As the accompanying picture shows, the 4-m has just become a well-ventilated telescope! The 4-m dome vent project was initiated to improve the delivered image quality (DIQ). Seeing measurements, combined with temperature readings at various points around the dome, have established that the local thermal environment is the largest culprit currently degrading the 4-m DIQ. Over the past two years we have attempted to tackle the thermal problems by (a) implementing a mirror cooling system, (b) installing a dome-air mixing fan, and now (c) installing dome vents. We expect that the dome vents will provide the largest improvement in the DIQ. The 22 panels provide 1600 ft² of opening; the dome slit provides an additional 2300 ft². Depending upon the wind direction, we expect that a 10 mph wind will provide 100-325 flushes of the dome per hour. This is about one-third the area-to-volume ratio that is provided by dome ventilation at the WIYN and 0.9m telescopes, and is essentially identical to that installed some years ago at the CTIO 4-m telescope.
The engineering and design of this project were carried out by Tony Abraham and Khairy Abdel-Gawad, with Bruce Bohannan providing project management. John Hoey and Ron Harris provided mechanical support, and Scott Bulau, David Stultz, and Guillermo Montijo provided electronic support. The mechanical installation was carried out by Central Facilities under the supervision of John Dunlop, with John Scott managing the mountain crews and contracted people.
Now that the dome vents are in use, our next significant effort will be to replace the primary mirror supports with an active system. Wavefront measurements suggest that astigmatism and coma add about 0.10-0.15" to the DIQ, depending upon location in the sky. We expect to be able to remove most of this by controlling the pressure in individual air bags, in much the same way that CTIO has improved the image quality at the 4-m Blanco telescope. We are planning to install the hardware for this project next summer when the primary is removed for aluminization. Further tests are underway to understand if we need to provide active support of the secondaries.
Phil Massey, Tony Abraham, Bruce Bohannan, Chuck Claver, George Jacoby, Richard Wolff