The ultimate goal of solar observing at any wavelength is to simultaneously perform high resolution 2D imaging and spectroscopy—so-called integral field spectroscopy. That is, each pixel in a 2D image will have its own high resolution spectrum with which to apply plasma diagnostics. At SXR wavelengths the pixels of a detector can have intrinsic energy resolution capability and options are discussed in section 5.3. For EUV and longer wavelengths, the solution for simultaneous imaging and spectroscopy is to reconfigure the 2D image such that the individual “pixels” can be processed through a standard slit–grating–detector set-up, and then re-constituted into an image by processing software. Section 7.1 describes a new instrument concept that will use this approach. This type of integral field spectroscopy has been well-tested for ground-based astronomy telescopes using microlens arrays, fiber optic cables and image slicers (e.g., Lee et al., 2001). The problem at EUV wavelengths is that photon fluxes are low and the reconfiguration of the input image requires extra optical elements that would reduce the instrument throughput.

The next-best approach to integral field spectroscopy is imaging slit spectroscopy, whereby the Sun is imaged through a narrow slit and the image is dispersed in the direction perpendicular to the slit with a grating. This leads to a two-dimensional image on the detector that has wavelength in one direction and spatial information in the other. The first solar imaging slit spectrometer in the EUV was HRTS, a rocket experiment flown several times between 1975 and 1992. The first spacecraft imaging slit spectrometers were CDS, SUMER, and UVCS on SOHO. No slit spectrometers in the SXR region have yet been flown. The key disadvantage of slit spectrometers is that, to build up two-dimensional images, the slit has to be scanned step-by-step to build up a raster. A solar active region typically has a size around 200^′′ and so a scan with a 1^′′ slit with 30 s exposure times would take 100 min. Because of this, a slit spectrometer is best used in tandem with a coronal imaging instrument such that the spectrometer observes a relatively small field-of-view and the evolution can be placed in context with the active region images.

Without a slit, an imaging spectrometer produces so-called “overlappograms”: each emission line in the spectrum will yield its own image of the Sun which then appear side-by-side in the detector image, spaced by the wavelength separations of the lines. This was most famously done with the S082A instrument on Skylab in the 1970's (Figure 1). The CDS and EIS instruments were both flown with “slot” options, i.e., slits that were significantly wider than the spectral resolutions of the instruments, thus producing rectangular images at the location of each line. For strong, relatively isolated lines the images are mostly pure and can be used for scientific analysis (e.g., Ugarte-Urra et al., 2009). As shown later in this article, slitless spectrometer designs are being considered for several mission concepts mainly to get around the limited field-of-view problems of slit spectrometers.

Spectrometers with no intrinsic spatial resolution are used for solar irradiance measurements and consequently require accurate radiometric calibration. Spectral resolution is usually low, but the EVE instrument (Woods et al., 2012) on SDO achieved resolutions of 100–1,000 in the 100–1,000 Å range, which has been sufficient to measure Doppler shifts in some circumstances (Hudson et al., 2011). At SXR wavelengths, solar flares are usually the main focus and, since the emission largely comes from a single compact source, then valuable spectroscopic measurements can be made without the need for imaging. In recent years the development of very compact X-ray spectrometers means that solar SXR spectra can be obtained with low-cost missions, as described in section 9.1.

