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Dome Venting: the Path to Thermal Balance and Superior Image Quality Steven E

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Dome Venting: the Path to Thermal Balance and Superior Image Quality Steven E Dome venting: the path to thermal balance and superior image quality Steven E. Bauman*a, Tom Benedict, Marc Baril, Derrick Salmona, Tom Vermuelena, Grant Matsushigea ,Larry Robertsa, Jean Charles Culiandrea&b, Rene Racinec, aCanada-France-Hawaii-Telescope Corporation, Kamuela, HI, USA 96743 bIBaldor-Reliance, University of Hawaii, Honolulu, HI USA 96822 ABSTRACT The Canada France Hawaii Telescope operates a 3.6m Optical/Infrared telescope on the summit of Mauna Kea. As an effort to improve delivered image quality in a cost-effective manor, a dome venting project was initiated to eliminate local contributions to 'seeing' that exist along the optical path and arise to a large extent due to temperature gradients throughout the dome volume. The quality of images delivered by the CFHT telescope is adversely affected by variations in air temperature within the telescope dome. Air temperature differences arise through the air’s contact with large structures whose temperatures differ from ambient air as a result of their large thermal inertias and the consequent inability of these structures to follow rapid air temperature changes. The addition of vents to the CFHT dome is intended to facilitate the passive flushing of interior air by the local wind, thereby greatly reducing air temperature variations, a process that has been successfully demonstrated to improve image quality at other telescope facilities and supported by recent water tunnel tests by CFHT staff. The dome venting project is an effort to add a series of large openings “vents” in the skin of the dome with the purpose of allowing free stream summit winds to flush out “stagnant air”. The term “stagnant air” implies to thermally mixed air from the inside of the dome environment that, for one reason or another, has been heated or cooled by surfaces in the dome environment. For the keywords, select up to 8 key terms for a search on your manuscript's subject. Keywords: Dome venting, dome flushing, dome ventilation, flushing stagnate air, dome thermal balance, improved image quality, 1. INTRODUCTION The dome vent system consists of twelve (12) identical vent units, six on either side of the dome slit shutter track centered at angles of 71.25, 86.25, 101.25, 116.25, 131.25 and 146.25 degrees from the dome slit centerline between vertical structural internal web members and an ice deflector mounted on the exterior dome surface immediately above each vent Each vent opening measures approximately 6.5 ft. (2m) wide by 18 ft. (5.5 m) high, each consisting of a vertically-acting weather “rollup” main door and internal louver units to direct and control the flow of air entering the vent openings. The vent width fills the available horizontal separation between adjacent internal vertical structural web members. Vent height was set by estimates of the total vent area desired from water tunnel testing and computational fluid dynamic simulations. Once the vent size needed was determined to achieve the required flushing, structural analysis was performed on the dome structure and skin to verify supports and suggested vent locations would not adversely affect the as build structural integrity. The dome vent unit doors use a simple Somfy electric motor and interconnecting slats to form a “mat” door. Position and feedback are communicated via proximity sensors at various locations, closed, open, 1/3 open and 2/3 open. The door motor is controlled using an Allen Bradley Micrologix PLC and PBC IEC electrical contactors. The louver system also uses a Somfy electric motor but smaller with built in RS-485 communication protocol which allows for one (1) degree adjustment of motor blade angle configurations. In house software was designed by CFHT’s Tom Vermulen and flashed onto a Raspberry Pi unit which provides the control and feedback of the individual louver systems. The vent unit project was completed with help from M3 for structural design and analysis work, CAID for fabrication, assembly, and shipment of the vent units, Nexus steel for steel work and vent erection, Tucson rolling shutters supplied louver and door systems, and Steel Tech provided onsite equipment and logistics. Begin the Introduction two lines below the Keywords. The manuscript should not have headers, footers, or page numbers. It should be in a one-column format. References are often noted in the text1 and cited at the end of the paper. 