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A white-light externally occulted coronagraph is a telescope designed to view the very faint emission of the solar corona in the region close to the sun. It uses the Lyot principle to mitigate the effects of diffraction. The first such telescope flew on a spacecraft in 1971 and showed the dynamic nature of the corona. Since that mission, six other such coronagraphic telescopes have been flown, whose designs evolved to meet the requirements of the mission. This article describes the latest two coronagraphs and compares their capabilities and their designs.
© 2015 Optical Society of America
1. INTRODUCTION
The white-light corona is observed during a total eclipse of the sun, when the moon blocks the disk emission, thus revealing the very faint emission from the corona. This occurs only a few times a year, but it reveals a highly structured scene. The emission arises from photospheric light that is scattered by free electrons in the coronal plasma in the Thomson scattering process [1]. These electrons are guided by the solar magnetic field out into the corona and beyond as structures called streamers and plumes.
In 1930, Bernard Lyot developed an internally occulted coronagraph, analyzing the sources of stray light [2,3]. In this way he was able to capture the diffracted light generated at the edges of the aperture and occulter, enabling the routine observation of the corona from the ground. This was quite an advance, but it had the limitation of only being able to observe the inner corona out to about 1 solar radius from the limb. The idea for an externally occulted coronagraph was proposed [4] to be able to see the extended corona, by blocking direct sunlight from impinging on the objective lens.
The Naval Research Laboratory (NRL), after using a series of sounding rockets to develop a small externally occulted coronagraph [5], was selected to fly a small coronagraph [6] on the seventh of NASA’s Orbiting Solar Observatories, which was launched in September 1971 into a low-Earth orbit. In subsequent years, six missions have been flown that included the white-light coronagraph [7–12]. Three of these missions were developed by the NRL, and three by the High Altitude Observatory.
In this paper, we describe the last two missions, one for the ESA/NASA Solar and Heliospheric Observatory (SOHO) mission [11] launched in 1995 and the other for the NASA STEREO mission launched in 2006. These two missions had unique orbits into interplanetary space, having left the Earth’s environment, whereas all the others were in low-Earth orbits.
The ESA/NASA SOHO [13] was developed by the European Space Agency and launched by NASA on December 2, 1995. Routine observations began in March 1996 after the spacecraft was placed in a halo orbit about the L1 Lagrangian point, from Earth and thus 1% of the way to the Sun. A complement of 12 remote sensing and in situ instruments have been observing the Sun and the solar wind. The primary scientific objectives were to study the solar interior and the heating of the solar corona, but it quickly added the objective of studying the dynamical nature of the Sun. The mission is still in operation.
SOHO was an advance in studying the Sun, because the satellite was always in sunlight and thus the day/night cycle characterized by most low-Earth orbits was not present. The evolution of the corona and the sudden change frequently associated with coronal mass ejections (CMEs) could be clearly observed. The very stable thermal environment enabled excellent solar pointing performance. The low energetic particle environment reduced the damaging effects of radiation.
The NASA Solar Terrestrial Relationships Observatory (STEREO) [14] was developed and launched by NASA on October 26, 2006. Two virtually identical spacecraft were launched into orbits about the Sun, separating from Earth at a rate of about 22° of ecliptic longitude per year. One of the spacecraft (STEREO-A) is drifting ahead of Earth in its orbit about the Sun, and the other (STEREO-B) is drifting behind Earth. The primary objective of this mission was to take observations of coronal mass ejections from multiple viewpoints to determine the physics of their initiation, their structure, and their interplanetary effects.
2. WHITE-LIGHT CORONAGRAPH OPTICAL STRATEGY
The primary objective of a white-light coronagraph is to observe the faint emission of the solar corona while pointed at the solar disk. The concepts devised by Lyot [2,3] to capture light diffracted at various apertures, and extended by placing an apodized occulter in front of the objective lens [15,16] to block direct solar illumination, have been used very successfully.
The Large Angle and Spectrometric Coronagraph (LASCO) on the SOHO satellite is a suite of three telescopes [11] consisting of three visible-light coronagraphs with nested fields of view, called C1, C2, and C3, that can record coronal emissions over a range of 12 orders of magnitude from 1.1 to 30 solar radii . The Sun Earth Connection Coronal and Heliospheric Investigation (SECCHI) on the STEREO spacecraft is a suite of five telescopes, called EUVI, COR1, COR2, HI1, and HI2, [12] recording coronal emissions in the inner heliosphere from the solar disk to the orbit of Earth. A comparison of the primary characteristics of the LASCO/C3 and the SECCHI/COR2 coronagraphs is shown in Table 1. The layout of the SECCHI/COR2 coronagraph is shown in Fig. 1. An image from C3 is shown in Fig. 2.
