Chapter 3: EUV/FUV High-Throughput Spectroscopic Telescope
3.1. Performance requirements for the spectrometer
The Solar-C mission provides an ideal platform for a EUV/FUV high-throughput highresolution spectrometer (may be called EUVS hereafter). Performance requirements are
summarized below for a breakthrough spectroscopic instrument, which can be realized withrecent technology advancements.
1. To operate in both spectroscopic and imaging modes with high temporal resolution(cadence). This is required for quantitatively understanding the physics of dynamical
phenomena. One order of improvement in effective area is required. Thus, thetemporal resolution at a slit position can be shortened down to 0.5 sec for active region
plasma and 1 sec for quiet Sun plasma.
2. To perform EUV/FUV spectroscopy with high spatial resolution. The spectrometer isrequired to resolve structures equal to 0.5” resolution or better. The target is 0.3 arcsec,
which is roughly equivalent to the width of chromospheric spicules observed with
Hinode/SOT.
3. To observe a field of view as large as possible. The best resolution may be restricted
to ~5x5 arcmin2 (a medium size active region), but lower resolution should cover alarger field of view.
4. To perform EUV/FUV spectroscopy with high spectral resolution. The spectrometer isrequired to determine Doppler velocities to an accuracy of ~ 2 km/s from spectral line
shifts and non-thermal motions as small as 20 km/s from line widths.
5. To make measurements with spectral lines from a broad temperature range and withadequate plasma diagnostics covering chromospheric, transition region, low coronaand flare temperatures. The observations must cover all relevant temperatures andstructures in order to track the flow of energy from the chromosphere into the corona.
6. To perform good observations with low-scattering optics. This performance is needed
to explore “faint” reconnection outflow regions, coronal holes, and off-limb
observations.
7. To ensure precise co-alignment with the data from other onboard telescopes. Coalignment must meet the high spatial resolution of the spectrometer.
The performance requirements summarized above are determined from the science goals in
section 1. Table 3.1 summaries instrument performance required for the Solar-C spectrometer,
compared with the previous spectrometers. It is obvious that the Solar-C spectrometer will
have significant improvements on the temporal resolution, spatial resolution, and temperature
coverage. Some of the performance required for the Solar-C spectrometer are discussed below
to show possible discovery spaces and new sciences.

3-1

Table 3.1. Performance of Solar-C spectrometer in comparison with other spectrometers
Temporal resolution fora slit position *1
Spatial resolution

Solar-C spectrometer
0.5 - 10 sec for AR
1 -10 sec for QS~0.3 arcsec (target)~0.16 arcsec (pixel size)
4.1 – 7.2

Hinode/EIS
10 - 20 sec for AR
30 – 60 sec for QS
3-4 arcsec
1arcsec (pixel size)
4.9, 5.6-7.2

SoHO/SUMER
10 – 20 sec for AR line*2
2 arcsec
1arcsec (pixel size)
4.1-5.8

Primary temperature
coverage (log10 K)*1 The values given here are typical exposure duration for multiple numbers of spectral lines covering a wide
temperature range.
*2 SUMER can record spectral lines in only one of spectral windows at the same time. If spectral lines inanother spectral window are measured, the grating is needed to move before the exposure.

3.1.1. Spatial resolution requirement
One of key tasks of EUVS is to precisely measure plasma conditions, such as temperature,density, and velocity of elementary structures of the magnetized atmosphere in all
temperature regions from the chromosphere through transition region and into the corona.
Such measurements made simultaneously with different spectral lines allow us to trace theflow of energy from the chromosphere up into the corona. In the photosphere the energy isobserved to be in plasma structures with a scale length of about 0.3″ or about 200 km. Movies
made in the Ca II H-line from Hinode SOT clearly reveal that the chromosphere is dominatedby a multitude of thin (~200 km), dynamical, jet-like extrusions called spicules. Since SOT
can carry out only imaging observations for the chromosphere without spectroscopic
capabilities, the proposed spectrometer will be the first instrument to diagnose the plasma
properties of these thin, dynamical structures and to follow these structures upwards throughthe atmosphere into the corona. As the magnetic field may expand with height, the overall
size of structures may increase as they rise above the chromosphere/transition region, but thewidth of spicules may be preserved into the transition region and corona. Coronal loops are
well-known coronal magnetic structures, but elementary structures are not yet fully resolvedin the corona with the currently existing imaging telescopes (0.5
″ at the highest resolution).
Spectroscopic measurements in the EUV have suggested that only 10% of the volume ofcoronal loops is filled with hot plasma, when they are observed with the spatial resolution of
2-3 arcsec (pixel size 1 arcsec) (Warren et al. 2008). Thus, measuring the plasma parameterswith 0.3 arcsec, i.e., 10% of the 1 arcsec volume, can fully resolve elementary structures
hidden in the magnetic loops. Understanding the magnetic sub-structure implied by the small
filling factor is crucial for determining energy flow in coronal loops and ultimately forunderstanding the physics of coronal heating.
In addition to its own scientific contribution, the spectrometer observations shall haveclose scientific synergy with the other proposed telescopes. One of the important science
goals is to understand the roles of waves propagating through the atmosphere in heating andactivating dynamical motions in the plasma. It is important to determine whether or not
sufficient energy remains in the waves that penetrate all the way to the corona to make a
substantial contribution to the heating budget of the solar corona and/or to the acceleration of
the solar wind. It is also critical to identify propagating MHD waves and their modes. For thispurpose, observations of magnetic field fluctuations are required in addition to observationsof intensity and velocity fluctuations. The proposed spectrometer can provide the observationof intensity and velocity fluctuations. Magnetic field measurements are available only for the
low atmosphere, i.e., in the photosphere and chromosphere, and they will be provided by the
3-2

