IRSIS : an IFU prototype in fluoride glassDavid HORVILLE1, Jean Michel KRIEG1, Thibaut LE BERTRE1, Louis D'HENDECOURT2, Emmanuel DARTOIS2

1 Observatoire de Paris 2 Institut d’Astrophysique Spatiale

Abstract: IRSIS (Infra-Red Spectroscopic Imaging Survey) is a project of a Small Satellite for Astronomy & Astrophysics developed by several indianlaboratories under the leadership of Prof. S.K. Gosh from the Tata Institute of Fundamental Research (Mumbai/Bombay).

This scientific experiment will carry out unique infrared spectro-imaging observations in the 1 to 6 µm wavelength range. The payload consists of a 30 cm telescope and of a two-channel spectrograph (SW : 1 to 3 µm and LW : 3 to 6 µm) equipped with two 1024x1024 array detectors. The spectral resolution is ~ 100, and the spatial resolution ~ 18 arcseconds. The field-of-view covered by the instrument is 10 arcminutes by 10 arcminutes.

A key element of the instrument is a fiber bundle that converts a two-dimension image in the focal plane of the telescope into several parallel slits located at the spectrograph entrance (IRSIS anamorphoser, or Integral Field Unit –IFU-). In the present concept, one-thousand optical fibers are coupled to a micro-lens array at one end (focal plane), and at the other end are arranged in four slits. Both the microlens array and the fibers are made of a fluoride glass that transmits light efficiently in the 1 to 6 µm range.

In this poster, we present the first phase of development of the IRSIS anamorphoser. In cooperation with LeVerreFluoré, a company based in Rennes, we have developed a prototype consisting of a 10x10 micro-lens array coupled to a bundle of 100 fibers.

This study was supported by CNES (Centre National d’Etudes Spatiales).

In orbit, the IFU will be exposed to high energy radiations. The amount of radiations expected for a 4 years mission in a low altitude polar orbit (h=800km) has been estimated(OMERE simulations) and an equivalent glass (AFG450) has been exposed at the accelerator of Orsay to the same quantity of radiations (2 ×1014 protons of 1 MeV/cm2). No effect has been detected (see figure on the left).

Infrared spectra before and after ion irradiation (left panel) and the transmittance at low temperature (right panel, 77 and 17K) with respect to ambient (296K) were measured using an FTIR spectrometer coupled to a high vacuum chamber at IAS.

The hexagonal shape of the microlenses is well respected, the measurements are in accordance with the specifications. The edges of the microlenses are well defined. Only the central 10x10 microlenses are coupled to a fiber.

259µm

296µm120.5°

118.99°

Low FRD : the input beam F-ratio is 3.4 and the average output of thefiber beam F-ratio is 3.3

Summary: An Integral Field Unit prototype has been developed for IRSIS. The present phase-A study has proven the concept of an infrared IFU in fluoride glass. The manufacturer (Le Verre Fluoré) has demonstrated its competence and its ability to develop an adequate product for IRSIS.

IRSIS preliminary concept

IFU input with microlens array The 100 fibers are rearranged in one exit slit.

Scientific Objectives: IRSIS will cover large areas of the sky in spectro-imagery, with a goal of 50%. In this survey mode it will detect all point sources down to magnitude K ~ 14 (L ~ 13). It will provide spectra from 1.6 to 5.5 µm, at a resolution of 100, that will allow us to characterize all detected sourcesdirectly without necessity of a follow-up. Deeper integrations by repeated observations in selectedareas will also be possible.

In addition, it will provide images in narrow spectral bands of extended sources, for instance in thePAH band at 3.3 µm or in the hydrogen Brackett and Paschen lines. Thanks to the low level of background emission which can be reached in Space, extended structures around AGB stars haverecently been revealed by satellites such as Spitzer, Herschel and GALEX (e.g. see GALEX image ofMira's tail on the left). In the near-infrared range WISE is expected to reveal also such structures, witha spatial resolution of 6 arcsec, in the broad bands centered at 3.4 and 4.6 µm. By resolving them spectrally, IRSIS will allow us to investigate in details these extended emissions.

IRSIS will thus be capable of addressing a wide variety of astrophysical topis. It will help to understandthe processes at work in the interstellar medium and in circumstellar environments. It will allow us toclassify stars of different types, in particular of late-type (AGB stars in the Galaxy and in the Magellanic Clouds, M-dwarfs in the Solar Neighborhood, etc.), and emission-line stars.

Short Wavelength (SW) channelThe curvature of the spectra has been minimized by curving the IFU exit slit.

Long Wavelength (LW) channel