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.

0

20

40

60

80

100

120

140

160

180

0 30 60 90 120 150 180slit-to-wind angle ( ° )

vent

ing

time

to c

ross

ing

time

ratio

actual CFHT dome

smaller vents

larger vents

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. The triangular horizontal sections repeat from the third gore section every other gore until the opposite arch girder is met, the other side on the dome is a mirror image about the centerline of the dome shutter.

Table 1. Margins and print area specifications.

The bottom of the vent apertures opening (~2m wide by 5.5m high) begins 84” above the 5th floor catwalk. There are a couple of reasons for this: Again the external door is present in the first gore section, various electrical enclosures are present from the 5th floor catwalk to about head height, and for safety reasons it eliminates the possibility for someone to fall through a vent opening.

Table 1. Rendering of the inside of the dome showing the external access door and existing electoral enclosures.

3.2 Structural analysis

The structural analysis of the dome was performed by M3 engineering. The proposed locations of the vent units and existing geometry set the requirements for the structural analysis to be performed. The analysis included the skeletal rib members of the dome, connections to the existing steel dome outer skin, and the vent unit enclosures combined with the present dome geometry.

Table 1. Finite element analysis model of the dome structure, courtesy of M3 engineering.

External access door

Electrical enclosures

84”

It just so happens that the weight of the skin removed (1,400lbs) to achieve a cut opening for the new vent unit enclosures (2,600lbs) was only about a 1,200lb difference from the initial configuration. Therefore the effect of additional weight per vent unit added to the dome/ring girder unit is negligible under 1997 Uniform building code, Vol 2 Structural Engineering Design Provisions.

The vent unit was checked for wind load, seismic load, dead loads, live loads, and ice loads. None of these loading cases exceeded the maximum or allowable stress or deflection in any design case.

4. VENT DESIGN OVERVIEW 4.1 Vent unit enclosure

Each identical vent unit configuration consists of a steel frame weldment assembly fabricated using ¼ inch steel plate, as discussed early the frame width is designed to fit between the vertical structural members of the dome. The exterior mounting flange that interfaces with the outer dome skin overlaps the cutout opening 4 inches of the sides and 6 inches on the top and bottom.

Figure 2. Figure

Each vent unit has a dual control system to allow additional air flow into the dome. An external roll-up door opens/closed to allow flushing of the dome, the outer door can be configured all the way open, 2/3 open, and 1/3 open to facilitate flushing control and reduce telescope shake during high winds. The other method uses internal louvers to throttle the flow or control its direction.

4.2 External door

The outer roll-up door is located on the interior of the steel framed enclosure and consists of interconnected slats that span the width of the vent unit opening. The slats travel up/down the outer radius of the enclosure in between guide rails on either side of the opening. The guide rails have felt seals with one seal on the front and one in the back for a total of four seals per vent unit. The slats are made of extruded aluminum with a double wall extruded 55mm profile with wind locks. The wind locks mount to the side of the slats and travel inside the guide rail to keep the slat for movement back and forth with in the track. They have a wind rating of 120psf and are approved for Dade County Florida hurricanes.

Figure 3. The side profile of a aluminum slat and a guide rail (raw alum) with felt seals (gray) on each side of the slat (white) with wind locks (black) installed.

The motor is a single 120VAC Somfy tubular motor rated for 60,000 cycles; it contains a manual override which can be cranked by hand or with a manual drill in the case of power failure. The position of the bottom slat of the door is controlled by using 5 proximity sensors located at the top (open), bottom (closed), 1/3 open, 2/3 open, and compressed (slats compressed for wind loads). The motor also has built in motor limits in the case of sensor or software failure. The outer door takes around 1min to open or close.

Minor maintenance of the roll-up door is made by removing a top cover which hides the roll-up drum and motor. All service work including slate replacement and motor replacement is performed at the top of the shutter door unit’s header box. Upon removal of the door cover plate and header cover plate, all components can be service and replaced by a

maintenance crew consisting of two people. The heaviest component would be the slat roll, weighing in at 217lbs, or 109lbs per person.

4.3 Internal louvers

The internal louver system consists of three independent motorized louver systems that house either 8 (middle & bottom) or 7 (top) louver blades mounted in a rectangular frame that mounts to the main steel enclosure. Each louver assembly, top, middle, or bottom can be removed independently from the enclosure for major repairs and to allow access to the outer door components if needed. Minor repairs can be made with the framework left in place; each 8 inch aluminum extruded airfoil blade has mounting shafts incorporated into the blade extrusion on either side which rotate in a bronze sleeve mounted to the main frame. The bottom louver blade on each framework differs from the other louvers as it incorporates the motor on one side of the blade. The louver system is controlled by a single 120VAC Somfy tubular motor rated for 60,000 cycles. The smaller motors are IP addressable and have built in control functionality for infinite position control and positive position feedback. Communications are established via RS 485 telephone handset cables which are funneled to a Raspberry PI. Internal high level software created by CFHT’s Tom Vermeulen, allows control of each louver system independently.

4.4 Proximity sensors

As discussed earlier the position of the bottom slat of the door is controlled by using 5 proximity sensors located at the top (open), bottom (closed), 1/3 open, 2/3 open, and compressed (slats compressed for wind loads). The sensors provide feedback to the PLC system for slat position. If a sensor fails there is no backup method for indicating the motor position, internal motor limits are used to allow the door to be closed or open all the way. The sensors are Omron TL-W20ME1 inductive NPN-NO 3-wire unshielded proximity sensors. The sensors are mounted in non metallic conduit

boxes, which will not interfere with the sensor and are chemical and UV resistant. Ferrous indicator plates are mounted to the slats to indicate slat position.

Figure 4. Proximity sensor and the locations of the sensors.

4.5 Vent unit control

Each

4.6 Vent unit software

Each

5. DOME VENT INSTALLATION 5.1 Fabrication

The

5.2 Assembly

Each

5.3 Shipping

The

5.4 Installation

The