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Chapter 3. Outline of the LASCO Instrument
In 1930 B. Lyot (Ref. 1) invented the coronagraph, which views the solar corona outside of times of total solar eclipse. This instrument is esentially a telescope with an occulting disk in the focal plane to eclipse the image of the solar disk, and with other features to reduce stray sunlight to a level where the corona surrounding the occulting disk can be observed. Because of the residual stray sunlight within the instrument, the corona can be observed only to about 1.3 Rsun from the center of the Sun, and then most succesfully in the coronal emission lines. J.W. Evans (Ref. 2) modified the basic Lyot design by placing a circular disk at a substantial distance in front of the entrance aperture, to eclipse the Sun externally and to shade the entrance aperture from direct sunlight. This reduces the instrumental stray light by several orders of magnitude, to a level where the outer corona can be viewed by rocket or satellite borne coronagraphs at altitudes where skylight is absent.
R. Tousey (Ref 3) first flew an externally occulted coronagraph on a sounding rocket in 1963. Since then, several externally occulted coronagraphs, all based ultimately on the earlier work of Lyot and Evans, have been flown on satellites for long-term continuous coronal observations: OSO-7 (1971-72) (Ref 4), Skylab (1973-74) (Ref 5), P78-1 (1979-1985) (Ref. 4, 6), and the Solar Maximum Mission (1980-1989) (Ref. 7). These coronagraphs brought improved spatial resolution, time resolution, and mission duration, but were limited to observing the corona over a limited range of elongation.
3.1 Optical configuration
A wide field-of-view for LASCO was chosen because of the vast range of distance over which coronal activity influences the solar wind. Within this domain, the K corona brightness varies by about eight orders of magnitude, and relevent spatial scales vary from features the size of photospheric granules (1-2 arc sec), to coronal holes, streamers, and explosive plasma ejections that frequently exceed the size of the Sun itself. To cover these extended ranges of brightness and spatial scale, the LASCO field- of-view is divided into three concentric annular rings, covered by three independent optical systems, each optimized for its observing range, and miniaturized to fit into a single instrument package of reasonable size.
The two outer telescopes, C2 and C3, are both externally occulted. This design, however, has a basic limitation. For the necessary distance from the occulting disk to the objective lens, such an instrument can only provide images of the corona from a distance well beyond the Sun's limb (&>1.5 Rsun). The spatial resolution at the inner edge is poor, because most of the objective lens is shadowed by the external occulter, which results in very small effective apertures at the inner edge of the field-of-view. In addition, because of practical size limitations on the instrument length, the objective lens aperture usually cannot exceed a few centimeters.
The LASCO design overcomes these problems by covering the inner corona, from 1.1 Rsun to 3 Rsun, by the C1 mirror version of the classic Lyot coronagraph, without an external occulter, thus preserving the full resolution of the instrument over its whole field-of-view. The C2 telescope images the corona from 2 Rsun to 6 Rsun, overlaping the outer field-of-view of C1 from 2 Rsun to 3 Rsun. The C3 telescope extends the field-of-view to 32 Rsun, from a previous maximum of around 10 Rsun with the SOLWIND coronagraph. The overlapping is essential for intercalibration of the three telescopes, including cross calibration on orbit, and to assist in later reconstruction of composite wide-field images. Table 3-1 summarizes the design parameters of the three coronagraphs.
LASCO will be the first space borne coronagraph with spectrometric capabilities. The C1 telescope is equipped with a Fabry-Perot interferometer that can take monochromatic images over the whole field-of-view with a spectral resolution of 0.07 nm. By stepping the bandpass across a spectral feature, line profiles can be recorded by a 1024x1024 pixel CCD camera.
C1 is internally occulted to obtain coronal images with full instrumental resolution over the entire field-of-view, as close to the solar limb as possible. The resolution of C1 is determined by the detector pixel size, not the theoretical optical resolution of the 4 cm objective mirror, approximately 3 arc sec. The 1024x1024 pixel CCD and 3 Rsun field-of-view give a pixel scale of 5.6 arc seconds. The spatial resolution, taken as two pixels, is approximately 11 arc sec.
Over most of their coverage, the spatial resolutions (taken as 2 pixels) of C2 (23 arc sec) and C3 (112 arc sec) are also set by the detector pixel scale, except toward the inner part of their ranges. As discussed, the external occultors cause vignetting at the inner edges of the fields-of-view, and the spatial resolution is degraded.
3.2 Stray Light Test Results
Independent tests of an initial configuration of C1 were carried out at the Max-Planck-Institut fur Aeronomie in Lindau and at the Institute d'Astrophysique in Paris. Different test methods produced very similar stray light levels: 10E-6 Bsun at 1.1 Rsun, and 10E-7 Bsun at 3 Rsun, where Bsun is the disk average solar brightness. It was demonstrated that only the scattered light from the first mirror contributes to the total instrument stray light at these levels. C1 was then equipped with improved versions of the earlier mirrors. The stray light measurements for C1 shown in Figure 3-1 are from system level tests performed at NRL and at MPAe, and show an improvement by a factor of two over the results for the earlier mirrors.
Stray light levels for C2 shown in Figure 3-1 are based on component measurements, not on system tests. This version of C2 had an inner field limit of 1.5 Rsun, which has been increased to 2.0 Rsun in the flight version. Stray light levels for C3 are based on system tests at NRL, and are an order of magnitude better than the levels suggested in the original proposal. The large field-of-view of C3 (16 degrees total) makes these measurements extremely difficult. When comparing the C2 or C3 stray light levels with the average equatorial coronal brightness at any radial distance, it must be remembered that both telescopes are increasingly vignetted toward their inner field limits, and so their CCD detectors would not see the coronal brightness level of the equatorial curve, but the vignetted brightness level.
