David F. Buscher

Diffraction-limited imaging with partially redundant masks. I. Infrared imaging of bright objects

The utility of partially redundant pupil geometries has been studied in the context of near-infrared speckle imaging with ground-based telescopes. Using both numerical simulations and experimental data collected with a 4-m-class telescope, we find that the decrease in redundancy resulting from apodizing the telescope pupil results in an enhancement of the quality of reconstructed images at high light levels. This improvement in imaging fidelity is particularly valuable when short-term variations in the statistics of the atmosphere make the seeing calibration of speckle interferograms difficult. However, the use of an apodizing mask necessarily restricts the faintest source that can be imaged, leading to a loss in sensitivity of one to two magnitudes. For many of the brighter near-infrared astrophysical sources in the sky that have been the subject of previous speckle-imaging studies, the use of a partially redundant pupil is expected to enhance the fidelity of the imaging procedure considerably.

Weather trends at the Magdalena Ridge Observatory

There have been astronomical observatories on Magdalena Ridge in south-central New Mexico since the late 1960s. Magdalena Ridge is relatively flat, at an average elevation of 10,560 feet (3220 meters) with a north-south length of 3/4 of a mile. In 2000 the Magdalena Ridge Observatory began site testing for two new facilities: a 2.4-meter optical telescope and a 10-element optical interferometer. As part of that testing, meteorological instrumentation was deployed at several locations across the mountain. As a result, we have an 18 year history of regular experience with the environment, including weather and cloud cover data for much of this time period. We present trends in the basic meteorological parameters: temperature, humidity, barometric pressure, wind speeds and directions, and cloud cover. Diurnal temperatures ranges vary from 15 C° in the spring when it is largest to 10 C° in the summer months when it is smallest. Barometric pressure varies more in the spring and fall than in the summer. Annual rain fall levels vary greatly with an average of about 10 inches of rain per year. The snow amounts have traditionally been very hard to measure as the area is partly above the tree line and wind-blown snow can leave parts of the region barren while other parts have a foot or more of snow. Winds speeds are typically 10 to 20 miles per hour. Wind speeds have been measured above 100 mph (45 m/s), with wind gusts as high as 125 mph (56 m/s), though this is primarily a spring phenomenon. The wind direction is predominately out of the Southwest. Wind speeds at the 2.4-meter telescope location are frequently 2 times as high wind speeds at the optical interferometer site due to the differences in terrain to the West of the two sites. An optical allsky camera has been in operation on the Ridge from 2003 to 2012 with nightly sequences of images obtained on most nights when the winds were less than 15 m/s and the humidity below 90%. Analysis of this imagery shows that a majority of the nights would be useable for astronomical observations. We present an overview of statistics of the site and discuss how these statistics will be used for defining appropriate operational windows for the Magdalena Ridge Observatory Interferometer.

Diffraction-limited imaging with partially redundant masks: II. Optical imaging of faint sources

In a recent paper [J. Opt. Soc. Am. A 9, 203 (1992)] the benefits of pupil apodization were examined for the near-infrared imaging of bright sources. In the current paper we extend these considerations to optical speckle imaging, in which photon noise rather than detector readout noise is important. We demonstrate that a one-dimensional pupil geometry (i.e., a thin slit) has several advantages over an unapodized aperture when faint sources are being observed through atmospheric turbulence. The use of a slit aperture does not decrease the signal-to-noise ratios of the power-spectrum and bispectrum measurements, and in many cases it increases them, despite the large reduction in signal level. The disadvantage of this apodization is a reduction in Fourier-plane coverage, which must be compensated for by observations with the slit aligned at several position angles. The performance of many of the current generation of photon-counting imaging detectors deteriorates at the high counting rates that can be experienced even when one is observing sources that are approaching the limiting magnitude of the speckle imaging technique. Under such conditions, we recommend the use of an apodized pupil, in contrast to the current preference for employing a neutral-density filter to reduce the detector count rate.