Direct illumination LED calibration for telescope photometry

Accepted by Nuclear Inst. and Methods in Physics Research, A NIMAA calibration method for telescope photometry, based on the direct illumination of a telescope with a calibrated light source regrouping multiple LEDs, is proposed. Its purpose is to calibrate the instrument response. The main emphasis of the proposed method is the traceability of the calibration process and a continuous monitoring of the instrument in order to maintain a 0.2% accuracy over a period of years. Its specificity is to map finely the response of the telescope and its camera as a function of all light ray parameters. This feature is essential to implement a computer model of the instrument representing the variation of the overall light collection efficiency of each pixel for various filter configurations. We report on hardware developments done for SNDICE, the first application of this direct illumination calibration system which will be installed in Canada France Hawaii telescope (CFHT) for its leading supernova experiment (SNLS)

Spectral calibration of SNDICE

SNDICE primary goal was to provide a photometric calibration of the CFH telescope in order to supplement its astronomic calibration based on reference stars. In particular it is intended to uniformize the response of the telescope over the very large field of the Megacam instrument and over well defined segments of the optical spectrum corresponding more or less to the filter set used for the SNLS experiment. The study of spectral features ofthe LED sources used by SNDICE came in this perpective mainly as a way to quantify second order corrections taking into account the shape of the spectral distribution around its central wavelength value. The spectrophotometric bench presented here did not need to have a precision better than a few percent for this task and consequently it is just an adaptation of our photometric bench presented elsewhere. It could be roughly described, as its photometric counterpart, as a «direct illumination LED calibration

The DICE calibration project: design, characterization, and first results

We describe the design, operation, and first results of a photometric calibration project, called DICE (Direct Illumination Calibration Experiment), aiming at achieving precise instrumental calibration of optical telescopes. The heart of DICE is an illumination device composed of 24 narrow-spectrum, high-intensity, light-emitting diodes (LED) chosen to cover the ultraviolet-to-near-infrared spectral range. It implements a point-like source placed at a finite distance from the telescope entrance pupil, yielding a flat field illumination that covers the entire field of view of the imager. The purpose of this system is to perform a lightweight routine monitoring of the imager passbands with a precision better than 5 per-mil on the relative passband normalisations and about 3{\AA} on the filter cutoff positions. The light source is calibrated on a spectrophotometric bench. As our fundamental metrology standard, we use a photodiode calibrated at NIST. The radiant intensity of each beam is mapped, and spectra are measured for each LED. All measurements are conducted at temperatures ranging from 0{\deg}C to 25{\deg}C in order to study the temperature dependence of the system. The photometric and spectroscopic measurements are combined into a model that predicts the spectral intensity of the source as a function of temperature. We find that the calibration beams are stable at the $10^{-4}$ level -- after taking the slight temperature dependence of the LED emission properties into account. We show that the spectral intensity of the source can be characterised with a precision of 3{\AA} in wavelength. In flux, we reach an accuracy of about 0.2-0.5% depending on how we understand the off-diagonal terms of the error budget affecting the calibration of the NIST photodiode. With a routine 60-mn calibration program, the apparatus is able to constrain the passbands at the targeted precision levels.Comment: 25 pages, 27 figures, accepted for publication in A&

LSST: from Science Drivers to Reference Design and Anticipated Data Products

(Abridged) We describe here the most ambitious survey currently planned in the optical, the Large Synoptic Survey Telescope (LSST). A vast array of science will be enabled by a single wide-deep-fast sky survey, and LSST will have unique survey capability in the faint time domain. The LSST design is driven by four main science themes: probing dark energy and dark matter, taking an inventory of the Solar System, exploring the transient optical sky, and mapping the Milky Way. LSST will be a wide-field ground-based system sited at Cerro Pach\'{o}n in northern Chile. The telescope will have an 8.4 m (6.5 m effective) primary mirror, a 9.6 deg$^2$ field of view, and a 3.2 Gigapixel camera. The standard observing sequence will consist of pairs of 15-second exposures in a given field, with two such visits in each pointing in a given night. With these repeats, the LSST system is capable of imaging about 10,000 square degrees of sky in a single filter in three nights. The typical 5$\sigma$ point-source depth in a single visit in $r$ will be $\sim 24.5$ (AB). The project is in the construction phase and will begin regular survey operations by 2022. The survey area will be contained within 30,000 deg$^2$ with $\delta<+34.5^\circ$, and will be imaged multiple times in six bands, $ugrizy$, covering the wavelength range 320--1050 nm. About 90\% of the observing time will be devoted to a deep-wide-fast survey mode which will uniformly observe a 18,000 deg$^2$ region about 800 times (summed over all six bands) during the anticipated 10 years of operations, and yield a coadded map to $r\sim27.5$. The remaining 10\% of the observing time will be allocated to projects such as a Very Deep and Fast time domain survey. The goal is to make LSST data products, including a relational database of about 32 trillion observations of 40 billion objects, available to the public and scientists around the world.Comment: 57 pages, 32 color figures, version with high-resolution figures available from https://www.lsst.org/overvie

Southern African Large Telescope Spectroscopy of BL Lacs for the CTA project

In the last two decades, very-high-energy gamma-ray astronomy has reached maturity: over 200 sources have been detected, both Galactic and extragalactic, by ground-based experiments. At present, Active Galactic Nuclei (AGN) make up about 40% of the more than 200 sources detected at very high energies with ground-based telescopes, the majority of which are blazars, i.e. their jets are closely aligned with the line of sight to Earth and three quarters of which are classified as high-frequency peaked BL Lac objects. One challenge to studies of the cosmological evolution of BL Lacs is the difficulty of obtaining redshifts from their nearly featureless, continuum-dominated spectra. It is expected that a significant fraction of the AGN to be detected with the future Cherenkov Telescope Array (CTA) observatory will have no spectroscopic redshifts, compromising the reliability of BL Lac population studies, particularly of their cosmic evolution. We started an effort in 2019 to measure the redshifts of a large fraction of the AGN that are likely to be detected with CTA, using the Southern African Large Telescope (SALT). In this contribution, we present two results from an on-going SALT program focused on the determination of BL Lac object redshifts that will be relevant for the CTA observatory

