Academic literature on the topic 'Astrometric – instrumentation: detectors'

The Hubble Space Telescope (HST) will have the capability of making relative astrometric measurements with an accuracy of four to ten times better than similar ground based measurements. The instruments of choice will be the Fine Guidance Sensors (FGS’s). Details of the instrumentation have been given elsewhere (c.f. Jefferys, 1980). Each FGS consists of two interferometer detectors which look at a small aperture located within a larger field of view (FOV). The raw data consist of encoder readings, from “star selectors”, and error signals derived from the interferometer transfer functions. The star selector positions determine the position of the small aperture within the pickles. The precision is always less than 0.001 arcsec, with an expected accuracy of 0.002 to 0.003 arcsec per observation. Under optimum conditions, the best expected accuracy is 0.0016 arcsec per observation. An observation will consist of the relative positions of several objects in one pickle. Thus the highest accuracy observations will be relative observations within a relatively small field of view.

Context. The detection of forming planets in protoplanetary disks around young stars remains elusive, and state-of-the-art observational techniques provide somewhat ambiguous results. The pre-transitional T Tauri star LkCa 15 is an excellent example. It has been reported that it could host three planets; candidate planet b is in the process of formation, as inferred from its Hα emission. However, a more recent work casts doubts on the planetary nature of the previous detections. Aims. We test the potential of spectro-astrometry in Hα as an alternative observational technique to detect forming planets around young stars, taking LkCa 15 as a reference case Methods. LkCa 15 was observed with the ISIS spectrograph at the 4.2 m William Herschel Telescope (WHT). The slit was oriented towards the last reported position of LkCa 15 b (parallel direction) and 90° from that (perpendicular). The photocenter and full width half maximum (FWHM) of the Gaussians fitting the spatial distribution at Hα and the adjacent continuum were measured. A wellknown binary (GU CMa) was used as a calibrator to test the spectro-astrometric performance of ISIS/WHT. Results. A consistent spectro-astrometric signature is recovered for GU CMa. However, the photocenter shift predicted for LkCa 15 b is not detected, but the FWHM in Hα is broader than in the continuum for both slit positions. Our simulations show that the photocenter and FWHM observations cannot be explained simultaneously by an accreting planet, but the lack of photocenter shift alone could still be consistent with an emitting planet with contrast ≳5.5 mag in Hα or ≲6 mag in the adjacent continuum. In turn, both spectro-astrometric observations are naturally reproduced from a roughly symmetric Hα emitting region centered on the star and extent comparable to the orbit originally attributed to the planet at several au. Conclusions. The extended Hα emission around LkCa 15 could be related to a variable disk wind, but additional multi-epoch data and detailed modeling are necessary to understand its physical nature. Optical spectro-astrometry carried out with mid-size telescopes is capable of probing small-scale structures in relatively faint young stars that are not easily accessible with state-of-the-art instrumentation mounted on larger telescopes. Therefore, spectro-astrometry in Hα is able to test the presence of accreting planets and can be used as a complementary technique to survey planet formation in circumstellar disks.

The Flagstaff Astrometric Scanning Transit Telescope (FASTT) has been equipped with a new CCD detector, and further improvements in instrumentation have been made. An evaluation of its performance indicates that the telescope can determine accurate star positions, tied into the VLBI/VLA extragalactic reference frame, for stars as faint as V ∼ 17.5 mag. With further improvements, it is hoped this accuracy can be improved. A special scanning technique for stars at high declinations has been developed that allows scanning up to the pole. Positions tied into the extragalactic reference frame for FK5 and radio stars have been determined with the FASTT, and a comparison of these positions with their catalog values is given.

The effect of Intra-pixel sensitivity variation (IPSV) in charge-coupled devices (CCDs) can be important in astronomical applications. This thesis studies the IPSV in a front-illuminated three-phase EEV05-20 CCD used in the Automated Patrol Telescope (APT), from multiple points of view. To explore the detailed sensitivity variation within pixels, the CCD was scanned using a 4 \mu meter diameter light beam in four colour bands: B, V, R and I. The resulting images clearly show the IPSVs due to the CCD electrode structure, and its dependence on wavelength. Unexpected ghost images appear in the scan images that are most likely due to the charge transfer inefficiency (CTI) of the CCD. A correction procedure for the CTI effect is presented. Using the pixel response function (PRF) which was derived from the CCD scans, instrumental point spread functions (iPSFs) were calculated from dithered images observed by the APT. The accurate iPSFs allowed us to generate a variety of simulated images of APT observations, enabling us to analyse in detail the effect of IPSV on astronomical observations. One of the astronomical impacts of IPSV is on photometry. The IPSV effect on the precision for estimating star fluxes was studied using both observed and simulated images. The IPSV effect can be expressed as magnitude estimation error maps plotted against the fractional part of a star's coordinates. The IPSV effect introduces \pm 4% errors in star fluxes for observed images with the APT in V band. Another astronomical impact of IPSV is on astrometry. IPSV influences the precision for estimating star coordinates, and this was studied using a number of simulated images. The IPSV effect can be expressed as coordinate estimation error maps plotted against the fractional part of a star's coordinates. The IPSV effect introduces \sim 0.02 pixel errors in RMS for images observed with the APT in V band. The appearance of the unexpected ghost images in the CCD scans suggested that CTI might also affect observed images. We examined the effects on PSFs and photometry. The CTI effect does affect the shapes of PSFs, but only to a small fraction. Its effect on photometry is negligible.