Table 1. Comparison of LASCO/C3 and SECCHI/COR2 Properties
Fig. 1. SECCHI/COR2 optomechanical layout. The three optical lens assemblies are the O1 objective, the O2 field lens, and the O3 imaging lens. A description of the stray light rejection strategy is given in the text.
Fig. 2. White-light corona on 8 Dec. 1998. The white circle in the center indicates the size and position of the Sun behind the occulter. The occulter is at 3.75 , and the edge of the field is at 30 . The dark region in the lower left is the pylon holding the occulter. The radial structures are called streamers and are regions of enhanced electron density. The curved structure is an eruption of plasma and magnetic fields called a coronal mass ejection (CME).
Earlier coronagraphs only observed to either 6 or 10 (about or ). But we wanted to see the development of the CMEs into interplanetary space. The C3 field of view is 3.5–30 observing beyond the LASCO/C2 FOV of 2–6 [11]. The COR2 FOV is 2–15 [12].
The layouts of the COR2 telescope [12] and the C3 telescope [11] are very similar. Both systems use the classical design by Lyot modified for an external occulting of the entrance aperture, A1, as used on the Orbiting Solar Observatory #7 (OSO-7) coronagraph [6], and shown in Fig. 1. However, in these latter two cases we have placed the internal occulter in front of the field lens, at the conjugate point of the last disk in the external occuter assembly, in contrast to the placement of the internal occulter on the OSO-7 coronagraph [6]. Thus, the large diffracted signal is captured by the slightly larger internal occulter, greatly reducing the diffracted light sources at the external occulter. This reduces the scattering within the field lens. This concept, in which sources of stray light are focused onto a subsequent disk or aperture, very nearly eliminates that stray light source. This procedure is described below.
The initial disk of the triple-disk external occulter assembly (EODA) blocks the disk light, and the diffracted image is captured partly by the second disk, and again by the third disk, apodizing the diffraction pattern. The third disk is imaged by the objective lens, O1, onto a larger internal disk. The sunlight entering around the EODA is reflected back out of the telescope by the parabolic heat rejection mirror, which is focused midway at the front of the telesope opposite the pylon holding the EODA.
The first objective forms an image of the corona ahead of the internal occulter (IO). The field is defined by a field stop, A2. The coronal image is transferred to the A4 plane by the field lens, O2, which performs chromatic and geometric corrections. The O2 also forms a real image of the A1 aperture onto the A3 aperture, also called the Lyot stop. The A3 aperture is slightly smaller than the A1 aperture in order to capture the majority of the diffraction pattern from A1, and it determines the system entrance pupil. Ghost internal reflections within the O1 are focused by the O2 onto the Lyot spot, just after the A3. This occulter spot was attached to a glass plate at the proper position. Finally, the coronal image is transferred to the CCD detector by the relay or imaging lens, O3.
The glass type for the O1 objective for both C3 and COR2 was radiation-hard BK-7. The O2 and O3 lens types did not need to be radiation-hard, since damaging radiation energetic particles are absorbed by the first element and by the sidewalls. The rear half of the O1 doublet was also not radiation-hard. The choice of a radiation-hard lens for only the first objective and not for the interior lenses was effective, since the degradation of the entire C3 instrument was only about 0.35% per year derived from stellar transits [17]. The quality of the radiation-hard lenses is very good and does not increase the scattered light.
There are a few significant differences between LASCO/C3 and SECCHI/COR2. C3 had a 9 mm A1 aperture, whereas COR2 had a 34 mm A1. For C3 the O1 was a singlet lens, but for COR2 it was an air-spaced doublet. The doublet was chosen because it forms a better image of the diffracted light produced at the external occulter (EO) onto the IO, resulting in about 1/20 less spilling of the diffracted energy than with the singlet. For LASCO/C3 the Lyot spot was not necessary. Both of these changes were due to the inner limit of the field of view being at 2 for COR2, compared to 3.75 for C3.
The rejection of stray light is divided equally between the EO/IO and the Lyot principles applied to the optical system. That is, the EO/IO combination reduces the scattered light by about six orders of magnitude [18], and then the Lyot principles by another six orders [19], with a resultant stray light on the order of of the solar disk. These ratios can be adjusted by adjusting the amount of overocculting of the internal occulting disk in the EO/IO system or the A3 aperture in the A1/A3 system.
