John P. Lehan

Development of lightweight x-ray mirrors for the Constellation-X mission

Constellation-X is NASAs next major X-ray astronomical observatory. Its salient features are its very large effective X-ray collecting area (about 30,000 cm2 at 1 keV) and high resolution gratings and cryogenic detector systems. The large mirror effective area presents unique and unprecedented challenges in optical fabrication and metrology. In this paper we report on the development of very lightweight X-ray mirrors that address these challenges. We use a two-step mirror fabrication process: (1) slumping thin (0.4mm) flat glass sheets to generate high quality substrates that may have mid-frequency figure errors, and (2) reducing or eliminating the mid-frequency errors using an epoxy replication process. We discuss problems and the potential associated with each of these two steps. Based on our work to date, we expect that this technology to be able to meet the baseline Constellation-X requirements, i.e, 15 HPD (half-power diameter) at the observatory level. In the next few years, we will further advance this technology and expect it to reach the Constellation-X goal: 5 HPD at the observatory level.

An approach for alignment, mounting, and integration of IXO mirror segments

The telescope on the International X-ray Observatory (IXO) comprises nearly 15 thousand thin glass mirror segments, each of them is capable of reflecting board-band soft x-rays at grazing angles. These mirror segments form densely packed, two-staged shells, in a Wolter type I optical design, in which each pair of the mirrors focus x-ray onto the focal plane in two reflections. The requirement in angular resolution of the IXO telescope is 5 arc-seconds. This requirement places severe challenges in forming precisely shaped mirror segments as well as in aligning and mounting these thin mirrors, which are 200 to 400 mm in size and 0.4 mm in thickness. In this paper, we will describe an approach for aligning and mounting the IXO mirror segments, in which no active adjustment is made to correct for any existing figure errors. The approach comprises processes such as suspension of a mirror under gravity to minimize gravity distortion, temporary bonding onto a strongback, alignment and transfer to a permanent structure and release of mirror from the temporary mount. Experimental results and analysis in this development are reported.

Measuring the phase transfer function of a phase-shifting interferometer

In characterizing the performance of a phase-shifting interferometer, the dependence of the measured height on the spatial frequency is rarely considered. We describe a test mirror with a special height relief that can be used to measure the height transfer function for the interferometer in a fashion analogous to the measurement of the modulation transfer function for the optical imaging system. We fabricated the test mirror at the National Institute of Standards and Technology (NIST) using a lithography-based process. The test mirror has several patterns (reminiscent of moth antennae) with variable spacing in radial direction. We describe the fabrication of the test mirror and its application to test the performance of the interferometer.

Joint Dark Energy Mission optical design studies

We present the latest optical design concepts for the Joint Dark Energy Mission (JDEM). This mission will tightly constrain the cosmological parameters describing the accelerating expansion of the universe. The current candidate designs are based on extensive examination of the interplay of requirements for the leading techniques being considered for space borne observation: Baryon Acoustic Oscillation (BAO), Type Ia Supernovae (SN), and gravitational Weak Lensing (WL). All techniques require very large fields of view across the visible/near infrared spectrum; BAO uniquely requires a moderate dispersion wide field spectroscopy capability. Weak lensing requires very good stability and knowledge of the point spread function in order to enable detection of local variations in galaxy ellipticities caused by the intervening dark matter. SN imaging spectroscopy should be done to high photometric signal to noise in order to make best use of these standard candles. We have studied medium class and smaller, Probe class implementations enabling from one to three of these techniques. We describe two concepts that were submitted to the 2010 Astrophysics Decadal review as well as current concepts.

Spatially resolved height response of phase-shifting interferometers measured using a patterned mirror with varying spatial frequency

In the performance evaluation of phase-shifting interferometers for figure metrology, the height response, or height transfer function, is rarely taken into consideration, because in most applications smooth surfaces are measured and only the lowest spatial frequencies are of interest. For measurements with low uncertainty it is important to understand the height response as a function of the spatial-frequency content of a surface under test, in particular when it contains form-error components with frequencies at the high end of an interferometers spatial-frequency passband. A mirror with a patterned area of 140-mm diameter, consisting of several subpatterns with varying spatial frequency, was used to evaluate the spectral response. Our goal was to develop a method for efficient mapping of the spectral response over the circular field of view of a phase-shifting interferometer. A new way of representing the dependence of the spectral response on the field of view of an interferometer is described.