Figure 2 illustrates the methods used to focus EUV and SXR radiation. The preferred method is to use normal incidence reflections from the mirror(s), but this is only possible above 50 Å. Below this wavelength the incoming radiation is simply absorbed by the surface. Between 50 and 500 Å normal incidence optics are only possible by applying multilayer coatings to the surfaces. These are alternating layers of a heavy and light element (Mb and Si are common choices) which lead to strongly enhanced reflectivity over a narrow wavelength range (typically 10–20% of the central wavelength). The first solar telescopes to use multilayer coatings were flown on sounding rockets in the mid-to-late 1980's (Underwood et al., 1987; Walker et al., 1988) and sub-arcsecond spatial resolution was demonstrated (Golub et al., 1990). Multilayer coatings are ideal for solar imaging as they naturally yield a narrow bandpass that can be tuned to specific emission lines in the EUV spectrum. The first multilayer imaging telescope flown on a spacecraft was EIT (Delaboudinière et al., 1995) on SOHO, launched in 1995, followed by TRACE in 1998, EUVI on the STEREO spacecraft in 2006, SWAP on the Proba-2 spacecraft in 2009 (Seaton et al., 2013), AIA on the SDO spacecraft in 2010 (Lemen et al., 2012), SUVI on the GOES-16 and 17 spacecraft in 2016 and 2018 (Vasudevan et al., 2019), and EUI on Solar Orbiter in 2020 (Rochus et al., 2020). Use of multilayer coatings for spectroscopy was pioneered with the SERTS rocket program (Davila et al., 1993) and the first spacecraft spectrometer to use them was EIS on Hinode (Culhane et al., 2007). The best spatial resolution currently possible with normal incidence optics in the EUV and SXR is around 0.1^′′, which is limited by the cost associated with the figuring of large mirrors. The Hi-C sounding rocket achieved 0.3^′′(Kobayashi et al., 2014).

Above 500 Å, broadband optical coatings can be used, giving high and relatively uniform sensitivity over a wide wavelength range, which is particularly valuable for spectroscopy. Examples include SUMER on SOHO and IRIS.

Below 50 Å direct imaging requires grazing incidence optics. As illustrated in Figure 2, the incoming radiation is focused off a pair of mirrors that are usually implemented in a cylindrical design (a Wolter-type telescope), with multiple concentric cylinders increasing the effective area. The XRT on Hinode is an example of this design. The disadvantage of grazing incidence is that irregularities on the optical surfaces are magnified, leading to worse imaging performance than normal incidence optics. XRT achieved a 2^′′ resolution, which compares with 0.3^′′ for the Hi-C sounding rocket telescope (Kobayashi et al., 2014), which is currently the best performance for a solar EUV multilayer telescope.

Collimators are the least attractive option for imaging, but are often necessary at X-ray wavelengths. At their crudest they achieve spatial resolution simply through restricting the field-of-view. A more sophisticated approach was used by RHESSI (Lin et al., 2002), which had two collimating grids. By rotating the spacecraft continuously every 4 s, a modulation pattern is built up that can be inverted to yield an image of the Sun. A spatial resolution up to 2^′′ can be achieved this way, but the method has limitations due to low dynamic range and a difficulty of resolving multiple sources. Note that a collimator approach is necessary in the hard X-ray wavelength region, hence grazing incidence optics were not an option for RHESSI.

A step beyond normal incidence optics is the use of Fresnel Zone Plates (FZPs) or Photon Sieves, which could yield spatial resolutions in the 10's of milliarcsecond range, i.e., an order of magnitude improvement over the best currently achieved in the EUV. The FZP is an idealized diffractive optic consisting of a circular plate with concentric rings cut into it. The placement of the rings is chosen to enable constructive interference of the transmitted light to yield a focused image. In practice, the rings are replaced with circles of dots or small slits—hence the name “photon sieve”—and they were first discussed in terms of X-ray imaging by Kipp et al. (2001). The key drawback with photon sieves is that the telescope needs a very long focal length of 100 m or more to achieve the highest spatial resolution. This requires a high-precision, formation-flying mission. For example, to yield sharp images, the transverse displacements of the optics spacecraft must be maintained to the size of a detector pixel, which is typically about 10 μm. This technology is becoming available now and, along with the photon sieve technology, is actively pursued by NASA (e.g., Davila, 2011; Calhoun et al., 2018). In terms of spectroscopy, a photon sieve is of interest because an adjustment of the focal length can lead to a sampling of different parts of an emission line, thus revealing red-shifted or blue-shifted plasma. The first solar mission to employ a photon sieve will be VISORS, a CubeSat project funded by the NSF and led by the University of Illinois at Urbana-Champaign with a launch date in 2023. It will consist of two CubeSats flying in formation. One will host the detector and the other the photon sieve. The He ii 304 Å line is targeted and a spacecraft separation of 40 m coupled with a 75 mm sieve diameter will yield a spatial resolution of, at best, around 0.1^′′.

