A Fiber Link between the Okayama 188-cm Telescope and the High-Dispersion Spectrograph, HIDES

Abstract

We have developed a high-efficiency fiber link (HE mode) connecting Okayama 188-cm telescope to the high-resolution spectrograph, HIDES, in order to improve the throughput of the observing system while keeping its radial-velocity measurement precision. In the HE mode, a multi-mode optical fiber collects stellar light within a 2.7-arcsecond diameter field of view (FOV) at the Cassegrain focus, which is twice as large as that of the typical seeing size at our observing site. An image slicer is used at the other end of the link, just in front of the entrance of HIDES, to keep the proper high spectral resolution for such a wide FOV. We have found that the maximum total throughput of the HE mode observations at around 550 nm reaches as high as 9%. Its practical throughput is also found to be more than twice that of the conventional slit observations. We have also confirmed that its reciprocal resolution is over 50000, as designed. The minimum available wavelength, which is determined from an inter-order gap of the echelle spectrum, is about 440 nm, where the gap becomes as narrow as 5 pixels on the CCD detector for the red cross-disperser of HIDES. From measurements of the rms errors of the radial velocity of some stars over one year, the long-term precision is as good as a few m s$ ^{-1}$. The fiber modal noise that could degrade the observed spectra may not be significant in the HE mode, as long as the signal-to-noise ratio of a spectrum is lower than 1500.