New stray light test facility and initial results John Fleming *, Frank Grochocki, Tim Finch, Stew Willis, Paul Kaptchen Ball Aerospace & Technologies Corp., P.O. Box 1062, Boulder, CO, USA 80306-1062 ABSTRACT BATC has developed a new stray light test facility (SLTF) and performed initial tests demonstrating its capabilities. The facility interior is nearly all black and is a Class 5 cleanroom. Coupled with a double cylindrical chamber that reflects the specular light away from the instrument under test, the stray light control in the facility is excellent. The facility was designed to be able to test a wide variety of instruments at a range of source angles from in-field to large off-axis angles. Test results have demonstrated PST performance below 1E-9. Keywords: Scatter, stray light, contamination, Rayleigh scattering 1. Introduction For over ten years BATC has performed stray light tests on all star trackers prior to delivery. These tests were performed in a small facility with HEPA filtered air, but scatter from airborne particulates was the dominant source of noise during these tests. Even though the facility had black walls and moveable light traps that adequately controlled stray light paths, more advanced trackers and other large instrument programs drove us to build a far more capable facility that could test larger instruments in a cleaner environment. 2. Facility Requirements A single project at BATC drove the need to build the SLTF, but the goal of the effort was to build a facility that could handle a wide variety of instruments now and well into the future. Since these future requirements are largely unknown, many assumptions on test requirements were made, and cost of meeting the requirements was always a vital consideration. We also studied the limitations of our existing facility and documented designs and results from other stray light test facilities. 1,2,3,4,5 In the old facility, scatter from airborne particulates was a large source of error. Scatter from airborne particulates dominated molecular (Rayleigh) scattering. We wanted to reduce airborne particulates to a level that the background was dominated by molecular scattering. Our calculations indicated that we would be strongly Rayleigh dominated operating at Class 5. A Class 5 facility with electro-static discharge protection (ESD) also would allow us to test virtually any hardware built at BATC. Meeting the cleanliness, ESD, and stray light requirements for all materials in the facility was the most challenging part of the effort. The distance from the last mirror in the source assembly to the unit under test limits the minimum angle that can be tested where scatter from the source optic cannot contribute. We initially had a requirement to set this limitation at 1 for an 18 diameter beam. This would require a facility that is on the order of 100 long. Considering limitations of existing facilities available at BATC and cost, we settled on a 58 long facility with a distance from the last source mirror to the aperture of the instrument under test of 46. For an 18 wide beam, the minimum angle where source scatter is not a factor is at 1.9 o. Considered another way, the beam size would have to be stopped down to 9.6 diameter to limit source scatter to less than 1 from the source. BATC builds a wide variety of instruments and telescopes with a wide range of apertures and entrance baffle sizes with the current maximum size of 6.5 meters for the James Webb Space Telescope. Clearly we could not build the facility to meet all possible needs. Again we chose to build the facility to allow testing of the majority of our instruments. For larger instruments, larger optics and stages could be borrowed from other company facilities or sub-aperture stray light tests could be performed. The chosen design will allow testing with up to an 18 diameter beam. * Email: jfleming@ball.com, phone: 303-939-4019 Optical System Contamination: Effects, Measurements, and Control 2008, edited by Sharon A. Straka, Proc. of SPIE Vol. 7069, 70690O, (2008) 0277-786X/08/$18 doi: 10.1117/12.798920 2008 SPIE Digital Library -- Subscriber Archive Copy Proc. of SPIE Vol. 7069 70690O-1
