2 Instrument characteristics

The DENIS instrument has been described in detail by Copet et al. ([1999]). A sketch of its main optical components is displayed in Fig. 1. It is located at the Cassegrain focus of the ESO 1 m telescope at La Silla Observatory (Chile). After reflection from the two telescope mirrors, the light beam goes through a field lens at the telescope focus, covered by a protective blade, both of CaF₂ and uncoated. Then a dichroic splits the i beam in reflection from the J/$K_{\rm s}$ beam in transmission.

Figure 1: Optics of the DENIS instrument

The i beam has two more reflections from coated mirrors before entering the objective (3 CaF₂ and 2 silica coated lenses), then goes through the Gunn i filter, a shutter, the cryostat entrance window (BK7) and arrives at the Tektronix 1 K CCD detector, cooled to 180 K.

The J/$K_{\rm s}$ beam is reflected off a microscanning mirror (uncoated Al), then J is reflected by a second dichroic and a coated mirror before entering the J objective (3 CaF₂ and 2 fused silica coated lenses), then the cryostat entrance window (coated fused silica), the filter and finally the NICMOS-3 detector, both cooled to 80 K.

The $K_{\rm s}$ beam is transmitted by the second dichroic, then reflects off two more coated mirrors, passes through the $K_{\rm s}$ objective (4 ZnS-Cleartran and 1 fused silica coated lenses), the cryostat entrance window (coated fused silica), the filter and the other NICMOS-3 detector.

We have tried to obtain response curves for all these elements. When this was not possible, a reasonable estimate of the transmission was adopted. Filter response curves as provided by Barr Associates (U.S.A.) for J and $K_{\rm s}$, at the nominal detector temperature (77 K), and by MTO-France for the Gunn i filter (at ambient temperature) are displayed in Fig. 2. The full system response curves (atmosphere, optics, filter and detector) are shown in Fig. 3.

Figure 2: The 3 DENIS filter response curves

Figure 3: The complete DENIS system normalized response curves (atmosphere, optics, filters, detectors)