2 ALFA: An overview

  ALFA is a sodium laser guide star (LGS) adaptive optics (AO) system installed at the Calar Alto 3.5-m telescope, and is currently available to guest observers. One major part of the system is a dye ring laser pumped by an Ar⁺ laser, providing 3.8W output at 589.2nm with a bandwidth of 10MHz (Quirrenbach et al. 1997). The laser is fed via a beam-relay to the launch telescope mounted beside the main mirror. Here a diagnostics bench (Rabien et al. 1998) provides an analysis of the beam quality before the laser is projected into the atmosphere. With good conditions the laser power is sufficient to produce a m_V=9 mag artificial guide star by sodium fluorescence in the mesosphere at a height of 90km. The adaptive optics bench is the other major component (Glindemann et al. 1997; Hippler et al. 1998; Wirth et al. 1998), and is based on a Shack-Hartmann wavefront sensor preceeded by a set of hexagonal lenslet arrays which allow interchange between 6, 18, and 30 subapertures. A set of up to 32 Zernike modes or Karhunen-Loeve functions are derived and applied to a 97 actuator deformable mirror. While correcting on a natural guide star, the wavefront sensor can determine all modes including tip-tilt; when using a laser guide star, a separate tracker camera is used to measure the tip-tilt from a nearby star.

ALFA is the instrument on which a number of experiments have been carried out since 1998 in conjunction with the European network on Laser Guide Stars for 8-m class Telescopes (Foy 1999). These include determining tip-tilt from a LGS, monitoring the Na layer profile and scattered light, and simultaneous SCIDAR/ALFA measurements of atmospheric turbulence.

During the last 6months significant progress has been made in the quality of correction that can be achieved with ALFA, even for mediocre atmospheric conditions. This is a necessity at the Calar Alto observatory since the seeing is often 1$^{\prime\prime}$ or worse. For a number of observing programmes, ALFA is now able to compete effectively with other adaptive optics systems.

Some progress has also been made with the laser guide star, although this is still limited by non-ideal observing conditions, being particularly sensitive to atmospheric transmission. We have been able to close the loop on the laser and correct the field around the galaxy UGC1347 with an improvement in both peak intensity and FWHM of a factor of 2.5. There remain two main restrictions to regular observations with the laser. One of these is the beam jitter, which can often throw the LGS spots outside the centroiding regions on the wavefront sensor. The second is the LGS size ($\sim$2.5$^{\prime\prime}$), the cause of which we are currently investigating; an intended fibre link to replace the beam relay between the laser and the telescope may overcome this. As well as these we are implementing automatic control algorithms for laser tuning and focussing, and also WFS focussing. These should increase the observing efficiency by a large margin.

Updates on progress with the system can be found on-line at http://www.mpe.mpg.de/www_ir/ALFA/ALFAindex.html.