447 research outputs found
Tales within Tales and Cutoffs within Cutoffs: What Sets the Mass Scale for Galaxies?
Please answer ``yes'' or ``no'': 1. Does the mass function for clusters of
galaxies cut off exponentially? 2. Does the luminosity function for galaxies
cut off exponentially? 3. Is the dependence of virial velocity on galaxy
luminosity a power law? 4. Does the velocity function for galaxies cut off
exponentially?Comment: 10 pages, no figures, contribution to the MPA/ESO/MPE/USM conference
"Lighthouses of the Universe", Sunyaev et al. (eds.), ESO Astrophysics
Symposia, Springer Verla
Generic Misalignment Aberration Patterns in Wide-Field Telescopes
Axially symmetric telescopes produce well known "Seidel" off-axis third-order
aberration patterns: coma, astigmatism, curvature of field and distortion. When
axial symmetry is broken by the small misalignments of optical elements,
additional third-order aberration patterns arise: one each for coma,
astigmatism and curvature of field and two for distortion. Each of these
misalignment patterns is characterized by an associated two-dimensional vector,
each of which in turn is a linear combination of the tilt and decenter vectors
of the individual optical elements. For an N-mirror telescope, 2(N-1) patterns
must be measured to keep the telescope aligned. Alignment of the focal plane
may require two additional patterns. For N = 3, as in a three mirror
anastigmat, there is a two-dimensional "subspace of benign misalignment" over
which the misalignment patterns for third-order coma, astigmatism and curvature
of field are identically zero. One would need to measure at least one of the
two distortion patterns to keep the telescope aligned. Alternatively, one might
measure one of the fifth-order misalignment patterns, which are derived herein.
But the fifth-order patterns are rather insensitive to misalignments, even with
moderately wide fields, rendering them of relatively little use in telescope
alignment. Another alternative would be to use telescope pointing as part of
the alignment solution.Comment: 50 pages, 13 figures, Accepted for Publication in PAS
Correlated random fields in dielectric and spin glasses
Both orientational glasses and dipolar glasses possess an intrinsic random
field, coming from the volume difference between impurity and host ions. We
show this suppresses the glass transition, causing instead a crossover to the
low phase. Moreover the random field is correlated with the inter-impurity
interactions, and has a broad distribution. This leads to a peculiar variant of
the Imry-Ma mechanism, with 'domains' of impurities oriented by a few frozen
pairs. These domains are small: predictions of domain size are given for
specific systems, and their possible experimental verification is outlined. In
magnetic glasses in zero field the glass transition survives, because the
random fields are disallowed by time-reversal symmetry; applying a magnetic
field then generates random fields, and suppresses the spin glass transition.Comment: minor modifications, final versio
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