The AFOE: A Spectrograph for Precise Stellar Radial Velocity Measurements

The Advanced Fiber-Optic Echelle (AFOE) spectrometer is a fiber-fed, bench-mounted echelle spectrograph, located at the 1.5m (60") telescope of the Whipple Observatory, near Tucson Arizona.

With a maximum resolution of 56,000, the spectrograph configuration provides a total coverage of 150 nm, typically between 392 and 664 nm, over some twenty four orders. The spectrum is recorded on a continuously cooled, thinned-chip, back-illuminated, 2048 by 2048 pixel CCD detector (15um pixels.)

This spectrograph is designed specifically to perform precise stellar radial velocity measurements, and has been optimized for asteroseismology and extra-solar planet detection. The spectrograph is in a thermally controled enclosure, itself in a thermally controled room. Critical distances in the spectrograph (i.e., from the slit to the collimator, and from the camera to the detector) are maintained by very low expansion coefficient spacers.

Short term stability (i.e., on the order of several hours) is achieved by recording a Thorium-Argon emission-line spectrum simultaneously adjacent to the stellar spectrum. To further increase the instrumental precision, a double-fiber scrambler (Brown, 1990) can be used, in which a pair of fibers is coupled together using a pair of microlenses, separated by the common focal length; in this arrangement the off-axis angle in the first fiber is mapped into radial position on the face of the second, and then effectively scrambled in the second fiber.

Long term stability is provided by an absorption cell filled with iodine vapor (Horner, 1996), which superposes a dense spectrum of molecular iodine lines upon the stellar spectrum in the wavelength range between 500 nm and 620 nm.

What does a spectrum taken with the AFOE look like?

A detailed description of the AFOE can be found in Brown et al., 1994.


  1. High precision Doppler measurements via echelle spectroscopy, Brown, 1990, in CCDs in Astronomy, ed. G Jacoby (San Francisco, ASP), ASP Conf. Ser. Vol. 8, p. 335.
  2. The AFOE: A spectrograph for precise Doppler studies, Brown et al., 1994, P.A.S.P., 106, 1285-1297.
  3. A Search for Pulsations in Four Late-Type Giants, Horner, 1996, Ap. J., 460, 449.
  4. Determining the AFOE's Radial Velocity Precision with Solar Observations Horner, S. D., Brown, T. M., Kennelly, E. J., Noyes, R. W., Korzennik, S. G., Nisenson, P., Krockenberger, M., 1995, BAAS, 187, 7006.
Sylvain G. Korzennik (
Last revised Apr 23 1997