OSCIR Hot News

Status of OSCIR and Telescope Chopping Commissioning at Gemini South

8 October 2001

During September 2001, the OSCIR mid-infrared imager was operated at Gemini South for the first time. Having served as a visitor instrument at Gemini North from June 2000 through July 2001, OSCIR became the first science instrument to be mounted on both Gemini telescopes.

The installation of OSCIR on Gemini South was a straightforward process, and after acquiring and focusing on the first star of the run with the Gemini Acquisition Camera, a well-focused stellar image immediately appeared near the center of the OSCIR detector. The OSCIR dewar and on-telescope electronics were installed at the Gemini South cassegrain focus using the same mounting equipment as used on Mauna Kea without any modifications, demonstrating the compatibility of the mechanical interfaces at the two telescopes. The electrical interface used to control the chopping secondary from the OSCIR instrument electronics, as well as the Visitor Instrument Interface used by OSCIR to command telescope motions and read telescope pointing data, also functioned identical to their Gemini North counterparts.

OSCIR quickly obtained images of stars that were nearly diffraction-limited. Unfortunately, poor weather during most of the run limited further careful testing of the image quality and instrument sensitivity.

The testing revealed two significant problems with the Gemini systems. The chopping secondary waveform proved to be of very high quality at duty cycles (percentage of the chop cycle for which the waveform is flat) up to 60%. However, at higher duty cycles the waveform required several tens of seconds to stabilize after the chopping was activated, an undesirably long period of time. The Gemini North chopping system typically stabilizes in a few seconds at duty cycles of about 80%, so there is a clear difference between the two systems. The Gemini Engineering staff is currently working to understand this difference and solve the problem in the southern system.

A second problem was an inability to close the automatic focus servo loop at the correct focus for OSCIR. The internal optics of the 2x2 Peripheral Wavefront Sensor commonly used for fast guiding and focus control while chopping are known to be slightly out of proper focus, and this will be corrected during the telescope shutdown in November, before the next OSCIR run.

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Last update October 8, 2001; Tom Hayward