Sloan Digital Sky Survey Telescope Enclosure: Flow Visualization

Sloan Digital Sky Survey Telescope Technical Note 19940310

Abstract

The telescope enclosure for the Sloan Digital Sky Survey 2.5 meter telescope is the first roll-off design used on a telescope of this size. The roll-off design eliminates dome-induced image degradation because the telescope enclosure is rolled downwind of the telescope during observations.

Flow visualization techniques were used to study a 1/72 scale model of the enclosure in a water tunnel. The overall slope and the vegetation present at the telescope site were represented in the model. Results indicate that flow separates at the leading edge of the telescope platform but that the turbulent boundary layer remains below the level of the telescope and should not contribute to image degradation. A close fitting telescope wind and light baffle is intended to reduce wind-induced tracking error and scattered light from off-axis sources. A model of a portion of the baffle was examined, also. The baffle is very effective in reducing flow velocity downstream of the baffle. Although the velocity of the flow through openings in the baffle is high, these jets diffuse within 0.1 to 0.2 meters of the baffle. The resulting flow should have little effect on telescope tracking while providing excellent flushing of the telescope light path.

Introduction

The preservation of image quality is one of the most important design criteria for a telescope enclosure. Image quality is adversely affected by the presence of fluctuations of air temperature in the light path. Pressure fluctuations, e.g., due to turbulent flow, do not significantly degrade images in ground-based telescopes. Ideally, the air in the telescope light path is continually replaced by ambient air before it has the opportunity to be warmed or cooled by the surfaces near the telescope.

A telescope enclosure represents a significant cost of a telescope project. Unfortunately, a poor telescope enclosure design can degrade the image quality of an otherwise excellent site. A better understanding of the interaction of the fluid and the enclosure should lead to improvements in the design of the telescope enclosures and preservation of the intrinsic image quality of the telescope site.

The SDSS 2.5-m telescope enclosure is designed to roll away from the telescope leaving the telescope totally exposed to the prevailing wind (see Sloan Digital Sky Survey telescope enclosure: design in these Proceedings). Large doors at each end provide access for servicing and allow the building to pass over the telescope. These doors remain open during operation. This reduces the effect of the building on upstream air flow. Mitigation of wind-induced tracking error is provided by a wind baffle (Figure 1) that fits closely around the telescope and that is also a component in the telescope light baffling system. The wind baffle is driven in two axes separately from the telescope and is supported by the telescope building. The wind baffle drive systems are electronically slaved to the telescope axes. The wind baffle is covered with panels (H. H. Robertson model 5100) that are 25% porous to the wind. This allows excellent flushing of the telescope optical path while reducing wind-induced forces on the telescope. (These panels have the desirable property of providing excellent light baffling.) The end of the wind baffle is closed except for an annular opening that provides clearance for light to enter the telescope. The inner edge of the annular opening is defined by a circular plate that is supported from the rest of the wind baffle by narrow vanes.

Figure 1. The SDSS 2.5-m telescope enclosure. The telescope enclosure rolls away from the telescope on crane rails to expose the telescope to the wind. A wind and light baffle, driven and supported independent of the telescope, fits closely around the telescope.

Description of the Experiments

This research was performed using the 13 m long water tunnel of the Department of Aeronautics and Astronautics of the University of Washington. The test section is 3 m long and 0.76 m square. The maximum flow speed is 0.6 m/s. The flow speed used was 10 cm per second. This scales to 7.2 m/s (16 mph). The Reynolds number is the most important parameter affecting flow in this system. In particular, typical wind speeds are high enough that buoyancy effects are negligible in a reasonable telescope enclosure design. The Reynolds number is

Re = VD/µ

where V is the flow velocity, D is a characteristic dimension, and µ is the fluid kinematic viscosity (0.9E-6 m^2/s for water and 18E-6 m^2/s for air). Typically, the Reynolds number for a scale model is too small because of the dependence of V and D on the scale. The lower viscosity of water partially offsets the effect of the scale change, making a water tunnel an ideal environment for examining flow characteristics around sharp edged structures.

The Reynolds number for the model system is about 4400 based on the width of the wind baffle. For the actual structure and typical wind speeds, the Reynolds number will be 10^6. Fortunately, for a shape which is not streamlined, flow patterns do not change much once the Reynolds number is above several thousand. In several cases, we increased the flow speed to about 50 cm/s to insure that our results were not sensitive to the Reynolds number.

