Improving the image quality was the major driver in almost all the projects handled last year by the Mechanical Engineering Section. We finished three major projects: the 1.5-m Dome Ventilation Doors, the 4-m Primary Mirror Cooler/Extractor system, and the f/14 Fast Guider Box.
The first ideas on how to modify the 1.5-m dome started to take shape in mid 1995. At that time we evaluated louvers, fans, openings, sliding doors, etc. Finally we adopted a door of a guillotine type, replicated 32 times at the spring line around the dome. Each door has its own motor and limit switches. A mechanical engineer hired with MACHO funds did the design work and produced the shop drawings. Inhouse fabrication of the components started in early 1996, and installation on the dome started in October. Four mechanics worked every day, during mornings only, in order to not disturb the astronomical observations. The final electrical installation was finished in April 1997, and the doors have been in routine use since then.
In 1994, the 4-m telescope was equipped with a system that blows cold air onto the primary mirror. Not too long after, a complementary idea was proposed to take advantage of the existing ducts to the mirror: Why not install another extractor connected to the cold air duct, with an appropriate valve, and reverse the flow of the air during the night? This would flush the face of the mirror with dome ambient air. This air from the mirror section would be dumped outside the building. In 1995, several tests were performed using a temporary installed extractor. Improvement of the image was evident, especially when the dome air temperature was above the mirror temperature. So, we decided to go ahead with a permanent system. Design work for a more robust and easytooperate system started in March 1996, and the project was completed in April 1997. The system has been in routine use since that day.
In addition to this air extractor, we installed 12 medium capacity fans inside the primary mirror central light baffle. These fans are installed at a level slightly below the optical surface of the mirror, and located in such a way to extract air from the long tube of the baffle and to force it to the main duct of the mirror extractor system. The idea is to promote a downward laminar circulation of air inside the baffle to dissipate any cell of hot air produced by heat coming from instrumentation at the Cassegrain location. Hot air bubbles in this area have a large negative effect on the image quality.
Soon after the Tip-Tilt Secondary Mirror was completed on the Blanco 4-m telescope, construction started on the Fast Guider Box. The guider box, installed at the f/14 Cassegrain focus, takes light from the secondary mirror and splits it between a science instrument and a fast CCD camera by means of a dichroic mirror. This camera is mounted on a stiff xy carriage that allows the observer to select the appropriate guiding star. The Guider Box accommodates the Infrared Spectrometer, CIRIM, CCOB, and in future, Phoenix. Design work started in January 1996; the project was completed in August 1997.
This year, the major tasks that will keep this Section busy are all related to significant instrumentation upgrades at the Blanco 4-m telescope: the Hydra installation, the Hydra ADC corrector, and modifications to the prime focus pedestal and cage in anticipation of the arrival of Mosaic.
Hydra is scheduled to arrive to CTIO in September this year. In the meantime, construction has started on the fiber bundles, the ADC corrector, comparison lamps system, and bench mounted spectrograph.
Mosaic is scheduled to arrive at CTIO sometime in early 1999. The prime focus pedestal will be modified before the end of 1998 to accept the weight of this instrument and to comply with the demanding requirements of precision and repeatability of the focus motion. Also, the prime focus cage will be modified to fit the size of the instrument and to facilitate installation.
Gabriel Perez (gperez@noao.edu),
Andres Montane (amontane@noao.edu),
Fabian Collao (fcollao@noao.edu)