Directions in VLBI technology

Magnetic tape recording, disc recording, and global electronic networks are advancing rapidly, promising both evolutionary and revolutionary impacts on the way VLBI is conducted. In this paper, we will examine the state-of-the-art technologies that may have application to VLBI and consider the current and projected costs in each of these areas.     

The Hayabusa Spacecraft Asteroid Multi-band Imaging Camera (AMICA)

The Hayabusa Spacecraft Asteroid Multi-band Imaging Camera (AMICA) has acquired more than 1400 multispectral and high-resolution images of its target asteroid, 25143 Itokawa, since late August 2005. In this paper, we summarize the design and performance of AMICA. In addition, we describe the calibration methods, assumptions, and models, based on measurements. Major calibration steps include corrections for linearity and modeling and subtraction of bias, dark current, read-out smear, and pixel-to-pixel responsivity variations. AMICA v-band data were calibrated to radiance using in-flight stellar observations. The other band data were calibrated to reflectance by comparing them to ground-based observations to avoid the uncertainty of the solar irradiation in those bands. We found that the AMICA signal was linear with respect to the input signal to an accuracy of ?1% when the signal level was <3800 DN. We verified that the absolute radiance calibration of the AMICA v-band (0.55 μm) was accurate to 4% or less, the accuracy of the disk-integrated spectra with respect to the AMICA v-band was about 1%, and the pixel-to-pixel responsivity (flat-field) variation was 3% or less. The uncertainty in background zero level was 5 DN. From wide-band observations of star clusters, we found that the AMICA optics have an effective focal length of 120.80 ± 0.03 mm, yielding a field-of-view (FOV) of 5.83° × 5.69°. The resulting geometric distortion model was accurate to within a third of a pixel. We demonstrated an image-restoration technique using the point-spread functions of stars, and confirmed that the technique functions well in all loss-less images. An artifact not corrected by this calibration is scattered light associated with bright disks in the FOV.     

Long-term degradation of optical devices on the Moon

Forty years ago, Apollo astronauts placed the first of several retroreflector arrays on the lunar surface. Their continued usefulness for laser ranging might suggest that the lunar environment does not damage optical devices. However, new laser ranging data reveal that the efficiency of the three Apollo reflector arrays is now diminished by a factor of 10 at all lunar phases and by an additional factor of 10 when the lunar phase is near full Moon. These deficits did not exist in the earliest years of lunar ranging, indicating that the lunar environment damages optical equipment on the timescale of decades. Dust or abrasion on the front faces of the corner-cube prisms may be responsible, reducing their reflectivity and degrading their thermal performance when exposed to face-on sunlight at full Moon. These mechanisms can be tested using laboratory simulations and must be understood before designing equipment destined for the Moon.     

In-flight calibration of the Cassini imaging science sub-system cameras

We describe in-flight calibration of the Cassini Imaging Science Sub-system narrow- and wide-angle cameras using data from 2004 to 2009. We report on the photometric performance of the cameras including the use of polarization filters, point spread functions over a dynamic range greater than 10⁷, gain and loss of hot pixels, changes in flat fields, and an analysis of charge transfer efficiency. Hot pixel behavior is more complicated than can be understood by a process of activation by cosmic ray damage and deactivation by annealing. Point spread function (PSF) analysis revealed a ghost feature associated with the narrow-angle camera Green filter. More generally, the observed PSFs do not fall off with distance as rapidly as expected if diffraction were the primary contributor. Stray light produces significant signal far from the center of the PSF. Our photometric analysis made use of calibrated spectra from eighteen stars and the spectral shape of the satellite Enceladus. The analysis revealed a shutter offset that differed from pre-launch calibration. It affects the shortest exposures. Star photometry results are reproducible to a few percent in most filters. No degradation in charge transfer efficiency has been detected although uncertainties are large. The results of this work have been digitally archived and incorporated into our calibration software CISSCAL available online.     

Thermal disturbances caused by lander shadowing and the measurability of the martian planetary heat flow

A measurement of the martian planetary heat flow requires the determination of the subsurface temperature gradient, which is affected by surface insolation. I investigate the propagation of thermal disturbances caused by lander shadowing and derive measurement requirements for in situ heat flow experiments. I find that for short term measurements spanning 180 sol, a measurement depth of at least 2 m is needed to guarantee a stable thermal environment directly underneath the lander for Moon-like thermal conductivities of . For extremely large conductivities of , this depth needs to be increased to 4 m, but if the probe can be deployed outside the lander structure, the respective depths can be decreased by 1 m. For long term measurements spanning at least a full martian year heat flow perturbations are smaller than 5% below a depth of 3 m directly underneath the lander. Outside the lander structure, essentially unperturbed measurements may be conducted at depths of 0.5 and 1.5 m for thermal conductivities of 0.02 and , respectively.     

