Minimize SST

SST (Space Surveillance Telescope) — providing ground-based SSA (Space Situational Awareness) Services

While space is increasingly crucial to modern society, it is also increasingly hazardous for satellites. Space is congested with tens of thousands of man-made objects as well as micro-meteors, asteroids and other natural satellites. Space is contested by a range of man-made threats that may have adverse effects on satellites. 1)

Current deep space telescopes do not provide a comprehensive picture of all objects in orbit around the Earth. Existing search telescopes have relatively narrow fields of view and cannot reliably detect and track faint objects, including small objects in geosynchronous orbits (roughly 36,000 km high). There may be as many as hundreds of thousands of additional pieces of debris and asteroids that are too faint to track with current sensors.

The SST (Space Surveillance Telescope) program of DARPA (Defense Advanced Research Projects Agency) aims to enable ground-based, broad-area search, detection, and tracking of faint objects in deep space for purposes such as space mission assurance and asteroid detection. SST offers improvements in determining the orbits of newly discovered objects and provides rapid observations of events that may only occur over a relatively short period of time, like a supernova. SST's innovative design allows for a short focal length, wide field of view, and a compact optical train. The SST mirrors are some of the steepest aspherical curvatures ever to be polished and allow the telescope to have the fastest optics of its aperture class. These features combine to provide orders of magnitude improvements in deep space surveillance.

The telescope achieved first light in February 2011. In 2013, the U.S. Secretary of Defense and Australian Minister of Defence signed an MOU agreeing to relocate the Space Surveillance Telescope from the White Sands Missile Range in New Mexico to Harold E. Holt Naval Communication Station in Western Australia. Australia offers a uniquely beneficial vantage point for operational testing and demonstration of SST's enhanced algorithms and camera. After the move, SST will be owned by the United States Air Force, but operated and maintained by Australia. It will be a dedicated sensor in the U.S. SSN (Space Surveillance Network).

Currently, the SST program is developing enhanced small-object detection algorithms, a new, advanced wide-field camera, and faster search operations in preparation for relocation to Harold E. Holt Naval Communication Station. SST is also leading the community in developing and applying tactical sensor tip-and-cue techniques, as well as applying the vast data archive to refine orbital debris density models. SST has also been prolific at observing asteroids, contributing to the discovery of more than 1,300 new asteroids, and providing more than 5,500,000 asteroid observations to the international community via the IAU (International Astronomical Union) Minor Planet Center. The telescope is also supporting the astronomical community by collecting data on behalf of the Large Synoptic Space Telescope consortium and NASA Orbital Debris Program Office.

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Figure 1: Left: Photo of the SST structure including the wide field camera; Right: Photo of the SST enclosure at White Sands Missile Range, New Mexico (image credit: DARPA)

SST on its way to Australia:

• Oct. 18, 2016: At a mountaintop event in New Mexico today, DARPA is marking the formal transition of its SST ( Space Surveillance Telescope) from an Agency-led design and construction program to ownership and operation by U.S. AFSPC (Air Force Space Command), which has announced plans to operate the telescope in Australia jointly with the Australian government. Taking place at SST's high-altitude perch at White Sands Missile Range, the event will include remarks by DARPA Deputy Director Steven Walker and senior AFSPC and Royal Australian Air Force leadership. 2)

"We're proud to celebrate SST's groundbreaking technological accomplishments and, more importantly, the collaboration between DARPA and the Air Force that has made them possible," said Lindsay Millard, DARPA program manager for SST. "By enabling much faster discovery and tracking of previously unseen or hard-to-find small space objects, this optical telescope is poised to revolutionize space situational awareness and help prevent potential collisions with satellites or the Earth itself."

In addition to Millard and Walker, scheduled speakers at today's ceremony include Maj. Gen. Nina Armagno, Director of Strategic Plans, Programs, Requirements and Analysis, Headquarters Air Force Space Command; Air Commodore Sally Pearson, Director of General Surveillance and Control, Royal Australian Air Force; and Eric Evans, director of MIT Lincoln Laboratory, the FFRDC (Federally Funded Research and Development Center) leading the team that built SST.

SST has moved space situational awareness from seeing only a few large objects at a time through the equivalent of a drinking straw, to a "windshield" view with 10,000 objects at a time, each as small as a softball. SST can search an area larger than the continental United States in seconds and survey the entire geosynchronous belt within its field of view—one quarter of the sky—multiple times in one night.

But of broader technical significance is that SST has developed many technological firsts that are helping redefine what telescopes can do. For example, SST uses the most steeply curved primary telescope mirror ever made. This mirror enables SST to collect more light to see images across a wider field of view than any other space surveillance telescope. To hold this mirror, SST uses an innovative Mersenne-Schmidt design, which enables much more compact construction than traditional telescopes.

