  Typical astronomical spectrographs have a resolution ranging between a few
hundred to 200.000. Deconvolution and correlation techniques are being employed
with a significance down to 1/1000 th of a pixel. HeAr and ThAr lamps are
usually used for calibration in low and high resolution spectroscopy,
respectively. Unfortunately, the emitted lines typically cover only a small
fraction of the spectrometer's spectral range. Furthermore, their exact
position depends strongly on environmental conditions. A problem is the strong
intensity variation between different (intensity ratios {>300). In addition,
the brightness of the lamps is insufficient to illuminate a spectrograph via an
integrating sphere, which in turn is important to calibrate a long-slit
spectrograph, as this is the only way to assure a uniform illumination of the
spectrograph pupil.
  Laboratory precision laser spectroscopy has experienced a major advance with
the development of optical frequency combs generated by pulsed femto-second
lasers. These lasers emit a broad spectrum (several hundred nanometers in the
visible and near infra-red) of equally-spaced "comb" lines with almost uniform
intensity (intensity ratios typically <10). Self-referencing of the laser
establishes a precise ruler in frequency space that can be stabilized to the
10e-18 uncertainty level, reaching absolute frequency inaccuracies at the
10e-12 level per day when using the Global Positioning System's (GPS) time
signal as the reference. The exploration of the merits of this new technology
holds the promise for broad-band, highly accurate and reproducible calibration
required for reliable operation of current and next generation astronomic
spectrometers.
