Eric Mentzell

Optical Design of WFIRST-AFTA Wide-Field Instrument

The WFIRST-AFTA Wide-Field Infrared Survey Telescope TMA optical design provides 0.28-sq°FOV Wide Field Channel at 0.11” pixel scale, operating at wavelengths between 0.76-2.0μm, including a spectrograph mode (1.35-1.95μm.) An Integral Field Channel provides a discrete 3”x3.15” field at 0.15” sampling.

Wide field instrument preliminary design for the Wide Field InfraRed Survey Telescope

We present the Wide Field Infra-Red Survey Telescope (WFIRST) wide field instrument concept based on the reuse of a 2.4m telescope recently made available to NASA. Two instrument channels are described, a wide field channel (~0.8x0.4degrees, 300Mpix, imaging and spectroscopy over 0.76-2.0um), and an integral field unit (3x3 arcsec, 1Mpix, R{2pixel} ~100 over 0.6-2.0um). For this mission concept, the telescope, instruments, and spacecraft are in a geosynchronous orbit and are designed for serviceability. This instrument can accomplish not only the baseline exoplanet microlensing, dark energy, and infrared surveys for WFIRST, but can perform at higher angular resolution and with deeper observations. This enables significant opportunities for more capable general observer programs. The emphasis on achieving very good imaging stability is maintained from the previous work.

GRIS: The grating infrared spectrometer

The Grating Infrared Spectrometer (GRIS) is an echelle grating, prism cross dispersed, spectrometer designed for the 2.3 m Steward Observtory telescope. The cross dispersed format utilizes a NICMOS 3 HgCdTe detector array for observations in the 0.86-2.5 micron spectral region. An echelle grating, ruled on both sides, provides resolutions of 3449 and 9439 per slit width respectively. Rotation of the grating achieves several functions: 1/2 pixel spectrum stepping for full Nyquist sampling, a change in the wavelength regions falling on the detector, and the means for changing grating sides.

Tracking near-infrared fringes on BETTII: a balloon-borne, 8m-baseline interferometer

We present the design of a fringe tracking system for the Balloon Experimental Twin Telescope for Infrared Interferometry (BETTII). BETTII is a balloon- borne, far-infrared, 8 m-baseline interferometer with two 50 cm siderostats. Beams from the two arms are combined in the pupil plane to enable double-Fourier, spatio-spectral interferometry. To maintain the phase stability of the system, we need to actively correct of the optical path difference (OPD) between the two arms. The fringe-tracking system will work in the near-infrared and will use a reference star within the field of view to achieve two goals: overlap the beams coming from the two siderostats, and track the location of the central fringe packet, which is a measure of the OPD. The fringe tracker will share most of the optical train with the science instrument. This system is part of the overall control architecture that feeds fast steering tip/ tilt mirrors and a warm delay line to ensure proper beam combination and OPD control for the science instrument. This paper investigates the different sources of perturbations that are expected at float altitude, and derives the sensitivity of the fringe-tracking system. We show progress on validating our design using a visible light, broadband Mach-Zehnder interferometer that was developed at NASA/GSFC. This system demonstrates the viability of our OPD determination approach and provides a means of testing and characterizing several OPD determination and control algorithms.

Enhanced MgF 2 and LiF over-coated Al mirrors for FUV space astronomy

Astronomical observations in the far--ultraviolet (FUV) spectral region are some of the more challenging due to the very distant and faint objects that are typically searched for in cosmic origin studies such as origin of large scale structure, the formation, evolution, and age of galaxies and the origin of stellar and planetary systems. The problem is compounded by the very limited option of reflecting coatings to use at FUV wavelengths and the modest reflectivity offered by those coatings such as Al+MgF2 [typically 82 % at Lyman-alpha, 1216 Α) that are used on reflecting surfaces of FUV instrumentation. Improved reflective coatings for optics, particularly in the ultraviolet part of the spectrum, could yield dramatically more sensitive instruments and permit more instrument design freedom. This paper will present recent advances in reflectance performance for Al+MgF2 mirrors optimized for Lymanalpha wavelength by comparing an ambient or ”cold” deposition with another where the deposition temperature for the MgF2 layer is done at elevated temperatures. We will also consider this improved procedure for the deposition of LiF over-coated Al mirrors in order to realize similar reflectivity gains at even shorter wavelengths.

