VELC: India's Solar Emission Line Coronagraph
- VELC is an innovative, internally occulted Lyot-style coronagraph that integrates imaging, spectroscopy, and spectropolarimetry for detailed solar corona analysis.
- It utilizes multiple channels (5000 Å continuum, Fe XIV 5303 Å, Fe XI 7892 Å, Fe XIII 10747 Å) to diagnose coronal temperatures, densities, and magnetic fields.
- Onboard processing with adaptive CME detection and telemetry management enables high-cadence observations of rapid coronal events with enhanced precision.
The Visible Emission Line Coronagraph (VELC) is the prime coronagraphic payload on the Aditya-L1 mission, India’s first dedicated solar observatory at the Sun–Earth L1 point. It is an internally occulted coronagraph that combines visible-continuum imaging of the corona from $1.05$ to with simultaneous spectroscopy in three coronal forbidden lines—Fe XIV $5303$ Å, Fe XI $7892$ Å, and Fe XIII $10747$ Å—over $1.05$ to ; the $10747$ Å channel also supports spectropolarimetry for coronal magnetic-field measurements. Across its design papers and early in-orbit reports, VELC is presented as an instrument optimized for close-to-limb observations, high cadence, and telemetry-aware operation, with particular emphasis on coronal mass ejections (CMEs), plasma diagnostics, and coronal magnetometry (Singh et al., 27 Mar 2025, Priyal et al., 2023, Patel et al., 2018).
1. Mission role and observational domain
Aditya-L1 operates from a halo orbit around the Sun–Earth L1 point, where uninterrupted solar viewing and atmosphere-free conditions permit continuous monitoring of the low corona. Within that platform, VELC is the payload that targets the innermost observable corona starting near , a height range that ground-based systems find difficult because of sky brightness and atmospheric scattering. The instrument is explicitly intended to address coronal diagnostics of temperature, velocity, and density, the origin and dynamics of CMEs, coronal heating, drivers of space weather, and direct measurements of coronal magnetic fields (Singh et al., 2022, Priyal et al., 2023).
The continuum channel alone creates a severe data-rate problem. The onboard-CME-detection study states that full-resolution, full-cadence continuum observations at L1 would exceed $1$ TB day0, and therefore motivated an onboard event detector that transmits preferentially the images containing CME activity. That same study frames one of VELC’s primary objectives as the study of CME dynamics in the inner corona using images with spatial sampling of approximately 1 arcsec pixel2 over 3–4, a 5 Å passband centered at 6 Å, and a nominal cadence capability of 7 s after onboard co-adding (Patel et al., 2018).
The later payload overview preserves that basic observational logic but places it in a broader operational context. Continuum images can be obtained at variable intervals depending on the data volume that can be downloaded, while the spectroscopic channels are designed for simultaneous observations with independent exposure times and cadences. This establishes VELC not as a single-mode coronagraph, but as a coordinated imaging, spectroscopy, and spectropolarimetry system whose observing programs are constrained as much by telemetry and onboard processing as by photon statistics (Singh et al., 27 Mar 2025).
2. Optical architecture and instrument channels
VELC is described as an internally occulted reflective coronagraph, or internally occulted Lyot-style system, built to suppress the solar-disk light while admitting coronal signal very close to the limb. The 2025 payload overview describes the primary mirror as an off-axis concave paraboloid with clear aperture 8–9 mm and focal length $5303$0 mm, the secondary mirror as a concave spherical mirror with a central elliptical hole that functions as the internal occulter, the tertiary mirror as the element that rejects disk light to deep space, and a Collimator Lens Assembly that re-images the entrance aperture onto a Lyot stop for diffraction control. Downstream optics include a quaternary mirror, dichroic beam splitters, imaging and spectrograph lens assemblies, a grating used in multiple spectral orders, and a Linear Scan Mechanism (LSM) that translates the coronal image across a four-slit assembly (Singh et al., 27 Mar 2025).
