Gabriel Prieto

Giant Magellan Telescope site testing: summary

Cerro Las Campanas located at Las Campanas Observatory (LCO) in Chile has been selected as the site for the Giant Magellan Telescope. We report results obtained since the commencement, in 2005, of a systematic site testing survey of potential GMT sites at LCO. Meteorological (cloud cover, temperature, pressure, wind, and humidity) and DIMM seeing data have been obtained at three potential sites, and are compared with identical data taken at the site of the twin Magellan 6.5m telescopes. In addition, measurements of the turbulence profile of the free-atmosphere above LCO have been collected with a MASS/DIMM. Furthermore, we consider photometric quality, light pollution, and precipitable water vapor (PWV). LCO, and Co. Las Campanas in particular, have dark skies, little or no risk of future light pollution, excellent seeing, moderate winds, PWV adequate for mid-IR astronomy during a reasonable fraction of the nights, and a high fraction of clear nights overall. Finally, Co. Las Campanas meets or exceeds all the defined science requirements.

Giant Magellan Telescope site evaluation and characterization at Las Campanas Observatory

Las Campanas Observatory has been designated as the location for the Giant Magellan Telescope (GMT). We report results obtained since the commencement, in 2005, of a systematic site testing campaign at LCO. Meteorological (cloud cover, temperature, pressure, wind, and humidity) and DIMM seeing data have been obtained at three potential sites, and are compared with identical data taken at the site of the twin Magellan 6.5m telescopes. In addition, measurements of the turbulence profile of the free-atmosphere above LCO have been collected with a MASS/DIMM. We examine the contribution to the seeing arising from turbulence in the ground layer (defined here as below an altitude of 500 m) through the difference between the turbulence integrals in the full atmosphere (as measured by DIMM) and in the free atmosphere (as measured by MASS). Additionally, we consider photometric quality, light pollution, and precipitable water vapor at LCO.

A CDMAM Image Phantom Software Improvement for Human Observer Assessment

A software tool is presented to improve the features of CDMAM image phantom by University Hospital Nijmegen. This software tool ensures that the 4-alternative forced choice method of CDMAM is actually kept, even when is being scored by highly expertise observers familiar on the test object pattern. For digital images, the developed software tool automatically changes the image position of the four corners. It can be selected a fixed rotation angle or a random one, so making impossible that any observer is able to remember the exact corner position of the target disc inside any cell. Two alternative successful algorithms have been tested. ROC curve analysis obtained by 36 observers shows that both original and computer-modified images are indistinguishable. The ROC area was 0.507±0.024 for first algorithm and 0.522±0.026 for the second one, indicating that there was no statistical difference between real and computer-modified images for both of them.

Two campaigns to compare three turbulence profiling techniques at Las Campanas Observatory

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).

Giant Magellan Telescope site testing seeing and turbulence statistics

Cerro Las Campanas located at Las Campanas Observatory in Chile has been selected as the site for the Giant Magellan Telescope. We report results obtained since the commencement, in 2005, of a systematic site testing survey of potential GMT sites at LCO. Seeing data have been obtained at three potential sites, and are compared with identical data taken at the site of the twin Magellan 6.5m telescopes. In addition, measurements of the turbulence profile of the free-atmosphere have been collected. Co. Las Camapanas and the Magellan site are nearly identical in their seeing statistics, and apparently their average ground-layer characteristics.

Comparison of precipitable water vapour measurements made with an optical echelle spectrograph and an infrared radiometer at Las Campanas Observatory

We present simultaneous precipitable water vapour (PWV) measurements made at the Las Campanas Observatory in late 2007 using an Infrared Radiometer for Millimetre Astronomy (IRMA) and the Magellan Inamori Kyocera Echelle (MIKE) optical spectrograph. Opacity due to water vapour is the primary concern for ground based infrared astronomy. IRMA has been developed to measure the emission of rotational transitions of water vapour across a narrow spectral region centred around 20 μm, using a 0.1 m off-axis parabolic mirror and a sophisticated atmospheric model to retrieve PWV. In contrast, the MIKE instrument is used in conjunction with the 6.5 m Magellan Clay telescope, and determines the PWV through absorption measurements of water vapour lines in the spectra of telluric standard stars. With its high spectral resolution, MIKE is able to measure absorption from optically thin water vapour lines and can derive PWV values using a simple, single layer atmospheric model. In an attempt to improve the MIKE derived PWV measurements, we explore the potential of fitting a series of MIKE water vapour line measurements, having different opacities.

