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A consistency check for the calibration of 5303Å solar coronal emission line observations with Aditya-L1/VELC

This paper presents a novel radiometric calibration of the Aditya-L1/VELC instrument at 5303 Å using solar disk observations, which is subsequently validated by the consistent agreement between expected and measured counts from the bright star Sirius-A, thereby confirming the instrument's accuracy for converting coronal brightness into absolute physical units.

Original authors: V. Muthu Priyal, R. Ramesh, Jagdev Singh, K. Sasikumar Raja, P. Savarimuthu

Published 2026-06-09

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CC0 1.0

Original authors: V. Muthu Priyal, R. Ramesh, Jagdev Singh, K. Sasikumar Raja, P. Savarimuthu

Original paper dedicated to the public domain under CC0 1.0 (http://creativecommons.org/publicdomain/zero/1.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine you are trying to take a perfect photograph of a tiny, faint firefly (the Sun's corona) sitting next to a blindingly bright stadium floodlight (the Sun itself). If you point your camera directly at the floodlight, the sensor gets overwhelmed and the photo is ruined. If you try to photograph the firefly, the floodlight's glare washes it out.

This is the daily challenge for scientists studying the Sun's outer atmosphere. To solve this, India launched a space telescope called Aditya-L1, carrying a special camera named VELC. This paper is essentially a "quality control report" to make sure VELC's camera is measuring light accurately.

Here is how the scientists checked their work, explained simply:

1. The Problem: How do you calibrate a space camera?

Usually, to make sure a camera measures light correctly, you point it at a known "standard light bulb" (like a bright star) and compare what the camera sees to what you know the light bulb's brightness should be.

However, VELC is designed to look very close to the Sun. Pointing it at a distant star while the Sun is right next to it is tricky because the Sun's glare is so intense. Other space telescopes do this by looking at stars, but the team behind VELC wanted to try a new trick: using the Sun itself as the calibration tool.

2. The New Trick: The "Sunglasses" Method

To use the Sun as a reference, the scientists had to dim its light so it wouldn't burn the camera's sensor.

The Analogy: Imagine the Sun is a laser pointer. To look at it without hurting your eyes, you put on extremely dark sunglasses.
The Reality: VELC has a special "Neutral Density" filter (a high-tech, space-grade dark glass) that blocks 99.99% of the Sun's light.
The Test: They pointed the camera at the Sun's center through this filter. They measured exactly how much light got through. Since they knew the Sun's true brightness, they could calculate exactly how sensitive the camera is.

3. The Reality Check: The "Star" Test

To make sure their "Sunglasses Method" was actually working, they needed a second opinion. They used a very bright star called Sirius (the brightest star in our night sky).

The Setup: They turned the camera away from the Sun and pointed it at Sirius.
The Prediction: Based on their "Sunglasses Method" calibration, they did the math to predict: "If our camera is calibrated correctly, it should see exactly 19 units of light from Sirius."
The Result: The camera actually saw 23 units of light.

4. The Verdict: It Works!

Is 19 close enough to 23?

The Analogy: Imagine you are baking a cake and the recipe says it should take 20 minutes. You check the oven, and it says 23 minutes. Given that the oven takes a long time to heat up and the temperature fluctuates a bit, that 3-minute difference is totally acceptable.
The Science: The paper explains that the difference (4 units) is well within the margin of error, especially because photographing a distant star requires a very long exposure time (100 seconds), which introduces a little bit of "noise" or fuzziness.

Why This Matters

The paper concludes that the "Sunglasses Method" works. This is a big deal because:

It's a New Way: It proves you can calibrate a solar telescope using the Sun itself, rather than relying on distant stars which are hard to see when the Sun is in the picture.
It's Reliable: After two years of watching the Sun, the camera is still measuring light accurately.
Bonus Discovery: While looking at Sirius, they also measured how sharp the camera's focus is (its "Point Spread Function"). They found the camera can resolve details as small as 3.8 arcseconds (which is like seeing a coin from a few kilometers away).

In short: The scientists built a special camera to see the Sun's faint outer atmosphere. They tested it by dimming the Sun with a filter and checking the math against a bright star. The numbers matched up well enough to say, "Yes, our camera is telling the truth about how bright the Sun's atmosphere is."

