* Corresponding author. Nuclear Instruments and Methods in Physics Research A 444 (2000) 457}460 IR "lters for high responsivity cryogenic detectors N. Rando*, P. Verhoeve, P. Gondoin, B. Collaudin, J. Verveer, M. Bavdaz, A. Peacock Astrophysics Division, Space Science Department, European Space Agency, ESTEC, Keplerlaan 1, 2200 AG Noordwijk, The Netherlands Thermal Control and Life Support Division, Mechanical Systems Department, European Space Agency, ESTEC, Keplerlaan 1, 2200 AG Noordwijk, The Netherlands Abstract Modern cryogenic detectors, such as Superconducting Tunnel Junctions and Transition Edge Sensors, provide single photon counting performance, medium to high energy resolution, high count rates and good photon collection e$ciency over a wide wavelength range. In order to avoid background limited performance, it is necessary to shield the detectors from any thermal IR radiation originating from the surrounding warm surfaces. In this paper we analyse the contribution of the thermal radiation to the detector performance and describe the IR "lters used in the S-Cam camera and in other experimental con"gurations. Future detectors may require very severe attenuation of the IR #ux ('1 m). Solutions to this problem are proposed and their validity demonstrated with experimental results.  2000 Elsevier Science B.V. All rights reserved. Keywords: Electro-optical instrumentation; Cryogenic detectors; IR "lters 1. Introduction Modern cryogenic photon detectors provide single photon counting performance from NIR to X-ray energies [1,2]. In the case of a tantalum- based STJ, responsivities are of order 10 e/eV, resolving power E/E is of order 20 at "300 nm and quantum e$ciency is about 70% in the visible range. Typical operating temperatures are about 300 mK for Ta and Nb-Al based STJs and of order 100 mK for Transition Edge Sensors (TES). In or- der to avoid background limited performance, it is necessary to reject any thermal radiation from warmer elements in the Field Of View (FOV) of the detectors. Such an issue is particularly challenging in the case of instruments targeting the lower photon energy range (e.g. visible range). While soft X-ray and UV applications can take advantage of very thin Al "lters [3], instruments operating in the visible range must provide high IR rejection com- bined with an adequate optical throughput. As predicted by the Planck distribution, the number of photons emitted by a grey-body is very large when compared to the low count rates expected from typical signal sources (e.g. faint astronomical ob- jects). This is the case of S-Cam, an astronomical camera for ground-based astronomy in the visible range, recently installed at the Nasmyth focus of the William Herschel Telescope, in La Palma, Canary Islands (Spain) [4,5]. The e!ects of IR radi- ation applied to the cryogenic detectors are of two- fold nature: (1) they may represent a heat load, increasing the detector temperature; (2) they 0168-9002/00/$ - see front matter  2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 9 9 ) 0 1 4 2 2 - 9 SECTION IX.