3. POLARIZATION ANALYSIS
To help distinguish between the scattering resulting from Thomson scattering from electrons (K-corona) and the scattering from interplanetary dust (F-corona), we have used the traditional technique [5] of measuring the degree (and angle) of linear polarization from three polarizer positions oriented 120° apart. The degree of polarization of Thomson scattering is described well by the theory. The dust scattering is unpolarized close to the Sun and increases with increasing height. The angle of both is perpendicular to the radial direction, and hence the departure from radiality can be used to give a measure of the accuracy of the measurement. The angular subtense of the Sun, as seen by the scatterer (the electron or dust particle), will result in the same variance in the scattered direction.
The LASCO/C3 and the SECCHI/COR2 used slightly different techniques to measure the polarization. The C3 used three linear polarizers of type HN-38 cemented onto a piece of plate glass. The three polarizers were oriented with the transmission direction at 0° and . A fourth position in the wheel contained just clear glass in order to obtain an unpolarized image. The wheel was located after the relay lens in the converging part of the beam.
In preparation for the STEREO mission, a study was conducted comparing a polarizer of type HN42E with a Polarcor. It was found that the Polarcor was more uniform and had a higher contrast ratio than the HN42E. Figure 3 shows relative histograms of the contrast ratios (the relative transmission when crossed to parallel) of the two types of polarizers.
Fig. 3. Comparison of contrast ratios for Polarcor and HN32E polarizers. The red curve (Polarcor) has its peak contrast ratio about lower than the black curve (HN32E).
The COR2 used a single Polarcor polarizer in a hollow-core motor. This was a lower-mass solution than the mechanism used in C3, but the polarizer is always in the beam. The Polarcor is quite uniform across the field. In operations the filter was rotated to the same 0° and positions to do the polarization analysis. To obtain an unpolarized image, images were taken at 0° and 90° and summed on board, and then the sum was sent to the ground.
Both techniques yielded similar results and have worked well in orbit. However, one of the C3 polarizers became damaged when SOHO was accidentally mispointed from Sun center by about 90° in the summer of 1998. That mispoint caused the instrument’s temperature to become extremely cold—it was estimated that the temperature was on the order of , well below the qualification temperature. One of the resultant failures was that one of the polarizers was damaged. The other two polarizers showed no damage. The change in operations was then to use the two good polarizers and the clear position to perform the polarization analysis.
4. DETECTOR AND CAMERA
Both instruments use a CCD [20] as the detector. The previous coronagraph missions used film [7], a secondary emission cathode vidicon [6,8,9], or a CCD [9]. Table 2 compares the characteristics of the LASCO/C3 and SECCHI/COR2. The CCD [20] is a solid-state sensor based on silicon, which is less massive and requires simpler control electronics than the vidicon. It is thus the more obvious choice for modern space instrumentation.
Table 2. Comparison of CCD/Camera Properties
The differences between the CCDs for C3 and COR2 reflect more the development of the technology in the past 10 years and the availability of devices with space heritage. For low-light-level applications, the larger pixel size of the LASCO CCD is better, but it was not available. The choice of a 14 bit analog-to-digital converter rather than a 16 bit converter was dictated by the power drawn by the 16 bit converters. Backside devices have a higher quantum efficiency (QE) and uniform subpixel response. The number of readout ports provides a degree of redundancy but isn’t a hard requirement. LASCO has used the same readout port for the entire 20-year mission after determining which readout port yielded the lowest noise. Similarly, COR2 has used the same readout port for its 10-year mission. The operating temperature was selected to mitigate the effects of radiation damage, particularly the degradation of charge-transfer efficiency. At this temperature the dark current generation is virtually nonexistent.
The use of CCDs in both C3 and COR2 significantly improved the dynamic range and sensitivity over the earlier film and vidicon instruments. The stability of the detectors over time is excellent. The instruments were fully calibrated on the ground prior to flight [11,21]. Determining the on-orbit degradation of the photometric sensitivity utilizes stellar transits of many stars and their inherent stability over the many years of operations. After the first 8 years of operations, C3 had lost about 0.35% per year [17], and recent unpublished results indicate that trend persists. The analysis of the COR2 degradation is currently under way, but it seems to be even less than C3.
5. DISCUSSION
Both LASCO and SECCHI have been extraordinarily successful. Observations from LASCO and SECCHI are immediately published on the Web and are used by the scientific community and citizen scientific community. For example, SOHO has discovered 3000 comets, making it the most prolific comet discoverer in history, and most of these discoveries are from the LASCO instrument. The observations of the very dynamic CMEs added a new dimension to the objectives of the SOHO mission, which was originally the study of the quiet Sun. It has firmly cemented the CME as the origin of the major geomagnetic storms at Earth.
Funding
National Aeronautics and Space Administration (NASA) (S-13631-Y, S-92835-D, NNG10PH19I); Office of Naval Research (SP033-02-43).
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