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Chanan, Gary

Publications and source records attributed to Chanan, Gary.

11 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS2018

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.

Martinek, Stephen J.↗
NASA NTRS2017

Keck Segment Surface Artifacts: Impacts on Segment Phasing and Image Quality

We show that the segments of the Keck telescopes su er from small but signi cant surface artifacts that limit segment phasing accuracy. Both the segment surface artifacts themselves and the limits they place on segment phasing have important implications for adaptive optics, high contrast imaging, and seeing limited observations. These surface artifacts are likely by-products of the segment polishing, most likely the Ion Beam Figuring (IBF) process that constitutes the nal step of the segment surface guring. The surface artifacts di ract a signi cant amount of light out past 3.5 arcseconds and generate chromatic e ects that limit segment phasing accuracy. We present on-sky measurements of the di racted intensity. Segment surface roughness measurements are also presented, which show that surface roughness is not the source of the problem. These results have direct relevance to segment manufacturing for the Thirty Meter Telescope (TMT) and other segmented mirror telescopes.

Brock, Neal↗
NASA NTRS2016

The Alignment and Phasing System for the Thirty Meter Telescope: Risk Mitigation and Status Update

Alignment and Phasing System (APS) is responsible for the optical alignment via starlight of the approximately 12,000 degrees of freedom of the primary, secondary and tertiary mirrors of Thirty Meter Telescope (TMT). APS is based on the successful Phasing Camera System (PCS) used to align the Keck Telescopes. Since the successful APS conceptual design in 2007, work has concentrated on risk mitigation, use case generation, and alignment algorithm development and improvement. Much of the risk mitigation effort has centered around development and testing of prototype APS software which will replace the current PCS software used at Keck. We present an updated APS design, example use cases and discuss, in detail, the risk mitigation efforts.

Optical Alignment↗
NASA NTRS2010

Shack-Hartmann Phasing of Segmented Telescopes: Systematic Effects from Lenslet Arrays

The segments in the Keck telescopes are routinely phased using a Shack-Hartmann wavefront sensor with sub-apertures that span adjacent segments. However, one potential limitation to the absolute accuracy of this technique is that it relies on a lenslet array (or a single lens plus a prism array) to form the subimages. These optics have the potential to introduce wavefront errors and stray reflections at the subaperture level that will bias the phasing measurement. We present laboratory data to quantify this effect, using measured errors from Keck and two other lenslet arrays. In addition, as part of the design of the Thirty Meter Telescope Alignment and Phasing System we present a preliminary investigation of a lenslet-free approach that relies on Fresnel diffraction to form the subimages at the CCD. Such a technique has several advantages, including the elimination of lenslet aberrations.

segmented mirrors↗
NASA NTRS2004

Experimental Verification of Dispersed Fringe Sensing as a Segment Phasing Technique using the Keck Telescope

Dispersed fringe sensing (DFS) is an efficient and robust method for coarse phasing of segmented primary mirrors (from one quarter of a wavelength to as much as the depth of focus of a single segment, typically several tens of microns). Unlike phasing techniques currently used for ground-based segmented telescopes, DFS does not require the use of edge sensors in order to sense changes in the relative heights of adjacent segments; this makes it particularly well suited for phasing of space-borne segmented telescopes, such as the James Webb Space Telescope. We validate DFS by using it to measure the piston errors of the segments of one of the Keck telescopes. The results agree with those of the Shack-Hartmann-based phasing scheme currently in use at Keck to within 2% over a range of initial piston errors of +/-16 (mu)m.

algorithm↗
NASA NTRS2004

Experimental Verification of Dispersed Fringe Sensing as a Segment Phasing Technique using the Keck Telescope

Dispersed Fringe Sensing (DFS) is an efficient and robust method for coarse phasing of segmented primary mirrors (from a quarter of a wavelength up to the depth of focus of a single segment, typically several tens of microns). Unlike phasing techniques currently used for ground-based segmented telescopes; this makes it particularly well-suited to the phasing of space-borne segmented telescopes, such as the James Webb Space Telescopes (JWST). In this work we validate DFS by using it to measure the pistons of the segments of one of the Keck telescopes; the results agree with those of the Shack-Hartmann based phasing scheme currently in use at Keck to within 2% over a range of initial piston errors of +/-16 microns.

dispersed fringe sensors↗
NASA NTRS1988

Prospects for solar flare X-ray polarimetry

A review of the literature indicates that no experimental evidence has yet been found for solar flare X-ray polarimetry. A satellite instrument is described that has sufficient sensitivity at high energies to detect the predicted solar flare polarization. It is shown that the instrument sensitivity for a moderate event approaches polarization levels of 1 percent in each of seven energy bins spanning the 10-100 keV range for integration times as short as 10 s.

Chanan, Gary↗
NASA NTRS1986

Impulsive phase solar flare X-ray polarimetry

The pioneering observational work in solar flare X-ray polarimetry was done in a series of satellite experiments by Tindo and his collaborators in the Soviet Union; initial results showed high levels of polarization in X-ray flares (up to 40%), although of rather low statistical significance, and these were generally interpreted as evidence for strong beaming of suprathermal electrons in the flare energy release process. However, the results of the polarimeter flown by the Columbia Astrophysics Laboratory as part of the STS-3 payload on the Space Shuttle by contrast showed very low levels of polarization. The largest value (observed during the impulsive phase of a single event) was 3.4% + or - 2.2%. At the same time but independent of the observational work, Leach and Petrosian (1983) showed that the high levels of polarization in the Tindo results were difficult to understand theoretically, since the electron beam is isotropized on an energy loss timescale. A subsequent comparison by Leach, Emslie, and Petrosian (1985) of the impulsive phase STS-3 result and the above theoretical treatment shows that the former is consistent with several current models and that a factor of approximately 3 improvement in sensitivity is needed to distinguish properly among the possibilities.

Chanan, Gary↗