Marsha J. Wolf

Ages and Metallicities of Extragalactic Globular Clusters from Spectral and Photometric Fits of Stellar Population Synthesis Models

Spectra of galaxies contain an enormous amount of information about the relative mixture of ages and metallicities of constituent stars. We present a comprehensive study designed to extract the maximum information from spectra of data quality typical in large galaxy surveys. These techniques are not intended for detailed stellar population studies that use high-quality spectra. We test techniques on a sample of globular clusters, which should consist of single stellar populations and provide good test cases, using the Bruzual-Charlot high-resolution stellar population synthesis models to simultaneously estimate the ages and metallicities of 101 globular clusters in M31 and the Magellanic Clouds. The clusters cover a wide range of ages and metallicities, 4 Myr < tage < 20 Gyr and -1.6 < [Fe/H] < +0.3, estimated by other methods in the literature. We compare results from model fits to both the spectra and photometry and find that fits to continuum-normalized spectra over the entire range available, typically 3500 A to 1 μm for this sample, provide the best results. For clusters older than 1 Gyr we agree with literature ages to 0.16 dex (35%) and [Fe/H] to 0.12 dex. For younger clusters we agree with literature ages to 0.3 dex (63%) but cannot constrain the metallicity. It is particularly important to use the entire continuum-normalized spectrum to avoid problems with model continua for young objects and to break age-metallicity degeneracies of broadband photometry. Our required S/N is 15-30 A-1 for 20% age uncertainties and 30-55 A-1 for 10% uncertainties over spectral resolutions of Δλ = 5-25 A. This technique should work well for the age-metallicity parameter space expected for early-type galaxies at z ~ 1, although individual galaxy spectral S/N may require the co-addition of a few like objects.

L dwarfs found in sloan digital sky survey commissioning data. II. Hobby-Eberly Telescope observations

Low-dispersion optical spectra have been obtained with the Hobby-Eberly Telescope of 22 very red objects found in early imaging data from the Sloan Digital Sky Survey (SDSS). The objects are assigned spectral types on the Two Micron All Sky Survey (2MASS) system and are found to range from late M to late L. The red and near-infrared colors from SDSS and 2MASS correlate closely with each other, and most of the colors are closely related to spectral type in this range; the exception is the i*-z* color, which appears to be independent of spectral type between about M7 and L4. The spectra suggest that this independence is due to the disappearance of the TiO and VO absorption in the i band for later spectral types, the presence of strong Na I and K I absorption in the i band, and the gradual disappearance of the 8400 ? absorption of TiO and FeH in the z band.

Volume phase holographic (VPH) grisms for infrared and optical spectrographs

Highly efficient Volume phase holographic (VPH) gratings do not lend themselves to use in existing spectrographs except for grism spectrographs where VPH grisms can be designed that disperse but do not deviate the light. We discuss our program to outfit existing spectrographs [the Imaging grism instrument (IGI) on the McDonald Observatory Smith Reflector, and the Hobby-Eberly Telescope Marcario Low Resolution Spectrograph (LRS)] with efficient VPH grisms. We present test data on sample gratings from Ralcon Development Lab, and compare them to theoretical predictions. We have created a simple test bench for efficiency measurements of VPH gratings, which we describe. Finally we present first results from the use of VPH grisms in IGI and the LRS, the latter being the largest grism ever deployed in an astronomical spectrograph. We also look forward to using VPH grisms in the LRS infrared extension, which covers the wavelength range from 0.9 to 1.3 microns.

Multi-object spectroscopy on the Hobby-Eberly Telescope low-resolution spectrograph

The low resolution spectrograph (LRS) is the first facility instrument on the 9.2m Hobby-Eberly Telescope (HET). The LRS has three operational modes: imaging, long-slit spectroscopy and multi-object spectroscopy (MOS). We present the design and early operations performance of the LRS MOS unit, which provides 13 slitless, each 1.3 arcsec by 15 arcsec, on 19.6 arcsec centers, within the 4 arcmin field of view of the HET. This type of remotely configurable unit was chosen over the more conventional slit masks due to the queue scheduling of the HET, and the instruments remote location at the prime focus of the telescope. A restricted envelope around the HET focus at the LRS port forced a very compact design. The MOS unit has miniature mechanisms base don custom cross- roller stages and 0.25 mm pitch lead-screws. Geared stepper motors with 10 mm diameters drive the 13 axes at 0.8 micron per step. The precision of the mechanism is far greater than required by the HET plate scale of 205 microns per arcsec, but result in a robust unit. The slitlets were fabricated at the University of Texas by shadow-masking the slit area with a wire and vacuum depositing aluminum onto the silica substrates. Both sides are then coated with MgF2 which serves as an antireflection coating and a protective layer. Web-based software is available for optimizing the orientation of the MOS unit and the placement of slitlets on objects in the field. These setup scan be down loaded to the unit for configuration outside of the beam while the HET is slewing to its next target in the queue, or while the LRS is used in imaging mode for setup on faint objects. The preliminary results presented here are from one commissioning run with the MOS, where the unit appears to be meeting performance specifications.

