Peter Christian Zimmer

The pattern speeds of m51, m83 and ngc 6946 using co and the tremaine-Weinberg method

In spiral galaxies in which the molecular phase dominates the ISM, the molecular gas as traced by CO emission will approximately obey the continuity equation on orbital timescales. The Tremaine-Weinberg method can then be used to determine the pattern speed of such galaxies. We have applied the method to single-dish CO maps of three nearby spirals, M51, M83, and NGC 6946, to obtain estimates of their pattern speeds: 38 ± 7, 45 ± 8, and 39 ± 8 km s-1 kpc-1, respectively, and we compare these results to previous measurements. We also analyze the major sources of systematic errors in applying the Tremaine-Weinberg method to maps of CO emission.

Are curved focal planes necessary for wide-field survey telescopes?

The last decade has seen significant interest in wide field of view (FOV) telescopes for sky survey and space surveillance applications. Prompted by this interest, a multitude of wide-field designs have emerged. While all designs result from optimization of competing constraints, one of the more controversial design choices is whether such telescopes require flat or curved focal planes. For imaging applications, curved focal planes are not an obvious choice. Thirty years ago with mostly analytic design tools, the solution to wide-field image quality appeared to be curved focal planes. Today however, with computer aided optimization, high image quality can be achieved over flat focal surfaces. For most designs, the small gains in performance offered by curved focal planes are more than offset by the complexities and cost of curved CCDs. Modern design techniques incorporating reflective and refractive correctors appear to make a curved focal surface an unnecessary complication. Examination of seven current, wide FOV projects (SDSS, MMT, DCT, LSST, PanStarrs, HyperSuprime and DARPA SST) suggests there is little to be gained from a curved focal plane. The one exception might be the HyperSuprime instrument where performance goals are severely stressing refractive prime-focus corrector capabilities.

Near-field calibration of an objective spectrophotometer to NISTradiometric standards for the creation and maintenance of standardstars for ground- and space-based applications

NIST-calibrated detectors will be used by the ground-based 100mm diameter Astronomical Extinction Spectrophotometer (AESoP) to calibrate the spectral energy distributions of bright stars to sub-1% per 1nm spectral resolution element accuracy. AESoP will produce about a hundred spectroradiometrically calibrated stars for use by ground- and space-based sensors. This will require accurate and near-continuous NIST calibration of AESoP, an equatorially mounted objective spectrophotometer operating over the wavelength range 350nm – 1050nm using a CCD detector. To provide continuous NIST calibration of AESoP in the field a near-identical, removable 100mm diameter transfer standard telescope (CAL) is mounted physically parallel to AESoP. The CAL transfer standard is calibrated by NIST end-to-end, wavelength-by-wavelength at ~ 1nm spectral resolution. In the field, CAL is used in a near-field configuration to calibrate AESoP. Between AESoP science observations, AESoP and CAL simultaneously observe clear sub-apertures of a 400mm diameter calibration collimator. Monochromatic light measured simultaneously by AESoP and CAL is dispersed by the objective grating onto the AESoP pixels measuring the same wavelength of starlight, thus calibrating both wavelength and instrumental throughput, and simultaneously onto a unique low-noise CAL detector providing the required throughput measurement. System sensitivity variations are measured by vertically translating the AESoP/CAL pair so that CAL can observe the AESoP sub-aperture. Details of this system fundamental to the calibration of the spectral energy distributions of stars are discussed and its operation is described. System performance will be demonstrated, and a plan of action to extend these techniques firstly into the near infrared, then to fainter stars will be described.

Space-based photometric precision from ground-based telescopes

Ground-based telescopes supported by lidar and spectrophotometric auxiliary instrumentation can attain space-based precision for all-sky photometry, with uncertainties dominated by fundamental photon counting statistics. Earths atmosphere is a wavelength-, directionally- and time-dependent turbid refractive element for every ground-based telescope, and is the primary factor limiting photometric measurement precision. To correct accurately for the transmission of the atmosphere requires direct measurements of the wavelength-dependent transmission in the direction and at the time that the supported photometric telescope is acquiring its data. While considerable resources have been devoted to correcting the effects of the atmosphere on angular resolution, the effects on precision photometry have largely been ignored. We describe the facility-class lidar that observes the stable stratosphere, and a spectrophotometer that observes NIST absolutely calibrated standard stars, the combination of which enables fundamentally statistically limited photometric precision. This inexpensive and replicable instrument suite provides the lidar-determined monochromatic absolute transmission of Earths atmosphere at visible and near-infrared wavelengths to 0.25% per airmass and the wavelengthdependent transparency to less than 1% uncertainty per minute. The atmospheric data are merged to create a metadata stream that allows throughput corrections from data acquired at the time of the scientific observations to be applied to broadband and spectrophotometric scientific data. This new technique replaces the classical use of nightly mean atmospheric extinction coefficients, which invoke a stationary and plane-parallel atmosphere. We demonstrate application of this instrument suite to stellar photometry, and discuss the enhanced value of routinely provably precise photometry obtained with existing and future ground-based telescopes.

