2 Optical and filtering systems

2.1 Optical system

In order that future versions of the photometer can accomodate small arrays of bolometers on each channel, imaging cold optics have been designed for Diabolo. Using lenses rather than mirrors, the system is compact enough that two (and possibly three in a next version) large throughput channels fit into a small portable dewar. The sky is imaged through a cold pupil lens onto a cold focal plane lens. For each channel, the light is then fed by another lens onto the bolometer and its associated Winston cone. The lenses are made of quartz (of index of refraction 2.14) with anti-reflection coatings adapted to each wavelength. As in the PRONAOS-SPM photometer (Lamarre et al. 1994), the optical plate is sustained below the cryostat by three pillars and contains the optical and filtering systems (Fig. 2). It is shielded by a 1.8 K screen covered with eccosorb. The cryogenic plate (Fig. 3), which is in direct contact with the lHe cryostat (pumped to 1.8 K), receives the dilution fridge (Sect. 3) which provides cooling of the two bolometers (Sect. 4). Ray-tracing was done including considerations on diffraction in order to optimize the parameters of the lenses and cones (with limited use of ASAP software). Care was taken to underilluminate the secondary and primary mirrors to reduce sidelobe levels (the photometer effectively uses 2 m out of the 2.6 m of the primary mirror of the Testa Grigia telescope).

Figure 4: Details of the filters that are used in Diabolo Channel 1. For each plot, the element transmission is shown as a function of the wavenumber. Except for the atmospheric transmission and the bolometer cut-off, the data are actual measurements interpolated onto a common grid (a constant level extrapolation was done to wavelengths larger than 3 mm). The entry block, situated next to the cold pupil, contains one C103 (from IRLabs) at 77 K on the penultimate cryostat screen, and a series of filters on the 1.8 K stage after the lens: another C103 filter, a diamond powdered polyethylene filter and 2 by 3 resonant capacitive grids (made at IAS) to insure a sharp submillimetre cutoff (F1 and F2 in Fig. 2). The dichroic beam splitter is made of 3 resonant capacitive grids each deposited on a mylar substrate and separated by 383 $\,\mu {\rm m}\,$(it was measured only around 5 cm-1). The 2 bandpass filters are made of free-standing metal mesh (platinum, copper and silver alloy, made in IKI, Moscow) of 10 $\,\mu {\rm m}\,$thickness. The bolometer cut-off comes from diffractive effects from the entrance Winston cone before the bolometer (Harper et al. 1976). The atmospheric transmission is for 3 mm precipitable water vapour (cut below 2 cm-1). The last 2 bottom figures show the global transmission in linear and logarithmic (resp.) scale, including the atmospheric transmission

Figure 5: Details of the filters that are used in Diabolo Channel 2. See previous figure for explanations

In inverse propagation mode, the beam exiting the photometer has a 5.6 f ratio and the useful diameter of the exit (plane parallel high-density polyethylene) window is 27.5 mm. This matches the bolometer throughput of $15 \rm\,mm^2~sr$, well above the diffraction limit for both channels which is 2.3 and $6.4\rm\,mm^2~sr$ in channel 1 and 2.

2.2 Filtering system

We have devised a filtering system in order to select the appropriate wavelengths while avoiding submillimetre radiation that would load the bolometers. This system does not rely on the atmosphere to cut unwanted radiation. Figures 4 and 5 summarise the different filters, which are all at 1.8 K temperature except for the first infrared cutoff filter (77 K). Measurements were done on each element separately at room temperature only and at normal incidence. In the submillimetre up to 1.8 mm, this was accomplished with a Fourier Transform Spectrometer, with a 0.3 K bolometer as the detecting device at the Institut d'Astrophysique Spatiale (IAS) facility. Several measurements around 2 mm were done with a heterodyne receiver and a carcinotron emitter at the Meudon Observatory facility (DEMIRM). Once all the measured transmissions are multiplied together we find an overall expected photometer transmission which is a factor 2.5 larger than the transmission deduced from point-source measurements. A large fraction of the discrepancy can be attributed to the optical elements that were not included in the calculation: the cryostat entrance window and the lenses, as well as to some diffractive optical losses.