Hobby-Eberly Telescope segment alignment maintenance system

A sensing and control system for maintaining the optical alignment of the ninety-one 1-meter diameter hexagonal segments forming the Hobby-Eberly Telescope (HET) primary mirror array has been developed by NASA - Marshall Space Flight Center (Huntsville, AL) and Blue Line Engineering (Colorado Springs, CO) and implemented. This Segment Alignment Maintenance System (SAMS) employs 480 edge sensors to measure the relative shear motion between each segment edge pair and compute individual segment tip, tilt and piston position errors. Error information is sent to the HET primary mirror control system, which then corrects the physical position of each segment every 90 seconds. On-site installation of the SAMS sensors, ancillary electronics and software was completed in September 2001. Since that time, SAMS has undergone engineering testing. The system has operated almost nightly, improving HETs overall operational capability and image quality performance. SAMS has not yet, however, demonstrated performance at the specified levels for tip, tilt, piston and Global Radius of Curvature (GRoC) maintenance. Additional systems development and in situ calibration are expected to bring SAMS to completion and improved operation performance by the end of this year.

Polarization shearing laser interferometer for aligning segmented telescope mirrors

The Center of Curvature Alignment Sensor (CCAS) was the original instrument installed in the center of curvature (CoC) tower on the Hobby-Eberly Telescope (HET) for aligning the 91 primary mirror segments. The CCAS is a polarization shearing interferometer with HeNe and diode laser sources that illuminate the HET primary mirror with polarized coherent light. Returns from each mirror segment focus back at the CoC and can be viewed on a faceplate at the front of the instrument for coarse alignment of the primary mirror, or sent into the interferometer for fine alignment. Inside the interferometer, Wollaston prisms separate the HET primary mirror image into two polarization components which are spatially shifted by the distance of one mirror segment. This overlaps images of segments with their neighbors to generate interference fringes. The beam is then split into 4 legs, each of which introduces phase shifts to the polarization. Fringe patterns shifted by 0, 90, 180, and 270 degrees are recorded on each leg by a CCD camera. The intensity in each pixel is measured and used in the standard 4-bucket algorithm to calculate the relative phase shift between the two mirror segments, and thus their tip/tilt misalignment. Segment piston is determined from the location of the peak in the fringe contrast function, using all four camera images and light at four laser diode wavelengths. Although the CCAS has recently been replaced with a Shack-Hartmann sensor for mirror alignment on the HET, its operation and performance are described. Under less environmentally challenging conditions, such as laboratory or space-based applications, this instrument could be used for aligning segmented mirrors to high precision.

The Hobby-Eberly Telescope Completion Project

The Hobby-Eberly Telescope (HET) is a fixed-elevation, 9.2-m telescope with a spherical primary mirror and a tracker at prime focus to follow astronomical objects. The telescope was constructed for

LRS-J: instrument design and characterization

13.9M over the period 1994-1997. A number of telescope performance deficiencies were identified and corrected following construction. Remaining problems included: 1) Dome seeing, 2) inadequate initial mirror segment alignment accuracy, and 3) mirror segment misalignment with time. The HET Completion Project was created in May 2001 to attack these problems and to identify and solve the next tier of problems. To address dome seeing, large louvers were installed and in operation by May 2002. Efforts are also underway to eliminate or suppress heat sources within the dome environment. To address segment alignment accuracy, a prototype Shack-Hartmann device, the Mirror Alignment Recovery System (MARS), was built and is in routine use at HET. The Segment Alignment Maintenance System (SAMS) is in early operation and has markedly improved telescope performance. Two Differential Image Motion Monitor (DIMM) telescopes were brought into regular operation in July 2001 to quantify atmospheric seeing at HET. As these improvements have been implemented, telescope image quality has improved significantly. Plans are in place to address additional performance issues.

Hobby-Eberly Telescope low-resolution spectrograph J-band camera

LRS-J is a 142 mm f/1 near infrared (J-Band) camera designed as a drop in replacement for the optical camera installed on the Hobby-Eberly Telescope (HET) multi-object low resolution spectrograph (LRS). The Hawaii-1RG (H1RG) based instrument is liquid nitrogen cooled, but it mates to the warm LRS making use of the existing longslit and multi-object (MOS) units as well as the existing optical collimator. LRS-J utilizes a molecular beam epitaxy (MBE) based Hawaii-1RG (H1RG) detector. The design is a fully cryogenic catadioptric Maksutov-type, with the detector at the internal focus. This configuration produces excellent images, but presents particular challenges in the mounting of the detector. This is the first time that such an arrangement has been used in an astronomical instrument with an infrared detector. By replacing the conventional optical grisms with two 170 mm diameter near-IR VPH grisms, LRS-J covers the 0.9-1.3 μm bandpass with R ~ 1750-2000. We present the opto-mechanical design of LRS-J including the thermally self-compensating corrector doublet mount, and 100 μm/turn cryogenic mirror adjusters, FEA optimized vacuum housing, and custom Dewar. We also characterize the electrical and thermal connections necessary to mount the detector head in this unusually small inverted arrangement.

The NEID precision radial velocity spectrometer: port adapter overview, requirements, and test plan

This paper presents the design of a near IR camera for the 9.2 m Hobby-Eberly Telescope (HET) Low Resolution Spectrograph (LRS), which will cover the wavelength range 0.85 to 1.35 micrometers . The LRS-J, an upgrade to the existing LRS, replaces the optical camera with an f/1 camera optimized for the J-band. The instrument design is strongly motivated by the desire to observe galaxies at 1 < z < 2, where the principal strong spectral features used to measure redshifts are shifted into the J-band. Since we are primarily interested in wavelengths up to 1.35 micrometers , mating the cryogenically cooled camera to the warm LRS spectrograph does not result in enough thermal background emission to compromise its performance. LRS-J represents a rapid and cost-effective way to enable multi-object near-IR spectroscopy on a very large telescope.