Direct Illumination Led Calibration for Ia Supernovae Photometry

A telescope calibration method for type Ia supernovae photometry is proposed. It is based on the direct illumination of a telescope with a calibrated multi-LED light source. Its description includes traceability of the calibration process and check standard to maintain 0.2% accuracy over years period. Although opto-electronics read-out and control could be built from commercial off-the-shelf components, we report on developments relevant for large systems. Their requirements are tailored on a proposal for the SNLS experiment performed on the Megaprime instrument installed in CFHT for which our R&D is performed. Similar LED systems of larger size are working in high-energy physics optical calorimeters

Angular Alignment and Control of SNDICE with the Canada-France-Hawaii Telescope

The SNDICE calibrated light source (“LED-head”) is mounted on an altitude-azimuth (alt-az) motion, itself installed on the south-east platform fixed to the CFHT superior part of the dome in a overhanging position. The altitude and the azimuth motors are hold together by a right angle bracket. The azimuthal plate has been shimmed horizontally using a level. Both motors have an elementary step of 1°/400= 9”. The optical axis of the LED-head is materialized by a parallel beam, the “artificial planet” (AP), which angular aperture is 40”

Dual Gain Clamp and Sample ASIC

This report describes the Dual Gain Clamp and Sample (DGCS) ASIC designed in LPNHE for the SNAP project, in a collaboration with Lawrence Berkeley Laboratory aiming to develop a completely integrated, space qualified, CCD readout electronics. This chip, which uses AMS 0.35 CMOS technology, was designed in order to optimize the first stage of a chain able to digitize a CCD output signal in the full 2 to 250,000range. It has to operate at low temperature (140 K) and to tolerate an integrated radiation dose of 100 kRad. We shall present here the electronic qualification tests, passed successfully by our ASIC. Its performances have been analyzed in the two classical modes of CCD readout, dual-integrator and clamp-and-sample, but we have pushed forward the analysis of the latter mode because we intend to use it for our next generation ASIC

CABAC : A CCD Clocking and Biasing Chip for LSST Camera

The aim of CABAC, is to provide to the LSST camera CCDs the necessary parallel and serial clocks, the power supply of the output amplifiers (OD) and most of the biases, excluding the HV substrate bias. The LSST focal plane is made of 189 large (4k*4k pixels) and highly segmented (16 outputs) CCDs. Each pixel will be read at a speed of 550kHz for a total readout (3.2 Gpixels) of 2s. To achieve this speed, each CCD will be driven by two CABAC in order to provide the necessary large current to move the 4004 lines of the sensor. CABAC will be implemented on a large REB (Raft Electronics Board) located inside the cryostat, the nominal operating temperature will be -26C. One CABAC is designed to provide : 4 parallel clocks of more than 15V amplitude, rise and fall time of less than 2us on a 66nF capacitive load, and a 8 bit programmable output current capability of 300mA max. 4 serial clocks with more than 15V amplitude, rise and fall time of 60ns on a capacitive load of 300pF, and a 8 bit programmable output current capability of 16mA max. 2 output drains, 8 bit programmable voltage from 13 to 36V with a nominal output current of 16mA each. 3 high level biases, 8 bit programmable from 13 to 36V 2 low level biases, 8 bit programmable from 0 to 5V Finally, for monitoring purpose, an internal dual multiplexer can provide 2 of any CABAC output, including the temperature sensor, and 6 external inputs. A deactivable internal pulser can inject pulse inside the CCD RD (reset drain) allowing electronic calibration of the readout chain. In order to save power during the 15s exposure, CABAC can be put in standby mode. This mode reduces the supply current by a factor of 10 for the clocks circuitry, and can set the OD voltages to a programmable value in order to reduce the glow inside the CCDs. Programation of CABAC is done via a SPI bus. CABAC is designed in CMOS H35 from AMS vendor, encapsulated in a QFN100 package with bottom pad, it is currently under tests

LSST camera readout chip ASPIC: test tools

Open AccessInternational audienceThe LSST camera will have more than 3000 video-processing channels. The readout of this large focal plane requires a very compact readout chain. The correlated ''Double Sampling technique'', which is generally used for the signal readout of CCDs, is also adopted for this application and implemented with the so called ''Dual Slope integrator'' method. We have designed and implemented an ASIC for LSST: the Analog Signal Processing asIC (ASPIC). The goal is to amplify the signal close to the output, in order to maximize signal to noise ratio, and to send differential outputs to the digitization. Others requirements are that each chip should process the output of half a CCD, that is 8 channels and should operate at 173 K. A specific Back End board has been designed especially for lab test purposes. It manages the clock signals, digitizes the analog differentials outputs of ASPIC and stores data into a memory. It contains 8 ADCs (18 bits), 512 kwords memory and an USB interface. An FPGA manages all signals from/to all components on board and generates the timing sequence for ASPIC. Its firmware is written in Verilog and VHDL languages. Internals registers permit to define various tests parameters of the ASPIC. A Labview GUI allows to load or update these registers and to check a proper operation. Several series of tests, including linearity, noise and crosstalk, have been performed over the past year to characterize the ASPIC at room and cold temperature. At present, the ASPIC, Back-End board and CCD detectors are being integrated to perform a characterization of the whole readout chain