UV-Visible-IR telescope with a spatial resolution of″ 0.2
or better. The UV-Visible-IR
telescope is designed to determine the modes and amplitudes of waves in the low atmospherejust after the excitation of the waves, and the proposed spectrometer can continue to trace
their propagation into the transition region and corona. With these coordinated observations,
we will explore for the first time the mode conversion in waves, which is expected in theatmospheric layer where the plasma beta is unity. Since the plasma beta changes from high(β>1) in photosphere and low chromosphere to low (β << 1) in the corona, we can observe
how the waves are evolved as a function of height in the stratified atmosphere. With having
high spatial resolution never achieved so far, we can newly explore behaviors of waves andturbulence in small scale, which are useful information for physically understanding roles ofwaves in the atmosphere. We may have new information concerning turbulent cascade andphase mixing of Alfvenic waves.
Considering these new discovery spaces, the proposed spectrometer should have thecapability of resolving structures with scales smaller than 350 km (0.5″). Ideally, it would bebetter to resolve even smaller structures, say, 0.3″ or ~200 km.
3.1.2. Temporal resolution requirement
Since the structures to be resolved with EUVS are small and dynamic, a high time cadenceis necessary and consequently a high-throughput instrument is required. As a goal of the
instrument design, the effective area of the spectrometer is set to be at least 10 times larger
than previous instruments (EIS onboard Hinode, SUMER and CDS onboard SoHO). Thismeans an improvement of at least an order of magnitude in temporal resolution if observing
with the same sampling scale (~1″ arcsec). There is also an improvement even whenobserving with higher spatial resolution (0.5″ arcsec) than achieved with previous instruments.
In the case of EIS observations, typical integration times of an exposure are 10 to 20 s for
active region emission lines and 30-60 s for the quiet Sun (Note that shorter exposures are
possible in very limited numbers of brighter lines). SUMER observations are carried out with10 to 20 s integration times (shorter exposures would be possible in the brighter lines but the
exposure time is limited by the available telemetry rate). With having high throughput
performance and bright spectral lines, the integration times of the proposed spectrometer canbe shortened down to 1-0.5 s. The sit-and-stare measurements at a slit position provide timeseries of spectral data with temporal resolution of 0.5-1 s. The field of view with 15 arcsec
width can be repeatedly measured with 25-50 sec cadence, if the slit step is 0.3 arcsec. The
spectral maps for 100 arcsec field of view are available every ~200 s, if the slit is stepped per
0.6 arcsec.
As discussed in section 1, numerical simulations suggest that low-frequency (<5mHz)
magnetoacoustic waves may provide a significant source of the energy necessary for
maintaining the chromosphere. Moreover, the outgoing low-frequency (<1mHz) Alfven
waves may provide a major contribution to the heating and acceleration of the plasma as aresult of the nonlinear generation of the compressive waves and shocks in the corona.
Observationally, various kinds of oscillations and waves have been detected with Hinode data.
For example, with analyzing EIS spectral data, upwardly propagating and standing slow
magnetoacoustic waves are reported in active regions (Kitagawa et al. 2010) and coronalholes (Banerjee et al. 2009). When the waves propagates along a coronal loop with the soundspeed (~150 km/s in the corona), the proposed spectrometer can scan the entire coronal loop
about 5 times with 0.6 arcsec step during the time when the waves propagates from one end tothe other end of the loop. Thus, such data will allow us to trace the waves from the excitation
to the dissipation. The speed of Alfven waves is much higher than the sound speed in the

