Sloan Digital Sky Survey Telescope Technical Note 19970403
Matthew Buffaloe, Russell Owen and Walter Siegmund
University of Washington
The Sloan Digital Sky Survey operates in both imaging and spectrographic modes. In the spectrographic mode, 640 optical fibers transport light from a plug-plate mounted at the telescope surface of best focus to a pair of spectrographs. The optical fibers and the plug-plate are part of an assembly called a spectrograph cartridge. Nine spectrograph cartridges are being constructed. This will allow plug-plates to be plugged and unplugged during the day and rapid exchange of cartridges at night.
A total of 5760 optical fibers are required to populate the nine cartridges. To facilitate assembly, testing, record keeping and maintenance, the optical fibers are organized into fiber harnesses, each containing twenty individual fibers. On one end, fibers are terminated in individual 2 mm diameter plugs. On the other end, all twenty fibers terminate in a single V-groove block.
C Technologies, Inc. of Cedar Knolls, New Jersey, manufactured 322 fiber harnesses. This is enough to populate nine cartridges and to provide spare harnesses. A fiber tester was developed that illuminates the plug end of each fiber with an f/5 beam (the f-ratio of the 2.5-m telescope) and measures the amount of light exiting each fiber in an f/4 beam (the f-ratio of the spectrographs). It is calibrated by mating the light source module to the detector module with no intervening optical fiber (Figure 1). Fibers are tested by plugging them one at a time into the source module. A motorized stage properly positions the V-groove block with respect to the detector module for each fiber (Figure 2). A microscope built into the light source module allows the plug end of each fiber to be inspected for contamination and flaws. Also, the position of the fiber relative to the illuminating spot can be checked (Figure 3).
Two fiber testers were assembled. This allowed each harness to be tested by C Technologies as manufactured and by the University of Washington upon delivery.
Figure 1: Fiber tester calibration. The light source and the detector modules are mated at the left end of the optical rail. In this configuration, the light is coupled directly to the detector with no intervening optical fiber. This provides a reference value for subsequent fiber measurements. Other features of interest are the linear stage (right end of optical rail), the stabilized lamp (behind and coupled to the light source module via an optical fiber), the pico ammeter and the linear stage controller (on shelf above lamp).
Figure 2: Testing fibers. One of the twenty fibers in a harness is plugged into the light source module. The V-groove block, at the other end of the harness, is mounted on the linear stage. The linear stage moves the V-groove block in order to align the transmitting fiber with the detector. Upon completion of this measurement, the next fiber is plugged into the source module and the process is repeated.
Figure 3: Visual inspection. Russell Owen uses the microscope built into the light source module to inspect the end of a fiber for contamination or damage. This is done for each fiber before it is tested. Even a small amount of contamination affects the throughput of a fiber.
The throughput distribution of fibers measured at the University of Washington is tight (Figure 4). Fused silica has a refractive index of 1.46 at 550 nm. Consequently, fiber throughput cannot be higher than 93% because of the dielectric reflection losses at the two ends of the uncoated fiber. This accounts for the abrupt cutoff on the right (Figure 4). The standard deviation of the distribution is such that if the distribution were Gaussian, no measurements would fall below about 90.5%. Instead, a long tail on the left is apparent. This indicates a non-Gaussian distribution exists.
Figure 4: Measured throughput of the fibers in 318 harnesses that satisfied testing by the University of Washington. The sharp cutoff at 93% is due to dielectric reflection losses at the two air-silica interfaces. A long non-Gaussian tail on the left side of the distribution is apparent.
Figure 5: Measured throughput of the fibers in 321 harnesses that satisfied testing by C Technologies. The smaller standard deviation of the University measurements may be due to a difference in the cleaning technique used.
Figure 6: Throughput scatter plot. Each point represents one of 6220 fiber measurements. The throughput measured by C Technologies is plotted versus that measured at the University of Washington. This plot is consistent with nearly all fiber throughputs being intrinsically identical with scatter associated with measurement error. A few fibers have lower throughput in measurements made by C Technologies or the University but not both. We attribute this to contamination or stress that was present for only one measurement or mismatching of fibers between the two measurements. The last group of fibers was measured to have significantly lower throughput by both groups. These are presumably intrinsically low-throughput fibers. Less than 1% of the fibers fall into this category.
Figure 6
: Throughput scatter plot. Each point represents one of 6220 fiber measurements. The throughput measured by
C Technologies
is plotted versus that measured at the University of Washington. This plot is consistent with nearly all fiber throughputs being intrinsically identical with scatter associated with measurement error. A few fibers have lower throughput in measurements made by
C Technologies or the University but not both
. We attribute this to contamination or stress that was present for only one measurement or mismatching of fibers between the two measurements. The last group of fibers was measured to have significantly lower throughput by both groups. These are presumably intrinsically low-throughput fibers. Less than 1% of the fibers fall into this category.
In that absence of measurement error and variable contamination or stress, measurements performed by C Technologies and the University would be identical and would fall along the diagonal of Figure 6. The scatter seen in the Figure is consistent with most fiber throughputs being identical and the scatter being due to measurement error. About 1% of the measurements fall on the diagonal trend line below and left of the main cluster. Presumably, these are fibers with intrinsically lower throughput. Also exceptional are a few fibers with significantly lower throughput measured by C Technologies or the University but not both. These may be attributed to contamination on the ends of the fibers or stress that affected only one measurement or mismatching of fibers between the two measurements.
The fiber harness contract specifies that the mean throughput of the fibers of each harness be greater than 90% and that the throughput of the worst fiber be better than 87%. The best fibers in each harness approach the 93% maximum possible transmission discussed above (Figure 7). The histogram of throughputs for the worst fibers in each harness (Figure 8) shows that only a few harnesses have fibers that are measured to have less than 90% throughput. (The data plotted in Figures 7 though 10 were obtained at the University of Washington. Only those harnesses that satisfied the throughput criteria based on UW measurements are included.)
Figure 7: Maximum throughput. The throughput for the best fiber in each harness is plotted. The sharp cutoff at 93% is due to dielectric reflection losses at the two air-silica interfaces. The data plotted in Figures 7 though 10 were obtained at the University of Washington.
Figure 8: Minimum throughput. The throughput for the worst fiber in each harness is plotted. Harnesses are rejected if this value falls below 87%.
Figure 9: Average throughput. This graph shows the mean fiber throughput for each harness. Harnesses are rejected if this value falls below 90%.
The mean throughput for each harness is very consistent (Figure 9). The standard deviation of the means is 0.22% and the range for 318 harnesses is only 2.1% peak-valley. Almost all of the harnesses measured have mean throughputs 1.0% or higher than the specification of 90%. The histogram of standard deviations for the harnesses (Figure 10) shows that the harnesses have fairly uniform throughput with few outliers.
Figure 10: The standard deviation of the fiber throughputs for each harness is plotted. The throughputs for most of the harnesses are very uniform, i.e., the standard deviation is less than 0.5%.
We are grateful to John Phelps of C Technologies, Inc. for supplying measurements of the fiber harnesses made by personnel at C Technologies.
Date created: 4/16/97 Last modified: 12/30/98 Copyright © 1997, 1998 Walter A. Siegmund Walter A. Siegmund
siegmund@astro.washington.edu