CCDs are the standard sensors used for EUV and SXR imaging spectroscopy. They are usually used in a back-illuminated configuration and are directly sensitive to the incoming photons. At wavelengths above around 500 Å, however, the sensitivity declines and it is common to use a microchannel plate to convert the photons to clouds of electrons. These can then be detected directly with an anode detector, such as for the SUMER and UVCS instruments on SOHO (Kohl et al., 1995; Wilhelm et al., 1995), or converted to visible photons via a phosphor screen and then detected with a CCD, which was done for CDS on SOHO (Harrison et al., 1995).

The readout time of CCDs is slow. For example the 4k × 4k detectors of the AIA instrument on SDO take about 3 s. CMOS sensors (also referred to as Active Pixel Sensors, APS) have a much faster readout performance and are likely to be used more extensively in the coming decade. Like CCDs, they can be used in a back-illuminated configuration, as demonstrated by EUI on Solar Orbiter (Rochus et al., 2020). A further advantage is that they are much more robust to harsh radiation environments, hence they have been used for the imaging instruments on board Solar Orbiter and PSP, which are operating far beyond the protective bubble of the Earth's magnetosphere.

Both CCD and CMOS detectors offer energy resolution in the SXR region, as the detectors are able to measure the numbers of electron-hole pairs created in the silicon by the incoming photons (The Lyman-α line of Fe xxvi produces almost 2,000 pairs, for example). The spectral resolution is limited to about 100 eV. Combining a CCD or CMOS with a focusing telescope enables imaging spectroscopy and examples are discussed in sections 9.4, 9.6.1, and 9.6.2.

The compact, silicon X-ray detectors from the company AMPTEK have proven very successful for obtaining moderate resolution, disk-averaged solar soft X-ray spectra from a package with dimensions ~15 mm. A detector was first flown on NASA's NEAR asteroid mission in 1996 in order to measure the solar X-ray flux. This measurement is often important for planetary science missions as solar X-rays illuminate the body's surface and the resulting fluorescent X-ray spectrum leads to valuable composition data. Similar detectors were also flown on SMART-1, Chandrayaan-1 (both lunar missions) and the Mercury MESSENGER mission. The solar data have proven valuable for solar flare studies (e.g., Dennis et al., 2015). The most recent planetary spacecraft with a solar X-ray spectrometer is the Indian Chandrayaan-2 mission (Mithun et al., 2020), which was launched in 2019.

The AMPTEK detectors are either silicon PIN (Si-PIN) or silicon drift detectors (SDD) and they both come in an all-in-one configuration referred to as X-123. SDDs are more expensive but allow higher count rates and slightly better spectral resolution. Spectral coverage is typically 1–20 keV and resolution is about E/ΔE = 30–50, which is sufficient to resolve spectral features from which element abundances can be derived (Dennis et al., 2015; Moore et al., 2018).

In terms of solar missions, the SphinX instrument (Gburek et al., 2013) on board the Russian CORONAS-Photon mission used an earlier version of the AMPTEK detector, while the X-123 was flown on the MinXSS-1 and 2 Cubesats (Moore et al., 2018) in 2016 and 2018, although the latter failed before data could be obtained. New missions are described in section 9.1.

