The present status of AOS development at KOSMA is discussed. A study of a new generation of AOS using the new Bragg-cell material "Rutil" is on the way, which is supposed to lead to spectrometers in the range of 4 GHz total bandwidth at an resolution of 2-3 MHz. A second alternative for a 4 GHz bandwidth spectrometer has been developed as an engineering model for the HIFI instrument aboard the ESA cornerstone mission "Herschel". It consists of an array-AOS with 1 GHz bandwidth of each of the four AOS bands at a resolution of 1 MHz. A hybrid system for an input between 4 and 8 GHz is setup, and various laboratory tests have demonstrated that this system is well suited for large bandwidth applications like with HIFI. For eventual future demand of even larger bandwidth, details of a new optical method for Rf-analysis are discussed. It consists of a modulated laser with one or two Fabry-Perot etalons to analyze the frequency distribution of the resulting laser sidebands. A bandwidth of several 10 GHz at moderate resolution can be achieved.
FLITECAM is a facility class instrument on SOFIA, operating form 1-5 microns. A 1024 by 1024 InSb ALADDIN II array will be used in a fully refractive optics system that provides three optical modes; imaging at a plate scale of 0.47 inch by 0.47 inch per pixel and afield of view of 8 feet, grism spectroscopy with moderate resolutions of about R equals 1000- 20000 depending on the slit width used, and a pupil viewing mode. The plate scale is achieved by using an f/4.8 back-end camera behind an f/4.9 collimator. The 8 foot field of view of SOFIAs f/19.6 telescope translates to a focal plane aperture of 114mm. The collimator optics are therefore quite large with a diameter of almost 170mm. Between the pupil image produced by the collimator and the f/4.8 camera, a ste of three lense will be inserted to achieve a pupil viewing mode. The optical train is entirely refractive, with triplets or split triplets using combinations of LiF, BaF2, ZnS, and ZnSe. The design of an optics system with this wide a wavelength range of more than two octaves and the large field of view present a challenging project. We demonstrate that our optical design meets the performance requirements with certain allowed lens-fabrication and misalignments built in.
A study of the average upper atmospheric conditions has been carried out in order to optimize the scientific return from SOFIA. By examination of atmospheric data from satellite missions, we found that at typical SOFIA flight altitudes (between 37,000 and 45,000 ft), it can be an advantage to fly north, as the water vapor overburden and the frequency of cloud occurrence is less than if the flights were centered above Moffett Field, CA, which will be the base for SOFIA. It has also been shown that for certain science projects, the amount of time on target can be considerably extended.
As a facility class instrument on SOFIA, FLITECAM will be developed at the UCLA Infrared Imaging Detector Laboratory. Its primary purpose is to test the SOFIA telescope imaging quality from 1.0 to 5.5 microns, using a 1024 X 1024 InSb ALADDIN II array. Once the telescope test flights are finished, FLITECAM will be available to the science community. FLITECAM's field of view of 8' in diameter, with a plate scale of 0.47' per pixel, is one of the largest available for any facility camera. Grisms are available to produce moderate resolution of R approximately equals 1000 - 2000, depending on the slit width, with direct ruled ZnSe grisms. The detector readout electronics will be provided by Mauna Kea IR Inc. and is able to operate the detector array at all its planned operation modes, including occulations, telescope-nodding, high-speed shift-and-add, and optionally chopping at the longer wavelengths. Here we present our design approach to achieve those specifications. We also discuss the most important tests FLITECAM will carry out and give examples of science projects on SOFIA. For the latter, we present a preliminary list of filters which is expandable and open for discussions within the science community.
Large bandwidth acousto-optical spectrometers have now reached a very high level of maturity. They achieve very compatible results in comparison with other spectrometer types like filterbanks, autocorrelators, and chirp transform spectrometers. In addition, AOS are rather simple in design, have little complexity and can be designed for space applications very easily. A new generation of broad-band AOS, the array-AOS, consists of four parallel 1 GHz spectrometers built into one optical unit. Tests results in the laboratory as well at a radio-observatory are very promising. For example, the Allan variance minimum time has been found above 1000 seconds. In comparison it can be shown that the AOS spectra are less affected by instrumental noise or baseline distortions due to platforming effects as they are visible with most hybrid autocorrelators. For future applications of acousto-optics the development of cross-correlators seems to be feasible. First steps in this new direction are on the way.
Acousto optical spectrometers (AOS) have become an attractive alternative to filterbanks or autocorrelators for applications in radioastronomy and in heterodyne as well as in laboratory spectroscopy. Due to continuous improvements, AOSs have now achieved a performance and reliability level that makes this technology applicable for airborne or spaceborne missions. A first fully space qualified AOS was built at the University of Cologne for the Submillimeter Wave Astronomy Satellite (SWAS) to be launched in fall 1995. The SWAS-AOS has a large bandwidth of 1.4 GHz covered by 1365 channels, with a center frequency of 2.1 GHz. Only 11 mW rf white noise input power is required for simultaneous saturation of all channels. The design is optimized for very high stability and allows operation within a temperature range from minus 5 to plus 30 degrees Celsius at temperature variations of up to 2 degrees Celsius/hour. The total weight is 7.2 kg including electronics, the power consumption is 5.4 watts including data pre-averaging electronics and dc-dc converter losses. The performance was verified also after complete vibrational and thermal vacuum environmental testing. For future projects with large bandwidth requirements or with multichannel systems the AOS technology can also be used to fabricate array spectrometers. Such array AOS offers the unique option to multiply the available bandwidth without multiplying the hardware accordingly. Especially for spaceborne applications this is an extremely useful development because weight, power consumption as well as costs increase only very moderately. At present the first prototype with four independent 1 GHz channels is in development. This array AOS will have a total bandwidth of 4 GHz covered by 4000 channels, and will be available in 1996.