Comparison of Adaptive Resonance Theory Neural Networks for Astronomical Region of Interest Detection and Noise Characterization

While learning algorithms have been used for astronomical data analysis, the vast majority of those algorithms have used supervised learning. In a continuation of the work described in Young et ah [18] we examine the use of unsupervised learning for this task with two types of Adaptive Resonance Theory (ART) neural networks. Using synthetic astronomical data from SkyMaker[2], [3] which was designed to mimic the dynamic range of the CTI-[14] telescope, we compared the ability of the ART-1 neural network[4] and the ART-1 neural network with category theoretic modiflcation[9], [11] to detect regions of interest and to characterize noise. We show a difference in the geometries of the templates created by each architecture. We also show an analysis of the two architectures over a range of parameter settings. The results provided show that ART neural networks and unsupervised learning algorithms in general should not be overlooked for astronomical data analysis.

Surface layer turbulence measurements on the LSST site El Peñon using microthermal sensors and the lunar scintillometer LuSci

Results from determining the optical turbulence profile (OTP) on the LSST site, El Peñon, located on Cerro Pachón (Chile) are presented. El Peñón appears to be an excellent observatory site with a surface layer seeing contribution on the order of 0.15” with most of this seeing being produced below 20m. These measurements also helped to confirm that the telescope is elevated high enough above ground. As part of the LSST site characterization campaign, microthermal measurements were taken in order to determine the contribution of the surface layer turbulence to the atmospheric seeing. Such measurements are commonly used for this purpose where pairs of microthermal sensors mounted on a tower measure atmospheric temperature differences. In addition, the lunar scintillometer LuSci was installed on El Peñon for short campaigns near full moon for the same purpose. LuSci is a turbulence profiler based on measuring spatial correlation of moonlight scintillations. The comparison of the results from both instruments during simultaneous data acquisition showed a remarkable temporal correlation and very similar mean OTPs.

Improved spherical aberration corrector for fast spherical primary mirrors

In the quest to design large and extremely large telescopes, one of the first questions encountered is that of basic optical configuration and shape of the primary mirror. Spherical mirrors have a number of advantages in production, testing and alignment but suffer from substantial spherical aberration, thereby requiring some form of corrective optics. Many designs for spherical aberration correctors are present in the literature, but each has its strengths and weaknesses. We present the design for a new spherical aberration corrector which is believed to offer higher performance with less complexity than previous approaches. The new design is substantially more compact and uses slower optical surfaces than most axially symmetric designs. It can scale to accommodate apertures as large as 100m, and adapts equally well to post prime focus and Cassegrain-like focus applications.

The second-generation CCD/Transit Instrument (CTI-II) precision astrometric and photometric survey

We are implementing the second-generation CCD/Transit Instrument (CTI-II), a unique 1.8-m imaging astrometric and photometric telescope. We discuss design aspects of CTI-II, including the optical system, structure, focal plane mosaic and the detector readout system that allows precise astrometric and photometric measurements. The scientific design drivers for the imaging telescope include discovery and measurement of motion and distance for late M, L and T stars, synoptic photometric monitoring of active galactic nuclei (AGN), and discovery and near real-time spectroscopic followup of distant supernovae and AGN outbursts. These projects drive the design of the wide field-of-view stationary telescope that employs the time-delay and integrate (TDI) readout mode for CCD detectors to produce a deep, multicolor image of the sky every clear night. Nightly observation of the same strip of the sky produces the time domain photometric and repeated astrometric measurements required by the science drivers. The telescope, its focal plane mosaic and the data system all incorporate unique and innovative elements that support an unbiased survey of the sky with intensive time-domain sampling. We review these aspects of the project, and describe steps taken to support the astrometric and photometric precision required by the scientific mission of the telescope.

LIDAR for measuring atmospheric extinction

The Georgia Tech Research Institute and the University of New Mexico are developing a compact, rugged, eye safe lidar (laser radar) to be used specifically for measuring atmospheric extinction in support of the second generation of the CCD/Transit Instrument (CTI-II). The CTI-II is a 1.8 meter telescope that will be used to accomplish a precise timedomain imaging photometric and astrometric survey at the McDonald Observatory in West Texas. The supporting lidar will enable more precise photometry by providing real-time measurements of the amount of atmospheric extinction as well as its cause, i.e. low-lying aerosols, dust or smoke in the free troposphere, or high cirrus. The goal of this project is to develop reliable, cost-effective lidar technology for any observatory. The lidar data can be used to efficiently allocate observatory time and to provide greater integrity for ground-based data. The design is described in this paper along with estimates of the lidars performance.

Alternative design for extremely large telescopes and options to use the VATT for ELT design demonstration

Abstract A variety of optical designs for extremely large telescopes (ELTs) can be found throughout the technical literature. Most feature very fast primary mirrors of either conic or spherical figure. For those designs with conic primary mirrors, many of the optical approaches tend to be derivatives of either the aplanatic Cassegrain or Gregorian systems. The Cassegrain approach is more common as it results in a shorter optical system, but it requires a large convex aspheric secondary mirror, which is extremely difficult and expensive to test. The Gregorian approach is physically longer and suffers from greater field curvature. In some design variations, additional mirrors are added to reimage and possibly flatten a Cassegrain focus. An interesting alternative ELT design uses a small Cassegrain system to image the collimated output of a Gregorian-Mersenne concentrator. Another alternative approach, currently in favor for use on the European ELT, uses three powered mirrors and two flat mirrors to reimage a Cassegrain focus out the side similar to a Nasmyth system. A preliminary examination suggests that a small, fast primary mirror, such as that used on the VATT, might be used for a subscale prototype of current ELT optical design options.