3-3transition region and corona, but the scan speed of the proposed spectrometer can allow us to
explore the properties of propagating Alfven waves, if we choose slit scan step size properly.
Moreover, owing to higher temporal resolution, we will search for higher frequency waves(~1Hz) that cannot be explored with the currently existing spectrometers. If higher frequency
waves are found in the transition region and corona with sufficient flux, high frequency waves
may be a possible candidate for making the bright corona in active regions.
High temporal cadence performance with high spatial resolution has the potential to resolve
fine dynamical structures in the coronal loops. Hinode EIS has revealed excess non-thermalline broadening at the footpoints of coronal loops, which is interpreted as the existence ofhighly blue-shifted small-scale flows at the base of coronal loops (Hara et al. 2008). Nonthermal line broadening is known in spectral lines from the transition region and corona, and
we guess that the broadening is created by turbulent nature in coronal loops. Having short
integration time is definitely useful to distinguish each of turbulent flows, providing new
information on interpreting non-thermal line broadening.
Higher temporal cadence would provide strong tools in exploring coronal holes. Existing
spectrometers need much longer integration times to meaningfully record spectral data incoronal holes, where the FUV and EUV coronal emissions are much fainter than in active
regions. It is important to identify the source region of the fast solar wind in polar coronalholes. Because of very long integration times (several minutes for a slit position and several
hours for mapping a region), it is difficult with existing instruments to confidently identify thesource regions. A substantial improvement in the throughput performance can much reduce
the time needed to spectrally map a region, getting closer to the timescales for the evolutionof the structures. Also, waves with periods from 7 to 20 min have been detected in plumes
and inter-plumes above the limb in the polar regions (DeForest & Gurman 1998; Ofman et al.
2000; Banerjee et al. 2000, 2009). A high throughout performance could allow a search forhigher frequency waves in coronal holes and a search for wave signatures in line widths andvelocity.

3.1.3. Broad temperature coverage
As one of the top-level science goals, we will attempt to understand how the energy istransported through elementary magnetic structures from the chromosphere to the transition
region, and from the transition region into the corona. Furthermore, magnetic energy
dissipation events are well observed over the entire solar atmosphere, not only in the outercorona which is filled with fully ionized, collisionless plasma, but also in the chromospherewhich consists of weakly ionized plasma. In order to capture the entire picture observationally,
it is important to measure the plasma conditions and behaviors in different temperature layers
simultaneously.
SoHO/SUMER records spectral lines at longer wavelengths in the FUV, where many
spectral lines originating from the transition region and chromosphere are available, while
only few coronal lines are observed. In contrast, Hinode EIS observes spectral lines at shorter
wavelength in the EUV, where many spectral lines from the corona and flare plasma are
available but the number of spectral lines is limited for exploring the transition region andchromosphere. Extensive coordinated observations by the two instruments have been obtainedsince the launch of Hinode, providing significant new results. However, we realize that it is
impossible to achieve high accuracy in the co-alignment of the spectral data from SUMER
and EIS. Also, since the spectral data are acquired for a single slit position and it takes a longtime to map a large field-of-view, it is nearly impossible for two different slit spectrographs
to obtain data at the same location on the Sun and at the same time. However, observations at

3-4

the same locations and times are critical for understanding the dynamical behavior andevolution of the plasma in the observed targets, because the timescales for the evolution of
structures are quite short.
Therefore, it is a requirement that the next generation advanced EUV/FUV spectrometer
capture spectral data over the entire temperature domain of the solar atmosphere at the same
locations and times. This performance can much improve and ensure accurate co-alignment
among different spectral lines. With this improvement, we can discuss with high confidence
the relationships among the observational signatures at different temperatures. Elementarymagnetic structures of the order of 0.3″ seen in the chromosphere can then be traced into thetransition region and corona. It is also possible to trace the flow of energy through static
elementary magnetic structures over the solar atmosphere and quantitatively investigate therole of waves as a transporter of energy for heating the corona and energizing the solar wind.
3.1.4. Necessity of low scattering optics
Low-scattering optics should be considered when the spectrometer is designed andfabricated to allow us to accurately record low intensity data. A single-mirror, off-axis
telescope is mandatory for off-limb observations, to avoid stray light from the bright solar
disk. Many important science discoveries may be hidden in low emission measure areas suchas reconnection outflow regions, which are much fainter than flare loops. As another example,
low intensity emission in coronal holes observed off-limb may hold the key for understanding
solar wind acceleration and shock formation in the high corona.
The reconnection region science discussed in section 1.3.4.2 is an excellent example forstressing the importance of low-scattering performance. Probing the reconnection region itself
is not easy because intense flare loops are formed near the reconnection region due tochromospheric evaporation. According to the model calculation given in section 1.3.4.2, thetotal intensity of the reconnection outflow region is about one thousandths of the total
intensity of the bright post-flare loops, when observed in spectral lines from Fe+17 to Fe+23.
This estimate tells us that low-scattering optics are necessary for investigating the flarereconnection outflow region.

