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PubMed2005

X-ray telescope onboard Astro-E. III. Guidelines to performance improvements and optimization of the ray-tracing simulator.

We present a detailed study of the performance of the Astro-E x-ray telescope (XRT) onboard the Astro-E satellite. As described in preceding papers the ground-based calibrations of the Astro-E XRT revealed that its image quality and effective area are somewhat worse than that expected from the original design. Conceivable causes for such performance degradation are examined by x-ray and optical microscopic measurements at various levels, such as individual reflectors, sectors, and quadrants of the XRT and their alignments. We can attribute, based on detailed measurements, the degradation of the image quality to a slope error in the individual reflectors and the positioning error of reflectors. As for the deficit of the effective area, the shadowing of x rays within the XRT body is the dominant factor. Error budgets for the performance degradation of the Astro-E XRT are summarized. The ray-tracing simulator, which is needed to construct the response function for arbitrary off-axis angles and spatial distributions of any celestial x-ray sources, has been developed and tuned based on the results of detailed measurements. The ray-tracing simulation provides results that are consistent within 3% with the real measurement except for large off-axis angles and higher energies. We propose, based on knowledge obtained from all the measurements and simulations, several plans for future developments to improve the performance of the nested thin-foil mirrors.

Journal Article↗
PubMed2005

Analytic diffraction analysis of a 32-m telescope with hexagonal segments for high-contrast imaging.

Large segmented telescopes cannot be modeled accurately with fast-Fourier-transform techniques since small features such as gaps between the segments will be inadequately sampled. An analytic Fourier-transform method can be used to model any pupil configuration with straight edges, including tolerance analysis and some types of apodization. We analytically investigated a 32-m segmented primary with 18 hexagonal segments for high-contrast imaging. There are significant regions in the image in which extrasolar planets could be detected. However, the hexagonal profile of the pupil was not as useful as expected. The gaps between the segments, the secondary obscuration, and the secondary spiders must be as small as possible and their edges must be apodized. Apodizing the edges of the individual segments reduced the useful regions in the image since the gaps appeared to be wider.

Journal Article↗
PubMed2005

Robust determination of optical path difference: fringe tracking at the infrared optical telescope array interferometer.

We describe the fringe-packet tracking system used to equalize the optical path lengths at the Infrared Optical Telescope Array interferometer. The measurement of closure phases requires obtaining fringes on three baselines simultaneously. This is accomplished by use of an algorithm based on double Fourier interferometry for obtaining the wavelength-dependent phase of the fringes and a group-delay tracking algorithm for determining the position of the fringe packet. A comparison of data acquired with and without the fringe-packet tracker shows a factor of approximately 3 reduction of the error in the closure-phase measurement. The fringe-packet tracker has been able so far to track fringes with signal-to-noise ratios as low as 1.8 for stars as faint as mH = 7.0.

Journal Article↗
PubMed2005

Millimeter wave reimaging optics for the 100 m Green Bank telescope.

Large bolometer arrays capable of operating at millimeter wavelengths are being built for astronomical use. For optimal sensitivity, high-quality optics with wide fields of view are needed. We report on the design of reimaging optics for use on the 100-m Green Bank telescope with a 64-element bolometer array.

Journal Article↗
PubMed2005

Fast computation of the narcissus reflection coefficient for the Herschel far-infrared/submillimeter-wave Cassegrain telescope.

Placement of a scatter cone at the center of the secondary of a Cassegrain telescope greatly reduces Nareissus reflection. To calculate the remaining Narcissus reflection, a time-consuming physical optics code/such as GRASP8 is often used to model the effects of reflection and diffraction. Fortunately, the Cassegrain geometry is sufficiently simple that a combination of theoretical analysis and Fourier propagation can yield rapid, accurate results at submillimeter wavelengths. We compare these results with those from GRASP8 for the heterodyne instrument for the far-infrared on the Herschel Space Observatory and confirm the effectiveness of the chosen scatter cone design.

Journal Article↗
PubMed2006

Optical design of interferometric telescopes with wide fields of view.

The performance of wide-field multiple-aperture imaging systems is dominated by easily understood, low-order errors. Each aperture produces an individual image, each pair of apertures produces a set of fringes under a diffraction envelope, and the system bandwidth produces a coherence envelope. For wide-field imaging, each of these elements must be coincident in the image plane as the field angle changes. We explore the causes of image degradation, derive first-order rules for preserving image quality across field, and give an example design that enforces some of the rules to achieve a relatively wide-field interferometric imaging telescope.