The four-slit spectrograph is a defining architectural feature. The slits are reported as $5303$1m wide and separated by $5303$2 mm, enabling simultaneous sampling at four coronal locations. The LSM moves the image across the slits in steps that are multiples of $5303$3m, and the spectroscopic field can be scanned out to approximately $5303$4. In the visible spectroscopic channels the detector sampling is $5303$5 arcsec pixel$5303$6, whereas the infrared channel uses $5303$7 arcsec pixel$5303$8. The continuum plate scale is reported as $5303$9 arcsec per pixel in the detector-calibration paper, while the 2025 payload overview quotes $7892$0/pixel for the continuum imager (Mishra et al., 2024, Priyal et al., 2023, Singh et al., 27 Mar 2025).
| Channel | Primary measurement | FOV |
|---|---|---|
| 5000 Å continuum | Coronal imaging and onboard CME detection | $7892$1–$7892$2 |
| Fe XIV 5303 Å | Spectroscopy | $7892$3–$7892$4 |
| Fe XI 7892 Å | Spectroscopy | $7892$5–$7892$6 |
| Fe XIII 10747 Å | Spectroscopy / spectropolarimetry | $7892$7–$7892$8 |
The detector suite comprises three sCMOS detectors for the continuum, $7892$9 Å, and $10747$0 Å channels, and one InGaAs detector for the $10747$1 Å channel. Thermo-vacuum calibration led to the recommendation that the sCMOS detectors be operated at $10747$2C and the InGaAs detector at $10747$3C at the spacecraft level. The detector electronics are partitioned into Detector Proximity Electronics, Digital Control and Processing Electronics, and Payload Power Electronics, with the digital chain also responsible for onboard processing including CME detection (Mishra et al., 2024).
3. Spectroscopy, spectropolarimetry, and diagnostic methodology
VELC’s spectroscopic channels were selected to span complementary coronal temperature regimes: Fe XIV $10747$4 Å is formed near $10747$5, Fe XI $10747$6 Å near $10747$7, and Fe XIII $10747$8 Å near $10747$9. Synthetic profile studies using CHIANTI 8.0, realistic instrumental scattered light, and detector noise were used to optimize slit width. After testing $1.05$0m, $1.05$1m, and $1.05$2m slits, the adopted compromise was $1.05$3m, which the study states provides sufficient signal-to-noise ratio under different solar conditions while maintaining detector headroom, especially in the IR channel. For the green line, reliable recovery of a $1.05$4 km s$1.05$5 Doppler shift at $1.05$6 was obtained when the line-peak SNR was at least about $1.05$7, using the standard relation $1.05$8. The same modeling also showed that a continuum-channel CME detection can be used operationally to trigger an exposure and gain change in the IR channel, because CME-enhanced Fe XIII signals can otherwise exceed the InGaAs full well in high gain (Patel et al., 2021).
The polarimetric subsystem is centered on Fe XIII $1.05$9 Å and is intended to retrieve full Stokes maps 0 for the inner corona. Its architecture uses a rotating quarter-wave plate as modulator and a dual-beam analyzer based on a PBD–HWP–PBD combination with a wedge plate to reduce aberrations. The 2024 calibration paper reports a rotation rate of 1 RPM, a full rotation time of 2 ms, and 3 Hall sensors that define 4 sectors for synchronized sampling. The forward model is written as 5, with demodulation through the pseudo-inverse 6, and laboratory calibration produced explicit modulation and demodulation matrices for the ordinary and extraordinary beams. Efficiency proxies derived from the measured modulation matrices are reported as 7, 8, 9 for the O-beam and $10747$0, $10747$1, $10747$2 for the E-beam (Narra et al., 2024).