The primary source is a Newport 1000W Xenon Arc lamp. Depending on beam size, the source and the collimating optics can provide irradiance at the test article of 10 to 40% of the solar irradiance. This source is used to simulate solar flux so perfect collimation is not required. A more important consideration is uniformity of the flux over the instrument aperture. For some programs and tests such as near field or in-field stray light testing excellent collimation is required. This is best accomplished with a laser and spatial filtering. For these applications our collimating mirror is an off-axis parabola. For a future upgrade we hope to have access to output from a tunable dye laser. For stray light control we borrowed some features from the stray light facility at Space Dynamics Laboratory at Utah State University in Logan, Utah. 1 Key to this stray light control is the double cylindrical chamber (DCC) design with the walls made with black acrylic. The specular black walls of the DCC are vital to the stray light performance of the facility. Our cylindrical chamber is 18 wide at its longest dimension and is entirely compatible with ESD safe Class 5 cleanroom operation. Star tracker stray light tests are performed as one of the last tests before delivery. These tests are performed with the flight focal plane. Tracker detectors have a high dynamic range but still do not approach the dynamic range attainable with a photodiode. Since the detector would saturate if the solar source were imaged to the focal plane, it is not possible to measure the point source rejection ratio (PSRR) directly. Instead we measure the irradiance at the entrance aperture with a NIST calibrated photodiode to determine the irradiance at the instrument relative to the solar irradiance. For other instruments testing at a lower level of integration is planned. For these tests a photodiode will be used at the telescope focal plane and the dynamic range on the order of 10 9 will enable a direct measurement of the PSRR. 3. Design Implementation The layout of the facility is shown in Figure 1. For stray light control in the facility in addition to the DCC, the walls and floor are black, and the ceiling lights and HEPA filters have eggcrate style grids so that the white surfaces are only visible when viewed from angles within about 45 o from directly below. Light leaks into the facility are very well controlled and can be eliminated completely if necessary. This is not necessary for most tests since the stray light from the source far exceeds any leaks under doors. In addition, testing from a remote location is possible, but has so far not been necessary. The additional airborne contamination from personnel in the cleanroom and lights from computers required to run the test has not been a problem for testing to date. DCC Unit under test on rotary stage and linear rail Source Assembly Figure 1. Test Facility Layout - Dimensions of Facility are 58 long by 20 wide The entirely black facility poses a challenge for professional photography. Photos of the facility are shown in Figures 2 and 3. The facility is equipped with several other features to help control stray light. A revolving darkroom door is used to eliminate light leaks from the gowning area into the test facility. Black curtains are used to separate the source area from the DCC area. Apertures in both the intermediate baffle and the aperture into the DCC can be changed so that light illuminating the test article is reduced to only the size that fills the front aperture. The test article is mounted on an Aerotech rotary stage that will handle up to 1100 lbs. Proc. of SPIE Vol. 7069 70690O-2
Figure 2. Source Illumination of Test Article and Airglow Figure 3. Source Table Proc. of SPIE Vol. 7069 70690O-3
To assess the performance of the facility prior to testing, bi-directional reflectance distribution function (BRDF) and reflectivity tests were performed on sample coupons representing most critical surfaces. Figure 4 shows the BRDF of four key surface types in the facility. Figure 5 shows the total reflectivity of these surfaces. These measurements gave us confidence that the facility would meet our requirements. Most of the surfaces have fairly flat reflectivity over the measured wavelength range. The exception is the curtain material which becomes significantly more reflective at wavelengths above 700 nm. 1.00E+05 1.00E+04 1.00E+03 1.00E+02 DCC Walls Wall Type 1 Wall Type 2 Epoxy Floor BRDF (/sr) 1.00E+01 1.00E+00 1.00E-01 1.00E-02 1.00E-03 1.00E-04-90 -75-60 -45-30 -15 0 15 30 45 60 75 90 Angle from Specular Figure 4. BRDFs at 633 nm of key surface types used in the facility 20 18 16 14 Reflectance (%) 12 10 8 Curtain Facility Walls 6 4 2 Floor DCC Walls 0 250 500 750 1000 1250 1500 1750 2000 2250 2500 Wavelength (nm) Black Floor Black Curtain Wall Type 1 Wall Type 2 DCC Walls Figure 5. Total reflectivity of key surface types used in the facility Proc. of SPIE Vol. 7069 70690O-4
