FOCAL PLANE ARRAY AND OPTICS CONFIGURATION

The LWIRC is mounted on the optical bench of the IR module at the f/25 forward Cassegrain focus of the Keck I telescope. This module is situated within the volume of the telescope, on a pillar which rises from the hexagonal hole in the primary mirror [5]. It will sit alongside the instruments NIRC and LWS, and the guider camera. The IRE readout system sits beneath the optical bench in a refrigerated enclosure. Radiation from the f/25 gold coated chopping secondary mirror is reflected by a tertiary mirror through a ZnSe window into the camera dewar. Beyond this point all the optics are cooled to about 10 K.

The camera has recently been upgraded from a 20 $\times$ 64 pixel Si:As focal plane array (FPA) to a Boeing/Rockwell 128 $\times$ 128 pixel Si:As FPA. The array is a moderate flux device, with a well depth of $\sim$ 1 $\times$ 10⁷ electrons and a read noise of about 500 electrons. The pixel size is 75 $\times$ 75 $\mu$ m. The array has four output lines and it is read out at 2.4 $\mu$ sec/pixel, yielding a 100 Hz frame rate. The array is sensitive over the 4-25 $\mu$ m wavelength range, and it is anti-reflection coated for the 8-12 $\mu$ m region. The manufacturer's stated responsivity, $\eta$ G, where $\eta$ is the detective quantum efficiency, and G is the photoconductive gain, ranges from 0.5 to 5, depending on detector bias. Currently we are operating at an $\eta$ G of $\sim$ 0.7 with a bias of 1 V. A useful feature of this array is a partial frame integration mode. By inhibiting the collection of charge during the frame period, this mode acts as a neutral density filter and near room temperature sources can be used for calibration, where otherwise these sources would saturate the array.

The FWHM of the telescope diffraction pattern for 10 $\mu$ m radiation corresponds to an angle of 0.21 arcsec. LWIRC has three plate scales giving a variation in the sampling of the diffraction pattern shown in Table 1. The plate scales are characterized by the f-number of the light cone at the detector.

**Table:** Optical Plate Scales. The telescope diffraction spot at 10 $\mu$ m is 0.21 arcsec FWHM.
f/#	field of view	plate scale	PSF (FWHM)
	(arcsec)	(arcsec/pix)	(pixels)
f/30	6.6	0.05	4.1
f/15	13.2	0.10	2.06
f/7.5	26.4	0.21	1.03

The camera optics consist of a set of lens pairs with pupil stops in a Lyot configuration. Lenses and filters are mounted on turrets or wheels which are computer controlled. Figure 1 shows the three optical configurations. There are optical stops placed at the image plane of the telescope, the Lyot stop, and the camera focal plane, and the optical paths are additionally baffled. All surfaces are painted with a high emissivity black paint [6]. To change optics, the fore optic lens turret is rotated as is the pupil wheel; the rear optic is the same for all configurations. The Lyot system has very good rejection of stray light, and a coronagraph has been added to the f/15 configuration. The coronagraph is a simple obscuring disk located at the telescope focus, with an angular size of 1'', or 10 pixels at the array.

**Figure 1:** The three LWIRC optical configurations. The telescope focus is at the left side of the diagram, followed by the fore optic, the infrared filter with dielectric ``blocker", the pupil (the Lyot stop), the rear optic, and the detector. The rays drawn are those for the extreme corner of the array.
$\begin{figure}\begin{center} \epsfig{file=optics2sm2.eps,height=5.in,angle=-90}\end{center} \end{figure}$

**Figure 2:** Ray tracing spot diagrams for the f/30 and f/15 optical configurations. The pixel size is 75 $\times$ 75 $\mu$ m. From top to bottom the spots correspond to: the corner of the array, the center of an edge, $\sim$ mid-way to the corner, and on the optic axis.
$\begin{figure}\begin{center} \epsfig{file=spot2.eps,height=6.in,angle=-90}\end{center} \end{figure}$

Ray tracing spot diagrams for the f/30 and f/15 configurations are shown in Figure 2. The dominant optical problem is spherical aberration. For both these configurations, the spot size is smaller than a 75 $\mu$ m pixel, and the spot diagram for the f/7.5 configuration is exactly one pixel in size. Because all the lenses are made of anti-reflection coated ZnSe, the system suffers from chromatic aberration. The chromatic aberration over the range 7-13 $\mu$ m leads to an equivalent spot size less than a pixel for the f/30 system and the center portions of the f/15 and f/7.5 configurations. The corners of the array for the two lower f-number systems have spot sizes slightly larger than one pixel. The telescope can, of course, be refocussed for different filters within the N-band.

The camera carries a complement of infrared filters over the range 2-12.5 $\mu$ m [7]; these are shown in Table 2. Each of the mid-infrared filters is installed with a CaF₂ or BaF₂ blocking filter, as appropriate. At present, no filters for the 20 $\mu$ m band are available, nor are the ZnSe lenses appropriate here. CdTe lenses and 20 $\mu$ m filters could, however, be installed in the future. There is also a circular variable filter (CVF) with a 0.2 $\mu$ m bandwidth over the wavelength range 7-14 $\mu$ m, and a 10 $\mu$ m polarizer on a ZnSe substrate. The polarizer cannot be rotated within the camera dewar, but because the Keck telescope has an alt-azimuth mount, different polarization orientations for an object can be measured during the course of a night.

**Table 2:** Available Filters
Std. Silicate Set	7.9	8.8	9.9	10.3	11.7	12.5
(b.w. 1 $\mu$ m)
Astronomical Set	K	K-wide	L	M	10.3	N
	2-2.4	2-2.5	3.5-4.1	4.4-5	1.5 $\mu$ m b.w.	7.5-12.5
Other	Circular Variable Filter
	Polarizer