The magnetic-diagnostic framework combines Zeeman and Hanle sensitivity. The weak-field relation used for Fe XIII is written as
$10747$3
with $10747$4 for Fe XIII when $10747$5, and the Zeeman splitting is written as
$10747$6
The sensitivity-requirements study concludes that across $10747$7–$10747$8 the required fractional circular-polarization sensitivity is approximately $10747$9 to 0, while the modulation-and-demodulation study targets SNR 1 in Stokes 2 for line-of-sight field estimation and 3 in 4 and 5 for topology (Raja et al., 2021, Nagaraju et al., 2021).
4. Onboard processing, telemetry management, and ground pipeline
VELC’s most distinctive operational feature is its onboard CME detector for the continuum channel. The processing chain described for the 6 Å images begins with acquisition at 7 fps and 8 ms exposure, addition of 9 frames to form one $1$0 s-equivalent image, and averaging of $1$1 such images to form a $1$2 s-equivalent image. After optional spatial binning and temporal median filtering over three images, a successive difference image is formed as
$1$3
with $1$4 nominally $1$5 s but tunable from $1$6 s to $1$7 s. Detection then uses adaptive intensity thresholding
$1$8
followed by area thresholding through convolution with a square kernel of size $1$9, 00, or 01; a CME is flagged when the maximum of the convolved image exceeds a tunable convolution threshold 02. Persistence over five consecutive frames is required to avoid false triggers, and when detection ceases the system reverts to co-added images if there is no CME for three hours or until the next detection (Patel et al., 2018).
The principal result of that algorithm study is that telemetry can be reduced by at least 03 while maintaining an overall CME detection rate of at least 04 on VELC-like data. Bright and fast synthetic CMEs reached detection rates above 05 for sensitive settings, whereas narrow faint CMEs were substantially harder to retain. Tests on STEREO/SECCHI COR-1A showed that, for low-jitter days, a setting around 06 could yield roughly 07 transmitted data and approximately 08 reduction, while K-Cor tests demonstrated that strong atmospheric scattering drives high false-detection rates. The computational design is explicitly lightweight: frame addition, averaging, small-kernel convolution, re-binning, and simple statistics were chosen because more elaborate ground-based CME detectors were considered unsuitable for onboard memory and CPU budgets (Patel et al., 2018, Patel et al., 2018).
The ground segment follows a staged Level-0/Level-1/Level-2 architecture. Raw telemetry are decompressed, checked, and converted to FITS; detector-specific corrections include bias or dark subtraction, flat-fielding, bad-pixel handling, port and gain merging for the sCMOS cameras, orientation fixes, and time-tag correlation. Higher-level processing includes spectral-curvature correction, wavelength calibration through atlas matching, correction for narrow-band filter transmission, scattered-light modeling and subtraction, Gaussian fitting to derive line peak, centroid, and FWHM, and the construction of intensity, velocity, width, and temperature products. For continuum images the 2023 processing paper describes a daily minimum image 09, an azimuthally averaged background 10, and a contrast-enhanced CME image
11
which is then used for event analysis and tracking (Singh et al., 2022, Priyal et al., 2023).
5. Calibration and measured in-orbit performance
Thermo-vacuum calibration quantified the conversion gain, full-well capacity, and read noise of all detectors. At 12C, the sCMOS detector at 13 gain is reported with conversion gain 14 e15/DN, full well 16 ke17, and read noise 18 e19. At 20 gain the corresponding values are 21 e22/DN, 23 ke24, and 25 e26. For the InGaAs detector at 27C, low gain gives conversion gain 28 e29/DN, full well 30 ke31, and read noise 32 e33, while high gain gives 34 e35/DN, 36 ke37, and 38 e39. These values underlie the recommendation to operate the visible detectors at 40C and the IR detector at 41C (Mishra et al., 2024).
Radiometric calibration of the 42 Å channel was subsequently carried out in orbit using attenuated Sun-disk light rather than bright stars as the primary reference. The reported equivalent full-aperture disk-center count rate is
43
corresponding to an adopted solar spectral irradiance at 44 AU of
45
This yields an empirical sensitivity
46
A consistency check with Sirius-A gave an expected count rate of approximately 47 and a measured value of approximately 48. The same stellar observation provided a measured point-spread-function FWHM of approximately 49 in the 50 Å channel (Priyal et al., 8 Jun 2026).
