Gary A. Chanan

Chromatic effects in narrowband phasing of the Keck Telescope segments: theory and numerical simulations

The narrowband segment phasing algorithm that was originally developed at Keck was replaced many years ago by a broadband algorithm that, although slower and less accurate than the former, has proved to be much more robust. A thorough investigation into the lack of robustness of the narrowband algorithm has now shown that this results from systematic errors (∼ 20 nm on average) that are wavelength-dependent. We show that the seemingly continuous distribution of these chromatic errors in fact results from (at least) two independent causes. The largest and most problematic effects are due to “plateaus” of unremoved material that were covered by supports during the ion beam figuring of three of the segments, but other smaller chromatic effects are also shown to be present and these are not yet understood. If the purely chromatic effects can be eliminated, we show that the intrinsic accuracy of the narrowband algorithm is about 6 nm (surface).

Interferometric characterization of Keck segment edge errors

The Keck telescope segments were manufactured by stressed mirror polishing of large circular pieces of Zerodur that were then cut into hexagons and finished by Ion Beam Figuring (IBF). It has long been believed that this process results in segments with little or no edge effects. As a result, this same general approach is planned for segment manufacturing for the Thirty Meter Telescope (TMT) and the European Extremely Large Telescope (E-ELT). However, recent measurements at the Keck telescope suggest that at least some of the Keck segments have significant aberrations within 60 mm of the edge. These aberrations impact the telescope phasing and the overall telescope image quality. We present interferometric measurements of multiple Keck segments, characterizing the surface errors near the edges over spatial periods from ~5 cm down to ~1 mm. We show that the largest phasing and image quality effects are due to plateaus of unremoved material, left behind after IBF as a result of obscuration by the IBF supports. Apart from these plateaus, the edge quality is relatively good, though not as good as in the segment interiors. Some residual phasing and image quality effects remain, and these are not currently understood.

Spectroscopy of six X-ray-selected BL Lacertae candidates

Results of a continuing program aimed at extending the small list of X-ray-selected BL Lac objects are reported. High-quality spectra have been obtained of six faint blue objects that lie within the positional error boxes of X-ray sources discovered serendipitously by the Einstein Observatory. Three of the objects are found to be previously uncataloged low-red-shift quasi-stellar objects, including one formerly suggested as BL Lac candidate. Two are faint galactic stars, while the final object has a featureless spectrum, and thus remains a candidate. Although X-ray selection may ultimately be an effective means of discovering faint BL Lac objects, such sources are evidently rare at the X-ray flux levels attainable by the Einstein Observatory.

An X-Ray Selected List of QSOs: Implications for the QSO Luminosity Function and the Origin of the Diffuse X-Ray Background

For the past two years we have conducted a program aimed at optical identification of x-ray sources discovered serendipitously by the Einstein Observatory. We obtain spectroscopy of all visible objects in the positional error boxes of the new sources, and have thus far examined about 120 such boxes to a limiting flux of V=18.5 and.010 Einstein IPC counts/sec. Among our findings are the discovery of 50 previously unreported active galactic nuclei (QSOs or compact Seyferts), ranging in redshift from z=0.03 through z=1.40. This list of identifications has a distribution of red-shift with flux quite different from an optically or radio selected sample. The implications of these differences are twofold. First, new constraints are available on the form and evolution of the luminosity function of quasars. Secondly, there are interesting inferences regarding the contribution of QSOs to the diffuse x-ray background radiation. Previous studies of radio-selected QSOs have shown a strong correlation between radio and x-ray luminosity, thus providing a warning that conclusions regarding the composition of the background radiation based on such studies of preselected objects must be regarded with extreme caution. Our program avoids this basic selection effect, and provides new insight into the problem of the contribution of discrete sources to the background radiation.