An acrylic model of the telescope enclosure, telescope and wind baffle was used. The scale was 1/72. The model was mounted upside down on a flat base plate 1.58 m long. Wind at site is observed to flow parallel to the ground, i.e., up the slope. This relationship was maintained in the water tunnel by mounting the base plate and model with the proper orientation with respect to the flow direction. Since flow in the water tunnel is horizontal, this meant that the telescope enclosure model was installed at an angle. However, the photographs of Figures 2 to 4 were taken with the camera tilted and printed upside down so that the orientation of the building would appear normal.

Realistic modeling of a telescope site must account for the velocity profile at the site, which in this case includes the presence of trees. The velocity profile was modeled using molded plastic trees with approximately the same density and height distribution found at the site. Plastic screening was added to the forest to match the velocity profile. Horizontal velocity was measured at several heights and adjusted to approximate that measured at the site.

Fluid flow through and around the telescope was examined at various angles of the flow with respect to the building orientation. Only the range of angles from 0° to 45° were used (0° corresponds to the wind coming directly up-slope, west-southwest at the site). This is not a significant limitation since the wind direction at the site is most frequently from the corresponding quadrant and is associated with the best image quality. Also, the telescope orientation with respect to the building is fixed. The orientation chosen is believed to be the worst case for shielding of the telescope by the wind baffle, i.e., with the enclosure oriented at 0°, the telescope is pointed directly into the wind.

Three types of experiments were performed. In the first, dye was injected into the flow upstream of the model. Several probe heights were used for each model; these are chosen to examine the flow at various levels in and around the telescope. Streamlines flowing through and around the telescope do not efficiently examine stagnate spaces since the flow rarely enters those spaces. In the second type of experiment, dye was injected directly into the telescope wind baffle and fills the volume in front of the primary mirror. Observing the rate at which dye disappears from the wind baffle indicates the flushing rate. Dye remains in stagnant spaces longer than in spaces which are well flushed. In the third type of experiment, dye was again injected into the interior of the wind baffle and video taped. The tape was subsequently analyzed to determine flow velocities near the primary and secondary mirrors. Features on the model were used to determine the scale on a television monitor and the frame interval (1/30 second) provided timing.

In a separate set of experiments, a model of a portion of a wind baffle panel was examined. The model was approximately 0.2 m square and its scale was 1/4. Because of the limited extent of the model, and its low porosity, flow around the model perimeter tended to affect the flow immediately downstream. However, by controlling the width of the gap between the model and the base plate and by looking at flow near the center of the model, we were largely able to overcome these effects.

Results

In earlier studies, we have found that telescope buildings often cause streamlines to rise as they approach the telescope.1,2 Since the height of telescope buildings is driven by the desire to place the telescope high with respect to streamlines, a design that prevents streamlines from rising may be more cost effective than increasing the height of the telescope above the ground. Figure 2 shows dye tracing a streamline just above the leading edge of the telescope platform. The telescope building is at 0°. (The building orientation is described by the angle between the down-slope direction and the direction from which flow occurs.) No elevation of the streamline is apparent near the building. Most likely, this is due to the presence of the forest that causes a strong gradient of flow speed with height.

Figure 2. With the enclosure at 0°, dye traces flow coming up-slope with mild turbulence induced by the presence of trees. This and similar tests show that the volume surrounding the wind baffle will be continually flushed by the wind and that the stagnant volume just above the telescope platform will be small. This and the other postage stamp images are linked to detailed 40kB images.

The streamline shown encounters the telescope wind baffle near the level of the primary mirror. This and other tests show that the entire exterior of the wind baffle will be very well flushed by the wind. At the leading edge of the telescope platform, flow separates and a stagnation region develops over the telescope platform. Even so, because this region is not enclosed, flushing is very rapid. Still, we will want to minimize the production of warm or cold air in this region. This will be more important with the telescope pointed in the opposite direction since the thickness of the stagnation region increases downstream.

Figure 3. These photographs, taken 10 seconds apart, show the very rapid flushing of the wind baffle due to its porosity and low volume. The full-scale flow speed is 7.2 m/s (16 mph).