Journal Article↗
PubMed2002

Multiconjugate adaptive optics for large telescopes: analytical control of the mirror shapes.

We present an analytical algorithm for deriving the shapes of the deformable mirrors to be used for multiconjugate adaptive correction on a large telescope. The algorithm is optimal in the limit where the overlap of the wave-front contributions from relevant atmospheric layers probed by the guide stars is close to the size of the pupil. The fundamental principle for correction is based on a minimization of the sum of the residual power spectra of the phase fluctuations seen by the guide stars after correction. On the basis of the expressions for the mirror shapes, so-called layer transfer functions describing the distribution of the correction of a single atmospheric layer among the deformable mirrors and the resulting correction of that layer have been derived. It is shown that for five guide stars distributed in a regular cross, two- and three-mirror correction will be possible only up to a maximum frequency defined by the largest separation of the conjugate altitudes of the mirrors and by the angular separation of the guide stars. The performance of the algorithm is investigated in the K band by using a standard seven-layer atmosphere. We present results obtained for two guide-star configurations: a continuous distribution within a given angular radius and a five-star cross pattern with a given angular arm length. The wave-front fluctuations are subjected to correction using one, two, and three deformable mirrors. The needed mirror dynamic range is derived as required root-mean-square stroke and actuator pitch. Finally the performance is estimated in terms of the Strehl ratio obtained by the correction as a function of field angle. No noise has been included in the present analysis, and the guide stars are assumed to be at infinity.

Journal Article↗
PubMed2002

Tip-tilt error for extremely large segmented telescopes: detailed theoretical point-spread-function analysis and numerical simulation results.

We present an analysis of point-spread functions for segmented mirrors affected by random tip-tilt errors on each segment. In addition to Strehl ratio evaluation, this analysis considers key characteristics such as the intensity and the location of speckles and secondary peaks and the relative energy distribution between these features. We develop a method to describe the shape of a nonaveraged point-spread function and deduce the final expressions for ensemble-averaged characteristics. Based on Keck-type hexagonal segmentation geometry, our study is extended to an arbitrary number of segments, and we describe qualitatively the transition from the case of a mirror with few segments to that of a mirror with several hundred segments--prototype of the next generation of Extremely Large Telescopes.

Journal Article↗
PubMed2003

Analytical study of diffraction effects in extremely large segmented telescopes.

We present an analysis of the diffraction effects from a segmented aperture with a very large number of segments-prototype of the next generation of extremely large telescopes. This analysis is based on the point-spread-function analytical calculation for Keck-type hexagonal segmentation geometry. We concentrate on the effects that lead to the appearance of speckles and/or a regular pattern of diffraction peaks. These effects are related to random piston and tip-tilt errors on each segment, gaps between segments, and segment edge distortion. We deliver formulas and the typical numerical values for the Strehl ratio, the relative intensity of higher-order diffraction peaks, and the averaged intensity of speckles associated with each particular case of segmentation error.

Journal Article↗
PubMed2005

Polarimetric calibration of large-aperture telescopes. I. Beam-expansion method.

A concept is described for the high-accuracy absolute calibration of the instrumental polarization introduced by the primary mirror of a large-aperture telescope. This procedure requires a small aperture with polarization-calibration optics (e.g., mounted on the dome) followed by a lens that opens the beam to illuminate the entire surface of the mirror. The Jones matrix corresponding to this calibration setup (with a diverging incident beam) is related to that of the normal observing setup (with a collimated incident beam) by an approximate correction term. Numerical models of parabolic on-axis and off-axis mirrors with surface imperfections are used to explore the accuracy of the procedure.

Journal Article↗
PubMed2005

Polarimetric calibration of large-aperture telescopes. II. Subaperture method.

A new method for absolute polarimetric calibration of large telescopes is presented. The proposed method is highly accurate and is based on the calibration of a small subaperture, which is then extended to the full system by means of actual observations of an astronomical source. The calibration procedure is described in detail along with numerical simulations that explore its robustness and accuracy. The advantages and disadvantages of this technique with respect to other possible alternatives are discussed.

Journal Article↗
PubMed2005

Mach-Zehnder interferometer for piston and tip-tilt sensing in segmented telescopes: theory and analytical treatment.

