222 research outputs found
Optimal slit orientation for long multi-object spectroscopic exposures
Historically, long-slit spectroscopic observations were carried out using the
parallactic angle for the slit orientation if slit loss was an important
consideration (either to maximize the signal-to-noise or to do
spectrophotometry). This requires periodic realignment of the slit position
angle as the parallactic angle changes. This is not possible for multi-slit
observations where one slit position angle must be chosen for the entire
exposure. Common wisdom suggests using the parallactic angle at the meridian
(HA=0). In this paper, I examine what the best strategy is for long, multi-slit
exposures. I find that in extreme cases (very long exposure time) the best
choice is to orient the slit \emph{perpendicular} to the parallactic angle at
the meridian. There are two effects to consider: the increasing dispersion with
increasing airmass and the changing angle between the parallactic angle and the
slit. In the case of \emph{traditional} slit orientation, the two effects
amplify each other, thus rendering a significant fraction of the observation
useless. Using the perpendicular orientation, the two processes work against
each other, thus most of the observation remains useful. I will use, as an
example, our 8 hour Lockman Hole observations using the Keck telescope, but
generic methods are given to evaluate a particular observation. I also make the
tools available to the community.Comment: Accepted by A&A (20/06/2005
High-precision Absolute Distance and Vibration Measurement using Frequency Scanned Interferometry
In this paper, we report high-precision absolute distance and vibration
measurements performed with frequency scanned interferometry using a pair of
single-mode optical fibers. Absolute distance was determined by counting the
interference fringes produced while scanning the laser frequency. A
high-finesse Fabry-Perot interferometer(F-P) was used to determine frequency
changes during scanning. Two multiple-distance-measurement analysis techniques
were developed to improve distance precision and to extract the amplitude and
frequency of vibrations. Under laboratory conditions, measurement precision of
50 nm was achieved for absolute distances ranging from 0.1 meters to 0.7
meters by using the first multiple-distance-measurement technique. The second
analysis technique has the capability to measure vibration frequencies ranging
from 0.1 Hz to 100 Hz with amplitude as small as a few nanometers, without a
priori knowledge.Comment: 16 pages, 7 figures, 1 table, accepted for publication in Applied
Optic
Atmospheric dispersion correction: model requirements and impact on radial velocity measurements
Observations with ground-based telescopes are affected by differential
atmospheric dispersion when seen at a zenith angle different from zero, a
consequence of the wavelength-dependent index of refraction of the atmosphere.
One of the pioneering technology in detecting exoplanets is the technique of
radial velocity (RV), that can be affected by uncorrected atmospheric
dispersion. The current highest precision spectrographs are expected to deliver
a precision of 10 cm/s (e.g., ESPRESSO). To minimize the atmospheric dispersion
effect, an Atmospheric Dispersion Corrector (ADC) can be employed. ADC designs
are based on sky dispersion models that nonetheless give different results;
these can reach a few tens of milli-arcseconds (mas) in the sky (a difference
up to 40 mas); a value close to the current requirements (20 mas in the case of
ESPRESSO). In this paper we describe tests done with ESPRESSO and HARPS to
understand the influence of atmospheric dispersion and its correction on RV
precision. We also present a comparison of different sky models, using EFOSC2
data (between 600nm and 700nm), that will be used to improve on the design of
ADCs
An Imaging Fabry-Perot System for the Robert Stobie Spectrograph on the Southern African Large Telescope
We present the design of the Fabry-Perot system of the Robert Stobie
Spectrograph on the 10-meter class Southern African Large Telescope and its
characterization as measured in the laboratory. This system provides
spectroscopic imaging at any desired wavelength spanning a bandpass 430 - 860
nm, at four different spectral resolving powers ranging from 300 to 9000. Our
laboratory tests revealed a wavelength dependence of the etalon gap and
parallelism with a maximum variation between 600 - 720 nm that arises because
of the complex structure of the broadband multi-layer dielectric coatings. We
also report an unanticipated optical effect of this multi-layer coating
structure that produces a significant, and wavelength dependent, change in the
apparent shape of the etalon plates. This change is caused by two effects: the
physical non-uniformities or thickness variations in the coating layers, and
the wavelength dependence of the phase change upon refection that can amplify
these non-uniformities. We discuss the impact of these coating effects on the
resolving power, finesse, and throughput of the system. This Fabry-Perot system
will provide a powerful tool for imaging spectroscopy on one of the world's
largest telescopes.Comment: 17 pages, 14 figures, accepted for publication in The Astronomical
Journa
Simulation of Imaging Atmospheric Cherenkov Telescopes with CORSIKA and sim_telarray
Imaging Atmospheric Cherenkov Telescopes (IACTs) have resulted in a
breakthrough in very-high energy (VHE) gamma-ray astrophysics. While early IACT
installations faced the problem of detecting any sources at all, current
instruments are able to see many sources, often over more than two orders of
magnitude in energy. As instruments and analysis methods have matured, the
requirements for calibration and modelling of physical and instrumental effects
