T. Ingerson (tingerson@noao.edu)
The CTIO Hydra fiber positioner has proven to be robust and reliable. It positions fibers accurately and consistently. The bench spectrograph also performs as expected. Overall instrument performance has been compromised because it has been necessary to use the Loral 1K x 3K CCD with the Air Schmidt camera on Hydra. This limits users to a maximum of about 60 independent targets at only fair efficiency. This was a choice we made in order to get the Hydra functionality on the air as quickly as possible, rather than wait on the final camera.
Now that MOSAIC is operational, resources have been freed to allow work to proceed on commissioning the design detector for Hydra. This is a SiTe 2 x 4K in a 400mm f.l. bench-mounted Schmidt camera. This new camera/CCD combination is expected to allow Hydra/CTIO to use all its fibers simultaneously and with significantly higher efficiency and resolution than at present. The new camera and CCD will almost certainly become operational sometime next semester.
The large (300 mm/2") Hydra fibers have performed well to date. Slit plates
200 m and 100
m wide can be placed in front of the line of fibers in the
spectrograph to improve resolution, albeit with some loss of light.
The small (200 mm/1.3") fibers have been a disappointment. They proved to be quite brittle, apparently as a result of manufacturing problems. After they had been installed in Hydra, about half were found to have low or zero throughput. This means that even with the new camera/CCD combination, only about 60 independent targets will be selectable with the small fibers. Fortunately, the small fibers do not appear to break while being positioned, so we believe the number of targets that can be observed with the them will remain constant. This will provide enough fibers to be adequate for many programs.
The primary motivation for using the small fibers is to obtain better S/N by excluding sky. Use of 1.3" fibers is indicated only for very dim objects when seeing is better than about 0.7", which is uncommon. We expect almost all observers will request the large fibers. Thus, scientific use of Hydra now is little compromised by the small-fiber problem. The defective fibers could be replaced, but it would be expensive and it is not an easy job. At present there are no plans to do so.