Microcalorimeter detectors would be a breakthrough for solar SXR spectroscopy, with two orders of magnitude improvement in energy resolution over silicon detectors. They can be arranged in an array format, thus yielding simultaneous 2D imaging and high-resolution spectra when combined with an imaging telescope. Development has so far focussed on X-ray astronomy missions and the Japanese astrophysics spacecraft Hitomi was the first to successfully operate a microcalorimeter as part of the Soft X-ray Spectrometer (SXS). Sadly Hitomi perished after obtaining only a single observation of the Perseus galaxy cluster (Hitomi Collaboration et al., 2016). The SXS had a 6 × 6 2D pixel array with energy resolution of 7 eV between 0.3 and 12 keV (Takahashi et al., 2018), which is comparable to crystal spectrometers (section 9.2).

The high count rates from the Sun pose a problem for microcalorimeters compared to other astrophysical sources, and development progress lags behind that in Astrophysics. Some discussion of the scientific possibilities of microcalorimeters is given in the White Paper of Laming et al. (2010), while technical aspects are covered in Bandler et al. (2010) and Bandler et al. (2013).

The only integral field spectrometer for EUV wavelengths is SNIFS, which was selected for flight on a sounding rocket with a launch in 2024. It uses an array of “mirrorlets” to extract small portions of the input image and send them to a grating in such a manner that the final image on the detector appears as an array of 1D spectra, one for each mirror. To prevent the spectra from overlapping, the spectrum bandpass is restricted and the grating is rotated relative to the mirrorlet array so that the dispersion axis is tilted relative to the detector axes. An earlier concept was described in Chamberlin and Gong (2016), where these principles are explained in greater detail.

SNIFS will use a 72 × 72 mirrorlet array with each mirror corresponding to a spatial element of 0.45^′′. Hydrogen Ly-α at 1,216 Å is the principal target and the nearby Si iii 1,206 Å and O v 1,214 Å lines will also be observed. Note that the optical elements of the system are a primary mirror, the mirrorlet array, a focusing grating and a detector; the mirrorlets replace the slit of a standard imaging slit spectrometer design.

Section 5.3 highlighted Si-PIN and SDD detectors for enabling very compact, non-imaging SXR spectrometers. The next evolution of the MinXSS concept will be the Dual-zone Aperture X-ray Solar Spectrometer (DAXSS), to be flown on INSPIRESat-1 in 2021 (INPSIRESat is a series of satellites developed by a consortium of space universities led by the University of Colorado at Boulder). DAXSS was first flown on a sounding rocket (Schwab et al., 2020). The key advance over the MinXSS instruments is an increase in the size of the primary aperture to increase sensitivity to high-energy photons, and the insertion of a Kaplon filter behind the primary to attenuate the more intense flux of low-energy photons. The result is a more balanced distribution of low and high-energy photons.

Another compact X-ray spectrometer utilizing SDDs will be SoLEXS (Sankarasubramanian et al., 2017), which will be launched on the Indian Aditya-L1 spacecraft in 2022. Two SDDs will be used with apertures of different sizes in order to observe both large and small flares. Aditya-L1 will also carry HEL1OS, a disk-averaged hard X-ray spectrometer covering the 10–150 keV range.

Certain crystals can reflect X-rays at a specific wavelength, determined by the angle of incidence, as first determined by L. and W.H. Bragg in 1913. If the crystal is rotated to modify the incidence angle, then a spectrum can be built up. A single crystal will give access to a range ≈±40% of the central wavelength. The Flat Crystal Spectrometer on SMM featured seven different crystals on a rotating drum structure, enabling almost complete wavelength coverage from 1.4 to 22.5 Å.

A problem with this design is that the position of a flare within the crystal spectrometer's field-of-view affects the position of the spectra on the detector, thus the shift of a spectral line could be due to plane-of-sky motions or Doppler motions. The “dopplerometer” design resolves this ambiguity by using two identical crystals, but with opposite angles of incidence. Plane-of-sky motions will be in the same direction in both spectra, but Doppler motions will be in the opposite direction—see Figure 3 of Siarkowski et al. (2016).

This approach was first attempted with the DIOGENESS instrument (Sylwester et al., 2015) on the Russian CORONAS-F mission. It had four separate channels, each covering a different wavelength region. Each of the dopplerometer channels had two crystals attached to a shaft that could rock backwards-and-forwards, allowing rapid scans through the spectral range. DIOGENESS was only operational for a few weeks in 2001 but some flares were observed (Sylwester et al., 2015).

