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Space Surveillance Telescope: Focus and Alignment of a Three Mirror Telescope Space Surveillance Telescope: focus and alignment of a three mirror telescope The MIT Faculty has made this article openly available. Please share how this access benefits you. Your story matters. Citation Woods, Deborah Freedman. “Space Surveillance Telescope: focus and alignment of a three mirror telescope.” Optical Engineering 52, no. 5 (May 1, 2013): 053604. As Published http://dx.doi.org/10.1117/1.oe.52.5.053604 Publisher SPIE Version Final published version Citable link http://hdl.handle.net/1721.1/80363 Terms of Use Article is made available in accordance with the publisher's policy and may be subject to US copyright law. Please refer to the publisher's site for terms of use. Space Surveillance Telescope: focus and alignment of a three mirror telescope Deborah Freedman Woods Ronak Y. Shah Julie A. Johnson Alexander Szabo Eric C. Pearce Richard L. Lambour Walter J. Faccenda Downloaded From: http://opticalengineering.spiedigitallibrary.org/ on 08/07/2013 Terms of Use: http://spiedl.org/terms Optical Engineering 52(5), 053604 (May 2013) Space Surveillance Telescope: focus and alignment of a three mirror telescope Deborah Freedman Woods Abstract. The Space Surveillance Telescope (SST) is a three-mirror Ronak Y. Shah Mersenne-Schmidt telescope with a 3.5 m primary mirror. It is designed MIT Lincoln Laboratory to rapidly scan for space objects, particularly along the geosynchronous 244 Wood Street belt, approximately 36,000 km above the Earth. The SST has an unusually Lexington, Massachusetts 02420 short focal ratio of F/1.0 and employs a camera composed of curved E-mail: [email protected] charge-coupled devices to match the telescope’s inherent field curvature. The field-of-view of the system is 6 square degrees. While the unique sys- Julie A. Johnson tem design is advantageous for space surveillance capabilities, it presents Alexander Szabo a challenge to alignment due to an inherently small depth of focus and the MIT Lincoln Laboratory additional degrees of freedom introduced with a powered tertiary mirror. Stallion Range Center The alignment procedure developed for the SST at zenith pointing is dis- Building 34560 cussed, as well as the maintenance of focus and alignment of the system Socorro, New Mexico 87801 across a range of elevation and temperature conditions. Quantitative per- formance metrics demonstrate the success of the system alignment during the telescope’s first year of operation. © The Authors. Published by SPIE under a Eric C. Pearce Creative Commons Attribution 3.0 Unported License. Distribution or reproduction of this work in Richard L. Lambour whole or in part requires full attribution of the original publication, including its DOI. [DOI: 10 Walter J. Faccenda .1117/1.OE.52.5.053604] MIT Lincoln Laboratory 244 Wood Street Subject terms: telescopes; alignment; wavefront sensors; astronomy. Lexington, Massachusetts 02420 Paper 130113 received Jan. 22, 2013; revised manuscript received Apr. 12, 2013; accepted for publication Apr. 12, 2013; published online May 7, 2013. 1 Introduction instrumentation. Instead, we align SST’s optics using only The Space Surveillance Telescope (SST), sponsored by the intra- and extra-focal images of stars. Defense Advanced Research Projects Agency (DARPA) is a The alignment process for two mirror telescopes is well documented and demonstrated in operation.1–5 The tech- 3.5 m diameter Mersenne-Schmidt telescope with a curved 1 focal surface. The camera mosaic of 12 charge-coupled devi- nique described by McLeod for the case of a Ritchie- ces (CCDs) employs curved substrates built to the shape of the Cretian telescope is to first remove coma by a translation or telescope’s focal surface tiled across a 6 square degree field- tilt of the secondary mirror about its vertex. The coma pro- of-view (FOV). The telescope has an extremely short focal duced by miscollimation does not vary across the FOV and ratio, F/1.0, whose compact layout ensures a highly agile so identifying and correcting for coma observed on-axis is sufficient for coma correction for the full field. However, the pointing system. Figure 1 shows a model of the optical sys- astigmatism produced by miscollimation varies as a function tem. Table 1 lists the system parameters, including details of of field position. Alignment of the two mirror telescope is the CCD mosaic. The combination of wide FOV and pointing accomplished by observing the effects of astigmatism at performance enables SST to rapidly scan the entire sky. Its multiple off-axis field positions and applying translation and location on an 8000-foot peak in the White Sands Missile tilt to the secondary mirror to remove the astigmatism due to Range (WSMR) south of Socorro, New Mexico, USA, pro- miscollimation. Astigmatism that is intrinsic to the optical vides a dry environment with excellent seeing conditions. ’ prescription will present a recognizable signature that can The SST s mission is to perform space surveillance