have increased. In this article, a set of Monte Carlo simulation tools is
described that attempts to include all relevant effects for IACTs in great
detail but aims to achieve this in an efficient and flexible way. These tools
were originally developed for the HEGRA IACT system and later adapted for the
H.E.S.S. experiment. Their inherent flexibility to describe quite arbitrary
IACT systems makes them also an ideal tool for evaluating the potential of
future installations. It is in use for design studies of CTA and other
projects.Comment: 25 pages, 14 figures, accepted for publication in Astroparticle
Physic
The Effects of Atmospheric Dispersion on High-Resolution Solar Spectroscopy
We investigate the effects of atmospheric dispersion on observations of the
Sun at the ever-higher spatial resolutions afforded by increased apertures and
improved techniques. The problems induced by atmospheric refraction are
particularly significant for solar physics because the Sun is often best
observed at low elevations, and the effect of the image displacement is not
merely a loss of efficiency, but the mixing of information originating from
different points on the solar surface. We calculate the magnitude of the
atmospheric dispersion for the Sun during the year and examine the problems
produced by this dispersion in both spectrographic and filter observations. We
describe an observing technique for scanning spectrograph observations that
minimizes the effects of the atmospheric dispersion while maintaining a regular
scanning geometry. Such an approach could be useful for the new class of
high-resolution solar spectrographs, such as SPINOR, POLIS, TRIPPEL, and ViSP
Equations for solar tracking
Direct Sun light absorption by trace gases can be used to quantify them and
investigate atmospheric chemistry. In such experiments, the main optical
apparatus is often a grating or a Fourier transform spectrometer. A solar
tracker based on motorized rotating mirrors is also needed to direct the light
along the spectrometer axis, correcting for the apparent rotation of the Sun.
Calculating the Sun azimuth and altitude for a given time and location can be
achieved with high accuracy but different sources of angular offsets appear in
practice when positioning the mirrors. A feedback on the motors, using a light
position sensor closed to the spectrometer is almost always needed. This paper
aims to gather the main geometrical formulas necessary for the use of a widely
used kind of solar tracker, based on two 45{\deg} mirrors in altazimuthal
set-up with a light sensor on the spectrometer, and to illustrate them with a
tracker developed for atmospheric research by our group.Comment: 14 pages, 7 figures. Second version of the paper as published in
Sensors. Main correction: a rotation matrix converted to a reflection matrix.
Main addition: a discussion on how the control theory applies to this kind of
tracking syte
Accurate Ritz wavelengths of parity-forbidden [Fe II], [Ti II] and [Cr II] infrared lines of astrophysical interest
With new astronomical infrared spectrographs the demands of accurate atomic
data in the infrared have increased. In this region there is a large amount of
parity-forbidden lines, which are of importance in diagnostics of low-density
astrophysical plasmas. We present improved, experimentally determined, energy
levels for the lowest even LS terms of Fe II, Ti II and Cr II, along with
accurate Ritz wavelengths for parity-forbidden transitions between and within
these terms. Spectra of Fe II, Ti II and Cr II have been produced in a hollow
cathode discharge lamp and acquired using high-resolution Fourier Transform
(FT) spectrometry. The energy levels have been determined by using observed
allowed ultraviolet transitions connecting the even terms with upper odd terms.
Ritz wavelengths of parity-forbidden lines have then been determined. Energy
levels of the four lowest Fe II terms (aD, aF, aD and
aP) have been determined, resulting in 97 different parity-forbidden
transitions with wavelengths between 0.74 and 87 micron. For Ti II the energy
levels of the two lowest terms (aF and bF) have been determined,
resulting in 24 different parity-forbidden transitions with wavelengths between
8.9 and 130 micron. Also for Cr II the energy levels of the two lowest terms
(aS and aD) have been determined, in this case resulting in 12
different parity-forbidden transitions with wavelengths between 0.80 and 140
micron.Comment: Accepted for publication in A&A, 13 pages, 6 figures, 9 table
Astronomical spectrograph calibration with broad-spectrum frequency combs
Broadband femtosecond-laser frequency combs are filtered to
spectrographically resolvable frequency-mode spacing, and the limitations of
using cavities for spectral filtering are considered. Data and theory are used
to show implications to spectrographic calibration of high-resolution,
astronomical spectrometers
Real-time distance measurement immune from atmospheric parameters using optical frequency combs
We propose a direct and real-time ranging scheme using an optical frequency
combs, able to compensate optically for index of refraction variations due to
atmospheric parameters. This scheme could be useful for applications requiring
stringent precision over a long distance in air, a situation where dispersion
becomes the main limitation. The key ingredient is the use of a mode-locked
laser as a precise source for multi-wavelength interferometry in a homodyne
detection scheme. By shaping temporally the local oscillator, one can directly
access the desired parameter (distance) while being insensitive to fluctuations
induced by parameters of the environment such as pressure, temperature,
humidity and CO content
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