Flat Field Exposures Research Articles

An investigation into the effect of exposing XMM–Newton’s Optical Monitor to the light of Jupiter

Abstract After nearly 18 yr of successful and safe operations, on 2017 July 17 and 18, XMM–Newton’s Optical Monitor (OM) observed Jupiter – an object 10 magnitudes brighter than safe brightness limits – in its visual (V) filter. The object was exposed 40 arcsec from the nominal European Photon Imaging Cameras pn boresight in the negative Y direction, creating a patch of depleted sensitivity. Two exposures of 4000 s each in the V filter left the sensitivity depleted in an area 70 by 40 arcsec; with the decrease in throughput varying from 34 per cent in the V filter to 15 per cent in the ultraviolet bands, but reaching a depth of 45 per cent in flat-field exposures. The wavelength dependency suggests the majority of the detector damage is due to loss of sensitivity in the photocathode from damage inflicted by ion feedback, while up to 15 per cent could be due to gain depletion of the MCP. The physical mechanisms causing the damage to the detector are discussed as well as possible solutions and opportunities that exist for the future operation of the OM.

Spectroscopic flat-fields can be used for precision CCD gain and noise tests

Abstract One of the basic parameters of a charge coupled device (CCD) camera is its gain, that is, the number of detected electrons per output Analogue to Digital Unit (ADU). This is normally determined by finding the statistical variances from a series of flat-field exposures with nearly constant levels over substantial areas, and making use of the fact that photon (Poisson) noise has variance equal to the mean. However, when a CCD has been installed in a spectroscopic instrument fed by numerous optical fibres, or with an echelle format, it is no longer possible to obtain illumination that is constant over large areas. Instead of making do with selected small areas, it is shown here that the wide variation of signal level in a spectroscopic ‘flat-field’ can be used to obtain accurate values of the CCD gain, needing only a matched pair of exposures (that differ in their realisation of the noise). Once the gain is known, the CCD readout noise (in electrons) is easily found from a pair of bias frames. Spatial stability of the image in the two flat-fields is important, although correction of minor shifts is shown to be possible, at the expense of further analysis.

Pair creation energy and Fano factor of silicon measured at 185 K using [formula omitted] X-rays

The pair creation energy, ω and the Fano factor of silicon were measured using a CCD sensor and X-rays from an 55Fe source. The measurements were performed at a sensor temperature of 185K. The pair creation energy was measured for X-rays in the 1.7–6.5 keV range. The measured pair creation energy is ω=(3.650±0.009)eV at the MnKα line energy. The Fano factor at this energy is F=0.128±0.001. The agreement with theory and previous measurements is satisfactory. The system gain was obtained from flat field exposures using the Poisson distribution properties. These results and the details of our measurement procedure are presented below.

SU-E-I-102: Independent Implementation of AAPM TG-150 Draft Image Receptor Test Recommendations.

To determine whether a proposed suite of generic tests for digital radiography (DR) detectors could be reduced to practice. MATLAB software was developed to analyze images according to descriptions in a document drafted by the TG150 Detector Subgroup. Forprocessing images were acquired directly from the acquisition stations of three DR and one Computed Radiography system. Images included flat-field exposures at the manufacturer's calibration condition, twice the exposure, ½ the exposure, and a low exposure, plus three images of a lead bar pattern in different orientations, also at the calibration condition. The flat field images were analyzed to determine Detector Response; Gain Correction; Signal, Noise, and Signal-to-noise (SNR) Uniformity; SNR Magnitude; and Anomalous Detector Element (del) Identification. The program also allowed visual inspection for evaluation of collimation and non-uniformity. Bar pattern images were analyzed to evaluate spatial resolution by a variance method. Acquisition revealed a number of pitfalls. Some manufacturers have multiple calibration points. For-processing images are not directly available from all systems, and PACS may modify them from their original state. The orientation of the flat field with respect to the anodecathode axis may not be defined by the manufacturer. Care must be taken to ensure collimation outside the edges of detectors, or the software must exclude collimator shadows. The matrix size of images differs among manufacturers, so the size of the region of interest (ROI) for analysis varies from the default size of 100×100 dels, as does the number of ROIs. The approach for dealing with edges and ROIs may affect the numerical results. The detector response function may also affect the interpretation of results. The software successfully implements most of the detector tests recommended by TG150. Comparison of these results with those of the parallel effort will validate the draft test definition.