Observatory conceptual development for the Joint Dark Energy Mission

The Joint Dark Energy Mission (JDEM)1,2 is a proposed dark energy space mission that will measure the expansion history of the universe and the growth of its large scale structure. It is intended to provide tight constraints on the equation of state of the universe and test the validity of general relativity. Three complementary observational analyses will be employed: Baryon Acoustic Oscillations, Type 1a Supernovae and Gravitational Weak Lensing. An observatory designed for efficient accommodation of these techniques combines wide-field, diffraction-limited observations, ultra-stable point spread function, and spectroscopy. In this paper we discuss optical configurations capable of simultaneous wide-field imaging and spectroscopy, using either afocal or focal telescope configurations. Spectroscopy may be performed by an integral field unit (IFU), grism or prism spectrometer. We present a flowdown of weak lensing image stability requirements (the most demanding technique optically) to telescope thermo-mechanical stability limits, based on variations in the optical transfer function of combinations of Zernike modes, and the sensitivity of these mode combinations to thermo-mechanical drift of the telescope. We apply our formalism to a representative threemirror anastigmat telescope and find quantitative relations between the second moments of the image and the required stability of the telescope over a typical weak lensing observation.

Some considerations for precision metrology of thin x-ray mirrors

Determination of the shape of very thin x-ray mirrors employed in spaced-based telescopes continues to be challenging. The mirrors shapes are not readily deduced to the required accuracy because the mount induced distortions are often larger than the uncertainty tolerable for the mission metrology. In addition to static deformations, dynamic and thermal considerations are exacerbated for this class of mirrors. We report on the performance of one temporary mounting scheme for the thin glass mirrors for the Constellation-X mission and prospects for deducing their undistorted shapes.

Design and fabrication of refractive nulls for testing the segmented mirrors of the Constellation-X spectroscopy x-ray telescope (SXT)

We designed a refractive null lens for (visible) optical testing of the segmented mirrors for the Constellation-X spectroscopy x-ray telescope. We explored two solution families and identified the trade-offs. We also present some initial results of the realization of one solution family.

Toward a complete metrologic solution for the mirrors for the Constellation-X Spectroscopy x-ray telescope

We present an overview update of the metrologic approach to be employed for the segmented mirror fabrication for Constellation-X spectroscopy x-ray telescope. We compare results achieved to date with mission requirements. This is discussed in terms of inherent capability versus in-practice capability. We find that all the needed metrics for the mirrors are in hand but that they are currently limited by the mounting of the mirrors themselves.

Wide-field spectroscopy and imaging at two plate scales with a focal three mirror anastigmat

The key enabling element of the Joint Dark Energy Mission (JDEM)1,2,3 is a wide-field, high-magnification mixed spectroscopic and imaging telescope intended to study dark energy via measurement of the expansion history of the universe and the growth of large-scale structure. It is designed to provide tight constraints on the equation of state of dark energy and test the validity of general relativity. Complementary observation of Baryon Acoustic Oscillations (BAO), Type 1a Supernovae (SNe) and Gravitational Weak Lensing (WL) are under consideration for the mission. The science goals of this mission call for a high-resolution imaging survey and a spectroscopic survey of at least 10,000 square degrees. Signal to noise requirements of the Baryon Acoustic Oscillation (BAO) survey favor a prism disperser with a λ • d θ /d λ of roughly 200 arcsec and a coarse plate scale (~0.45arsec/pixel). The WL imaging survey seeks the shapes of galaxies, and therefore prefers a finer plate scale of ~0.1-0.23 arcsec/pixel. Accommodation all of these goals may be accomplished with an afocal telescope but the results of this study suggest that a focal telescope is also capable of achieving these goals. Discussed herein are several novel prism concepts designed for use in a focal three mirror anastigmat telescope (TMA). Multiple elements are used for aberration balancing and tailoring resolving power over the observational band. Several options for simultaneous or staggered imaging and spectroscopy as well as the required plate scale change with a focal TMA are presented.