84 research outputs found
Building a CCD Spectrograph for Educational or Amateur Astronomy
We discuss the design of an inexpensive, high-throughput CCD spectrograph for
a small telescope. By using optical fibers to carry the light from the
telescope focus to a table-top spectrograph, one can minimize the weight
carried by the telescope and simplify the spectrograph design. We recently
employed this approach in the construction of IntroSpec, an instrument built
for the 16-inch Knowles Telescope on the Harvard College campus.Comment: 17 pages including 7 figures, PASP, accepted (higher resolution
figures at http://cfa-www.harvard.edu/~sheila/introspec.ps.gz
A Detailed Thermal Analysis of the Binospec Spectrograph
Refractive optics in astronomical instruments are potentially sensitive to
temperature gradients and temperature transients. This sensitivity arises from
thermally dependent refractive indices, lens spacings, and lens dimensions. We
have therefore undertaken a detailed thermal analysis of Binospec, a wide-field
optical spectrograph under development for the converted MMT. Our goals are to
predict the temperature gradients that will be present in the Binospec optics
and structure under realistic operating conditions and to determine how design
choices affect these gradients. We begin our analysis by deriving thermal time
constants for instrument subassemblies. We then generate a low-resolution
finite difference model of the entire instrument and high-resolution models of
sensitive subassemblies. This approach to thermal analysis is applicable to a
variety of other instruments. We use measurements of the ambient temperature in
the converted MMT's dome to model Binospec's thermal environment. During
moderate conditions we find that the Binospec lens groups develop 0.14 C axial
and radial temperature gradients and that lens groups of different mass develop
0.5 C temperature differences; these numbers are doubled for the extreme
conditions. Internal heat sources do not significantly affect these results;
heat flow from the environment dominates. The instrument must be periodically
opened to insert new aperture masks, but we find that the resulting temperature
gradients and thermal stresses in the optics are small. Image shifts at the
detector caused by thermal deflections of the Binospec optical bench structure
are approx 0.1 pixel/hr. We conclude that the proposed Binospec design has
acceptable thermal properties, and briefly discuss design changes to further
reduce temperature gradients.Comment: 11 pages, to appear in PASP v114 Dec 200
A High Merger Fraction in the Rich Cluster MS1054-03 at z=0.83: Direct Evidence for Hierarchical Formation of Massive Galaxies
We present a morphological study of the galaxy population of the luminous
X-ray cluster MS1054-03 at z=0.83. The sample consists of 81 spectroscopically
confirmed cluster members in a 3 x 2 Mpc area imaged in F606W and F814W with
WFPC2. We find thirteen ongoing mergers in MS1054-03, comprising 17% of the L >
L* cluster population. Most of these mergers will likely evolve into luminous
(\sim 2 L*) elliptical galaxies, and some may evolve into S0 galaxies. Assuming
the galaxy population in MS1054-03 is typical for its redshift it is estimated
that \sim 50% of present-day cluster ellipticals experienced a major merger at
z < 1. The mergers are preferentially found in the outskirts of the cluster,
and probably occur in small infalling clumps. Morphologies, spectra, and colors
of the mergers show that their progenitors were typically E/S0s or early-type
spirals with mean stellar formation redshifts z* \gtrsim 1.7. The red colors of
the merger remnants are consistent with the low scatter in the color-magnitude
relation in rich clusters at lower redshift. The discovery of a high fraction
of mergers in this young cluster is direct evidence against formation of
ellipticals in a single ``monolithic'' collapse at high redshift, and in
qualitative agreement with predictions of hierarchical models for structure
formation.Comment: Added GIF version of Figure 1. At
http://www.astro.rug.nl/~dokkum/preprints/merger_fig1.eps.gz the PS file is
available. Accepted for publication in ApJ Letter
--PhotoZ: Photometric Redshifts by Inverting the Tolman Surface Brightness Test
Surface brightness is a fundamental observational parameter of galaxies. We
show, for the first time in detail, how it can be used to obtain photometric
redshifts for galaxies, the -PhotoZ method.
We demonstrate that the Tolman surface brightness relation, , is a powerful tool for determining galaxy redshifts from
photometric data.
We develop a model using and a color percentile (ranking) measure to
demonstrate the -PhotoZ method. We apply our method to a set of galaxies
from the SHELS survey, and demonstrate that the photometric redshift accuracy
achieved using the surface brightness method alone is comparable with the best
color-based methods.
We show that the -PhotoZ method is very effective in determining the
redshift for red galaxies using only two photometric bands. We discuss the
properties of the small, skewed, non-gaussian component of the error
distribution.
We calibrate from the SDSS to redshift, and tabulate the
result, providing a simple, but accurate look up table to estimate the redshift
of distant red galaxies.Comment: Accepted for publication in the Astronomical Journa
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