The Balloon Experimental Twin Telescope for Infrared Interferometry (BETTII): Progress Towards High Angular Resolution in the Far-Infrared

The Balloon Experimental Twin Telescope for Infrared Interferometry (BETTII) is an 8-meter baseline far-infrared interferometer to fly on a high altitude balloon. In this paper, we discuss the current status of the project, focusing on the optical and mechanical design elements.

Cryogenic metrology for the James Webb Space Telescope Integrated Science Instrument Module alignment target fixtures using laser radar through a chamber window

The James Webb Space Telescope Integrated Science Instrument Module utilizes two fixtures to align the Optical Telescope Element Simulator (OSIM) to the coordinate systems established on the ISIM and the ISIM Test Platform (ITP). These fixtures contain targets which are visible to the OSIM Alignment Diagnostics Module (ADM). Requirements on these fixtures must be met under ambient and cryogenic conditions. This paper discusses the cryogenic metrology involving Laser Radar measurements through a chamber window that will be used to link photogrammetry target measurements used during ISIM structure cryogenic verification and the ADM targets, including evaluation of distortion introduced from the window.

The Balloon Experimental Twin Telescope for Infrared Interferometry (BETTII): towards the first flight

The Balloon Experimental Twin Telescope for Infrared Interferometry (BETTII) is a balloon-borne, far-infrared direct detection interferometer with a baseline of 8 m and two collectors of 50 cm. It is designed to study galactic clustered star formation by providing spatially-resolved spectroscopy of nearby star clusters. It is being assembled and tested at NASA Goddard Space Flight Center for a first flight in Fall 2016. We report on recent progress concerning the pointing control system and discuss the overall status of the project as it gets ready for its commissioning flight.

Design and validation of the mounting structure for BETTII balloon-based telescope with thin-walled optics

Abstract. The NASA Balloon Experimental Twin Telescope for Infrared Interferometry (BETTII) system is designed to study the infrared emissions from star formation and active galactic nuclei through a double-Fourier Michelson interferometer located on a balloon at an altitude of 37 km. The BETTII external optics include a pair of identical beam-reducing, four-mirror telescopes, each with a 522-mm aperture, nonrotationally symmetric primary mirror. These telescopes were designed and assembled at the North Carolina State University Precision Engineering Consortium and are composed entirely of thin-walled aluminum components. The mounting structure is designed to be light weight and stiff to reduce thermal equilibration time in the rarified air at the edge of space and to maintain robust alignment of the optical elements. The mounts also prevent deformation of the large optical elements via custom-built kinematic Kelvin couplings and fixed-load clamps; the maximum form error of the optical surfaces are 300 nm RMS. This work details the design of the thin mirrors and mounting structure as well as validation of the mount assembly process, mount stiffness, and the kinematic couplings.

A dispersive backend design for the 'double-Fourier' interferometer BETTII

BETTII (Balloon Experimental Twin Telescope for Infra-red Interferometry) is designed to provide high angular resolution spectroscopic data in the far-infrared (FIR) wavelengths. The most significant limitation for BETTII is its sensitivity; obtaining spectral signal-to-noise ratio >5 in <10 minutes requires sources >13 Jy. One possible way to improve the signal-to-noise ratio (SNR) for future BETTII flights is by reducing the spectral bandwidth post beam-combination. This involves using a dispersive element to spread out a polychromatic point source PSF on the detector array, such that each pixel corresponds to a small fraction of the bandwidth. This results in a broader envelope of the interferometric fringe pattern allowing more fringes to be detected, and thereby improving the spectral SNR. Here we present the analysis and optical design of the dispersive backend, discussing the tradeoffs and how it can be combined with the existing design.