Figure 1: 2001 Wafer In Mount
Figure 2: Hornplate 2.1mm Top
Figure 3: Wafer on Wedding Cake
Figure 4: JFET close up
Figure 5: Bolocam CSO
The heart of Bolocam is a monolithic array of 144 spider web bolometers, both developed by Jamie Bock and collaborators at the Jet Propulsion Lab [see figure 1]. We can operate at 1.1, 1.4, or 2.1mm, the optimal wavelengths for detecting the Sunyaev Zel'Dovich effect from clusters of galaxies. After entering the dewar window, the raditation from the sky passes through a series of optical filters. Once we have only the wavelength band we need, the radiation is concentrated onto the bolometers using individual feedhorns, which you can see in this photograph [see figure 2]. The array is mounted inside the Bolocam dewar [see figure 3] and cooled to 0.25K using a three stage 3He refrigerator developed in collaboration with Simon Chase. The bolometers are AC biased, which produces highly stable output signals. All 144 bolometers are read out with their own electronics chain, resulting in some highly compact electronics. A JFET module, pictured here, is part of the read out electronics inside the dewar [see figure 4].
Bolocam mounts on the Cassegrain focus of the Caltech Submillimeter Observatory in Hawai'i [see figure 5]. With 144 bolometers, and an 8 arcminute field of view, Bolocam's strength is mapping speed. We are able to map large areas of sky to high sensitivity quickly. Because of our stable readout electronics there is no need for us to modulate the source by chopping the telescope. Our observing strategy takes advantage of this fact. We have two observing modes, raster and drift scan. For the raster mode we move the telescope in a raster pattern across the sky. In drift scan mode we park the telescope and let the rotation of the Earth move the sky in front of the telescope. This way we can make maps much faster than chopping on and off a single source. The science page discusses the types of sources we will be observing with Bolocam.