A study is presented of a Mach-Zehnder interferometer for the measurement of phasing errors of the type found in segmented telescopes. We show that with a pinhole much larger than the Airy disk and an optical path difference between the arms equal to a quarter of the wavelength, the interferometric signal is related to the second derivative of the wave front. In this condition the signal is produced mostly by the segmentation errors and is marginally sensitive to other aberrations including atmospheric turbulence. The signal has distinguishable symmetric and antisymmetric properties that are related to segment aberrations. We suggest using the antisymmetric component of the signal to retrieve piston, tip, and tilt. The symmetric component of the signal serves as an estimate of the measurement error. In this way we proceed with a study of the errors associated with the misalignment of the interferometer, the segment edge imperfections, and the nonaveraged atmospheric perturbations. The entire study is performed on a theoretical basis, and numerical simulations are used to cross check the analytical results.

Journal Article↗
PubMed2004

Superresolution in compensated telescopes.

We present a procedure for attaining resolution beyond the diffraction limit in ground-based telescopes. This procedure is based on the use of rotationally symmetric pupil plane filters that can be easily implemented in dynamic optical devices such as a deformable mirror of an adaptive-optics system. We show that a successful application of the technique requires partial compensation for atmospheric distortion by adaptive optics. Consequently, we derive the required level of compensation as a function of the atmospheric conditions. Finally, our results are checked using simulated data.

Journal Article↗
PubMed1992

Diagnostic value of telescoping plugged catheters in HIV-infected patients with pulmonary infiltrates.

To assess the diagnostic value of telescoping plugged catheters (TPC) in human immunodeficiency virus (HIV)-infected patients with pulmonary infiltrates (PI), we performed a prospective clinical study in 71 episodes of fever and PI in 66 HIV-infected patients (five patients had two different episodes of fever and PI). A control group of 12 HIV-infected patients with fever and normal chest roentgenogram was also studied. In all patients and prior to antibiotic treatment (except in mechanically ventilated patients), a TPC using quantitative cultures (cutoff point established at 10(3) CFU/ml) and a bronchoalveolar lavage (BAL) sampling were performed via fiberoptic bronchoscope. The overall incidence of bacterial pneumonia in the study group was 21 percent. The TPC cultures resulted in a microbiologic diagnosis of bacterial pneumonia in eight patients (11 percent) from the study group and in one patient (8 percent) from the control group. The TPC sensitivity in diagnosing bacterial infections was 53 percent and specificity was 76 percent. Negative predictive value was 85 percent and positive predictive value was 38 percent. By means of BAL, 35 episodes from the study group and two from the control group were diagnosed as nonbacterial or mycobacterial pulmonary infections. Considering TPC and BAL together, diagnosis was performed in 42 cases of PI (59 percent). Twenty percent (17/83) of HIV-infected patients suffered from bacterial colonization of their lower airways (a TPC culture greater than or equal to 10(3) CFU/ml without clinical evidence of bacterial infection). We conclude that the combined use of TPC and BAL may be useful in HIV-infected patients with PI, since this combined use allows the proper diagnosis of bacterial and nonbacterial infections, thereby increasing the overall diagnostic accuracy. To distinguish bacterial colonization from pulmonary infection in HIV-infected patients with PI, the cutoff point of quantitative cultures of TPC may be 10(4) CFU/ml.

Adult↗
PubMed1993

Diagnostic fiberoptic bronchoscopy in patients with community-acquired pneumonia. Comparison between bronchoalveolar lavage and telescoping plugged catheter cultures.

We compared the diagnostic value of quantitative cultures of bronchoalveolar lavage (BAL) and telescoping plugged catheter (TPC) samples in 40 patients with moderately severe community-acquired pneumonia (CAP). None had received antibiotics previously. BAL quantitative cultures were performed in 13 control patients without pneumonia. In 28 (70 percent) patients, TPC cultures yielded > or = 10(3) cfu/ml of a pathogenic bacterium in pure culture. In 27 of these cases and in four additional cases, BAL cultures yielded > or = 10(3) cfu/ml. BAL cultures from the control group were sterile in seven cases, yielded normal flora in three cases, and yielded microorganisms thought to be not significant in two cases. Microbiologic agreement was reached by both TPC and BAL for 32 (84 percent) of 38 microorganisms recovered. Quantitative cultures from both techniques correlated very well (r = 0.71, p < 0.0001). We conclude that both TPC and BAL have similar accuracy to determine etiologic diagnosis of CAP. There was good qualitative and quantitative agreement between both techniques.

Adolescent↗
PubMed1999

Diagnosis of nosocomial pneumonia in cancer patients undergoing mechanical ventilation: a prospective comparison of the plugged telescoping catheter with the protected specimen brush.

