J, this error is 0.2-0.3 mu for bright unsaturated images, which agrees quite well with measurements made with the Yale PDS microdensitometer. It is expected that, with further improvements in the positional accuracy of the PDS, it should be possible to reach the emulsion grain noise limit, providing that emulsion shifts or other large scale errors do not dominate. It is also shown that, with appropriate trimming, marginal distribution image centering algorithms can yield an accuracy only slightly poorer than that obtained with two-dimensional distributions.

Lee, J.-F.↗

NASA NTRS1979

Space telescope astrometry

A description of instruments used for astrometric observations are examined with special emphasis on the Fine Guidance Systems of the space telescope. The telescope consists of a 2.4 meter diameter primary and a secondary figured to the Ritchey-Chretien design that yields a f/24 focal plane. The function of the FGS is to keep the total guidance jitter within the ST requirement of + or - 0.0007 rms, and to maximize the probability of finding suitable guide stars with the available 210 arcmin. Attention is given to the focal plane arrangement and to the ray paths and output images from the Koester's prisms. Some projected scientific uses of the space telescope are given as well as beneficial areas of close cooperation between the space telescope and the ESA.

Van Altena, W.↗