In fact, SST is the largest telescope ever to use this design, which has also made it the quickest and most nimble large telescope in the world. SST's camera includes its own noteworthy inventions. SST developed the first-ever curved CCD (Charge Coupled Device), to provide clear imagery across its wide field of view because current digital cameras with flat CCDs are unable to record images from such highly curved mirrors without distortion. The camera has the fastest telescope camera shutter in the world and is able to take thousands of pictures a night. In recent months, and in preparation for SST's move to Australia, DARPA has upgraded its camera, faint-object detection algorithms, and search speed to make it even more effective.

SST's wide-open eye on the sky sees objects not just around the Earth but also in the solar system and universe beyond. NASA is already leveraging SST's capabilities to see very faint objects in a wide field of view to help provide warning of asteroids and other near-Earth objects. With 2.2 million asteroid observations in 2014, 7.2 million in 2015, and hopes for 10 million in 2016, SST has become the most prolific tool for asteroid observation in the world. SST has also discovered 3,600 new asteroids and 69 near-Earth objects, including four that carry a risk of possibly hitting the Earth.

"With its amazing capabilities, SST joins a prestigious list stretching back decades of game-changing SSA (Space Situational Awareness) programs on which DARPA and AFSPC have collaborated," said Brad Tousley, director of DARPA's Tactical Technology Office, which oversees SST. "DARPA looks forward to seeing what the Air Force will do with SST, and we will continue to work with them as DARPA pushes the technological envelope on space situational awareness with our Hallmark and Orbit-Outlook programs."

Australia offers a uniquely beneficial vantage point for operational testing and demonstration of SST's capabilities. From its new home, SST will provide key space situational awareness information from the southern hemisphere—an area of the geosynchronous belt that is currently sparsely observed—to the U.S. SSN (Space Surveillance Network) that AFSPC operates. SST will also continue to provide NASA and the scientific community with surveillance data on transient events such as supernovas, as well as potentially hazardous near-Earth asteroids.

"DARPA has worked closely with the Air Force since the Agency's creation to boldly invest in high-impact technologies, so the United States can be the first to develop and adopt the novel capabilities made possible by such work," Walker said. "In Air Force Space Command and the Royal Australian Air Force, we at DARPA could not ask for more qualified and enthusiastic partners and we know SST is going to be in good hands."

 


 

Some background of the SST development:

The SST (Space Surveillance Telescope) is a DARPA sponsored technology developed by MIT/ LL (Lincoln Laboratory) to significantly enhance the nation's capabilities in space situational awareness. The SST's mission is to provide timely observations of satellites and space debris, particularly at the geosynchronous region located at approximately 36,000 km from the Earth's surface. The proliferation of microsatellites and space debris underscores the importance of building and maintaining accurate and complete cataloging of space objects. Current knowledge of the space environment is key to safeguarding vital space resources. 3)

It is important to be able to search for and track small dim objects in space, such as satellite debris and microsatellites, which are in close proximity to other satellites and may endanger their continued functionality. It is particularly challenging to observe microsatellites in close proximity to satellites in geosynchronous orbits (at a distance of 36,000 km). At this distance, microsatellites may have brightness of less than 18th visual magnitude and are thus hard to detect by most telescopes capable of high search rates. 4)

The Space Surveillance Telescope (SST) was built specifically to address this challenging task, combining high search rate and high resolution, with a 3.5 m Mersenne-Schmidt design. Its f/1.0 optics offers a wide field of view (WFOV) for a large telescope (6 deg2) The large aperture, low f/#, and compact design of the mount allows SST to sweep the sky at very high rates, with low step and settle times, and monitor activity in the GEO-belt closely. The SST also performs astrometry on the stars, updating and refining the star catalog, in cooperation with the Naval Observatory.

One of the characteristics that enables this performance is the curved focal surface of the CCD (Charge Coupled Device) detectors. Most optical systems do not have a true focal plane, rather a curved focal surface. One traditional way to correct for this curvature is to limit the field of view, which is contrary to the primary objective of this telescope. Other methods to accommodate the natural curved focal surface involve additional optical elements, which complicate and compromise the optical performance. For the SST system, the project at MIT/LL has curved the thin-membrane CCD imaging devices to conform to a convex sphere of 5.44 m radius. This novel curved imaging surface allows SST to have only three reflective and one refractive element (the refractive element also functions as a window into the vacuum dewar containing the CCD array) and to achieve a point spread function only about one arcsec wide over its WFOV (Wide Field of View).

To curve the CCDs, large format detectors (2k x 4k pixels, each 15 µm square) are fabricated on silicon (Si) wafers and then subjected to a back-illumination (BI) process. The BI CCD detector, however, rests on a full-thickness carrier wafer, which needs to be thinned to provide the flexibility necessary for curving. After thinning, the total thickness of the imager is about 200 µm, which is thin enough to deform to a 5.44 m radius without fracturing or damaging the CCD. The curving of the device occurs while the flexible detector is pulled into a curved concave vacuum chuck and a convex mandrel is attached to the non-illuminated side with adhesive. This mounting technique is sufficient to meet the specification of <5 µm deviation root mean square (RMS) from the ideal spherical surface for a single curved CCD. After curving, 12 of the CCDs are aligned in the array, approximately 120 mm x 180 mm; the RMS deviation of the array from the ideal surface is 6 µm. The array is shown in Figure 2; the curvature is revealed by the apparent curving of the reflected fluorescent tube.