3.1.5. Substantial jump expected from the NASA IRIS mission
IRIS (Interface Region Imaging Spectrograph) is a NASA SMEX mission selected forlaunch in December 2012. This spectrograph observes several spectral lines in the 120 to 280
nm range with high spatial resolution (0.3″) and high cadence (1 to 5 sec), to discover how
the outer atmosphere is energized. The spectral lines observed with IRIS are suitable for
exploring the interface between the upper chromosphere and lower transition region, and the
IRIS mission is expected to open a window of discovery into a crucial gap in current solar
observational capabilities.
However, the spectrometer proposed here would provide a substantial step forward from
IRIS observations, because the Solar-C Plan B mission can provide unique observations thathave never been realized, allowing us to progress beyond the chromosphere-transition region
interface into the entire transition region and corona. The proposed spectrometer will be able
to observe spectral lines in the EUV and FUV that seamlessly cover all the temperatures fromthe chromospheric to the corona with high spatial resolution (0.5″ or better) and high cadence(1 to 5 s). Observations of this completeness are necessary for understanding coronal heating
and the formation of the solar wind. In addition, the proposed spectrometer observes several
spectral lines that originate from high temperature plasma in the 106-107 K temperature range,

3-5and these spectral lines are useful for diagnosing flares and active region dynamics in thecorona. Thirdly, and more important, is that EUV and FUV spectroscopic observations canbe carried out simultaneously with quantitative measurements of magnetic fields in thechromosphere and photosphere with 0.1″ to 0.2″, taken by the UV-Visible-IR telescope. This
would be an enormous help in associating an accurate magnetic geometry with the EUV-FUV
spectra. Finally, unlike IRIS, the single-mirror off-axis design of the telescope allows us toobserve faint structures in the corona far from the solar disk.

3.2. Technical feasibility of each key element
The strawman instrumentation draws heavily on current technology under developmentby the international EUV/FUV spectroscopic community. Technical feasibility of each key
technology is briefly described below.

3.2.1. Two element optical layout
In order to improve throughput performance, minimizing the number of optical elements
is essential; the spectrograph consists of two optical elements, i.e., an off-axis parabola
primary mirror and a concave grating. The two-element spectrograph design was firstdeveloped and flown for EIS onboard Hinode and it has since been refined to meet various
requirements for RAISE, VERIS and EUNIS rocket payloads.

3.2.2. The telescope mirror and mirror coating
For off-limb solar observations the single-mirror telescope greatly reduces stray light, as
compared to an on-axis, double-mirror design. In addition, a secondary mirror would not only
reduce throughput by adding another reflection but would also be vulnerable to degradationdue exposure to very high solar flux. Thus the decision is made for a single-mirror telescope.
Increasing the size of the primary mirror gives a significant improvement in throughput. If
the diameter of the mirror can be increased twice, the effective area is improved by a factor 4.
30 cm or larger diameter gives 4 times or larger throughput, as compared with EIS (diameter
15cm).
Driven by requirements for EUV photolithography, EUV optics with very low scatter anddiffraction limited performance are available. For example, the optics developed for the MET
EUV photolithographic system have a slope error of 0.3 to 1.0 microradians, andmicroroughness of 0.1 to 0.2 nm over spatial frequencies from 9 nm to 4 mm (Goldberg et al.
2005).
A single coating on the primary mirror – without division in sections for EUV and FUV
ranges - results in good performance across a broad wavelength range. The suggested coating
is described in Figure 3.1. It consists of a broadband Si/Mo multilayer with a 5 nm B4C
topcoat. The B4C topcoat is semi-transparent in the EUV but gives reasonable reflectance atlonger wavelengths as shown in Figure 3.1 (left). The single coating minimizes risk andcomplexity for the primary mirror associated with dividing the solar illuminated mirror into
segments with different coatings. Recent advances in stabilizing multilayer for higher
temperature performance will be incorporated into the multilayer coating design.