Use of the cross-correlation component of the multiscale structural similarity metric (R* metric) for the evaluation of medical images

PURPOSE The aim of the present work is to analyze the potential of the cross-correlation component of the multiscale structural similarity metric (R*) to predict human performance in detail detection tasks closely related with diagnostic x-ray images. To check the effectiveness of R*, the authors have initially applied this metric to a contrast detail detection task. METHODS Threshold contrast visibility using the R* metric was determined for two sets of images of a contrast-detail phantom (CDMAM). Results from R* and human observers were compared as far as the contrast threshold was concerned. A comparison between the R* metric and two algorithms currently used to evaluate CDMAM images was also performed. RESULTS Similar trends for the CDMAM detection task of human observers and R* were found in this study. Threshold contrast visibility values using R* are statistically indistinguishable from those obtained by human observers (F-test statistics: p > 0.05). CONCLUSIONS These results using R* show that it could be used to mimic human observers for certain tasks, such as the determination of contrast detail curves in the presence of uniform random noise backgrounds. The R* metric could also outperform other metrics and algorithms currently used to evaluate CDMAM images and can automate this evaluation task.

Giant Magellan Telescope site testing: PWV statistics and calibration

Cerro Las Campanas located at Las Campanas Observatory (LCO) in Chile has been selected as the site for the Giant Magellan Telescope. We report results obtained since the commencement, in 2005, of a systematic site testing survey of potential GMT sites at LCO. Atmospheric precipitable water vapor (PWV) adversely impacts mid-IR astronomy through reduced transparency and increased background. Prior to the GMT site testing effort, little was known regarding the PWV characteristics at LCO and therefore, a multi-pronged approach was used to ensure the determination of the fraction of the time suitable for mid-IR observations. High time resolution monitoring was achieved with an Infrared Radiometer for Millimeter Astronomy (IRMA) from the University of Lethbridge deployed at LCO since September of 2007. Absolute calibrations via the robust Brault method (described in Thomas-Osip et al.1) are provided by the Magellan Inamori Kyocera Echelle (MIKE), mounted on the Clay 6.5-m telescope on a timescale of several per month. We find that conditions suitable for mid-IR astronomy (PWV < 1.5 mm) are concentrated in the southern winter and spring months. Nearly 40% of clear time during these seasons have PWV < 1.5mm. Approximately 10% of these nights meet our PWV requirement for the entire night.

Automatic scoring of CDMAM using a model of the recognition threshold of the human visual system: R*

A software tool is presented for the automatic evaluation of the CDMAM phantom images that are currently used for the quality assessment of the image quality in mammography. This software tool is based on the use of the cross-correlation component of the index MS-SSIM*, R*, oriented to the recognition threshold of different image representations. Given an image sequence, whose images begin as unrecognizable and are gradually refined to include more information, the recognition threshold corresponds to first the image in the sequence in which an observer indentifies the content. We have validated our software tool by comparing our readouts with those obtained by a total of 4 expert observers in the evaluation of 8 CDMAM images. The correlations obtained between both readout sets are better than 0.99 and the range of useful correlation comprises diameters from 0.16 mm to 2.0 mm of the gold disks inside the CDMAM.

A SOFTWARE TOOL TO MEASURE THE GEOMETRIC DISTORTION IN X-RAY IMAGE SYSTEMS

A software tool is presented to measure the geometric distortion in images obtained with X-ray systems that provides a more objective method than the usual measurements over the image of a phantom with usual rulers. In a first step, this software has been applied to mammography images and makes use of the grid included into the CDMAM phantom (University Hospital Nijmegen). For digital images, this software tool automatically locates the grid crossing points and obtains a set of corners (up to 237) that are used by the program to determine 6 different squares, at top, bottom, left, right and central positions. The sixth square is the largest that can be fitted in the grid (widest possible square). The distortion is calculated as ((length of left diagonal - length of right diagonal)/ length of left diagonal) (%) for the six positions. The algorithm error is of the order of 0.3%. The method might be applied to other radiological systems without any major changes to adjust the program code to other phantoms. In this work a set of measurements for 54 CDMAM images, acquired in 11 different mammography systems from 6 manufacturers are presented. We can conclude that the distortion of all equipments is smaller than the recommendations for maximum distortions in primary displays (2%)