The next iteration of this type of instrument will be part of the SOLPEX instrument package (Steślicki et al., 2014; Sylwester et al., 2019) to be flown with the Russian KORTES experiment to the ISS in 2024 (see article by Kirichenko et al., under review in this issue). The Rotating Drum X-ray Spectrometer (RDS) will have crystals placed on an octagonal drum that rotates ten times per second to achieve high time resolution data of flares. High spectral resolution is enabled through the 1 μs readout time of SDDs. Radiation is incident on one of the outer halves of the drum, and detectors on the rotation and anti-rotation directions will enable dopplerometer spectra to be obtained. The combination of crystals is chosen to cover 0.4–23 Å.

Bent crystal spectrometers (BCS) take advantage of Bragg diffraction to yield high resolution spectra over a narrow wavelength region without the need for scanning. They have been flown on SMM (Culhane et al., 1981), Yohkoh (Culhane et al., 1991), and CORONAS-F (Sylwester et al., 2005). The ChemiX instrument (Siarkowski et al., 2016) on Interhelioprobe will use bent crystals in two modes. One is a standard configuration with four spectral channels that combined give spectral coverage over 1.5–8.8 Å. The other is a dopplerometer configuration with three spectral channels optimized to the H and He-like ions of iron, calcium and argon, respectively. Each channel has two crystals with angles of incidence in the opposite direction to yield the dopplerometer spectra. Because bent crystals are used, there is no need to rotate the crystals.

Imaging slit spectrometers have been very successful at EUV wavelengths, but none have been flown in the SXR range. This will change with the launch of the sounding rocket experiment MaGIXS (Kobayashi et al., 2018) in 2021. It will observe the 6–24 Å region without the need for scanning and have spatial resolution along the slit of 5^′′. The spectral resolution of 500 is not as good as that possible with the crystal spectrometers, but significantly better than for the silicon detector instruments.

Simultaneous coverage of the Fe xvii–xxiii lines with the H and He-like lines of oxygen through silicon will lead to excellent plasma diagnostics in the 3–10 MK temperature range. This is important for understanding the heating of solar active regions. The spectra would also be excellent for flare measurements, but this is unlikely for a 5-min rocket flight.

FOXSI is a sounding rocket experiment that was flown in 2012, 2014, and 2018. It was the first solar instrument to obtain solar images at hard X-ray wavelengths using focusing optics (Krucker et al., 2014), and the detectors yield modest spectral resolution. Wavelength coverage extends to the 4–10 keV SXR region, where spectral resolution was around 10–20. The data have been valuable for studying active region heating (Ishikawa et al., 2017; Athiray et al., 2020).

The FOXSI concept was proposed for the 2016 SMEX call, but was not selected after reaching Phase A. For the 2019 MIDEX call it was teamed with an EUV imager and the concept was named FIERCE, but it was not selected for Phase A. A key difference with the rocket experiments is that the spacecraft versions would have had extendable booms, extending the spectral coverage from around 20 keV up to around 70 keV.

The next FOXSI flight is planned for 2024 as part of a special flare campaign coordinated with another launch of the Hi-C EUV imager. The latter will include the COOL-AID EUV spectrometer (section 7.3.3).

CubIXSS is a CubeSat mission concept that is currently under Phase A review with NASA. It consists of two distinct instruments: a spatially-integrated SXR spectrometer, the Small Assembly for Solar Spectroscopy (SASS), that is an evolution of the MinXSS spectrometers, and the Multi-Order X-ray Spectral Imager (MOXSI). The latter has a set of five pinhole cameras, four of which yield full-disk images of the Sun in X-ray lines. The fifth camera feeds a transmission grating to yield full-disk, overlappogram spectra (see Figure 1). Further details are available in the White Paper of Caspi et al. (2017).