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3.2.3. Grating and grating coating
To realize seamless temperature coverage in the 104 to 107 K range, the spectrometer
needs to cover between 2 and 4 wavelength bandpasses over a range from 10 nm to 120 nm.
To accommodate this broad wavelength range, the grating is divided into two segments. The
ruling density, groove geometry and figure of each segment is optimized for the specificwavelength range. Each grating segment is coated individually. A process is envisioned
where each coated grating segment would be contacted/cemented to a common plate. This
plate would then be installed into the instrument.

Figure 3.1. Reflectivity of the proposed primary mirror coating. The red curve in the left
panel shows the performance of the coating from 50 to 120 nm. The right panel gives the
EUV coating performance.

3.2.4. Solar blind detectors for EUVS
Enhanced technology detectors with larger format and faster readout are underdevelopment for space flight. The intensified (CMOS-APS) active pixel sensor (flown on
NASA’s EUNIS and RAISE rocket flights) has the promise of exceptionally fast image
readout with quick access to specific pixel areas. The intensifier with opaque anode ensuresthat the detector is “solar blind” and focal plane filters, to suppress visible solar radiation, are
not needed. The opaque photocathode on the microchannel plates will ensure very highsensitivity in selected spectral ranges.
At shorter wavelengths (in the EUV), back side thinned, large format, fast readout silicon
based CCD and APS detectors are highly efficient, low-noise, fast-readout detectors. The
backside illuminated detectors will be fitted with focal plane filters to provide rejection of thevisible solar radiation scattered from the grating. The performance of currently available
intensified CCD and EUV backside illuminated CCDs are suitable for the EUVS mission
while similar CMOS-APS sensors are presently under development.

3.2.5. Removal of metal filters and thermal control concept
To achieve low scatter, higher throughput and broad wavelength coverage, the EUVS
strawman design has no prefilters in the front aperture. These prefilters have historically beenused to protect the multilayers from the incident solar radiation. With one solar constantilluminating the primary mirror, the temperature of the mirror is expected to increase to 50 °C
3-7

or 60 °C provided that a passively cooled plate is provided. Similar systems for NEXUS
instrument (proposed as a SMEX) and SUMER (flown on SOHO) have had very reasonableproperties, but a more detailed solution has yet to be found. A heat rejection mirror will beplaced in front of the slit plane to reject the majority of the solar radiation before the slit.

3.3. Strawman Instrument

3.3.1. Optical Design
The EUVS instrument schematic is shown in Figure 3.2. As shown in the schematic,
solar radiation is incident on the EUVS off-axis paraboloid primary mirror (φ~40cm). The
primary mirror focuses an image of the solar disk on the entrance slit and reflective slit jaws.
Solar radiation falling outside the instrument field of view is redirected out the front of the
instrument with a heat rejection mirror following a similar design used for the
SUMER/SOHO and the NEXUS instruments. A slit selection mechanism interchanges andprecisely places various slits in the telescope focal plane.

5m length

Primary Mirror

solar
radiation
Focal Plane Detectors

Entrance Slit

Grating

Figure 3.2. EUVS strawman instrument schematic
Solar radiation passing through the slit is reflected and dispersed by the elliptical,
variable line space grating onto the various focal plane detectors. The grating may be divided
into as many as four separate gratings to cover the various wavelength bands. The figure,ruling, and coating of each grating segment are optimized to meet the requirements of theparticular wavelength range. The spectra are recorded by large format, electronic detectors.

3.3.2. Effective Area
Results from a conservative first order effective area calculation for the strawman
instrument are given in Figure 3.3. The effective area was calculated using the expected
performance of a next-generation, broadband multilayer reflection coating for both mirror andgrating. A groove efficiency of 0.40 and detector efficiency of 0.5 were assumed. Each

3-8channel used 0.25 of the mirror total area. The calculated effective area is a factor of >10
higher than previous instrumentation as shown below. Further optimization and refinement of
this calculation would likely show considerable improvement. In particular, the use of only
two grating sections for the EUV and the FUV ranges will enhance by a factor two thethroughput.

Figure 3.3. Effective area of the EUVS strawman instrument. Previous comparable effective
areas for EIS, CDS and SUMER are plotted for comparison.

3.3.3. Selection of wavelength bands
Four spectral bands were chosen as representative for the purpose of developing thestrawman design. The bandpasses will be further optimized to meet the specific Solar-C
scientific requirements.

