A seeing measurement device for the PoET solar telescope

This paper describes the design, commissioning, and initial on-sky validation of a dedicated solar seeing monitor developed for the Paranal site to characterize daytime atmospheric conditions and optimize the observing performance of the PoET solar telescope.

The Gist

The Big Picture: Why Do Stars Twinkle?

Imagine you are looking at a campfire through a heat haze. The flames look like they are dancing and wobbling, even though the fire itself is steady. That "wobble" is caused by hot air rising and mixing with cooler air, distorting the light.

In astronomy, this is called "seeing." When astronomers look at stars or the Sun through a telescope, the Earth's atmosphere acts like that wobbly heat haze. It blurs the image, making it hard to see fine details. At night, this is annoying. During the day, when looking at the Sun, it's a major problem because the ground heats up even more, creating a lot of turbulence.

The Problem: The "PoET" Telescope

The scientists in this paper are building a new solar telescope called PoET (Paranal solar ESPRESSO Telescope). Think of PoET as a super-powerful camera designed to take incredibly sharp, high-definition photos of the Sun to help us understand other stars and find Earth-like planets.

However, PoET has a tricky feature: it can zoom in on tiny, specific spots on the Sun's surface. But if the "heat haze" (seeing) is bad, zooming in just makes a blurry mess. To get a clear picture, the telescope needs to know exactly how bad the air is right now so it can decide how much to zoom in.

The Analogy: Imagine trying to take a photo of a bird through a window. If the window is dirty or wavy, you can't get a sharp picture. You need a device that tells you, "Hey, the window is wavy right now, so don't zoom in too close, or the picture will be blurry."

The Solution: The "Shadow Band Ranger" (SHABAR)

To solve this, the team built a special device called SHABAR. It doesn't look like a normal telescope. Instead of a big lens, it's a long bar with six small "eyes" (sensors) looking at the Sun.

How it works (The Metaphor):
Imagine you are standing in a field with six friends, all holding flashlights pointing at the same cloud.

1. The Cloud: The Sun is the light source.
2. The Air: The atmosphere is the cloud.
3. The Shadows: As the light passes through the turbulent air, it creates tiny, flickering shadows on the ground (scintillation).

If you stand very close together, your shadows flicker in sync. If you stand far apart, your shadows flicker differently because the air turbulence is different at different heights.

SHABAR uses this principle. By measuring how the light flickers at different distances between its six sensors, it can mathematically "reverse engineer" the atmosphere. It figures out exactly how turbulent the air is at different heights, from the ground up to the sky.

The "Recipe" for the Device

The paper details how they built this device:

• The Eyes: They used simple, off-the-shelf light sensors (photodiodes) behind filters.
• The Brain: They built custom electronic boards to catch the tiny flickers in the light (AC signals) and the steady brightness (DC signals).
• The Software: They wrote a computer program that takes the flickering data and runs a complex calculation (an "inversion") to turn those flickers into a number called the Fried Parameter ($r_0$).
  • Simple translation: A high number means the air is calm (great for zooming in). A low number means the air is wobbly (better to zoom out).

The Test Run: "Commissioning"

The team didn't just build it in a lab; they took it to the top of a tower in La Palma (where the Swedish Solar Telescope is) to test it against an existing, trusted device.

• The Results: They watched the Sun for several days. As expected, the air was calmest in the morning and evening, but got very wobbly around noon when the ground was hottest.
• The Comparison: They compared their new device's readings with the old, trusted device. After adjusting for a slight difference in the color of light they were measuring, the two devices agreed perfectly. They were like two different thermometers giving the exact same temperature reading.

Why Does This Matter?

This device is the "traffic cop" for the PoET telescope.

• Before: Astronomers might guess the air conditions and risk taking blurry photos.
• Now: SHABAR tells them, "The air is calm right now! Go ahead and zoom in to the smallest 1-arcsecond spot." Or, "The air is wobbly; let's look at a bigger 55-arcsecond spot to get a clear image."

The Bottom Line:
This paper describes the successful creation and testing of a "weather station for light." By constantly monitoring the "wobble" of the Sun's light, this device ensures that the new PoET telescope can take the sharpest possible pictures of our star, helping scientists understand the physics of stars and the potential for life on other planets.

Technical Summary

1. Problem Statement

Ground-based astronomical observations, particularly those targeting the Sun, are fundamentally limited by atmospheric turbulence. This turbulence causes two primary effects:

• Seeing: Random fluctuations in the direction of incoming light, degrading image resolution.
• Scintillation: Stochastic variations in light intensity.

While nighttime seeing is well-studied, daytime seeing is often more severe due to solar heating of the ground, which increases turbulence in near-surface layers. The Paranal solar ESPRESSO Telescope (PoET) is a new instrument designed to feed the high-resolution ESPRESSO spectrograph at the ESO Very Large Telescope. PoET aims to study solar-type stars by observing the Sun as a proxy, capable of targeting specific regions on the solar disk with apertures ranging from 1 to 55 arcseconds.