3.3 Expected Images From the Coronagraphs
A green line coronal image was obtained with a prototype of C1 at the Sacramento Peak Observatory. Figure 3-2 shows the Fe XIV image with a nearby continuum image subtracted. The sky background, which dominated the instrumental stray light, was 40x10E-6 Bsun. At the inner edge of the field-of- view, the instrumental stray light in orbit will be less than 1 x 10E-6 Bsun, an improvement of a factor of 40 over the ground-based image, and images from orbit should be a vast improvement over the ground-based images. In Figure 3-3, an eclipse image by Koutchmy has been adapted to the C1 resolution of 11 arc seconds, to demonstrate the expected C1 image quality from orbit.
Similarly, Figure 3-4, an eclipse photo by Keller, shows an image of the corona as it is expected from C2. Note that the full resolution will be achieved only between 3 Rsun and 6 Rsun; resolution inside the 3 Rsun circle is reduced by vignetting. Since the preparation of this figure, the inner radius of the C2 field-of-view has been increased to 2 Rsun.
The corona beyond 10 Rsun has never been seen, and the images from C3 will give the first views of this region.
The individual, and overlapping, fields-of-view of the three LASCO telescopes are shown in Figure 3- 5 (low resolution) or Figure 3- 5 (high resolution) . The square blocks within which the fields of C1, C2, and C3 are shown give an approximate indication of the spatial coverage of the 1024x1024 CCD detector for each coronagraph.
3.4 Mechanical Design
The instrument mechanical design is conceptually similar to the solar instruments at many ground-based observatories. A "spar," or rigid structure (in the case of LASCO, the box containing the optics) provides mechanical support, alignment, and thermal control, and is serviced by a microprocessor-controlled electronics system. Within the spar structure is the cluster of three compact optical systems, each specially designed and optimized for its particular range of operation, and all held in precise co-alignment through proper mechanical and thermal design.
LASCO consists of two boxes. The first is the coronagraph optical box (COB). It contains the three optical systems and cameras, and provides alignment, mechanical support, and enclosure against contamination and unwanted stray light. It is mounted to the spacecraft instrument pylon with isostatic mounting legs. The rear set of these legs is equipped with motor drives, controllable in open-loop fashion by ground command, for the purpose of removing any launch-induced misalignments of the LASCO optical axis and the spacecraft pointing direction. The LASCO optical axis is taken to be the C2 optical axis.
The second box is the LASCO electronics box (LEB). It contains microprocessors for instrument control and image processing, memory, power conditioning circuitry, and the command and telemetry interface serving C1, C2, and C3, and the Extreme_ultraviolet Imaging Telescope (EIT).
The LEB supports three camera/data channels for the three LASCO telescopes, one for the EIT, and a command/data channel for the Fabry-Perot. The camera and Fabry-Perot electronics contain all of the functions required to operate the CCD and Fabry-Perot as a "smart" peripheral, with only high level commands from the LEB being required. Clock-driver, preamplifier, and conditioning circuits for the CCDs are part of the camera units, and are contained on the (plug-in) camera modules. Mechanism motor drivers are also identical in all four channels, with the only difference being the exact number of motors in each. Similarly, monitors are of similar type in each channel. A block-redundant CPU/power converter/memory subsystem approach is employed. Since either processor subsystem can receive the image data from any of the four cameras, considerable flexibility and redundancy exists. Motor control and status information are passed along one interface to the COB, and the camera control and video output are passed along a different interface. Thus, an operational restriction is that only one motor can be driven at the same time, and only one camera can be read out or commanded at the same time. Since in a typical five minute interval, a camera will be read out for about 30 seconds and mechanisms driven for about 30 seconds, neither restriction presents any difficulty to the LASCO/EIT operation.
3.6 Thermal Design
The thermal environments of both the COB and LEB are individually controlled. Design of the isostatic mounting legs for the COB, and the attachment of the LEB, includes thermal isolation. Thermal balance and stability are controlled by a multi-layer thermal blanket, Sun shields, radiators, and (in the case of the COB) small balance heaters. A critical item in the thermal design is the inclusion of a passive radiant cooler for each CCD detector chip. In order to minimize thermal dark current noise and to reduce the effects of radiation damage on the CCDs, the chips operate at about -80°C. Each camera has a suitable radiator structure as part of its design. The three are similar in concept, but the details of the shielding baffles depend on the environment and view angle for each camera.
1 Lyot, B. 1930, La couronne solair etudiee en dehors des eclipses, C.R. Acad. Sci. Paris 191, 834.
2 Evans, J. W. 1948, A photometer for measurement of sky brightness near the Sun, J.O.S.A. 38, 1083.
3 Tousey, R. 1965, Observations of the white-light corona by rocket, Ann. Astrophys. 28, 600.
4 Koomen, M. J., Detwiler, C. R., Brueckner, G. E., Cooper, H. W., and Tousey, R. 1975, White light coronagraph in OSO-7, Appl. Opt. 14, 743.
5 MacQueen, R. M., Gosling, J. T., Hildner, E., Munro, R. H., Poland, A. I., and Ross, C. L. 1974, The High Altitude Observatory white light coronagraph, Soc. Photo-Opt. Instrum. Eng. 44, 207.
6 Sheeley, N. R., Jr., Michels, D. J., Howard, R. A., and Koomen, M. J. 1980, Initial observations with the Solwind coronagraph, Ap. J. 237, L99.
7 MacQueen, R. M., Csoeke-Poeckh, A., Hildner, E., House, L., Reynolds, R., Stanger, A., Tepoel, H., and Wagner, W. 1980, The High altitude Observatory coronagraph polarimeter on the Solar Maximum Mission, Solar Phys., 65, 91.