3.3.4. Expected count rates and Temperature Coverage
The expected count rates for the emission lines in the four channels have been calculated
and are given in Appendix 3-A. The temperature coverage of the bandpasses is superb as
shown in Figure 3.4. The total counts in each line for quiet Sun, active regions and flares are
given for a spatial sampling of 0.33". Count rates larger than 100 counts/s will allow rasteringof quiet-Sun plasmas with 1-s exposure times in the hydrogen H I and the carbon C III lines.
Spatial binning and slightly longer integrations allow rapid scanning in a wide variety offainter lines (C II, He I, O V, Ne VIII, Fe IX, Fe X, etc.) and will yield extraordinary
3-9spectroscopic images of the quiet Sun. Active region count rates of several hundred counts/s
with 0.33" spatial resolution enable spectroscopic imaging of active region flows anddynamics. The EUVS line list includes high temperature, well isolated emission lines in flares
and active region plasmas.
Figure 3.5 compares the count rates expected for the Solar-C spectrometer with thecurrently operating SoHO/SUMER and Hinode/EIS and planned IRIS. It is obvious that theSolar-C spectrometer collects much more numbers of photons in 1 sec integration time, eventhough the spatial resolution is 0.33 arcsec.

Figure 3.4. Expected Count Rates. The expected count rates are plotted as a function ofionization temperature for a spatial sampling of 0.33". The plots show total counts in each line
plotted at their ionization temperature. Each asterisk corresponds to a specific emission line.
The top panel corresponds to typical quiet Sun; the middle panel shows typical active region
count rates, and the bottom panel shows flare plasma count rates.

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Active Region Plasma
10000

Solar-C/ EUVS
1000

Plan ned IRIS

100

Hinode/ EIS

10
SoHO/ SUMER

1
10 4

10 5
10 6
Temperature (K)10 7

Figure 3.5. Count rates for typical active regions. The Solar-C spectrometer is compared withcurrently operating SUMER and EIS and planned IRIS. The expected count rates are plotted
as a function of ionization temperature for the spatial sampling of each spectometer. Eachasterisk corresponds to a specific emission line observed with the Solar-C spectrometer.
Triangles, diamonds, and squares are for IRIS, SUMER, and EIS, respectively.

3.3.5. Improvements over previous instrumentation
Critical improvements over previous instrumentation are obtained with this strawman
design as follows:•••>18 improvement in spatially resolved area on the solar disk
>10 improvement in effective area
complete temperature coverage (104 – 107 K in active regions with Δ(logT) <0.3)

A summary table of the EUVS scientific characteristics is given in Table 3.2.

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Table 3.2. EUVS scientific summary characteristics.
Plate scale
Simultaneous field of view
Maximum raster width
Wavelength range
Exposure times
Velocity resolution
Temperature coverage~0.16"/pixel (sampling comparable to Hinode SOT)
Slit: 0.16", 0.32", 0.96"; Slot: 10", 40", 200"
Slit/slot length: 200" nominal, >300" extended+/- 75" nominal, +/- 200" extended
four channels (Å):130-250; 510-630; 690-810; 940-1060;
1-5 s with 0.33" spatial sampling
0.1-0.5 s in active regions with 1.0" spatial sampling
Doppler (centroid) shift measurement accuracy <3km/s
Turbulent velocity (width) <10km/s
104 – 107 in active regions with Δ(logT) <0.3

3.4. Strawman design details
Further details of the strawman design are given in Table 3.3. Provisions have been made in
the strawman design for a slit jaw camera imaging system and a possible closed loop servocontrol system using a correlation tracker system and an articulated primary mirror for image
stabilization. Solar image rastering is accomplished by tilting the primary mirror.

Table 3.3. EUVS strawman design – nominal performance
Telescope
Baseline or nominal valueType
off-axis paraboloid mirror, single coating optimized for both long and short bands.
optical properties 40cm usable aperture diameter, 400cm focal length, 30cm off-axis distance, <λ/80rms,<0.5nm microroughness, 19.4 microns/arcsecond plate scale.
Mirror blank
Lightweighted premium grade zerodur blank (or equivalent).
Stabilization<0.03 arcsecond jitter suppression, >5Hz bandwidth possible
Geometric blur ~0.26" at 75" roughly circular field of view,~0.52" at 150" roughly circular field of view
Slit/slot
Baseline or nominal value
assembly
General
6 slits/slot positions, front face with reflective coating, >327" length
selected widths Slits: 0.16", 0.32", 0.96"; Slots: 10", 40", 200"
Slit jaw imaging Baseline or nominal value
GeneralProjecting a high-quality image of the slit jaws onto a CCD camera.
Feeds a CCD camera used for correlation tracker system.
Design char.
field of view: 400" x 400"; plate scale: 0.2"/pixel; CCD format: 2K x 2K.
Filter bandpass Possible selections include: Ly-α line (121.6 nm), Tmin(160 nm), Mg II line (280 nm),
Ca II (393 nm), G-band (430.5 nm), Hα line (656.3 nm), TiO band (705.7 nm).
Correlation
If implemented is expected to be similar to that used on Hinode/SOT bandwidth (10-30
tracker system
Hz). Dedicated small-format CCD camera.
SpectrographBaseline or nominal value
characteristics
GeneralOptimized single reflection EUV spectrograph, elliptical substrate with variable line
spacing as required, grating figure and ruling individually optimized for the
bandpass/quadrant.
Typical blur: <1 pixel over majority of wavelength range for +/- 50" at slit center.
Coating selected and optimized for each quadrant.
Detector optimized for the bandpass/quadrant
EUV1 bandpass detector: 12kx2k mosaic CCD/APS, backside illuminated, 13.5 µm pixel size.
(130-250Å)stray light rejection: thin film metallic filter with supporting mesh.
spatial plate scale: 0.16"/pixel.
spectral plate scale: 0.010 Å/pixel, 17.7 km/s-pixel@170Å.
Grating: m~4.35, elliptical figure, 4000line/mm VLS ruling, -1 order.