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Figure 2: Twelve 2k x 4k CCDs mounted to a 5.44 m spherical surface. The curved reflection of the fluorescent bulb demonstrates that the focal surface is curved (image credit: MIT/LL)

After extensive testing, the array was installed in an observatory constructed for this purpose on White Sands Missile Range in New Mexico (Figure 3) and operated continuously since first light in February 2011. The telescope and mount are shown in Figure 4.

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Figure 3: Picture of SST facility on Atom Peak of White Sands Missile Range, New Mexico (image credit: MIT/LL)

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Figure 4: Two images of the SST. The telescope is a Mersenne-Schmidt system with powered primary, secondary, and tertiary mirrors. The compact layout achieved by the f/1.0 optics facilitates the mount gimbal's pointing performance (image credit: MIT/LL)

One unique aspect of the SST system design is that there are no dedicated focus actuators. The hydraulic support system does the focusing and also provides solid body motion of the mirror. The support systems of the secondary and tertiary mirrors provide sufficient range of motion to maintain focus over all orientations and temperatures. Each mirror has nominally four degrees of freedom: translation along the optical axis ("piston"), rotation about the elevation axis ("tip"), rotation along the elevation axis ("tilt") and translation in the direction perpendicular to the elevation axis ("decenter"). The mirror motion is achieved using a large number of hydraulic powered actuators. In principle, we move the tertiary to achieve most of the aberration reduction. The secondary is used next, and mostly in piston to alleviate spherical aberration. The primary mirror serves as a reference surface and is assumed to be fixed. The focal surface of the camera is fixed with respect to the tertiary mirror cell.

Focus and alignment of the SST is accomplished without a dedicated wavefront sensor or other permanent focusing instrumentation. Given the volumetric constraints of the narrow depth of focus and the need to have large amounts of the field of regard available for the primary mission, an alternative, image-based method was desired. We carry out system alignment based on estimation of the optical aberrations present in the system using defocused imagery. The "Donut" program provides the algorithms for measurement of optical aberrations from a single defocused star, which we adapt for use on the SST. 5) We then consult our Zemax model of the as-built optical system to determine appropriate mirror position shifts that compensate or remove the measured aberrations.

In summary, the project achieved focus and alignment of the three-mirror Mersenne-Schmidt type SST, a unique telescope system that has no predecessor or precedent for focusing methodology. Focus and alignment relied solely on measurements of optical aberrations observed in the imagery itself on a system that does not include a dedicated wavefront sensor.

The SST maintains a consistent spot size performance over a range of temperature and elevation conditions during a night's observing. The temperature model predicts the best focus position to within 25 µm (tertiary mirror location along the optical axis) when the system is in thermal equilibrium and is used to keep the telescope in focus as the ambient temperature varies over the course of a night. The elevation model successfully maintains in real-time the focus and alignment from zenith to horizon through the application of tertiary mirror adjustments.

The focus and alignment program for the SST has successfully brought the spot size down to 28 µm rms diameter during optimal seeing conditions. The minimum spot size compares favorably with the system performance prediction of 26 µm based on the Zemax model for the as-built optical system.

 


1) Lindsay Millard, "Space Surveillance Telescope (SST)," URL: http://www.darpa.mil/program
/space-surveillance-telescope

2) "DARPA to Transfer Space Surveillance Telescope to U.S. Air Force — Senior U.S. Air Force and Australian leadership to join the Agency in celebrating a program that is redefining what telescopes can do and could revolutionize space situational awareness," DARPA, Oct. 18, 2016, URL: http://www.darpa.mil/news-events/2016-10-18

3) Deborah F. Woods, Ronak Shah, Julie Johnson, Alexander Szabo, Eric C. Pearce, Richard Lambour, Walter Faccenda, "The Space Surveillance Telescope: Focus and Alignment of the Tree Mirror Telescope," Advanced Maui Optical and Space Surveillance Technologies (AMOS) Conference, Maui, HI, USA, Sept., 2012, URL: http://www.amostech.com/Technical
Papers/2012/NAEOSSAS/WOODS.pdf

4) "Space Surveillance Telescope," MIT/LL (Massachusetts Institute of Technology/Lincoln Laboratory), URL: https://www.ll.mit.edu/mission/electronics/ait/scientific-dod-imagers/space-surveillance-telescope.html

5) A. Tokovinin, S. Heathcote, "Donut: Measuring Optical Aberrations from a Single Extrafocal Image," PASP (Publications of the Astronomical Society of the Pacific), Vol. 118, pp: 1165–1175, August 2006 , URL: http://iopscience.iop.org/
article/10.1086/506972/pdf

 


The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (herb.kramer@gmx.net).