Proc. SPIE. 10562, International Conference on Space Optics — ICSO 2016
KEYWORDS: Staring arrays, Signal to noise ratio, Point spread functions, Astronomy, Sensors, Laser range finders, Fourier transforms, Charge-coupled devices, Modulation transfer functions, Astronomical imaging
The intrapixel response is the signal detected by a single pixel illuminated by a Dirac distribution as a function of the position of this Dirac inside this pixel. It is also known as the pixel response function (PRF). This function measures the sensitivity variation at the subpixel scale and gives a spatial map of the sensitivity across a pixel.
The reduction of systematic effects is necessary to improve the accuracy in imaging and astrometry. For example, in Euclid Mission which aims at carrying out accurate measurements of dark energy and quantifying precisely its role in the evolution of the Universe, systematic effects need at be controlled to a level better than 10<sup>-7</sup> (Euclid, Science Book). To achieve this goal, a high-level of knowledge of the system point spread function (PSF) is required. This paper follows the concept-paper presented at the last SPIE conference<sup>1</sup> and gives the recent developments achieved in the design of the test bench for the intrapixel sensitivity measurements. The measurement technique we use is based on the projection of a high spatial resolution periodic pattern on the detector using the self-imaging property of a new class of diffractive objects named continuously self-imaging gratings (CSIG) and developed at ONERA. The principle combines the potential of global techniques, which make measurements at once on the whole FPA, and the accuracy of spot-scan-based techniques, which provide high local precision.
Theia is an astrometric mission proposed to ESA in 2014 for which one of the scientific objectives is detecting
Earth-like exoplanets in the habitable zone of nearby solar-type stars. This objective requires the capability
to measure stellar centroids at the precision of 1x10<sup>-5</sup> pixel. Current state-of-the-art methods for centroid
estimation have reached a precision of about 3x10<sup>-5</sup> pixel at two times Nyquist sampling, this was shown at
the JPL by the VESTA experiment. A metrology system was used to calibrate intra and inter pixel quantum
efficiency variations in order to correct pixelation errors. The Theia consortium is operating a testbed in vacuum
in order to achieve 1x10<sup>-5</sup> pixel precision for the centroid estimation. The goal is to provide a proof of concept
for the precision requirement of the Theia spacecraft.
The testbed consists of two main sub-systems. The first one produces pseudo stars: a blackbody source is
fed into a large core fiber and lights-up a pinhole mask in the object plane, which is imaged by a mirror on the
CCD. The second sub-system is the metrology, it projects young fringes on the CCD. The fringes are created by
two single mode fibers facing the CCD and fixed on the mirror. In this paper we present the latest experiments
conducted and the results obtained after a series of upgrades on the testbed was completed. The calibration
system yielded the pixel positions to an accuracy estimated at 4x10<sup>-4</sup> pixel. After including the pixel position
information, an astrometric accuracy of 6 x 10<sup>-5</sup> pixel was obtained, for a PSF motion over more than 5 pixels.
In the static mode (small jitter motion of less than 1 x 10<sup>-3</sup> pixel), a photon noise limited precision of 3x10-5
pixel was reached.
This paper is devoted to the presentation of a new technique of characterization of the Intra-Pixel Sensitivity Variations (IPSVs) of astronomical detectors. The IPSV is the spatial variation of the sensitivity within a pixel and it was demonstrated that this variation can contribute to the instrument global error. Then IPSV has not to be neglected especially in the case of under-sampled instruments for high quality imaging and accurate photometry. The common approaches to measure the IPSV consist in determining the pixel response function (PRF) by scanning an optical probe through the detector. These approaches require high-aperture optics, high precision mechanical devices and are time consuming. The original approach we will present in this paper consists in projecting high-resolution periodic patterns onto the whole sensor without classic optics but using the self-imaging property (the Talbot effect) of a Continuously Self Imaging Grating (CSIG) illuminated by a plane wave. This paper describes the test bench and its design rules. The methodology of the measurement is also presented. Two measurement procedures are available: global and local. In the global procedure, the mean PRF corresponding to the whole Focal Plane Array (FPA) or a sub-area of the FPA is evaluated. The results obtained applying this procedure on e2v CCD 204 are presented and discussed in detail. In the local procedure, a CSIG is moved in front of each pixel and a pixel PRF is reconstructed by resolving the inverse problem. The local procedure is presented and validated by simulations.