Hemispherical infrared focal plane arrays: a new design parameter for the instruments

Abstract

In ground based astronomy, mainly all designs of sky survey telescopes are limited by the requirement that the detecting surface is flat whereas the focal surface is curved. Two kinds of solution have been investigated up to now. The first one consists in adding optical systems to flatten the image surface; however this solution complicates the design and increases the system size. Somehow, this solution increases, in the same time, the weight and price of the instrument. The second solution consists in curving artificially the focal surface by using a mosaic of several detectors, which are positioned in a spherical shape. However, this attempt is dedicated to low curvature and is limited by the technical difficulty to control the detectors alignment and tilt between each others.Today we would like to propose an ideal solution which is to curve the focal plane array in a spherical shape, thanks to our monolithic process developed at CEA-LETI based on thinned silicon substrates which allows a 100% optical fill factor. Two infrared uncooled cameras have been performed, using 320 x 256 pixels and 25 µm pitch micro-bolometer arrays curved at a bending radius of 80 mm. These two micro-cameras illustrate the optical system simplification and miniaturization involved by curved focal plane arrays. Moreover, the advantages of curved detectors on the optical performances (Point Spreading Function), as well as on volume and cost savings have been highlighted by the simulation of the opto-mechanical architecture of the spectrometer OptiMOS-EVE for the European Extremely Large Telescope (E-ELT).Keywords: infrared detectors / components design / curved retina / optical design / large field of view telescope / novel design / advanced detector / Si CMOS detectors / infrared focal plane arrays

Mechanical behavior of flexible silicon devices curved in spherical configurations

“…Briefly the focalization surface of a spherical lens is not a flat plane but a curved concave shape close to a spherical shape. By curving a Focal Plane Array (FP A), one can correct this aberration without extra corrective lenses [4]. This can be applied to terrestrial and spatial telescopes [5] and very compact high-performance cameras [6].…”

Flexible focal plane arrays for UVOIR wide field instrumentation

“…This achievement was done by adapting the curving process developed by Dumas and Fendler published in 2012 for bolometric arrays working at room temperature and at a wavelength of 11µm (cf Fig.1 middle). This group also worked on the optical design optimizations of the proposed instrumentation for the E-ELT (Fendler+ [8]) by use of curved focal planes, demonstrating a global increase in performance with the use of less component, less complex. They also demonstrated that a major issue in their process was the curving of convex detectors, for mechanical reasons, that degrades the shape and the performance.…”

“…As explained by Rim et al [4] or Fendler & Dumas [8][9] [10], the curvature of focal plane arrays leads to a drastic simplification of the optical systems by offering a new parameter in the optimizations. The field curvature, composed to first order by the Petzval curvature and the astigmatism aberration, no more has to be compensated by a set of complex optics flattening the field.…”

Flexible focal plane arrays for UVOIR wide field instrumentation

“…As explained by Rim et al or Fendler & Dumas [8][9] [10], the curvature of focal plane arrays leads to a drastic simplification of the optical systems by offering a new parameter in the optimizations. The field curvature, so called the Petzval surface, no more has to be compensated by a set of complex optics flattening the field.…”

“…This achievement was done by adapting the curving process developed by Dumas and Fendler published in 2012 for bolometric arrays working at room temperature and at a wavelength of 11µm ( Figure 1). This group also worked on the optical design optimizations of the proposed instrumentation for the E-ELT (Fendler+ [8]) by use of curved focal planes, demonstrating a global increase in performance with the use of less component, less complex. They also demonstrated that a major issue in their process was the curving of convex detectors, for mechanical reasons, that degrades the shape and the performance.…”