Stellar Photometry and Astrometry with Discrete Point Spread Functions

The key features of the MATPHOT algorithm for precise and accurate stellar photometry and astrometry using discrete Point Spread Functions are described. A discrete Point Spread Function (PSF) is a sampled version of a continuous PSF which describes the two-dimensional probability distribution of photons from a point source (star) just above the detector. The shape information about the photon scattering pattern of a discrete PSF is typically encoded using a numerical table (matrix) or a FITS image file. Discrete PSFs are shifted within an observational model using a 21-pixel-wide damped sinc function and position partial derivatives are computed using a five-point numerical differentiation formula. Precise and accurate stellar photometry and astrometry is achieved with undersampled CCD observations by using supersampled discrete PSFs that are sampled 2, 3, or more times more finely than the observational data. The precision and accuracy of the MATPHOT algorithm is demonstrated by using the C-language MPD code to analyze simulated CCD stellar observations; measured performance is compared with a theoretical performance model. Detailed analysis of simulated Next Generation Space Telescope observations demonstrate that millipixel relative astrometry and millimag photometric precision is achievable with complicated space-based discrete PSFs. For further information about MATPHOT and MPD, including source code and documentation, see http://www.noao.edu/staff/mighell/matphotComment: 19 pages, 22 figures, accepted for publication in MNRA

CRBLASTER: A Parallel-Processing Computational Framework for Embarrassingly-Parallel Image-Analysis Algorithms

The development of parallel-processing image-analysis codes is generally a challenging task that requires complicated choreography of interprocessor communications. If, however, the image-analysis algorithm is embarrassingly parallel, then the development of a parallel-processing implementation of that algorithm can be a much easier task to accomplish because, by definition, there is little need for communication between the compute processes. I describe the design, implementation, and performance of a parallel-processing image-analysis application, called CRBLASTER, which does cosmic-ray rejection of CCD (charge-coupled device) images using the embarrassingly-parallel L.A.COSMIC algorithm. CRBLASTER is written in C using the high-performance computing industry standard Message Passing Interface (MPI) library. The code has been designed to be used by research scientists who are familiar with C as a parallel-processing computational framework that enables the easy development of parallel-processing image-analysis programs based on embarrassingly-parallel algorithms. The CRBLASTER source code is freely available at the official application website at the National Optical Astronomy Observatory. Removing cosmic rays from a single 800x800 pixel Hubble Space Telescope WFPC2 image takes 44 seconds with the IRAF script lacos_im.cl running on a single core of an Apple Mac Pro computer with two 2.8-GHz quad-core Intel Xeon processors. CRBLASTER is 7.4 times faster processing the same image on a single core on the same machine. Processing the same image with CRBLASTER simultaneously on all 8 cores of the same machine takes 0.875 seconds -- which is a speedup factor of 50.3 times faster than the IRAF script. A detailed analysis is presented of the performance of CRBLASTER using between 1 and 57 processors on a low-power Tilera 700-MHz 64-core TILE64 processor.Comment: 8 pages, 2 figures, 1 table, accepted for publication in PAS

WFPC2 Observations of the Carina Dwarf Spheroidal Galaxy

We present our analysis of Hubble Space Telescope Wide Field Planetary Camera 2 observations in F555W (~V) and F814W (~I) of the Carina dwarf spheroidal galaxy. The resulting V vs (V-I) color-magnitude diagrams reach V ~ 27.1 mag. The reddening of Carina is estimated to be E(V-I) = 0.08 +- 0.02 mag. A new estimate of the distance modulus of Carina, (m-M)_0 = 19.87 +- 0.11 mag, has been derived primarily from existing photometry in the literature. The apparent distance moduli in V and I were determined to be (m-M)_V = 20.05 +- 0.11 mag and (m-M)_I = 19.98 +- 0.12 mag, respectively. These determinations assumed that Carina has a metallicity of [Fe/H] = -1.9 +- 0.2 dex. This space-based observation, when combined with previous ground-based observations, is consistent with (but does not necessarily prove) the following star formation scenario. The Carina dwarf spheroidal galaxy formed its old stellar population in a short burst (=< 3 Gyr) at about the same time the Milky Way formed its globular clusters. The dominant burst of intermediate-age star formation then began in the central region of the galaxy where stars formed for several billion years before the process of star formation became efficient enough in the outer regions of the galaxy to allow for the formation of large numbers of stars. There has been negligible star formation during the last few billion years. This observation provides evidence that at least some dwarf galaxies can have complex global star formation histories with local variations of the rate of star formation as a function of time and position within the galaxy.Comment: 23 pages (LaTeX+aaspp4.sty), 4 tables and 9 figures (Postscript, gzipped tar file). Postscript version of paper, tables, and full-resolution figures available at http://www.noao.edu/noao/staff/mighell/carina.html To appear in the Astronomical Journa

Sense of Place in the Anthropocene: A students-teaching-students course

Contemporary environmental education is tasked with the acknowledgement of the Anthropocene - an informal but ubiquitous term for the current geological epoch which arose from anthropogenic changes to the Earth system - and its accompanying socio-ecological implications. Sense of Place can be a hybridized tool of personal agency and global awareness for this task. Through the creation, execution and reflection of a 14-student students-teaching-students (STS) course at the University of Vermont in the Spring of 2019, Giannina Gaspero-Beckstrom and Ella Mighell aimed to facilitate a peer-to-peer learning environment that addressed sense of place, social justice and community engagement. The students-teaching-students framework is an alternative educational approach that supports the values and practices of the University of Vermont’s Environmental Program, as well as an intentional breakdown of the hierarchical knowledge paradigm. Using alternative pedagogies (predominately critical and place-based), we attempted to facilitate meaningful learning through creative expression, experiential education, community dialogue and personal reflection. Our intention with this was to encourage awareness and action

The Star Formation History of the Carina Dwarf Galaxy

We have analyzed deep B and V photometry of the Carina dwarf spheroidal reaching below the old main-sequence turnoff to about V = 25. Using simulated color-magnitude diagrams to model a range of star formation scenarios, we have extracted a detailed, global star formation history. Carina experienced three significant episodes of star formation at about 15 Gyr, 7 Gyr, and 3 Gyr. Contrary to the generic picture of galaxy evolution, however, the bulk of star formation, at least 50%, occured during the episode 7 Gyr ago, which may have lasted as long as 2 Gyr. For unknown reasons, Carina formed only 10-20% of its stars at an ancient epoch and then remained quiescent for more than 4 Gyr. The remainder (~30%) formed relatively recently, only 3 Gyr ago. Interest in the local population of dwarf galaxies has increased lately due to their potential importance in the understanding of faint galaxy counts. We surmise that objects like Carina, which exhibits the most extreme episodic behavior of any of the dwarf spheroidal companions to the Galaxy, are capable of contributing to the observed excess of blue galaxies at B = 24 only if the star formation occurred instantaneously.Comment: 23 pages of text, 20 figures, 8 tables. AJ, in pres