Project Documentation Technical Note TN-0013 Rev. D M1 Microroughness and Dust Contamination Robert Hubbard Systems Engineering November 2013 Advanced Technology Solar Telescope 950 N. Cherry Avenue Tucson, AZ 85719 Phone 520-318-8102 [email protected] http://atst.nso.edu Fax 520-318-8500 M1 Microroughness and Dust Contamination REVISION SUMMARY: 1. Date: 6 September 2002 Revision: A Changes: Initial release 2. Date: 22 October 2002 Revision: B Changes: Correct “gray area” in figures. Add additional information about the cause of small-angle scatter by dust. 3. Date: 28 October 2002 Revision: C Changes: Corrected figure 12 to include better low-angle data. Corrected cleanliness level from 230 to 240. Added additional clarification of dust discussion in section 5.2. Added van de Hulst reference. 4. Date: 6 November 2013 Revision: D Changes: Updated scattered light requirement per most recent SRD version. Updated format to conform with “modern” document template. TN-0013, Rev. D Page i M1 Microroughness and Dust Contamination Table of Contents 1. INTRODUCTION ............................................................................................. 1 2. SCIENCE REQUIREMENTS AND SKY BRIGHTNESS EXPECTATIONS ................... 2 3. THE ASAP MODEL.......................................................................................... 2 3.1 THE OPTICAL SYSTEM MODEL ............................................................................ 2 3.2 THE SOURCE MODEL ........................................................................................ 3 3.3 SCATTER MODELS AND IMPORTANT AREA SAMPLING .............................................. 5 4. MIRROR MICROROUGHNESS ........................................................................ 5 4.1 SCATTER DUE TO MICROROUGHNESS .................................................................. 5 4.2 BSDF AND THE HARVEY MODEL ........................................................................ 7 4.3 RMS MICROROUGHNESS ............................................................................... 10 4.4 REASONABLE ASSUMPTIONS, AND MICROROUGHNESS RESULTS ............................... 11 5. DUST CONTAMINATION .............................................................................. 13 5.1 THE MIE MODEL FOR SCATTER BY DUST PARTICLES .............................................. 13 5.2 RESULTS FOR A CLEAN MIRROR ........................................................................ 13 6. CONCLUSIONS AND RECOMMENDATIONS .................................................. 17 7. REFERENCES ................................................................................................ 18 TN-0013, Rev. D Page ii M1 Microroughness and Dust Contamination 1. INTRODUCTION The optical design of the Advanced Technology Solar Telescope (ATST) is an all reflecting, off-axis Gregorian telescope. Some of the considerations that drove this solution stem from the need to perform coronagraphic observations close to the sun’s limb. Even with this configuration, scattered light in the telescope optics has the potential to be the limiting factor on the quality of such observations at a site with low sky brightness. Care must be taken in the design, fabrication, and maintenance of the telescope to prevent this. If occulting is performed at prime focus, then the dominant instrumental stray-light contributor during coronal observations will be the microroughness of the primary mirror, and dust accumulating on that same surface. The is the result primary mirror being large, fully illuminated by the sun, and because of the relatively low scattering angles necessary to couple this unwanted light into the system. These stray- light sources are the subject of this report. Mirror microroughness is a static condition that will persist throughout the lifetime of the telescope, or at least during the lifetime of the primary mirror. It is a function of mirror polish applied during manufacture, so the project’s only opportunity to mitigate its effects comes in the specification and acceptance testing of this major telescope component. On the other hand, an aggressive roughness spec has the potential to significantly impact project cost, schedule, and risk management. The effects of mirror microroughness need to be well understood so that the project can make good decisions. Dust control, on the other hand, has a larger systems impact. It will be affected by site selection, enclosure design, ventilation, mirror-cell design, telescope support structure design, and a variety of operational issues such as mirror washing, cleaning, and recoating schedules. Once again we need to thoroughly understand the degradation of telescope performance due to dust contamination. In this case good data and