STUDY OBJECTIVES: Quantitative culture of protected samples of lower respiratory tract secretions obtained by a fiberoptic protected specimen brush (PSB) is widely accepted for the diagnosis of ventilator-associated pneumonia (VAP), but this diagnostic procedure is time consuming, expensive, and may give rise to iatrogenic complications, especially in cancer patients who often present with thrombocytopenia. The plugged telescoping catheter (PTC) could be a satisfactory alternative to the PSB in this setting. The aim of the present study was to evaluate the interest of the PTC to diagnose VAP in ventilated cancer patients. DESIGN: A prospective observational study. SETTING: A 15-bed medical-surgical ICU in a comprehensive cancer center. PATIENTS AND INTERVENTIONS: Over a 9-month period, 42 patients suspected of having bacterial VAP during mechanical ventilation underwent 69 bronchial samplings: a blinded PTC and a fiberoptic PSB were performed successively in each case. A positive culture for both sampling procedures was defined as the recovery of > or = 10(3) cfu/mL of at least one potential pathogen. The PSB result was taken as the reference standard. MEASUREMENTS AND RESULTS: The overall agreement between the techniques was 87% (60/69). PTC had a sensitivity of 67%, a specificity of 93%, a positive predictive value of 71%, and a negative predictive value of 91%. CONCLUSIONS: We conclude that the accuracy of the blinded PTC compares well with that of the PSB for the diagnosis of VAP in cancer patients. The sensitivity of the PTC observed herein, which is slightly lower than that described in previous studies, may be due to the blinded nature of the method: the indications for initial or secondary coupling with a directed sampling method in patients with suspicion of localized pneumonia remain to be determined.

Bacteria↗
PubMed2006

Agreement between quantitative cultures of postintubation tracheal aspiration and plugged telescoping catheter, protected specimen brush, or BAL for the diagnosis of nosocomial pneumonia.

BACKGROUND: The diagnosis of ventilator-associated pneumonia relies on protected specimen brush (PSB), BAL, and plugged telescoping catheter (PTC) procedures. In the particular setting of nosocomial pneumonia (NP) occurring in non-mechanically ventilated patients, no consensus exists on their use. When mechanical ventilation (MV) becomes mandatory, postintubation tracheal aspiration (PITA) could be a simple, fast, and cheap diagnostic tool. Our aim was to compare the diagnostic accuracy of PITA to that of PSB, BAL, or PTC in patients requiring MV for suspected NP. METHODS: Patients with a prior hospital stay of > or = 48 h who required MV for suspicion of NP were prospectively enrolled in the study. PITA was performed by sterile suction. Within 2 h, pulmonary samples were obtained by PSB, BAL, or blinded PTC, which are referred to hereafter as "reference methods" (RMs). The definite diagnosis of NP was made using a composite item of clinical, radiologic, and bacteriologic (ie, blood or pleural fluid cultures) patterns. The agreement between the quantitative microbiological results obtained with PITA and those of the RMs was assessed by the kappa-statistic. The sensitivity, specificity, and positive and negative likelihood ratios of PITA and RMs were calculated taking the definite diagnosis of NP as the reference. RESULTS: There were 44 cases (63.8%) of confirmed NP. The kappa-statistic was 0.71. The sensitivity, specificity, and positive and negative likelihood ratios were 77%, 84%, 4.80, and 0.27, respectively, for PITA, and 75%, 88%, 6.25, and 0.28, respectively, for RMs. CONCLUSIONS: PITA may be a reliable alternative to RMs in the particular setting of NP in newly mechanically ventilated patients.

APACHE↗
PubMed1989

Pulmonary infiltrates in immunocompromised patients. Diagnostic value of telescoping plugged catheter and bronchoalveolar lavage.

The usefulness of telescoping plugged catheter (TPC) together with bronchoalveolar lavage (BAL) in the same bronchoscopic act in the diagnosis of pulmonary infiltrates was studied in 113 fiberoptic bronchoscopic examinations performed on 96 immunocompromised patients. The TPC cultures detected pulmonary bacterial infections in 25 (22 percent) cases but showed a high frequency of false positive results (12 microorganisms, 27 percent). Bronchoalveolar lavage had an overall diagnostic yield of 49 percent (53 of 113 cases). Combining TPC and BAL diagnostic values, 78 of 113 pulmonary infiltrates (69 percent) were diagnosed. The results obtained by both techniques allowed us to modify the treatment in 35 (31 percent) cases. Combined, TPC and BAL show a good diagnostic yield in immunocompromised patients with pulmonary infiltrates. Both techniques should be performed as the first approach in the evaluation of these patients, and be done in the same bronchoscopic procedure.

Bacteria↗