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Register a new userA Binary Offset Effect in CCD Readout and Its Impact on Astronomical Data
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We have discovered an anomalous behavior of CCD readout electronics that affects their use in many astronomical applications. An offset in the digitization of the CCD output voltage that depends on the binary encoding of one pixel is added to pixels that are read out one, two and/or three pixels later. One result of this effect is the introduction of a differential offset in the background when comparing regions with and without flux from science targets. Conventional data reduction methods do not correct for this offset. We find this effect in 16 of 22 instruments investigated, covering a variety of telescopes and many different front-end electronics systems. The affected instruments include LRIS and DEIMOS on the Keck telescopes, WFC3-UVIS and STIS on HST, MegaCam on CFHT, SNIFS on the UH88 telescope, GMOS on the Gemini telescopes, HSC on Subaru, and FORS on VLT. The amplitude of the introduced offset is up to 4.5 ADU per pixel, and it is not directly proportional to the measured ADU level. We have developed a model that can be used to detect this binary offset effect in data and correct for it. Understanding how data are affected and applying a correction for the effect is essential for precise astronomical measurements.
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We present details of the design for the CCD readout electronics for the Subaru Telescope Prime Focus Spectrograph (PFS). The spectrograph is comprised of four identical spectrograph modules, each collecting roughly 600 spectra. The spectrograph modules provide simultaneous wavelength coverage over the entire band from 380 nm to 1260 nm through the use of three separate optical channels: blue, red, and near infrared (NIR). A camera in each channel images the multi-object spectra onto a 4k x 4k, 15 um pixel, detector format. The two visible cameras use a pair of Hamamatsu 2k x 4k CCDs with readout provided by custom electronics, while the NIR camera uses a single Teledyne HgCdTe 4k x 4k detector and ASIC Sidecar to read the device. The CCD readout system is a custom design comprised of three electrical subsystems: the Back End Electronics (BEE), the Front End Electronics (FEE), and a Pre-amplifier. The BEE is an off-the-shelf PC104 computer, with an auxiliary Xilinx FPGA module. The computer serves as the main interface to the Subaru messaging hub and controls other peripheral devices associated with the camera, while the FPGA is used to generate the necessary clocks and transfer image data from the CCDs. The FEE board sets clock biases, substrate bias, and CDS offsets. It also monitors bias voltages, offset voltages, power rail voltage, substrate voltage and CCD temperature. The board translates LVDS clock signals to biased clocks and returns digitized analog data via LVDS. Monitoring and control messages are sent from the BEE to the FEE using a standard serial interface. The Pre-amplifier board resides behind the detectors and acts as an interface to the two Hamamatsu CCDs. The Pre-amplifier passes clocks and biases to the CCDs, and analog CCD data is buffered and amplified prior to being returned to the FEE.
We present the single event effect (SEE) tolerance of a mixed-signal application-specific integrated circuit (ASIC) developed for a charge-coupled device camera onboard a future X-ray astronomical mission. We adopted proton and heavy ion beams at HIMAC/NIRS in Japan. The particles with high linear energy transfer (LET) of 57.9 MeV cm^{2}/mg is used to measure the single event latch-up (SEL) tolerance, which results in a sufficiently low cross-section of sigma_{SEL} < 4.2x10^{-11} cm^{2}/(IonxASIC). The single event upset (SEU) tolerance is estimated with various kinds of species with wide range of energy. Taking into account that a part of the protons creates recoiled heavy ions that has higher LET than that of the incident protons, we derived the probability of SEU event as a function of LET. Then the SEE event rate in a low-earth orbit is estimated considering a simulation result of LET spectrum. SEL rate is below once per 49 years, which satisfies the required latch-up tolerance. The upper limit of the SEU rate is derived to be 1.3x10^{-3}events/sec. Although the SEU events cannot be distinguished from the signals of X-ray photons from astronomical objects, the derived SEU rate is below 1.3% of expected non-X-ray background rate of the detector and hence these events should not be a major component of the instrumental background.
We report the radiation hardness of a p-channel CCD developed for the X-ray CCD camera onboard the XRISM satellite. This CCD has basically the same characteristics as the one used in the previous Hitomi satellite, but newly employs a notch structure of potential for signal charges by increasing the implant concentration in the channel. The new device was exposed up to approximately $7.9 times 10^{10} mathrm{~protons~cm^{-2}}$ at 100 MeV. The charge transfer inefficiency was estimated as a function of proton fluence with an ${}^{55} mathrm{Fe}$ source. A device without the notch structure was also examined for comparison. The result shows that the notch device has a significantly higher radiation hardness than those without the notch structure including the device adopted for Hitomi. This proves that the new CCD is radiation tolerant for space applications with a sufficient margin.
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One of the possible approaches to detecting optical counterparts of GRBs requires monitoring large parts of the sky. This idea has gained some instrumental support in recent years, such as with the Pi of the Sky project. The broad sky coverage of the Pi of the Sky apparatus results from using cameras with wide-angle lenses (20x20 deg field of view). Optics of this kind introduce significant deformations of the point spread function (PSF), increasing with the distance from the frame centre. A deformed PSF results in additional uncertainties in data analysis. Our aim was to create a model describing highly deformed PSF in optical astronomy, allowing uncertainties caused by image deformations to be reduced. Detailed laboratory measurements of PSF, pixel sensitivity, and pixel response functions were performed. These data were used to create an effective high quality polynomial model of the PSF. Finally, tuning the model and tests in applications to the real sky data were performed. We have developed a PSF model that accurately describes even very deformed stars in our wide-field experiment. The model is suitable for use in any other experiment with similar image deformation, with a simple tuning of its parameters. Applying this model to astrometric procedures results in a significant improvement over standard methods, while basic photometry precision performed with the model is comparable to the results of an optimised aperture algorithm. Additionally, the model was used to search for a weak signal -- namely a possible gamma ray burst optical precursor -- showing very promising results. Precise modelling of the PSF function significantly improves the astrometric precision and enhances the discovery potential of a wide-field system with lens optics.
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