Fiber optics have found use in astronomical spectrographs for nearly the past 15 years. In order to achieve optimal implementation of the fiber optics, one must know the performance characteristics of the fiber and its impact on how a fiber-fed spectrograph should be designed. I review here the fundamental properties of fiber optics as they relate to their use in such astronomical spectrographs covering the spectral band of 3000 to 11000 angstrom.
A comprehensive test facility has been set up at the Royal Greenwich Observatory for the evaluation of fibers in astronomical applications. Particular emphasis is placed on the performance of fibers at the fast focal-ratios delivered by current prime-focus systems. Recent work carried out at the facility has included a series of experiments on image-scrambling in fibers, and results from these are highlighted in the paper. A probable mechanism for the observed effects is described. Finally, characterization tests on some less-conventional fibers are reported, and possibilities for future work outlines.
We present a concept design for a versatile, high throughput fiber couple spectrograph for the 9-meter Hobby-Eberly telescope. This instrument, designated the medium resolution spectrograph (MRS) will cover the visible spectrum from 350 nm to 1100 nm and have a resolution range 300 < (lambda) /(Delta) (lambda) < 20000. It will reside in an environmentally controlled spectrograph room. Operating modes of the spectrograph will include single object, synthetic long slit, and multiple object spectroscopy. The dual requirements of spectral coverage and high resolution mandate that the MRS be a dual beam instrument.
The FLAIR multifiber spectroscopy system on the 1.2m UK Schmidt Telescope is the world's most powerful multi-object spectroscopy system in terms of area coverage and available fiber numbers. This has been achieved at modest cost providing an instrument of surprising flexibility, power, and robustness yet with an inherently simple design. During 1994 alone over 6500 individual target observations were made from just 30% of the telescope time. This paper describes the current status of the facility, problems encountered, modifications, and enhancements made over the last two years and those planned for the future. Continued innovation should maintain the facility's power and competitiveness for several years to come.
The prime focus of the William Herschel telescope (WHT) provides a field of one degree which is to be used for fiber spectroscopy. The WYFFOS spectrograph, based on a Baranne white pupil design, is located on one of the Nasmyth platforms of the telescope and is fed from prime focus by 126, 26 meter long fibers. The system is designed for a wavelength range of 350 nm to 1.1 microns using both transmission and reflection gratings. This paper describes the integration and testing of the spectrograph undertaken in the laboratory. The image quality and spectral resolving power have been measured. The scattered light and amount of cross-talk between adjacent fibers has been assessed. The provision of calibration illumination and facilities for back illumination of the fibers, a requirement needed by the fiber positioner, is discussed.
The KPNO fiber-fed, multi-object spectroscopic instrument, Hydra, has been moved from the Mayall to the WIYN telescope. Modifications to the instrument allow the fibers to align with the telescope exit pupil while lying along the curved focal surface. We also upgraded the manner in which the fibers are held in place around the focal plane in order to reduce neighboring fiber interactions beyond the pivot circle. In addition, the wavelength calibration assembly was modified to take advantage of extra room within the instrument. We developed guiding algorithms which utilize the field orientation probes (7-fiber coherent bundles). The bench spectrograph associated with Hydra was also moved over to the WIYN. Commissioning is currently underway at the time of this paper and is expected to be complete by mid-summer of 1995. We give a general description of the instrument, discuss initial efficiency and scattered light measurements, and comment on the performance of the guider.
It is suggested that multimode slab waveguide spectrographs currently under development for wavelength division multiplexing in optical communications might have an application in astronomy. The nature of these devices is described, and a survey of their development in the communications industry presented. The applicability of existing devices is investigated and found to be limited. However, it is shown that only a modest improvement in resolution (factor of 2-4), together with a realization of the potentially very high optical efficiency, could yield an astronomically useful device. The anticipated problems in developing such as instrument are outlined.
An idea for a photographic vertical circle (PVC) was proposed by M. S. Zverev in 1960. This instrument was used for the observation of the absolute declination of stars in Chile and Pulkovo. Later in 1980's modernization of the PVC was made. During its modernization fiber optics were used for improving the readings of the divided circle and making it photoelectrical instead of photographical. Therefore the precision of the observations were increased up to 0.05 inches.