Relative flux calibration for the Guoshoujing Telescope (LAMOST)

This paper presents a relative flux calibration method for the Guoshoujing Telescope (LAMOST), which may be applied to connect a blue spectrum to a red spectrum to build the whole spectrum across the total wavelength range (3700 ∼ 9000 Å). In each spectrograph, we estimate the effective temperatures of selected stars using a grid of spectral line indices in the blue spectral range and a comparison with stellar atmosphere models. For each spectrograph, stars of types A and F are selected as pseudo-standard stars, and the theoretical spectra are used to calibrate both the blue (3700 ∼ 5900 Å) and red spectrograph arms (5700 ∼ 9000 Å). Then the spectral response function for these pseudo-standard stars could be used to correct the raw spectra provided by the other fibers of the spectrograph, after a fiber efficiency function has been derived from twilight flat-field exposures. A key problem in this method is the fitting of a pseudo stellar continuum, so we also give a detailed description of this step. The method is tested by comparing a small sample of LAMOST spectra calibrated in this way on stars also observed by the Sloan Digital Sky Survey. The result shows that the Teff estimation and relative flux calibration method are adequate.

The XDSpres CL-Based Package for Reducing OSIRIS Cross-dispersed Spectra

ABSTRACTWe present a description of the CL-based package XDSpres, which aims at being a complete reducing facility for cross-dispersed spectra taken with the Ohio State Infrared Imager/Spectrometer, as installed at the SOAR telescope. This instrument provides spectra in the range between 1.2 μm and 2.35 μm in a single exposure, with resolving power of R ∼ 1200. XDSpres consists of two tasks: namely, xdflat and doosiris. The former is a completely automated code for preparing normalized flat-field images from raw flat-field exposures. Doosiris was designed to be a complete reduction pipeline, requiring a minimum of user interaction. General steps toward a fully reduced spectrum are explained, as well as the approach adopted by our code. The software is available to the community online.

The nature of thefibernoise with the FOCES spectrograph

The non-white noise noticed using the fiber-fed Foces spectrograph is recognized as the result of vignetting to an uneven distribution of monochromatic light in the spectrograph beam behind the fiber exit. This phenomenon – that can be assigned to every fiber-coupled spectrograph with vignetting of the light path behind the fiber – is qualitatively and quantitatively described and modeled. The ratio that seemed to be limited to ≈200 when surrounding the object exposure by two flatfield exposures is shown to reach the theoretical limit (defined by the linear range of the CCD-detector) of . This is achieved by using multiple flatfield exposures in combination with a fiber vibration device allowing non-harmonic movement of the fiber during exposure time.

Inhomogeneity in Spectral Transmission of the CoatedUBVRIFilters of BFOSC and its Influence on Photometry

In order to realize the Johnson UBV and Kron-Cousins RI photometric systems in BFOSC, glass filters were purchased according to the Bessell prescription. All the filters are anti-reflection coated on both sides to increase the transmission. By comparing two dome flat-field exposures taken through any one of these filters set in two orientations 90° apart, inhomogeneities in the filter transmission is clearly demonstrated. Using a PDS microphotometer, we have confirmed that the form of their spectral transmission curve varied from point to point on the filters. This kind of inhomogeneity cannot be eliminated by dividing by flat-fielding exposure and must be ascribed to inhomogeneities in the coating. This is not some accidental defect in coating, it is in the existing coating technique because all the filters (including the narrow-band interference ones) for the BFOSC and a Hβ filter made by another manufacturer showed a similar pattern. We found that for the studies of variable stars with amplitudes less than 0.05 mag. or studies aiming at establishing accurate C-M diagrams of star clusters, the usual uncoated Schott glass filters of Bessell's prescription are to be preferred.

The Coudé Echelle Spectrograph for the Xinglong 2.16-m Telescope

A brief description of the NAO coude echelle spectrograph mounted on the 2.16m telescope at Xinglong station is given. This echelle spectrograph is located at the coude focus with a prism cross disperser. The echelle image covers the spectral region from 330 to 1100nm displayed in 80 spectral orders in one exposure through two light beams. With a slit height of 2mm, spectral orders are separated by 15 to 23 pixels in blue region and by 7 to 19 pixels in red region. Alternatively, two additional resolution modes corresponding to dierent focal length cameras with resolving power R = 16000, 170000 in the blue beam and R = 13000, 170000 in the red beam could be provided by this spectrograph. The bias, dark, wavelength calibration, flat field and science exposure are described in details. The limiting magnitude for 1 hour exposure with an S/N ratio of 100 scales to V = 9.5 in the red path and to V = 7.2 in the blue path.

