1 Introduction

Near infrared imaging observations are usually made using a set of "standard'' filters, the most popular being J, H, K which match the 3 regions of clear atmospheric transmission at near infrared wavelengths, i.e. between 1.1 and 2.5 $\mu$m. Although these bands were first defined long ago (see Johnson 1962, 1964, 1968), no standard recipe exists for defining and manufacturing these filters. Consequently, different filters were used in different observatories and, in several cases, the filter transmission curve at the operating temperature of IR instruments ($\simeq$77 K) is not available (see e.g. Moro & Munari 2000).

Contrary to the broad-band filters at optical wavelengths, several of which can be manufactured using combinations of commercial coloured glasses, those for IR observations can only be produced by multi-layers coating. Although some firms have produced standard astronomical IR filters for many years, the stability of their characteristics is not always satisfactory. An instructive example is comparing the transmission curves of the standard J filters manufactured by the same company for ESO and for the TNG (dotted and dashed curves in Fig. 1, respectively).

Clarifying this situation has been one of the main aims of the MKO near-infrared filter consortium (Simons & Tokunaga 2002; Tokunaga et al. 2002). Thanks also to the organizational effort of A. Tokunaga, a large number of observatories throughout the world could recently purchase a uniform set of carefully defined, high quality filters at an affordable price.

Since direct measurements of the transmission of these filters at cryogenic temperatures are scarce or, for some of them, lacking altogether, we report here accurate spectral scans at 78 K for the set of filters purchased for the multi-mode IR instrument of the Italian 3.5 m telescope (TNG). The broad-band filters were purchased as part of the first MKO filter consortium in 1998 that used OCLI as the filter vendor. The narrow-band filters were purchased as part of a separate filter consortium organized in 2000. We plan to repeat these measurements roughly every year to monitor the aging of the coating layers (if any), the results will be made available in the TNG web page (www.tng.iac.es/IRfilters.html) where one can also find the data presented here in electronic form.

Figure 1: Overview of the transmission curves of the filters of Mauna Kea Observatories (MKO) near infrared filter consortium. The data were taken at a temperature of 78 K and with the broad and narrow band filters illuminated at an incidence angle of 5$^\circ$ and 3$^\circ$, respectively. For comparison, we also include the curve relative to the ``standard J'' filter of NICS measured in the same conditions (dashed line) as well as that of the ``standard J'' filter used in the SOFI and ISAAC instruments of ESO (dotted line). The data for the ESO filter were downloaded from the ISAAC web page.