One of the problems encountered when using single-mode fibers in a wideband interferometer is the dispersion of the waveguide. By optimizing the waveguide structure for an actual fluoride glass, a fiber can be made whose dispersion between 2 micrometers and 2.7 micrometers is reduced by more than two orders of magnitude. Simulated interferograms show that an interferometer using such a fiber could withstand a fiber length difference of 10m without a substantial degradation of its performances. This raises the possibility to consider all-fiber delay lines with several meters of optical path delay variation.
Principally because single-mode fibers are perfect spatial filters, they have great potential for ground-based interferometry. The author of this paper has recently shown how a novel multiple-core design can enhance the overall efficiency of a SM fiber many-fold when the fiber is coupled to atmospherically degraded wavefronts. We present theoretical results on the possible enhancement of signal-to-noise ratios in image reconstruction with interferometers using such fibers with a multiple-core geometry. Our results have strong implications for the feasibility of an all-fiber interferometry concept, both on the ground and in space.
In fiber optic Fourier transform spectrometry (FTS), fiber optic double Fourier interferometry (DFI) and, probably, in single-mode fiber-linked telescope array, fiber delay lines are needed to make the system all-fiber optic, which will results in many advantages. Fiber-stretching operation has been demonstrated to be one method in which a piece of fiber is stretched mechanically to generate optical path difference (OPD). Nevertheless, this fiber-stretching operation has two drawbacks. First, it introduces additional polarization and dispersion effects, both of which are stretching state and wavelength dependent. Second, in practive, its maximum delay is limited. In this paper, we describe a new type of fiber delay line in which the fiber-stretching device (we call it OPD generator) is used to generate continuous OPD, and, OPD stores made of optical switch and two segments fibers having different lengths are employed to save and release the OPD that the OPD generator produces. The main advantages of this kind of fiber delay line are: 1) dispersion in the OPD stores may be minimized because the fiber in an OPD store is free of external strain, and polarization effect can be eliminated; 2) fiber-stretching operation will introduce less dispersion and polarization effects; hence 3) large optical delay may be obtained with shorter fiber. At the same time, performance of this kind of delay line is analyzed and the characteristics are discussed.
The Infrared Optical Telescope Array (IOTA) is an interferometric facility currently observing in the near infrared bands at the Smithsonian Institution's Fred Lawrence Whipple Observatory in Arizona. The 45 cm siderostats can be moved on an L-shaped track allowing discrete bases ranging between 5 and 38 m. The capability to combine beams with fiber optics in the K band (2 micrometers <EQ (lambda) <EQ 2.4 micrometers ) has been demonstrated on the Fiber Link Unit for Optical Recombination (FLUOR) at Kitt Peak National Observatory, in which two existing 0.8 m telescopes have been coherently coupled by means of optical fibers. FLUOR is now set as a focal instrument of IOTA. It uses single-mode fluoride glass waveguides and couplers as a substitute for mirrors and beamsplitters to perform beam transportation and recombination. Processing the light in single-mode waveguides offers the possibility to self-calibrate each interferogram against the loss of fringe visibility induced by atmospheric turbulence, thus improving the accuracy of the fringe visibility measurements. The FLUOR unit can be operated as a Mach-Zehnder interferometer to produce zero-baseline spectra used in double-Fourier interferometry to obtain the visibility as a function of wavelength. In the current status, a N-S baseline of 21.2 m is used to observe late-type starts and derive their angular diameters.
Polarization effect on twin-core fiber interferometer (TCFI) has been observed for the first time. When using this effect on TCFI to measure the strain within a beam, we can highly improve the S/N ratio and the stability of the system. In this paper, we give some analyses and experimental results.
The MEFOS instrument is being regularly used at the ESO 3.6m telescope for multi-object spectroscopy. The instrument can access up to 29 targets at the 1 degree field of the prime focus. The 21m fiber bundle links a low resolution spectrograph located in the Cassegrain cage. MEFOS has been also used to evaluate the sky subtraction with optical fibers. This paper reports on the description of the final configuration of the instrument and on its performance. A brief study on the photometric and spectral stability of fibers along with the observations is also reported.