Flushing of the telescope wind baffle is very rapid due to its 25% porosity and low volume. Figure 3 shows that at 0°, flushing is nearly complete in 10 seconds. Flushing is only slightly slower at 45° and Table 1 shows flushing times at other angles which were examined.

Table 1. Model enclosure flushing times (as a function of azimuth angle)

   Azimuth   Run 1    Run 2     Comments
    Angle    (sec)    (sec)
      0°      17       19
     15°      24       25
     30°      22       24
     45°      23       24
     60°      28       30       Stagnation regions formed
     75°      24       25       on downstream bottom
     90°      23       24       corners of enclosure
    105°      19       16
    120°      13       14       Stagnation region formed
    135°      18       15       behind secnodary mirror
    150°      17       18
    165°      18       19
    180°      17       18
 

Video tape recordings of dye injected into the interior of the wind baffle were analyzed to determine the effectiveness of the wind baffle in mitigating wind-induced tracking error. With the building at 0°, fluid entered the wind baffle through the open end, passed near the edge of the secondary and rotated in a clockwise direction between the primary and secondary. The highest velocity measured was near the edge of the secondary. The average was 0.39 of the free stream flow velocity (see Table 2). This velocity reduction corresponds to a dynamic pressure reduction of about 1/7 and is a large decrease in the wind loading on the telescope. As mentioned above, this appears to be the worst case orientation of the telescope. At other orientations, the shielding will be even more effective. Wind-induced moments on the secondary truss and frame are a large portion of the total moment about the telescope altitude axis. These components are very well shielded by the wind baffle so that only the secondary and its baffles and support package are likely to be much affected by the wind.

The motivation for examining the detailed model of the wind baffle panel was to study how rapidly flow diffuses on the downstream side of the panel. Since the dynamic pressure of the wind goes as the square of the wind velocity, it is desirable that flow through the baffle diffuse rapidly to avoid wind-induced tracking error. Many flow diffusers produce persistent high velocity jets. This particular design avoids this problem. Figure 4 shows that diffusion is essentially complete, i.e., comparable to the spacing of panel openings, about 0.2 m (full scale) downstream of the panel.

Figure 4. Dye approaching this model of a portion of a wind baffle panel flows through the model and diffuses very rapidly downstream. The lack of a persistent jet demonstrates that this panel geometry is very effective at shielding downstream structures from the wind while providing 25% porosity. The full-scale thickness of the panel is 0.1 m.

Table 2. Velocities within the wind baffle (as a fraction of the free stream velocity)

   Location                  Velocity
   At edge of secondary         0.39
   In front of secondary        0.18
   In front of primary          0.21
 

Conclusions

The roll-off telescope enclosure, in combination with a close fitting independently driven wind baffle provides an excellent flow environment for a telescope. The stagnation region above the telescope platform is well flushed and is not likely to have any significant effect on image quality even with the telescope pointed downwind. The exterior of the wind baffle is continually flushed by the wind. Air flowing through the wind baffle panels and the open end of the baffle flush the interior of the baffle very rapidly. At the same time, the wind baffle provides excellent shielding of the telescope from wind-induced tracking error. The baffle panels provide good porosity and rapid diffusion downstream.

Acknowledgments

It is a pleasure to thank Bob Breidenthal and Charlie Hull of the University of Washington who variously contributed ideas, encouragement and assistance.

References

1. W.A. Siegmund, W. Wong, F.F. Forbes, C.H. Comfort, Jr., and S. Limmongkol, Flow visualization of four 8-m telescope enclosure designs, Proc. of S.P.I.E. 1236, p. 567, 1990.
2. F. Forbes, W.-Y. Wong, J. Baldwin, W. Siegmund, S. Limmongkol, and C. Comfort, Telescope Enclosure Flow Visualization, Proc. of S.P.I.E. 1532, 1991.

A paper similar in content to this note is published as "Sloan Digital Sky Survey telescope enclosure: flow visualization", C. H. Comfort, Mark Matheson, S. Limmongkol, W. A. Siegmund, Proc. of S.P.I.E., 2199, 1994, p.1074.

Date created: 3/10/94
Last modified: 5/26/99
Copyright © 1999, Walter A. Siegmund

Walter A. Siegmund

siegmund@astro.washington.edu