Adaptive optics with a Laser For Astronomy

Results with the laser: NGC 6764

May 1999

NGC 6764 is a spiral galaxy 32 Mpc away, classified as a LINER and displaying Wolf-Rayet stellar features in its nucleus. A plethora of observational results are given in Eckart et al. 1996 (ApJ, 472, 588).

To the left is shown a 3 x 3 arcmin field around the galaxy NGC 6764 (optical image from the digitised sky survey). The laser was pointed at the galaxy nucleus as indicated. The star used for tip-tilt correction has a magnitude of V=14.8 and lies 42 arcsec from the nucleus of the galaxy. The galaxy is too faint and the star too distant for either to be used as the reference for high order correction; this observation could only be achieved using the laser. An even fainter (V > 16 mag) PSF comparison star lies between the two, 25 arcsec from the galaxy.

We observed this galaxy in the K-band (2.2 microns) and co-added 10 second integrations to produce open and closed loop images. Cutouts from these, each 5 arcsec across, are shown below for the galaxy nucleus and the nearby comparison star. The top pair are the uncorrected images; next come the images when only the tip-tilt loop is closed; and at the bottom is the result when additionally the high order loop is closed on the laser guide star.

colour scale
low colourbar high
star open
galaxy open
star tiptilt
galaxy tiptilt
on LGS
star closed
galaxy closed

The seeing was measured as 0.93 arcsec from the open loop image. The tip-tilt correction made no measurable difference. Probably this is because the star was quite distant and also rather faint (at our limit) so the frame rate was only 10 Hz, and the bandwidth much less. However, the LGS did provide a good improvement, increasing the peak intensity on the star by a factor of 2.8 and reducing the FWHM to 0.55 arcsec (see the profiles shown below). At this resolution the galaxy nucleus is clearly resolved, having a FWHM of 1.26 arcsec - ie an intrinsic size of 1.13 arcsec, equivalent to 175 pc. With only a little more resolution it will be possible to determine whether this nucleus is made of a several individual star clusters.