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Building the world's largest optical solar telescope

A 4m high-powered solar telescope will dissect the origins and mechanisms of the sun's magnetic fields.
26 December 2007, SPIE Newsroom. DOI: 10.1117/2.1200712.0963

Four centuries ago, Galileo observed changing spots that blemished the ‘perfect’ face of the sun. One century ago, George Ellery Hale discovered that the spots are caused by intense magnetic fields, starting modern exploration of the sun as an active, dynamic star. We now view the solar surface as a ‘magnetic carpet'1 that constantly reweaves itself and controls solar irradiance and space weather. Understanding its structure and continual interaction with the solar atmosphere requires a large-aperture solar telescope capable of resolving a few tens of kilometers on the solar surface. Such a telescope must also perform accurate and precise spectroscopic and polarimetric measurements of magnetic fields in all layers of the solar atmosphere, including the faint (10−6 times the disk brightness) corona.2

The 4m Advanced Technology Solar Telescope (ATST) (see Figure 1), with its integrated high-order adaptive optics (AO) system, will enable observations at a resolution of 0.022arcsec at 430nm, enabling precise measurements of solar magnetic fields at their fundamental scales. This complex, high-performance optical system,3 which will include wavefront control,4 integrated polarimetry,5 and powerful post-focus instrumentation,6 will be the solar polarimetric microscope that will unravel many of the remaining mysteries of solar magnetism. ATST, slated for first light in 2015, will be the largest and most capable solar telescope ever built.

Figure 1. Cutaway view of the ATST and its adjacent Support and Operations Building. Image credit: Mark Warner, NSO/AURA/NSF.

The ATST design contains a large number of subsystems that push the limits of state-of-the-art technology. The optical configuration of the main telescope is an off-axis Gregorian design. The 4.24m-diameter, 75mm-thick, off-axis parabola solid meniscus primary mirror (M1), although requiring a nontrivial amount of polishing and mirror support,7 avoids the central obscuration and spider diffraction of conventional on-axis telescopes. In addition, it gives the required stray-light performance critical for observing coronal magnetic fields close to the solar limb and out to 0.5 solar radii above the limb.

The Gregorian design rejects 97% of the 13kW heat load in prime focus by a cooled heat stop. Thermal control of subsequent optical elements is essential to avoid self-induced seeing, and the use of the heat stop makes this a manageable, although challenging, problem. The cooled stop limits the field of view to 5arcmin, significantly reducing scattered light produced by the following optical elements. It also serves as an occulting element that can block the solar disk light, enabling coronal observations. Scattered light will be further reduced by frequently removing dust from the primary by in situ washing, another unique feature of the ATST.

The off-axis design provides space near the prime focus for placement of the required cooling equipment, while leaving the aperture unobscured. This is a great advantage for solar AO, which is needed for locking on low-contrast extended objects, such as granulation. An all-reflecting design passes sunlight at wavelengths from 300nm (near UV) to 28μm (far IR). Examining the unexplored far-IR wavelengths is very likely to lead to new discoveries.

In addition to the main telescope, 11 mirrors are required to integrate AO and multiconjugate AO (MCAO)8 into the optical path that relays the image to equipment on the co-rotating Coudé platform (see Figure 2). Science instruments9–12 will be located on the 16m-diameter rotating platform, which is modeled after the Dunn Solar Telescope (DST) and serves as an image de-rotator. This allows tremendous flexibility in adding and updating equipment as technology evolves, and combining instruments for complementary simultaneous observations.

Figure 2. The optical system involves 13 mirrors between the sun and the science instruments, from the 4m M1 to the 415mm M13 steering flat. M7 and M8 will convert to deformable mirrors (DMs) in a future MCAO plan. B/S: Beam splitter. Image credit: NSO/AURA/NSF.

To ensure diffraction-limited imaging performance, ATST employs several wavefront correction systems. A quasi-static alignment (QSA) system uses several off-axis, extended field wavefront sensors (WFSs) in order to keep the entire optical path aligned in closed-loop operation.13 The QSA also provides WFS information for the active optics system that compensates for M1 deformation due to gravitational and thermal distortions. The high-order AO uses a correlating Shack Hartmann sensor with 1236 subapertures operating at 2kHz to make up for atmospheric blurring and deliver diffraction-limited images at high Strehl ratio.

Thermal control of the ATST is crucial. A highly ventilated enclosure design14 is sloped to minimize surface areas normal or near-normal to the sun. Active skin-cooling systems will minimize shell seeing. Besides thermally controlling the telescope mirrors close to ambient air temperatures, flushing of M1 and M2 with a steady flow of ambient air is a critical part of the strategy to control telescope seeing. Large, controllable vent gates will optimize the airflow across M1 while protecting the interior from direct exposure to sunlight.

Building the world's best solar telescope also meant selecting the world's best site. After an extensive survey,15 a site next to the existing Mees Solar Observatory at Haleakalā, Maui, Hawai'i, was selected for its excellent seeing and dark IR skies with little seasonal variation. ATST is a potential new start in the National Science Foundation Major Research Equipment and Facility Construction (MREFC) account as early as fiscal year 2009, and could become operational as early as 2015.

A hint of ATST's discovery potential was recently demonstrated by the addition of AO to the 76cm DST and the 98cm Swedish Solar Telescope (SST). Both have provided substantially sharper images and diffraction-limited spectro-polarimetry of solar activity. ATST's AO will have a fourfold improvement in linear resolution over the SST. The ability to study the sun deep into the relatively unexplored thermal IR spectrum, combined with the capacity to measure the elusive coronal magnetic fields, will advance ground-based solar astronomy by a leap as big as those of Galileo and Hale.

The US National Science Foundation (NSF) funds ATST through the National Solar Observatory (NSO), which operates under a cooperative agreement between the Association of Universities for Research in Astronomy (AURA) Inc. and NSF.

Thomas Rimmele, Stephen Keil, Dave Dooling 
National Solar Observatory (NSO)
Sunspot, NM

Thomas Rimmele is a full astronomer with tenure at the NSO and a research professor at the New Jersey Institute of Technology/Rutgers University. He received his diploma and PhD at the University of Freiburg, Germany. He is project scientist for the ATST, ATST adaptive optics team leader, and chief scientist for the DST.

Stephen Keil graduated in 1969 from the University of California, Berkeley, and received his PhD in 1975 from Boston University. He has served as the director of the NSO since 1999. Prior to joining the NSO, he led the Air Force's Solar Environmental Disturbances task force for 16 years.

Dave Dooling joined the NSO at Sac Peak in October 2002. While assigned primarily to the ATST, he provides educational and public outreach support for all branches of the NSO. He holds an MS in space studies from the University of North Dakota.