Gain Stabilization of a Submillimeter SIS Heterodyne Receiver

We have designed a system to stabilize the gain of a submillimeter heterodyne receiver against thermal fluctuations of the mixing element. In the most sensitive heterodyne receivers, the mixer is usually cooled to 4 K using a closed-cycle cryocooler, which can introduce ~1% fluctuations in the physical temperature of the receiver components. We compensate for the resulting mixer conversion gain fluctuations by monitoring the physical temperature of the mixer and adjusting the gain of the intermediate frequency (IF) amplifier that immediately follows the mixer. This IF power stabilization scheme, developed for use at the Submillimeter Array (SMA), a submillimeter interferometer telescope on Mauna Kea in Hawaii, routinely achieves a receiver gain stability of 1 part in 6,000 (rms to mean). This is an order of magnitude improvement over the typical uncorrected stability of 1 part in a few hundred. Our gain stabilization scheme is a useful addition to SIS heterodyne receivers that are cooled using closed-cycle cryocoolers in which the 4 K temperature fluctuations tend to be the leading cause of IF power fluctuations.Comment: 7 pages, 6 figures accepted to IEEE Transactions on Microwave Theory and Technique

A Photonic mm-Wave Local Oscillator

A photonic millimeter wave local oscillator capable of producing two microwatts of radiated power at 224 GHz has been developed. The device was tested in one antenna of Smithsonian Institution's Submillimeter Array (SMA) and was found to produce stable phase on multiple baselines. Graphical data is presented of correlator output phase and amplitude stability. A description of the system is given in both open and closed loop modes. A model is given which is used to predict the operational behavior. A novel method is presented to determine the safe operating point of the automated system.Comment: 4 pages, 7 figures, to appear in the Proceedings of the 17th International Symposium on Space Terahertz Technology, held 10-12 May 2006 in Pari

Laminate polyethylene window development for large aperture millimeter receivers

New experiments that target the B-mode polarization signals in the Cosmic Microwave Background require more sensitivity, more detectors, and thus larger-aperture millimeter-wavelength telescopes, than previous experiments. These larger apertures require ever larger vacuum windows to house cryogenic optics. Scaling up conventional vacuum windows, such as those made of High Density Polyethylene (HDPE), require a corresponding increase in the thickness of the window material to handle the extra force from the atmospheric pressure. Thicker windows cause more transmission loss at ambient temperatures, increasing optical loading and decreasing sensitivity. We have developed the use of woven High Modulus Polyethylene (HMPE), a material 100 times stronger than HDPE, to manufacture stronger, thinner windows using a pressurized hot lamination process. We discuss the development of a specialty autoclave for generating thin laminate vacuum windows and the optical and mechanical characterization of full scale science grade windows, with the goal of developing a new window suitable for BICEP Array cryostats and for future CMB applications

MicroObservatory Net: A Network of Automated Remote Telescopes Dedicated to Educational Use

Many students have a deep interest in astronomy, but a limited opportunity to use telescopes to explore the heavens. The MicroObservatory Network of automated telescopes is designed to provide access to classroom teachers who wish their students to conduct projects over the World Wide Web. The intuitive interface makes it easy for even 10-year-olds to take pictures. Telescopes can be remotely pointed and focused: filters, field of view, and exposure times can be changed easily. Images are archived at the website, along with sample challenges and a user bulletin board, all of which encourage collaboration among schools. Wide geographic separation of instruments provides access to distant night skies during local daytime. Since “first light” in 1995, we have learned much about remote troubleshooting, designing for unattended use, and for acquiring the kinds of images that students desire. This network can be scaled up from its present capability of 240,000 images each year to provide telescope access for all US students with an interest in astronomy. Our WWW address is http://mo-www.harvard.edu/MicroObservatory