Optical distortions arising from inhomogeneous air in the telescope dome can be a non-negligible contributor to the delivered image quality (DIQ). Optimization of the dome environment is particularly important for first-class wide-angle imagers, such as the DECAM on the 4-m Blanco telescope, Omegacam at CFHT and, in the future, LSST. However, the standard method of comparing the DIQ with the site seeing does not single out the effect of the dome, being affected by contributions from other sources and by biases. We developed the dome seeing monitor for the Blanco telescope. It propagates the collimated 4-cm beam from the mirror cell to the top ring and back, parallel to the main beam. The angle-of-arrival fluctuations on this 10 m long path gives a quantitative estimate of the dome seeing. We describe the instrument and its software. The results for the Blanco telescope are presented. The median dome seeing is around 0.2 00. It shows the expected dependence on the temperature difference with the outside air and on the speed and direction of the wind.
In recent years the V. M. Blanco 4-m telescope at Cerro Tololo Inter-American Observatory (CTIO) has been renovated for use as a platform for a completely new suite of instruments: DECam, a 520-megapixel optical imager, COSMOS, a multi-object optical imaging spectrograph, and ARCoIRIS, a near-infrared imaging spectrograph. This has had considerable impact, both internally to CTIO and for its wider community of observers. In this paper, we report on the performance of the renovated facility, ongoing improvements, lessons learned during the deployment of the new instruments, how practical operations have adapted to them, unexpected phenomena and subsequent responses. We conclude by discussing the role for the Blanco telescope in the era of LSST and the new generation of extremely large telescopes.
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
A lunar scintillometer, LuSci, is an inexpensive and robust instrument which deploys a linear array of photodiodes
pointed toward the moon to measure scintillation produced by atmospheric turbulence. Covariances between the signals from the photodiodes are analyzed to derive estimates of the turbulence profile within a few hundred meters above the ground. Instrument parameters and phase of the moon are taken into account. This method has been used for site testing and monitoring. We present the development of a new LuSci instrument used to validate the ground-layer turbulence distribution measured from the laser wavefront sensor signals of the Ground Layer Adaptive Optics system at the MMT. The near-simultaneous measurements are used to characterize the performance of the GLAO system. We describe the instrument, its operation, approaches to data reduction, and use in performance characterization of a GLAO system.
Between February and April 2009 a number of ultrasonic anemometers, temperature probes and dust sensors were
operated inside the CTIO Blanco telescope dome. These sensors were distributed in a way that temperature and
3 dimensional wind speeds were monitored along the line of sight of the telescope. During telescope operations,
occasional seeing measurements were obtained using the Mosaic CCD imager and the CTIO site monitoring MASS-DIMM
system. In addition, also a Lunar Scintillometer (LuSci) was operated over the course of a few nights inside the
dome. We describe the instrumental setup and first preliminary results on the linkage of the atmospheric conditions
inside the dome to the overall image quality.
We present a new lunar scintillometer, LuSci. A simple and accurate way to determine the Ground Layer (GL)
turbulence profile is through measuring lunar and solar scintillation. The contribution of the first 10-100 m to
the total seeing is usually significant. Measuring the seeing in this GL is important to evaluate sites, especially
to set the height of future domes and to translate existing seeing data to higher domes. This holds in particular
to Antarctic sites where the GL seeing is dominant, with obvious implications for AO and interferometry. We
develop robust methods for turbulence profile restoration from LuSci data, incorporating the effect of lunar
phases. We present restored profiles from initial campaigns. We also extract a simple model for the wind profile
from the rich information present in the scintillation spectrum.
In preparation to characterize the Giant Magellan Telescope site and guide the development of its adaptive
optics system, two campaigns to systematically compare the turbulence profiles obtained independently with
three different instruments were conducted at Las Campanas Observatory in September, 2007 and January 2008.
Slope detection and ranging (SLODAR) was used on the 2.5-m duPont telescope. SLODAR measures the C2n
profile as a function of altitude through observations of double stars. The separation of the observed double
star sets the maximum altitude and height resolution. Ground layer (altitudes < 1 km) and free atmosphere
turbulence profiles are compared with those obtained with a lunar scintillometer (LuSci) and a multi-aperture
scintillation sensor (MASS), respectively. In addition, the total atmospheric seeing was measured by both
SLODAR and a differential image motion monitor (DIMM).
One of the main tools used in the TMT site testing campaign is the turbulence profiler MASS. We describe
empirical investigations and a side by side comparison of two MASS systems which were performed in order to
identify the accuracy of MASS turbulence data and its dependence on the instrument calibration. The accuracy
of the total seeing delivered by the TMT MASS systems is found to be better than 0"05. The combination of
MASS and DIMM allows to observe the seeing within the first few hundred meters of the atmosphere and can be
used to investigate possible correlations with meteorological parameters measured close to the ground. We also
compare the detection of clouds and cirrus by means of MASS data (LOSSAM method) with measurements of
the thermal emission of clouds using a net radiation sensor. These methods are compared with the visual cloud
detection using all sky cameras.