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EUV2 bandpass detector: 5kx2k CCD/APS, 17 µm pixel size with MCP intensifier with KBr(510-630Å)
photocathode.
stray light rejection: solar-blind intensifier
spatial plate scale: 0.20"/pixel.
spectral plate scale: 0.026 Å/pixel, 13.7 km/s-pixel@570 Å.
grating: m~4.35, elliptical figure, 2000line/mm VLS ruling, +1st order.
EUV3 bandpass detector: 5kx2k CCD/APS, 17µm pixel size with MCP intensifier with KBr(690-810Å)
photocathode.
stray light rejection: solar-blind intensifier.
spatial plate scale: ~0.20"/pixel
spectral plate scale: ~0.025 Å/pixel, 10.0 km/second-pixel @ 750Å.
grating: m~4.35, elliptical figure, 2000line/mm VLS ruling, -1st order.
EUV4 bandpass detector: 5kx2k CCD/APS, 17 µm pixel size with MCP intensifier with KBr(940-1060Å)
photocathode.
stray light rejection: solar-blind intensifier.
spatial plate scale: ~0.20"/pixel.
spectral plate scale: ~0.024 Å/pixel,7.2 km/s-pixel@1000 Å.
Grating: m~4.35, elliptical figure, 2000line/mm VLS ruling, +1st order.

3.5. Further optimization for better instrument
Considerable optimization activities remain for the spectrograph instrument. Science
goals and associated measurement requirements need further definition. The trades relating to
instrument wavelength ranges need further definition. The number of channels drives
instrument complexity. The wavelength ranges should be placed more conveniently on
various detectors. The size and f# of the primary mirror strongly drive the instrument massand envelope. The primary mirror size also drives the design of the articulated mirror
mechanism and associated requirements. The optical design may be further optimized for the
wavelength ranges and detectors selected. The coatings of the mirror and gratings could also
be optimized for the particular application.

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Appendix 3-A. EUVS count rates for the brightest lines
Count rates were calculated for the EUVS instrument using the CHIANTI spectral code forthe quiet Sun, active region and flare plasmas. The counts listed are for the total emission line
in a 0.33" spatial bin. The emission lines are sorted by channel and ionization temperature.
The ion and associated ionization temperature are included for reference. For the quiet Sun,
all lines with a total count rate of >10 counts/s are included. For active regions, emission
lines with a total count/s of >50 were tabulated. For flare plasmas, those emission lines withcount rates >10000 counts/s are included. Many valuable faint diagnostic lines have not beentabulated.
QUIET SUN INTENSITIES
Ion

Wvl.(A)

Tmax

Count/s

Fe IX
Fe X
Fe X
Fe XI

171.073174.531
177.240
180.408

5.90
6.10
6.10
6.10

86.9
36.4
16.4
14.1

He I
O V
Mg X

584.335
629.732
609.794

4.40
5.40
6.10

230.7
76.5
13.0

O III
N IV
O IV
O IV
Ne VIII
Ne VIII

703.854765.147787.710790.199770.410780.325

4.90
5.20
5.20
5.20
5.80
5.80

14.7
29.5
24.9
45.1
23.5
11.9

H I
H I
H I
C II
C II
C III
N III
O VI
O VI

949.745972.5381025.724
1036.339
1037.020
977.020991.5771031.914
1037.615

4.20
4.20
4.10
4.30
4.30
4.90
4.80
5.50
5.50

71.2
107.3
453.1
87.5
102.2
98.9
13.6
69.6
34.9

The lines counted in this table have all a count rate greater than
10.0000

3-14

ACTIVE REGION INTENSITIES
Ion

Wvl.(A)

