Altair at Gemini North : full sky coverage laser AO correction at visible wavelengths

Abstract

We present two recent upgrades to the Gemini North Adaptive Optics (AO) system, Altair. These two upgrades provide 100% sky coverage for low performance AO suitable for improving the natural seeing by factors of 25% to 3 from blue visible wavelengths (350 nm) through the near infrared (2.5 micron wavelengths). The first upgrade, dubbed LGS + P1 "Super Seeing" mode, allows correction of high order aberrations with an on-axis Laser Guide Star (LGS) while tip/tilt correction is performed with a more distant peripheral wavefront sensor (P1). Most currently operating LGS AO systems are limited in their sky coverage, primarily due to tip/tilt star availability. Although P1 provides sub-optimal tip/tilt correction due to its distance from the science source, its patrol radius allows operation in LGS + P1 mode anywhere in the sky from declinations of +70 degrees to -30 degrees. This mode was offered for science use at Gemini North in 2013A. We present typical performance and use from its first semester in science operation, with a factor 2 to 3 image quality improvement over seeing limited images. The second upgrade is the commissioning of the AO system to correct at visible wavelengths, which is expected to be completed in 2014. In this mode, Altair will feed the Gemini Multi-Object Spectrograph (GMOS), which is an optical imager as well as a long-slit, multi-slit and integral field unit spectrograph. We intend to replace the current Altair science dichroic with a sodium notch filter, passing only the 589nm wavelength light from the LGS to the AO system. The rest of the spectrum from 350 nm to the GMOS red cutoff at 1.1 microns is intended as science capable light. Tip/tilt correction will be performed close to the science target with the GMOS on-instrument wavefront sensor or with P1 as in the P1+LGS mode discussed above. We expect an image quality improvement of 25% in this mode over seeing limited observations. Since exposure time to reach a given signal-to-noise ratio scales roughly as the square of the image quality, these two upgrades represent a substantial efficiency improvement which is available to nearly all targets normally observed at Gemini North.7 page(s