Light pollution can create difficulties for astronomers attempting to observe faint objects in the night sky. Light
from a local small town can be just as intrusive as light from a large city in the distance. As the population
of the Earth increases, light pollution will become more of a problem, even in remote areas. The Thirty Meter
Telescope site testing program has measured light pollution at the candidate sites by using all sky cameras;
an analysis procedure enhances the all sky camera images to make the determination of the effects of the light
pollution. This paper summarizes the light pollution analysis procedure and current results, which are that light
pollution is currently unimportant for TMT to select a site for the final telescope location.
Seeing stability is an important criterion of site characterization. Two sites, with the same seeing statistics, could in
principle differ in their temporal stability and hence have their observatories perform differently. Temporal variability
can, however, be defined in several ways, all of which may determine the performance of the observatories in different
manner. In this paper, we propose three methods to measure variability each focusing on different applications: Selection
(maximization of observation time), Image quality (seeing variation within a given integration time) and finally
Scheduling (prediction of seeing fluctuation on a given time scale). We apply these methods to the seeing of the TMT
candidate sites to determine their stability properties.
The Thirty Meter Telescope (TMT) project has been collecting data on five candidate sites since 2003. This paper
describes the site testing portion of the TMT site selection program and the process and standards employed
by it. This includes descriptions of the candidate sites, the process by which they were identified, the site
characterization instrument suite and its calibration and the available results, which will be published shortly.
All Sky Cameras were deployed at all Thirty Meter Telescope (TMT) candidate sites. The images gathered
by these cameras were used to assess the cloud statistics for each site. We describe two methods that were
developed to do this, a manual method based on inspection of blue and red movies, and an automated method
based on photometric analysis of the images.
Differential Image Motion Monitors (DIMMs) have become the industry standard for astronomical site characterization.
The calibration of DIMMs is generally considered to be routine, but we show that particular care
must be paid to this issue if high accuracy measurements are to be achieved. In a side by side comparison of
several DIMMs, we demonstrate that with proper calibration we can characterize the seeing to better than ±0.02
The All-Sky camera used in the LSST and TMT site testing campaigns is described and some early results are shown. The All-Sky camera takes images of
the entire visible hemisphere of sky every 30s in blue, red, Y and Z filters giving
enhanced contrast for the detection of clouds, airglow and the near-infrared.
Animation is used to show movement of clouds. An additional narrow band filter
is centered on the most prominent line of the sodium vapor lamp spectra and is used to monitor any man-made light pollution near the site. The camera also
detects aircraft lights and contrails, satellites, meteor(ite)s, local light polluters,
and can be used for stellar extinction monitoring and for photometry of transient
astronomical objects. For outreach and education the All-Sky camera can show
wandering planets, diurnal rotation of the sky, the zodiacal light, and similar
The Thirty Meter Telescope (TMT) site testing team are developing a suite of instruments to measure the atmospheric and optical characteristics of candidate TMT sites. Identical sets of robotically operating instruments will be placed at each candidate site. The fully developed system will comprise of a combined MASS/DIMM. a SODAR, tower mounted thermal probes and a portable DIMM. These instruments have overlapping altitude coverage and provide a measure of the C2n profile from the ground up with sufficient resolution to make conclusions about the ground layer and high altitude turbulence characteristics. The overlapping altitude coverage is essential to ensure consistency between these very different instruments. In addition to checking for consistency in the overlap regions, procedures are being used to cross check between instruments, i.e. the calculation of the isoplanatic angle from both the MASS and DIMM and that the integrals of the C2n profiles from the MASS, SODAR and 30m tower gives the same r0 value as measured by the DIMM.
We discuss a variation of the traditional DIMM system in which we employ a continuous drift mode readout technique giving a maximum of nearly 300 samples per second.
Findings of our major equipment testing campaigns and first field deployment are presented that demonstrate our progress in developing a rigorous approach to site testing.
The Giant Segmented Mirror Telescope (GSMT), along with other proposed Extremely Large Telescopes (ELT's) with apertures over 20-m, is likely to impose rather different site selection criteria than those for existing large telescopes. Advantageously, remote-sensing techniques allow rather more objective comparisons than was possible in the past, and the general task is aided by numerical modeling and new ground-based measurement techniques. In recognition of the difficulty of the site-selection process, co-operation between the several ELT projects is the norm. A description is given of the site survey for the GSMT, begun in late 1999, and now part of the GSMT studies and evaluation project, run by the Associated Universities for Research in Astronomy (AURA) New Initiatives Office (NIO).