2. BACKGROUND & JUSTIFICATION 2.1 Dome venting – image quality studies The underlying justification for dome venting comes from a number of sources, but is based on the now well established fact that the delivered CFHT image quality - say as represented by MegaCam images - is considerably poorer than our site on Mauna Kea actually offers. Existing sources of localized air temperature differences are known to arise from a variety of sources: in-dome electrical power consumption, an overly warm or cold observing floor, radiative cooling of the exterior dome skin leading to skin / interior air temperature gradients and inner shell support structure cooling, and finally and perhaps most fundamentally, the lag experienced by the massive components in the telescope including the primary mirror and their ability to keep up with rapid air temperature changes. Figure 1. Graphs showing the effect of dome air temperature gradients on image quality Figure 2. Images showing representative overly warm and overly cold structures within the 5th floor observing environment showing the effect of dome air temperature gradients on image quality . Figure 3, 4, & 5. Graphs showing histograms of temperature difference between outside air and telescope structures of progressively lower thermal mass (i.e. the Primary mirror, the caisson central, and an upper telescope tube trusses). Dome venting will not substantially narrow these distributions, but rather will decrease the time air is in contact with the surfaces and therefore reduce air temperature differences. 2.2 Computational Fluid dynamic analysis The intent of this section 2.3 Water tunnel tests and flushing times The intent of this section 2.4 Venting comparison The upper curve in the Subaru plot is for a CFHT-type dome but with a back vent. The 3rd curve down is for the same dome with ~15% porosity side vents open. The bottom curve is for the current Subaru enclosure with all vents open. 180 160 140 120 100 actual CFHT dome 80 60 venting time to crossing time ratio 40 20 smaller vents larger vents 0 0 30 60 90 120 150 180 slit-to-wind angle ( ° ) CFHT UW results, Salmon et al. 2011 Subaru water tunnel tests, Ando et al 1991 3. DOME STRUCTURE 3.1 Dome Geometry The intent of this section is to provide a brief description of the CFHT dome structure which consists of two concentric shells and intervening steel trusses. The outer shell is made of vertical strips (gores) of 1/4 inch thick (6 mm) sheet steel. The inner shell consists of 2 inch thick foam insulation panels faced on both sides with one mm thick aluminum sheeting. The two shells are separated by a 24 inch (600 mm) air gap formed by steel truss work, the cavity between the inner and outer skins. The base of the dome is supported by a massive ring girder sitting on a series of bogie wheel carriages. The dome shutter and gantry crane are supported by a pair of arch girders that span the diameter of the dome and are separated by 21.5ft; however the clear aperture opening or slit is only 18 ft across. Figure X. The CFHT dome showing the arch girders on either side of the dome slit opening and the individual outer vertical gore sections that combine to make the hyper-hemispheric dome, the gores are alternating ¼ inch thick steel plates which overlap at their edges. Establishing the locations available for the vents was determined by the distance available between the overlapping gores sections of the dome. The location where the outer skins overlap and meet is the location where the vertical structure members for the dome exist. Alterations to the dome skeletal structure at this location would not be possible as it would compromise the structural integrity of the dome. Therefore it was decided that any vent units added to the dome would need to fit in the locations which dictated the width available for each unit. The first gore sections immediately next to the arch girders were eliminated because an access door to the outside of the dome is located in this position. Figure X. The CFHT dome showing the arch girders supporting the dome shutter and exterior access doors on either side of the arch girders. The horizontal structural members requiring removal for vent installation are shown at each dome skin vent opening. Each gore section spans 7.5°. The first gore section that interfaces with the Arch girder has 10 gauge Northwest joist F cord sections stitch welded to the exterior skin. The horizontal F chord section members begin at 10° below the spring line of the dome and are arrayed upward at evenly spaced 10° increments. They are arrayed at every other gore section around until the other arch girder is met. Figure X. The CFHT dome showing the arch girders on either side of the dome slit opening and the individual outer vertical gore sections that combine to make the hyper-hemispheric dome, the gores are alternating ¼ inch thick steel plates which overlap at their edges. The second and third gore sections from either side of the arch girder have inner and outer chords with 1.5” diameter tube web frames welded between the chords. These sections are joined by a bolted connection to the vertical frame members on either side of the gore section; in addition they are stitch welded to the exterior gore skin.
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