Technical Summary: Radiometric Calibration of Aditya-L1/VELC at 5303Å

Problem Statement
Quantitative studies of the solar corona require precise radiometric calibration to convert detector counts into absolute physical units of brightness. While ground-based coronagraphs face limitations in duration and weather, space-based instruments like the Visible Emission Line Coronagraph (VELC) on the Aditya-L1 mission offer continuous monitoring. However, performing absolute calibration in orbit is challenging because traditional laboratory light sources cannot be used. Previous space coronagraphs (e.g., LASCO on SoHO, METIS on Solar Orbiter) have relied on observations of bright stars or planets (like Jupiter) for photometric calibration. The VELC presents a unique opportunity to utilize the Sun itself as a calibration source, a method not previously employed for space solar coronagraphs in this manner, provided the intense disk light can be safely attenuated.

Methodology
The authors employed a dual-source approach to verify the radiometric calibration of the VELC's 5303Å channel:

Sun Disk Observation (Primary Calibration Source):
- Attenuation: To prevent detector saturation from the Sun's disk light (which is $\approx 10^6$ times brighter than the corona), a custom-built Neutral Density (ND) filter with a transmission of $\approx 0.64 \times 10^{-4}$ at 5303Å was placed over a 45 mm aperture within the 147 mm Entrance Aperture (EA).
- Pointing: The satellite was off-pointed by $\pm 16'$ from the Sun center to allow the attenuated disk light to enter the field of view without hitting the internal occulter.
- Measurement: Observations were conducted using slit 4. The measured detector count at the center of the Sun's disk was recorded, corrected for exposure time, gain, and the ND filter transmission, and normalized to the full EA size.
Sirius-A Observation (Consistency Check):
- Setup: The satellite was off-pointed by $135^\circ$ to align the occulter with Sirius-A, and the EA shutter was kept open (no ND filter). The star was off-pointed by $\pm 19'$ to appear in the field of view.
- Measurement: The detector count for Sirius-A was measured and corrected for exposure time and gain.
- PSF Measurement: The Point Spread Function (PSF) was derived from the spatial profile of Sirius-A.
Cross-Verification:
- The authors calculated the expected detector count for Sirius-A based on the calibrated Sun disk flux and the known flux ratio between the Sun and Sirius-A at 1 AU (assuming L1 flux is equivalent to Earth flux). This expected value was compared against the actual measured count for Sirius-A.

Key Contributions

Novel Calibration Approach: The paper demonstrates a successful radiometric calibration of a space-based coronagraph using the Sun's own disk light attenuated by an ND filter, offering an alternative to the star-based calibration methods used by other missions.
In-Orbit Stability Verification: The study validates the stability of the VELC instrument over a two-year operational period by comparing measurements taken at multiple epochs.
PSF Characterization: The spatial resolution (Full Width at Half Maximum) of the VELC 5303Å channel was measured directly using Sirius-A.

Results

Sun Disk Flux: The measured detector count corresponding to the center of the Sun's disk, after correcting for the ND filter and aperture ratio, is estimated at $\approx (1.70 \pm 0.12) \times 10^8$ counts sec $^{-1}$ . This corresponds to a solar flux at 1 AU of $\approx 3.0456 \times 10^6$ erg sec $^{-1}$ cm $^{-2}$ at 5303Å.
Sirius-A Consistency:
- Expected Count: Based on the Sun disk calibration and known stellar fluxes, the expected detector count for Sirius-A is $\approx 19 \pm 1.4$ counts sec $^{-1}$ .
- Measured Count: The actual measured count for Sirius-A is $\approx 23 \pm 0.4$ counts sec $^{-1}$ .
- Agreement: The authors note a difference of approximately 4 counts (a $\approx 17\%$ discrepancy relative to the Sirius measurement). However, they attribute this to the statistical uncertainty associated with the long 100-second exposure time required for the faint star (where the error margin is $\pm 4$ counts), concluding the agreement is "reasonable."
Spatial Resolution: The Point Spread Function (PSF) of the VELC 5303Å channel has a Full Width at Half Maximum (FWHM) of $\approx 3.8''$ (3 pixels).

Significance and Claims
The paper claims that the reasonable agreement between the expected and observed values for Sirius-A serves as a successful consistency check. This validates the radiometric calibration of the VELC using Sun disk light observations. The authors emphasize that this method is advantageous because it allows calibration data to be acquired using exposure times similar to those used for coronal observations, thereby avoiding the propagation of uncertainties associated with the long exposure times required for faint stellar targets. The results confirm that the VELC is functioning as expected and that converting coronal brightness measurements at 5303Å into absolute physical units via Sun disk calibration is a viable and robust approach for space-based solar coronagraphs.

1. The Problem: How do you calibrate a space camera?

2. The New Trick: The "Sunglasses" Method

3. The Reality Check: The "Star" Test

4. The Verdict: It Works!

Why This Matters

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