Methods for evaluating the performance of volume phase holographic gratings for the VIRUS spectrograph array

The Visible Integral Field Replicable Unit Spectrograph (VIRUS) is an array of at least 150 copies of a simple, fiber-fed integral field spectrograph that will be deployed on the Hobby-Eberly Telescope (HET) to carry out the HET Dark Energy Experiment (HETDEX). Each spectrograph contains a volume phase holographic grating as its dispersing element that is used in first order for 350 nm to 550 nm. We discuss the test methods used to evaluate the performance of the prototype gratings, which have aided in modifying the fabrication prescription for achieving the specified batch diffraction efficiency required for HETDEX. In particular, we discuss tests in which we measure the diffraction efficiency at the nominal grating angle of incidence in VIRUS for all orders accessible to our test bench that are allowed by the grating equation. For select gratings, these tests have allowed us to account for > 90% of the incident light for wavelengths within the spectral coverage of VIRUS. The remaining light that is unaccounted for is likely being diffracted into reflective orders or being absorbed or scattered within the grating layer (for bluer wavelengths especially, the latter term may dominate the others). Finally, we discuss an apparatus that will be used to quickly verify the first order diffraction efficiency specification for the batch of at least 150 VIRUS production gratings.Comment: 18 pages, 11 figures. To be published in Proc. SPIE, 2012, "Ground-Based and Airborne Instrumentation for Astronomy IV", 8446-20

Setting UBVRI Photometric Zero-Points Using Sloan Digital Sky Survey ugriz Magnitudes

We discuss the use of Sloan Digital Sky Survey (SDSS) ugriz point-spread function (PSF) photometry for setting the zero points of UBVRI CCD images. From a comparison with the Landolt (1992) standards and our own photometry we find that there is a fairly abrupt change in B, V, R, & I zero points around g, r, i ~ 14.5, and in the U zero point at u ~ 16. These changes correspond to where there is significant interpolation due to saturation in the SDSS PSF fluxes. There also seems to be another, much smaller systematic effect for stars with g, r > 19.5. The latter effect is consistent with a small Malmquist bias. Because of the difficulties with PSF fluxes of brighter stars, we recommend that comparisons of ugriz and UBVRI photometry should only be made for unsaturated stars with g, r and i in the range 14.5 - 19.5, and u in the range 16 - 19.5. We give a prescription for setting the UBVRI zero points for CCD images, and general equations for transforming from ugriz to UBVRI.Comment: 13 pages. 6 figures. Accepted for publication in the Astronomical Journa

Design and construction progress of LRS2-B: a new low resolution integral field spectrograph for the Hobby-Eberly Telescope

The upcoming Wide-Field Upgrade (WFU) has ushered in a new era of instrumentation for the Hobby-Eberly Telescope (HET). Here, we present the design, construction progress, and lab tests completed to date of the blue-optimized second generation Low Resolution Spectrograph (LRS2-B). LRS2-B is a dual-channel, fiber fed instrument that is based on the design of the Visible Integral Field Replicable Unit Spectrograph (VIRUS), which is the new flagship instrument for carrying out the HET Dark Energy eXperiment (HETDEX). LRS2-B utilizes a microlens-coupled integral field unit (IFU) that covers a 7"x12" area on the sky having unity fill-factor with ~300 spatial elements that subsample the median HET image quality. The fiber feed assembly includes an optimized dichroic beam splitter that allows LRS2-B to simultaneously observe 370 nm to 470 nm and 460 nm to 700 nm at fixed resolving powers of R \approx 1900 and 1200, respectively. We discuss the departures from the nominal VIRUS design, which includes the IFU, fiber feed, camera correcting optics, and volume phase holographic grisms. Additionally, the motivation for the selection of the wavelength coverage and spectral resolution of the two channels is briefly discussed. One such motivation is the follow-up study of spectrally and (or) spatially resolved Lyman-alpha emission from z ~ 2.5 star-forming galaxies in the HETDEX survey. LRS2-B is planned to be a commissioning instrument for the HET WFU and should be on-sky during quarter 4 of 2013. Finally, we mention the current state of LRS2-R, the red optimized sister instrument of LRS2-B.Comment: 22 pages, 12 figures, 2 tables. To be published in Proc. SPIE, 2012, "Ground-Based and Airborne Instrumentation for Astronomy IV", 8446-103. In v2, a note has been added indicating that this paper has been superseded by arXiv:1407:601

Mass production of volume phase holographic gratings for the VIRUS spectrograph array