Early in-orbit performance reports show a mixed but informative status. The 51 Å spectroscopy channel is operational in sit-and-stare and raster modes and has produced plausible Fe XIV line widths and radiances consistent with historical values. By contrast, the 52 Å emission line had not been detected in orbit despite extensive tests, and the 53 Å channel was reported to have elevated IR noise, likely EMI-related, making both channels unsuitable for routine science at that stage. The same report notes that, near 54 in Fe XIV spectra, the continuum background is approximately 55 counts pixel56 s57, the continuum-to-disk ratio is about 58, and the line signal contributes only about 59–60 of the total line-center counts, with 61–62 arising from background. Time-series operation also revealed spatial drift along the slit of about 63 and spectral drift of about 64 pixels before correction (Singh et al., 27 Mar 2025).
6. Scientific results, limitations, and broader significance
The first published VELC spectroscopy of CME onset used Fe XIV 65 Å observations from 16 July 2024. In that event, a sudden dimming began at approximately 13:18 UT near position angle 66 and heights 67–68. The reported spectroscopic signatures were a 69 decrease in peak intensity, a 70 increase in FWHM from about 71 Å to about 72 Å, a mean redshift of about 73 km s74, and a derived non-thermal velocity of approximately 75 km s76. The paper presents this as the first VELC spectroscopic capture of CME onset in the Fe XIV green line and uses it to constrain CME onset to roughly an hour before the first LASCO/C2 appearance (Ramesh et al., 2024).
A second early result used joint VELC and radio observations to constrain CME-driven shock formation. For the 27 May 2024 event, VELC detected a Fe XIV 77 Å enhancement at 78 at 07:04 UT, while a type II burst started essentially simultaneously at 79 MHz; under a two-fold Newkirk density model, that radio frequency maps to 80. The inferred shock–CME standoff distance was approximately 81, which the study interprets as direct evidence that a CME-driven shock formed as low as 82 (Kathiravan et al., 24 Feb 2026).
These results illustrate the scientific rationale for visible and near-IR emission-line coronagraphy more generally. The 2023 white paper on next-generation space-based visible and near-IR line observations argues that such lines are radiatively excited out to large helioprojective distances and remain diagnostically powerful beyond the effective range of many EUV diagnostics. It cites eclipse measurements showing Fe XI 83 nm observable to at least 84 and Fe X 85 nm to about 86, and frames visible/NIR line sets as uniquely valuable for electron and ion temperatures, magnetic fields, Doppler motions, ionic freeze-in distances, and CME thermodynamics. In that wider context, Aditya-L1/VELC occupies the inner-coronal end of a broader instrumental class: a space-based visible emission-line coronagraph that couples close-to-limb coverage to spectroscopy and, in principle, coronal spectropolarimetry (Boe et al., 2023).
The present limitations are instrument-specific rather than conceptual. Current reports describe elevated background and stray light in the Fe XIV channel, unresolved issues in the 87 Å and 88 Å channels, daily telemetry limits that constrain cadence and raster density, and the need for continuing in-flight refinement of calibration, drift correction, and polarimetric performance (Singh et al., 27 Mar 2025, Priyal et al., 2023). A plausible implication is that VELC’s long-term significance will depend not only on its original optical design but also on how effectively its calibration strategy, onboard event selection, and channel-specific mitigation mature through operations. Even in its current state, however, the 89 Å results already demonstrate the central premise of the instrument: direct spectroscopy of the low corona from about 90 can capture CME initiation and shock formation in a region that had been only sparsely sampled by routine spaceborne coronagraphy (Ramesh et al., 2024, Kathiravan et al., 24 Feb 2026).
