Daniel J. Schroeder

The imaging performance of the Hubble Space Telescope

Problems with the HST instantaneous imaging performance and pointing performance are discussed. Optical tests have clearly demonstrated that the HST suffers from spherical aberration. The top level specification was that 70 percent of the energy be focused in a 0.1 in. radius, but the present, and close to optimum, focus setting gives only about 16 percent. The pointing control system also is having problems with the results that the spacecraft achieves a stability of about 0.007 rms in quiescent periods, falling short of specification which requires that such performance be maintained for 24 hr. The finite guidance sensors are not guiding well on faint stars. There is a loss of sky coverage at high Galactic latitude, especially for the Wide-Field/Planetary Camera. The consequences of these defects for the scientific program are examined.

Echelle efficiencies: theory and experiment.

Comparisons of theoretical calculations and experimental measurements of echelle grating-efficiencies are given for R2 echelles used in three possible configurations: alpha > beta;alpha < beta and the quasi-Littrow mode. The throughput-resolution products for these various cases are also compared.

Characterization and Subtraction of Well-Exposed HST/NICMOS Camera 2 Point-Spread Functions for a Survey of Very Low Mass Companions to Nearby Stars

We are conducting a search for very low mass ( M ) companions of stars within 10 pc M ! 0.2 , of the Sun using the NICMOS infrared camera on the Hubble Space Telescope. The highly exposed images of each target star are subtracted from the NICMOS Camera 2 (NIC2) field using previously observed stars so that faint companions may be detected. We have characterized the NIC2 point-spread function (PSF) at large angles and measured encircled energies useful for point-source aperture photometry corrections. The Camera 2 PSFs have elliptical diffraction rings and asymmetric diffraction spike banding patterns that appear to be caused by a misalignment of the NICMOS cold mask relative to the telescope obscurations. The masks position appears to vary about a general offset, creating a time-dependent diffraction pattern. We have devised a procedure for selecting target star images that provide the best PSF subtractions. The ultimate detection limits of our survey are F110W ∼ 21 and F222M ∼ 19.5, roughly approximating J and K measurements, respectively.

DESIGN CONSIDERATIONS FOR ASTRONOMICAL ECHELLE SPECTROGRAPHS

Principles common to the design of all spectrographs using echelles as the main dispersing element and gratings for the cross dispersion are discussed. Consideration is given to the possibilities of a Cassegrain echelle spectrograph for an f/8 beam, and a comparison of its features with those of conventional grating spectrographs. Results are presented in a form which makes them useful for echelle spectrograph design for any telescope.

Scanning Spectrometer of the Gillieson Type

A new type of plane grating monochromator has been devised in which the grating is illuminated by convergent light. The main feature of this design is an arrangement by which the grating is simultaneously translated and rotated by means of a simple rotation about an axis displaced from the grating face. The motion is such that the spectrum remains in good focus on a fixed exit slit over a range of wavelengths from zero order to 5000 A in the third order for a grating with 600 lines/mm. This type of mount has the advantages of small aberrations and small astigmatism over the usable range. Because there are only two reflections, the instrument is best suited for studying problems where low light level might be of concern, such as for astronomical work or for use in the far ultraviolet.

An Echelle Spectrograph for Astronomical use

A Cassegrain spectrograph which uses a 73.5 groove/mm echelle grating as its main dispersing element has been designed and constructed at the University of Wisconsin. The optical design and, in particular, the effects of various aberrations are discussed in detail. The mechanical design is discussed in general terms and preliminary observations of various astronomical objects are given. Key words: echelle spectrograph

All-reflecting Baker-Schmidt flat-field telescopes

The theory of the Baker-Schmidt flat-field telescope with tilted reflecting corrector and an analysis of the performance of several different all-reflecting Baker-Schmidt systems is presented. A comparison is given between the performance of a flat-field Baker-Schmidt and an all-reflecting Schmidt telescope of similar focal ratio.

Optimization of Converging-Beam Grating Monochromators*

Design principles for the optimization of a version of the Monk–Gillieson grating monochromator are developed. Third-order aberrations for coma and astigmatism are both compensated for one wavelength, which can be chosen as wanted. The general relations for the orientations of the imaging mirror and the plane grating, with respect to each other and to the light-bundle axis for correct compensation of imaging errors, are developed. Expressions are given for the residual aberrations at other than the compensated wavelength. The results are applied to a typical mirror–grating combination.

THE VERY LOW MASS COMPONENT OF THE GLIESE 105 SYSTEM

Multiple-epoch, multicolor images of the astrometric binary Gliese 105A and its very low mass companion Gliese 105C have been obtained using the Hubble Space Telescopes Wide Field Planetary Camera 2 (WFPC2) and near-infrared camera and multiobject spectrometer (NICMOS). The optical and near-infrared colors of Gl 105C strongly suggest a spectral type of M7 V for that star. Relative astrometric measurements spanning 3 yr reveal the first evidence of Gl 105Cs orbital motion. Previous long-term astrometric studies at Sproul and McCormick Observatories have shown that the period of Gl 105As perturbation is ~60 yr. To satisfy both the observed orbital motion and Gl 105As astrometric period, Gl 105Cs orbit must have an eccentricity of ~0.75 and a semimajor axis of ~15 AU. Measurements of Gl 105As radial velocity over 12 yr show a linear trend with a slope of 11.3 m s-1 yr-1, which is consistent with these orbital constraints and a nearly face-on orbit. As no other faint companions to Gl 105A have been detected, we conclude that Gl 105C is probably the source of the 60 yr astrometric perturbation.

Wide Field Planetary Camera 2 Observations of Proxima Centauri: No Evidence of the Possible Substellar Companion

Two-epoch observations of Proxima Centauri using the Hubble Space Telescope (HST) Wide Field Planetary Camera 2 (WFPC2) are reported. Exposures of 10 and 40 s were recorded through the F1042M filter (λc ≈ 1 μm), permitting examination of the circumstellar region beyond 009 from Proxima Cen. No evidence of a substellar companion within 085 of Proxima Cen is seen, which counters the recently reported detection of a faint feature by Schultz et al. using HSTs Faint Object Spectrograph (FOS). A feature in the WFPC2 images having a relative brightness and a separation comparable to those of the FOS feature would have been detected with a signal-to-noise ratio of ~22. Moreover, if the FOS feature were a substellar companion, it should have appeared in our F1042M images to be about 3.7 mag fainter than Proxima Cen. Inspection of deep WFPC2 images of Proxima Cen through three filters indicates that the FOS feature is not a background object. Local enhancements of WFPC2s point-spread function suggest a possible instrumental origin for the FOS feature, but the singularity and apparent motion of the FOS feature complicate this notion.