The Visible Integral-field Replicable Unit Spectrograph (VIRUS) is a baseline array of 150 copies of a simple, fiber-fed integral field spectrograph that will be deployed on the Hobby-Eberly Telescope (HET). VIRUS is the first optical astronomical instrument to be replicated on an industrial scale, and represents a relatively inexpensive solution for carrying out large-area spectroscopic surveys, such as the HET Dark Energy Experiment (HETDEX). Each spectrograph contains a volume phase holographic (VPH) grating with a 138 mm diameter clear aperture as its dispersing element. The instrument utilizes the grating in first-order for 350-550 nm. Including witness samples, a suite of 170 VPH gratings has been mass produced for VIRUS. Here, we present the design of the VIRUS VPH gratings and a discussion of their mass production. We additionally present the design and functionality of a custom apparatus that has been used to rapidly test the first-order diffraction efficiency of the gratings for various discrete wavelengths within the VIRUS spectral range. This device has been used to perform both in-situ tests to monitor the effects of adjustments to the production prescription as well as to carry out the final acceptance tests of the gratings' diffraction efficiency. Finally, we present the as-built performance results for the entire suite of VPH gratings.Comment: 16 pages, 11 figures, 2 tables. To be published in Proc. SPIE, 2014, "Advances in Optical and Mechanical Technologies for Telescopes and Instrumentation", 9151-53. The work presented in this article follows from arXiv:1207:448

Characterization and calibration of the James Webb space telescope mirror actuators fine stage motion

The James Webb Space Telescope’s (Webb’s) deployable primary and secondary mirrors are actively controlled to achieve and maintain precise optical alignment on-orbit. Each of the 18 primary mirror segment assemblies (PMSAs) and the secondary mirror assembly (SMA) are controlled in six degrees of freedom by using six linear actuators in a hexapod arrangement. In addition, each PMSA contains a seventh actuator that adjusts radius of curvature (RoC). The actuators are of a novel stepper motor-based cryogenic two-stage design that is capable of sub-10 nm motion accuracy over a 20 mm range. The nm-level motion of the 132 actuators were carefully tested and characterized before integration into the mirror assemblies. Using these test results as an initial condition, knowledge of each actuator’s length (and therefore mirror position) has relied on software bookkeeping and configuration control to keep an accurate motor step count from which actuator position can be calculated. These operations have been carefully performed through years of Webb test operations using both ground support actuator control software as well as the flight Mirror Control Software (MCS). While the actuator’s coarse stage length is cross-checked using a linear variable differential transformer (LVDT), no on-board cross-check exists for the nm-level length changes of the actuators’ fine stage. To ensure that the software bookkeeping of motor step count is still accurate after years of testing and to test that the actuator position knowledge was properly handed off from the ground software to the flight MCS, a series of optical tests were devised and performed through the Center of Curvature (CoC) ambient optical test campaigns at the Goddard Space Flight Center (GSFC) and during the thermal-vacuum tests of the entire optical payload that were conducted in Chamber A at Johnson Space Center (JSC). In each test, the actuator Fine Step Count (FSC) value is compared to an external measurement provided by an optical metrology tool with the goal of either confirming the MCS database value, or providing a recommendation for an updated calibration if the measured FSC differs significantly from the MCS-based expectation. During ambient testing of the PMSA hexapods, the nm-level actuator length changes were measured with a custom laser deflectometer by measuring tilts of the PMSA. The PMSA RoC fine stage characterization was performed at JSC using multi-wave interferometric measurements with the CoC Optical Assembly (COCOA). Finally, the SMA hexapod fine stage characterization test was performed at JSC using the NIRCam instrument in the “pass-and-a-half” test configuration using a test source from the Aft-Optics System Source Plate Assembly (ASPA). In this paper, each of these three tests, subsequent data analyses, and uncertainty estimations will be presented. Additionally, a summary of the ensemble state of Webb’s actuator fine stages is provided, along with a comparison to a Wavefront Sensing and Control (WFSC)-based requirement for FSC errors as they relate to the optical alignment convergence of the telescope on-orbit

A Petal of the Sunflower: Photometry of the Stellar Tidal Stream in the Halo of Messier 63 (NGC 5055)

We present surface photometry of a very faint, giant arc feature in the halo of the nearby spiral galaxy NGC 5055 (M63) that is consistent with being a part of a stellar stream resulting from the disruption of a dwarf satellite galaxy. This faint feature was first detected in early photographic studies by van der Kruit (1979); more recently by Mart\'inez-Delgado et al. (2010) and as presented in this work, the loop has been realized to be the result of a recent minor merger through evidence obtained by deep images taken with a telescope of only 0.16 m aperture. The stellar stream is confirmed in additional images taken with the 0.5 m of the BlackBird Remote Observatory and the 0.8 m of the McDonald Observatory. This low surface brightness structure around the disk of the galaxy extends ~29 kpc from its center, with a projected width of 3.3 kpc. The stream's morphology is consistent with that of the visible part of a "great-circle" stellar stream originating from the accretion of a ~10^8 M_sun dwarf satellite in the last few Gyr. The progenitor satellite's current position and fate are not conclusive from our data. The color of the stream's stars is consistent with Local Group dwarfs and is similar to the outer regions of M63's disk and stellar halo. We detect other low surface brightness "plumes"; some of these may be extended spiral features related to the galaxy's complex spiral structure and others may be tidal debris associated with the disruption of the galaxy's outer stellar disk as a result of the accretion event. We differentiate between features related to the tidal stream and faint, blue features in the outskirts of the galaxy's disk previously detected by the GALEX satellite. With its highly warped HI gaseous disk (~20 deg), M63 represents one of several examples of an isolated spiral galaxy with a warped disk showing strong evidence of an ongoing minor merger.Comment: 16 pages, 10 figures, 3 tables, Accepted for publication in The Astronomical Journa