The core challenge is that atmospheric seeing at Paranal typically limits resolution to 2–3 arcseconds ($r_0 \approx 3\text{--}4$ cm). To maximize scientific return and avoid contaminated spectra, PoET requires a dedicated, real-time method to characterize daytime seeing conditions to dynamically select the optimal observation aperture.

2. Methodology

The authors developed a dedicated instrument, $SHABAR_P$ (SHAdow Band Ranger for PoET), based on the correlation between solar scintillation and atmospheric seeing established by Seykora (1993).

Instrument Design

• Opto-mechanical: The device consists of six independent scintillometer units mounted on a 50 cm bar. Each unit contains a neutral density filter, a bandpass filter, a field stop, and an imaging lens focusing sunlight onto a planar silicon PN photodiode. The units are separated by distinct baselines to sample turbulence at different effective altitudes.
• Electronics: Each channel uses a transimpedance amplifier to separate the signal into DC (mean intensity) and AC (fluctuations/scintillation) components using high-pass and low-pass filters (crossover at 1 Hz). The AC gain is adjustable (10x, 50x, or 100x) to optimize the dynamic range of a 24-bit digitizer.
• Tracking: The assembly is mounted on a motorized telescope mount controlled by open-source software (N.I.N.A.) to ensure continuous solar tracking.

Data Processing & Inversion

The software pipeline processes time-series data (AC and DC signals) acquired at 1000 Hz:

1. Normalization: Signals are normalized by electronic gains.
2. Correlation: Pairwise correlations of AC signals between detector pairs are computed every 2 seconds (yielding 15 distinct covariance measurements).
3. Inversion: Using a Kolmogorov turbulence model and sensitivity kernels, the pipeline inverts the covariance data to retrieve the vertical profile of the refractive index structure parameter, $C_n^2(h)$.
4. Derivation: The Fried parameter ($r_0$) is calculated by integrating the turbulence profile, which is then converted to a seeing value (in arcseconds) at a reference wavelength (520 nm).

3. Key Contributions

• Instrument Development: Designed and built a custom, multi-baseline scintillometer ($SHABAR_P$) specifically for daytime solar monitoring at the Paranal site.
• Validation Pipeline: Developed a robust software inversion pipeline to convert scintillation covariance data into quantitative seeing metrics ($r_0$).
• Cross-Validation: Successfully validated the new instrument against the established $SHABAR_S$ system installed at the Swedish Solar Telescope (SST) on La Palma.
• Wavelength Scaling: Demonstrated the necessary correction for comparing seeing measurements taken at different wavelengths ($\lambda_{PoET} = 565$ nm vs. $\lambda_{SST} = 520$ nm) using the scaling law $r_0 \propto \lambda^{6/5}$.

4. Results

• Laboratory Testing: Gain characterization tests confirmed the stability and uniformity of the six detector channels, validating the normalization procedures.
• On-Sky Commissioning: The instrument was tested at the SST site (La Palma) from July 26–31, 2025.
  • Diurnal Trends: Results showed expected diurnal variations, with lower seeing values (better conditions) in the early morning and late afternoon, and higher values (worse conditions) near solar noon due to thermal convection.
  • Performance: The average seeing measured was 1.2 arcseconds, with a range of 0.7 to 2.2 arcseconds. An anomaly on July 28th (higher seeing) was correctly attributed to cloudy conditions.
• Instrument Comparison:
  • When comparing $SHABAR_P$ against $SHABAR_S$, the raw $r_0$ values differed due to wavelength.
  • After applying the theoretical wavelength scaling factor ($r_{0,PoET} \approx 0.66 \times r_{0,SST}$), the two instruments showed excellent agreement in both absolute seeing levels and temporal variability.
  • Pipeline cross-validation using SST covariance data confirmed that the new inversion algorithm introduces no systematic bias.

5. Significance

This work provides a critical operational tool for the PoET solar telescope. By delivering continuous, high-fidelity measurements of daytime atmospheric turbulence, $SHABAR_P$ enables:

• Optimized Aperture Selection: Operators can dynamically choose the best aperture size (1–55 arcseconds) based on real-time seeing conditions, ensuring the highest possible spectral resolution and signal-to-noise ratio.
• Scientific Fidelity: Accurate seeing characterization is essential for disentangling true stellar signals (such as granulation and oscillations) from atmospheric noise in ultra-high-precision radial velocity measurements.
• Future Solar Astronomy: The successful deployment and validation of this non-telescopic seeing monitor set a precedent for future solar observatories requiring precise turbulence monitoring to support high-resolution spectroscopy.

In conclusion, the $SHABAR_P$ instrument is a robust, operationally ready solution that directly supports the scientific goals of the PoET project by mitigating the impact of atmospheric turbulence on solar observations.