good analysis needs to be in hand early to guide the design process in many areas. Buffington and Jackson (UCSD Center for Astrophysics and Space Sciences) have already done work on these and other stray-light topics relevant to ATST 1. On the subject of primary mirror microroughness they note that something like one percent of the Total Integrated Scatter (TIS) from microroughness will enter the coronagraphic field of view, and warn that, “Specifying, manufacturing, testing and certifying [the primary mirror] could prove a significant challenge for ATST.” This report attempts to further quantify Buffington and Jackson’s microroughness results over a range of realizable mirror-polish parameters. Results will also show how scatter levels due to primary mirror microroughness will vary with limb distance. On the subject of scattering by dust particles, Buffington and Jackson note that there are modeling challenges associated with the absence of small-angle observational results in the visible. The power-law relationship between scattered light and scatter angle cannot continue indefinitely if TIS is to remain finite, and must roll off at small angles. But since Buffington and Jackson were uncertain where, exactly, this roll-off begins, and because the details of the angular dependence in this unobserved region are critical to the performance of ATST, they were only able to establish a relatively broad range of dust- contamination predictions. In their worst-case scenario, there is little chance of ever being able to do state-of-the-art coronagraphic observations without resorting to heroic measurements. Even at the more optimistic end of their parameter space, dust control of ATST would appear to be challenging. This report attempts to extend that work, and further clarify the impact of dust on the primary mirror. It will support the view that their “optimistic” parameters are probably the correct ones, and further explore the issue of dust accumulation rates in this context. This will lead to recommendations for mirror- cleaning strategies. TN-0013, Rev. D Page 1 of 18 M1 Microroughness and Dust Contamination 2. SCIENCE REQUIREMENTS AND SKY BRIGHTNESS EXPECTATIONS The science requirement for stray light in the ATST dictate that scattered light must be controlled to less -6 than 2510 of the solar disk irradiance at R/Rsun = 1.1 and = 1 m. This is derived from a desire to keep instrumental scatter comfortably below scatter due to the earth’s atmosphere, presuming the best day at a good coronal site. While we have been unable to find direct sky-limited irradiance measurements at -5 1.1 Rsun, estimates abound. LaBonte reports median values for Haleakala of 1×10 , with some observations as low as one or two millionths2. This, however, corresponds to measurements in an annular -6 ring spanning a range between 1.6 and 4.4 Rsun at 0.53 m. Jacques Beckers uses a value of 6.3×10 at 3 -6 1.1 Rsun at 0.5 m in the Clear studies . David Elmore recently reported values of order 5×10 at 1.5 Rsun at 0.7 m (for Mauna Loa Solar Observatory) and estimated that we could expect values closer to 40×10-6 at 1.1 Rsun at 1 m. Many other observations of the sun obtained with telescopes, instruments, and sites not optimized for coronal observations show values very much higher than a part in 105. Analysis by Barducci et al. attempts to separate out instrumental scatter from that due to the atmosphere4. Their results for the Donate Solar Tower in Arcetri suggest values of 250 to 1000 millionths for the sky-brightness component at that admittedly non-coronal site. It is not surprising that there is considerable variation in these estimates because of the difficulty of the measurement and the wide variation in site-to-site and day-to-day sky brightness. In any event, the instrumental scatter predictions that follow should be viewed in the context of possible sky-brightness values at 1.1 Rsun in a range between a few millionths to few tens of millionths of the solar irradiance. 3. THE ASAP MODEL The analysis that follows was performed using version 7.1 of the Advanced Systems Analysis Program (ASAP) sold by the Breault Research Organization. Because the scatter models used in this analysis all have analytical representations, it is possible in principle to perform these calculations by just “doing the integrals” of the scatter functions over the appropriate solid angles. ASAP obtains its results by performing a Monte-Carlo style simulation involving a large number of geometrical rays. ASAP is fundamentally a non-sequential ray-tracing engine. The term “non-sequential” in this context means that rays proceed through even complex three-dimensional system models making no assumptions about the order in which objects will be encountered. This allows us