Particle sieving and sorting under simulated martian conditions

We report on sorting of small grained material under simulated martian conditions in order to better understand the nature of particle movement in the acquisition-to-analysis chain for future martian missions. We find that triboelectric charging when material is sieved is a major phenomenon that has to be understood and mitigation strategies explored in order to be able to successfully move particles under these types of conditions while minimizing cross sample talk. In different experimental set-ups, we have observed such phenomena as caking of the sieve, adhesion of particles to hardware, clodding of dry fines, and electrostatic repulsion. These phenomena occur when different experimental testing is performed with varied configurations and environmental conditions. Identifying these electrostatic effects can help us understand potential bias in the analytical instruments and to define the best operational protocols to collect samples on the surface of Mars. These experiments demonstrate the need for end-to-end system testing under the most realistic environmental conditions and platforms before mission configurations can be demonstrated before launch.     

Spectral Extraction Algorithm for LAMOST Two-dimensional Fiber-spectroscopic Images

This paper describes the method of spectral extraction to be used for the data processing of LAMOST's two-dimensional fiber-spectroscopic images. The effect of the selection of sampling points on the result of spectral extraction is analyzed. Based on the characteristics of Gaussian distribution functions and some experiments, the selection range of sampling points is defined, and the problem that some negative flux values appear in the result of spectral extraction is resolved. In addition, aiming at the problem that rather large errors exist in the result of spectral extraction under strong noise background, an improved extraction method based on the frequency-domain filtering is proposed. By using the FFT and low-pass filter, the sharp noises affecting the real fiber profiles are removed at first, before the normal spectral extraction is performed. Experiments and tests are made by using the simulative data provided by the data processing system of the LAMOST telescope, and the results indicate that the improved extraction method is feasible and effective.     

A powerful new infrared array camera for the NASA Infrared Telescope Facility

We have just finished building a 1.0–5.5 m infrared array camera for the NASA Infrared Telescope Facility based upon the SBRC 256×256 InSb array. After a brief overview of the capabilities of the camera (named NSFCAM), we present some of the first images obtained with this new camera, including results obtained with our realtime shift & add speckle imaging mode.     

Astronomy with a mid-infrared echelle spectrograph

Irshell, a mid-infrared echelle spectrograph developed through a collaboration between the University of Texas, the University of California Space Sciences Laboratory, and Hughes Aircraft, has been in regular use for astronomical observations for about five years. It is optimized for high spectral resolution (R10,000) observations of narrow spectral regions (/1/300) and compact sources (a few arcsec), and is sensitive in the 5–25 m region. A wide variety of astronomical objects have been studied. Observing techniques have been developed to achieve maximum sensitivity in those observations for which Irshell is best suited and to allow observations of wide spectral regions and extended objects. We discuss the observing techniques and show some of the data obtained.     

Probing Titan’s lower atmosphere with acousto-optic tuning

Narrow-band images of Titan were obtained in November 1999 with the NASA/GSFC- built acousto-optic imaging spectrometer (AImS) camera. This instrument utilizes a tunable filter element that was used within the 500- to 1050-nm range, coupled to a CCD camera system. The images were taken with the Mount Wilson 2.54-m (100 in.) Hooker telescope, which is equipped with a natural guide star adaptive optics system. We observed Titan at 830 and 890 nm and at a series of wavelengths across the 940-nm window in Titan’s atmosphere where the methane opacity is relatively low. We determined the absolute reflectivity (I/F) of Titan and fit the values at 940 nm to a Minnaert function at Titan’s equator and at −30° latitude (closer to the subsolar point) and obtained average values for the Minnaert limb-darkening slope, k, of 0.661 ± 0.007 and 0.775 ± 0.018, respectively. Comparison with models suggests that the equatorial value of k is consistent with rain removal of haze in the lower atmosphere. The higher value of k at −30° is consistent with the southern hemisphere being brighter than the equator. However, the fits are not unique. The data and models at 890 are consistent with no limb brightening or darkening at this wavelength either at the equator or at −30°.