Dose response of two storage-phosphor systems for intra-oral radiography

We evaluated the dose response of two storage-phosphor systems, DenOptix and Digora, for intra-oral radiography. Dose response curves were generated using uniform flat field exposure. The dynamic range of both systems was assessed. Root mean square (RMS) granularity curves were also generated to compare noise levels at different exposures. For DenOptix, curves were obtained by activating and inactivating the automatic range control (ARC) setting. For Digora, curves were generated at different calibration exposures. Moreover, tube voltage and tube current dependencies in both systems were examined.

Effects of Shutter Timing on CCD Photometry

CCD-shutters take some time to open and close. This results in a difference between the recorded and real exposure times. The pixel-to-pixel pattern δ(x, y) in which this difference varies depends on the characteristics of the device. These timing errors affect photometric measurements in two ways: directly, since the recorded exposure time is not the real one, and indirectly, via the flatfielding process, because the shutter effect is mixed up with the flatfield pattern. The direct effect may be negligible when exposure times are long. The indirect effect is present irrespective of the exposure times of the frames, if flatfield exposure times are short.

Technical note: compensation for field non-uniformity on a mammographic X-ray unit.

A well observed phenomenon in diagnostic radiology is that of the anode heel effect. The heel effect is caused by target absorption of X rays emitted from an electron impact site at angles more nearly parallel to the anode face (Curry et al, 1984). This has the consequence of producing a non-uniform exposure profile in the image plane (Figure 1). Other non-uniformities may be caused by imperfections in any pre-filtration foils (IPSM, 1989). Although non-uniformity is tolerated and to a large extent is of no clinical concern, there are applications where a uniform field exposure is required. One such area is where quantitative measurements need to be made at different spatial positions. Additionally it may prove advantageous to use a flat field exposure in mammographic examination of women with very large breasts where areas towards the breast periphery may get a significantly lower exposure than at the chest wall, reducing image contrast in that region. As part of a Department of Health evaluation into the use of computed radiography (CR) in mammography (see, for example, Takeno et al, 1987) the authors have been investigating the mammographic imaging performance of CR using quantitative measurement techniques. One such technique is noise power spectrum analysis. Because this technique requires uniform exposure distribution over a large field area (12 cm × 12 cm) it was necessary to develop a technique to compensate for X-ray beam non-uniformity on a clinical mammography unit (Philips Diagnost UC).

CCD Photometry of NGC 2453

We have undertaken a detailed photometric study of the open cluster NGC 2453 in Puppis with a view to determining accurately its distance modulus and to investigate its possible association with the PN NGC 2452. We have observed the core region of the cluster in V and I photometric bands using a Thomson-CSF TH 7882 CCD chip with the format 384×576 pixels, at the f/13 cassegrain focus of the 102-cm telescope at Vainu Bappu Observatory, Kavalur, India. Each image frame covers a field of 2′.3 × 3′.4 of the sky. Bias frames and flat field exposures on the twilight sky were obtained as per standard practice.

The Design and Construction of a Charge-Coupled Device Imaging System

view Abstract Citations (23) References (10) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Design and Construction of a Charge-Coupled Device Imaging System Gudehus, D. H. ; Hegyi, D. J. Abstract The design and construction of a charge-coupled device (CCD) imaging system optimized for the detection of low surface brightness objects is discussed in detail. In particular, we describe the design of the Dewar and the CCD electronics, as well as the choice of CCD operating voltages, clock timings, and operating temperature. In addition, we discuss the measurement of readout and spatial noise for an RCA SID536l2 thinned CCD (320 by 512 pixels). For this chip, we find a readout noise of 64 electrons and obtain a well depth in excess of 350000 electrons with essentially no blemishes or bleeding of charge on bright star images. The threshold level for trailing of charge is 300 electrons in the center of the chip and is essentially zero in the corner of the chip which is read out first. On broadband exposures, fringing from night-sky lines is typically seen only in the infrared (∼ 8700 Å) at about 2% of the night- sky level. Our twilight flat-field exposures taken on different nights are consistent with each other at a level varying from one part in 100000 to a few parts in 10000 on the large scale, and the zero level is stable to ± 10 electrons over a night. The system is linear over its entire range, and we can currently do photometry to better than 0.8% using standard stars. Publication: The Astronomical Journal Pub Date: January 1985 DOI: 10.1086/113720 Bibcode: 1985AJ.....90..130G full text sources ADS |