and be separated from the astigmatism due to miscollimation.1 space situational awareness. That is, the telescope provides Theoretical expectations for alignment of three mirror the position with respect to the stars and the brightness of telescopes have been described in recent literature. satellites and space debris, particularly at the geosynchro- Thompson et al.6 discuss the optical aberrations generated nous region located at approximately 36,000 km from the by misalignment of a three mirror anastigmatic (TMA) tele- ’ Earth s surface. MIT Lincoln Laboratory developed the pro- scope, such as for the future James Webb Space Telescope. gram and L-3 Brashear built the telescope optics and mount. The misalignment-induced aberrations that will most signifi- The telescope achieved first light in February 2011 and is cantly affect the image quality are coma and astigmatism. currently transitioning from a commissioning to an opera- Thompson et al. note that while coma can be effectively tional phase. canceled by a combination of mirror tilt and decenter, the While the unique design of the SST is advantageous for resulting mirror positions will not generally remove astig- space surveillance, it creates a challenge to alignment as a matism due to misalignment. The astigmatism must be mea- result of the inherently small depth of focus and the addi- sured at multiple field positions to be fully characterized. In tional degrees of freedom introduced with a powered tertiary addition, they describe a field-asymmetric aberration that is mirror. Furthermore, the installation of the large CCD array intrinsic to the TMA optical design. Characterization of this at the prime focus of the telescope does not allow us to field-asymmetric aberration aids alignment. Plans for align- use a dedicated wavefront sensor or other special focusing ment of the three-mirror Large Synoptic Survey Telescope Optical Engineering 053604-1 May 2013/Vol. 52(5) Downloaded From: http://opticalengineering.spiedigitallibrary.org/ on 08/07/2013 Terms of Use: http://spiedl.org/terms Woods et al.: Space Surveillance Telescope: focus and alignment of a three mirror telescope employed at the SST to align the telescope at zenith pointing are similar in spirit to the techniques described for two and three mirror telescopes. We obtain a quantitative measure of the magnitude and direction of optical aberrations manifest in the system wavefront using defocused image techniques. The algorithms of the DONUT program, which we adapt for use with the SST, generate a measurement of optical aberra- tions from a single defocused star.9 Comparison of the DONUT measured aberrations with the sensitivity analysis generated by a Zemax model of the as-built optical system leads to predictions for the required adjustments to mirror alignment. We accomplish the focus and alignment of the SST using a five-step iterative process to find the optimal Fig. 1 Optical layout of the SST. Light enters the system from the left mirror positions at a given temperature for the zenith point- in this image. ing case. After achieving optimal focus and alignment at zenith pointing, we then develop an elevation dependent focus Table 1 Space Surveillance Telescope system parameters, includ- and alignment model to correct for gravity sag. The eleva- ing the optical system and the camera. tion dependent corrections are applied real-time to maintain focus and alignment across all elevation angles. We likewise develop a model to describe the temperature dependent focus Telescope corrections. Real-time adjustments to focus as a function of mount structure temperature ensure that the system remains Diameter (actual) 3.50 m in focus as the temperature environment varies over the Diameter (effective) 2.90 m course of the night. In this paper we describe the method that we developed Effective focal length 3.49 m for the focus and alignment of the SST and present quan- titative measures of the alignment performance. Section 2 Nominal field-of-view 3 × 2 deg contains an introduction to the measurement and representa- tion of wavefront aberrations; Sec. 3 describes the Zemax Camera sensitivity analysis; Sec. 4 provides details on the focus and alignment method to achieve best focus at zenith point- Sensors (thinned, backside illuminated) 2 K × 4 K CCD ing, including adjustments for different elevation angles in Sec. 4.1 and for temperature in Sec. 4.2. Section 5 provides Pixel size 15 μm 0.89 arcsec performance results and Sec. 6 discusses applicability to other systems. We conclude in Sec. 7. Mosaic 6 × 2 chips 2 Measurement and Representation of Effective size 12288 × 8192 pixels Wavefront Aberrations The telescope mirror alignment is accomplished through the Readout time 0.65 to 1.30 s measurement of optical aberrations present in the system. The shape of the wavefront produced by an optical system, Exposure time 0.025 to 10 s Wðρ; θÞ, carries with it information on the optical aberrations Filter Open (0.4 to 1.0 μm) of the system that generated it.
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