Seeing Limited Mode

To date, almost all science observations with LUCI have been done in seeing-limited mode with an off-axis guide star used to close the guiding and collimation loops, known as the ACTIVE telescope mode. In ACTIVE mode the off-axis AGw uses a user selected guide star to keep the telescope on source and collimated.

The off-axis guider sits in front of LUCI. The patrol field is shown above, shaded in green. The probe can vignette the LUCI field of view, so one should be cautious about using guide stars within about an arcmin of the side or top edges of the LUCI field of view. Guiding is typically done through an r’ filter with exposure times of 2 seconds. Guide stars brighter than r ∼ 12th mag can saturate on a clear night with excellent seeing, while stars fainter than r ∼ 16.5 mag may be a problem on nights with poor-seeing.

In monocular observations guiding is done by moving the telescope mount, as is done for other telescopes. In binocular observing it is the telescope optics on each side that make the guiding corrections so that any ongoing observation on the other side is not disturbed. Moving the mount is faster than moving the optics, so with overheads the guider cycles every 4 (monocular) to 8 (binocular) seconds.

Active correction of the telescope collimation uses the same guide star as for guiding. A beam-splitter separates the shorter wavelengths (r’) for guiding and the longer wavelengths (i+z) for the Shack-Hartmann wavefront sensor. The WFS exposes for 30 seconds to integrate over atmospheric and guiding effects to get a good measure of the collimation of the optics, cycling about every 45 seconds. Thus we recommend deep observations use a minimum dwell time on any on-sky position of about a minute to allow sufficient time for the telescope to stay well-collimated.

The AGw will automatically follow the selected guide star when dithering but sometimes there will be a need to dither far enough that the guide star is outside of the patrol field. The OT will help users visualize observations, helping with the selection and optimization of dithers (see OT and discussions LUCI GS Selection for details).

In cases where the guide star is not reachable by the AGw probe, the telescope will automatically fall back to open-loop TRACK mode. As long as the guide star is recovered on a subsequent dither (within 5min or so), the telescope will resume guiding and collimating.