Tmax

Count/s

Fe VIII
Fe VIII
Fe VIIIFe IX *Fe IX
Ni XI
Ni XIINi XIINi XIIINi XIIINi XIVNi XIV
Fe X
Fe X
Ni XV
Fe X
Fe XI
Fe XI
Fe X
Fe XI
Fe XI
Fe X
Fe XII
Fe XI
Fe XI
Fe XIIFe XIIFe XIIFe XIII

167.486
168.173168.544168.647
171.073148.374
152.154154.162
157.729
164.150
170.500
171.370
174.531175.263
176.741
177.240178.060
180.408180.441181.137
182.169
184.537186.887
188.232
188.299
192.394
193.509
195.119202.044

6.00
6.00
6.00
6.10
6.10
6.20
6.20
6.20
6.30
6.30
6.30
6.30
6.20
6.20
6.40
6.20
6.20
6.20
6.20
6.20
6.20
6.20
6.20
6.20
6.20
6.20
6.20
6.20
6.20

71.0
118.0
58.7
59.8
1671.6
56.3
73.4
50.4
125.3
90.8
72.2
108.2
1408.2
153.2
71.2
637.8
57.7
900.6
62.9
59.0
128.8
110.7
91.5
195.6
71.0
108.4
212.0
277.7
85.1

He I
O IV
O V
Si XII
Si XI
Mg X
Mg X

584.335
554.513629.732520.666
580.920609.794624.943

4.40
5.20
5.40
6.30
6.20
6.20
6.20

936.4
50.4
822.3
94.8
51.6
1010.1
613.9

O III
O II
O II
O V
N IV
S V
O IV
O IV
Mg IX
Ne VIII
Ne VIII

703.854718.506
718.568
760.446765.147786.470787.710790.199706.036770.410780.325

4.80
4.60
4.60
5.40
5.20
5.20
5.20
5.20
6.10
6.20
6.20

125.8
134.1
86.7
80.1
195.3
67.6
184.4
334.0
70.6
636.5
319.7

H I
H I
H I
C II
C II
S II

949.745972.5381025.724
1036.339
1037.020
1045.765

4.20
4.20
4.20
4.40
4.40
4.40

143.0
336.1
1441.1
95.0
103.3
54.8

3-15

C III
N III
N III
C II
O VI
O VI
Si VIIIFe XVIII

977.020989.799991.5771010.373
1031.914
1037.615
944.467974.860

4.70
4.80
4.80
4.50
5.50
5.50
6.10
6.70

2728.3
86.6
159.3
53.1
1258.7
631.0
57.7
68.3

The lines counted in this table have all a count rate greater than
50.0000

3-16

FLARE INTENSITIES
Ion

Wvl.(A)

Tmax

Count/sFe XXIIIFe XXII
Fe XXII
Fe XXIV

132.906
135.791
156.019
192.029

7.20
7.10
7.20
7.20

16711.910684.5
10742.9
41466.6

He I
O III
O IV
O IV
O V
Mg X
Fe XX
Fe XIX584.335
599.590554.513609.829629.732
609.794567.867592.236

4.50
4.90
5.20
5.20
5.30
6.80
7.10
7.00

22806.821647.016856.3
13597.1
123868.6
15972.914098.6
24262.3

O III
O III
O III
O III
O III
O III
O II
O II
N IV
O V
S V
O IV
O IV
O IV
Fe XX
Fe XXI

702.337702.838
702.896702.900
703.851703.854718.506
718.568
765.147760.446786.470787.710790.112790.199721.559786.162

4.90
4.90
4.90
4.90
4.90
4.90
4.70
4.70
5.10
5.30
5.20
5.20
5.20
5.20
7.10
7.10

15646.4
14272.3
12071.4
19085.9
19602.958808.9
17162.6
11081.7
61374.2
26905.2
26186.6
71908.2
13861.8128603.8108802.5
16011.5

H I
C III
N III
N III
C II
H I
H I
C II
C II
O VI
O VI
Fe XVIII

972.538
977.020989.799991.5771010.373
1025.724
1025.725
1036.339
1037.020
1031.914
1037.615
974.860

4.50
4.80
4.80
4.80
4.60
4.50
4.50
4.60
4.60
5.40
5.40
6.90

18466.2
834613.1
29670.652456.010053.1
72905.936616.4
24846.749448.2
67592.833837.3
116048.3

The lines counted in this table have all a count rate greater than
10000.0

3-17