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Acquisition of targets into the desired fiber is currently a two step process for bHROS. Light from the telescope reaches the instrument via a pick-off mirror that directs light from the telescope focal plane to the fiber. The pick-off mirror and fibers are placed in the focal plane as a GMOS-South cassette (like the GMOS-S IFU or MOS masks) that also contains a fiducial mask. By taking images with GMOS-S (with the bHROS cassette in place) it is possible to precisely center a target inside a fiducial box and then offset by a predetermined amount from the box to the request fiber.
Due to the current (in)accuracy of the OIWFS mapping, which limits the accuracy of offsets, the target must be centered on the fiber using the spiral peak-up process. Flux from the target is measured from the exposure meter while offsets are applied to the telescope by as much as 0.4 arcsec per offset step, resulting in a 1.4 arcsec search radius. A centroid algorithm is applied to the spiral data and an telescope offset necessary to center the target on the fiber is derived and applied.
The efficiency and accuracy of the peak-up process is highly dependent on atmospheric transparency and seeing. In good conditions, this procedure is very repeatable and efficient. As conditions degrade, both the accuracy of the result and the time required suffer.
Since only a small amount of light reaches the exposure meter, fainter targets require longer exposure meter integration times to achieve a signal large enough for the acquisition process. Targeting overhead estimates for a given target magnitude are given in the table below and are based upon the experience gained during the commissioning and science verification phases. These estimates do not include telescope slew time and guiding acquisition which is typically 5-10 minutes across the whole sky.
|Target Magnitude||Acquisition Overhead (minutes)|
Last update September 6, 2005; Randy Grashuis, Steven Margheim