4. Initial Test Results In preparation for the first test of a flight article in the new facility, a prototype star tracker was tested. The unit had been tested in the previous stray light test facility and could be correlated to other flight trackers that had been tested in the old facility and whose on-orbit stray light performance was also known. To calculate the point source transmittance (PST) the signal from the source at various off-axis angles was measured. In addition, at several angles of the tracker relative to the source, the tracker was pulled back out of the beam and the signal at the focal plane was again measured. This airglow was clearly dominated by Rayleigh scattering and other scatter pathways in the facility were insignificant. The PSTs are plotted in Figure 6 at three locations in the focal plane. 1.0E-05 Point Source Transmittance 1.0E-06 1.0E-07 1.0E-08 Star 1 Star 3 Star 5 1.0E-09 1.0E-10 20 25 30 35 40 45 50 55 60 Source Off-axis Angle (degrees) Figure 6. Prototype Star Tracker Point Source Transmittance The PSTs in Figure 6 are modestly higher than we measured previously on this tracker but are far more believable. Airglow measurements in the old facility had a different and probably higher stray light background than when the beam was centered on the tracker front aperture. In the new facility the technique for measuring airglow is much more accurate and almost certainly conservative. When the source beam is incident on the tracker, each point of the focal plane looks through a maximum area of the source beam. When the source is pulled back from the beam, the vertical field points are looking through a reduced area of the beam. This conservatism has not been a problem for star tracker testing and requirements compliance, but it could be an issue in future testing. This will be a more significant issue with large fieldof-view, high attenuation systems. Since the prototype tracker has very high attenuation and a FOV along the diagonal of greater than 10 o, there won t be many systems where this is an issue. FUTURE EFFORTS There are two key efforts that will occur in 2009. We will be testing a large instrument in the facility which will require significant modifications to our test stage. Due to the overall size of the instrument a new system for holding the instrument and rotating relative to the beam is required. The existing rotary stage will still be used, but the test table cannot because the table would place the aperture higher than the source beam. We also hope to get internal funding to move a Ti:Sapphire pumped tunable dye laser close to the facility and bring the light into the facility via a fiber. This would give us a high power monochromatic source that could be tuned over most visible wavelengths and into the near infrared. Proc. of SPIE Vol. 7069 70690O-5
SUMMARY A new stray light test facility has demonstrated excellent stray light control and ability to measure PSTs at below 1E-9. As stray light requirements have become more stringent, the importance of testing stray light instead of just using component level BSDFs and system level models has increased. This facility can be contracted for testing of non-batc hardware. ACKNOWLEDGEMENTS This effort would not have been possible without help from our key suppliers. Special thanks are due to CleanZone Technologies of Colorado Springs, CO for the clean room, Howell Construction of Denver, CO for the facility modifications, C&T Custom Fabrication of Berthoud, CO for the cylindrical chamber frame, Premium Powder Coating of Longmont, CO for the powder coating of the cylindrical chamber frame, Colorado Plastics of Boulder, CO for the acrylic walls and apertures, and Space Optics Research Laboratory for their off-axis parabola. REFERENCES: 1 J. C. Kemp, J. L. Stauder, S. Turcotte, H. O. Ames, Terrestrial Black Hole for measuring high-rejection off-axis response, Proc. SPIE 3122, 45-56 (1997). 2 I. T. Lewis, et. al., Stray light rejection in a WFOV star tracker lens, Proc. SPIE 1530, 306-324 (1991). 3 A. E. Lowman, J. L. Stauder, Stray light lessons learned from the Mars Reconnaissance Orbiter s Optical Navigation Camera, SPIE 5526, 240-248 (2004). 4 D. B. Leviton, et. al., White light stray light test of the SOHO UVCS, SPIE 3443, 50-60 (1998) 5 L. Blarre, A. Mestreau, Stray light characterization of optical systems, SPIE 2775, 279-286 (1996) Proc. of SPIE Vol. 7069 70690O-6