Why We Invest in Metrology
Manufacturing precision optics is one thing. Maintaining consistency across production is another.
Our metrology investment isn't about generating paperwork—it's about manufacturing excellence. We use laboratory-grade instruments to verify our processes, catch issues before they become problems, and ensure that every telescope we ship meets or exceeds its published specifications.
When a new coating run arrives, we characterize it on the Zygo before it enters production. When we assemble an etalon, we verify spectral performance on the monochromator. When something doesn't look right through the eyepiece, we have the tools to diagnose exactly what's happening and why.
But here's what matters most: individual component specifications don't tell the whole story. A telescope system with multiple precision optical elements—etalon, blocking filter, objective lens, double-stack modules—has tolerance interactions that can't be predicted from bench testing alone.
That's why every Lunt telescope is tested on the Sun itself before it ships. We swap blocking filters, adjust configurations, and match components until the complete system delivers the chromospheric detail our customers expect. The Sun is the ultimate test target, and we use it.
Our published specifications are conservative by design. They account for real-world tolerance stacking across all components. The actual design targets are tighter—but we only promise what we can guarantee in a complete, matched system.
SURFACE METROLOGY
Surface Figure Analysis: Zygo Verifire Laser Interferometer
Surface figure—the precise shape of an optical surface—determines how light behaves when it encounters that surface. For Fabry-Perot etalons, surface errors translate directly into spectral performance degradation. A "flat" surface that isn't actually flat produces an etalon that isn't actually narrowband.
Our Zygo Verifire interferometer measures surface figure with sub-nanometer precision across the full aperture. The system uses phase-shifting interferometry at 632.8nm with a 1200×1200 pixel detector, providing over 1.4 million measurement points per surface.
What We Measure:
Peak-to-Valley (P-V): The total range from highest to lowest point on the surface. Our production etalon substrates typically measure λ/70 or better P-V.
RMS (Root Mean Square): A statistical measure of overall surface roughness that accounts for the entire measured area, not just the extremes. Our typical production surfaces achieve 0.002λ RMS, equivalent to 1.3 nanometers.
Power: The spherical curvature component. Unlike many manufacturers who remove power from their specifications to make numbers look better, we report power-included values. Our surfaces are genuinely flat—not mathematically corrected.
| Measurement | Value |
|---|---|
| P-V | 0.014λ (λ/71) |
| RMS | 0.002λ (1.3nm) |
| Power | -0.002λ (negligible) |
| Measured on Zygo Verifire, 632.8nm, QPSI mode, 1200×1200 resolution Power included — this is the conservative measurement, not mathematically flattened. | |
Why RMS Matters More Than P-V
Peak-to-valley measurements can be misleading—a single dust particle or edge artifact can dominate the P-V value while the actual working surface is far better. RMS provides a statistically meaningful measure of the entire surface.
For etalon applications, RMS directly correlates with:
- Spectral transmission efficiency
- FWHM consistency across the aperture
- Ghost image suppression
- Overall filter contrast
Our sub-2nm RMS surfaces aren't achieved through luck or careful sample selection. They're the natural result of classical pitch polishing techniques that eliminate the mid-spatial-frequency errors inherent in CNC diamond polishing.
SPECTRAL METROLOGY
Spectral Performance Analysis: Sciencetech 9150 Monochromator
Surface quality is the foundation. Spectral performance is the proof.
Our Sciencetech 9150 monochromator—a 1.5-meter focal length research-grade instrument—provides laboratory-quality measurement of etalon spectral characteristics. This class of instrument is typically found in university research labs and national metrology institutes, not production facilities.
The recent upgrade to an 1800 lines/mm grating and high-sensitivity photomultiplier tube gives us 0.09 angstrom spectral resolution—fine enough to accurately characterize our narrowest hydrogen-alpha filters without instrument broadening affecting the measurement.
What We Measure:
Central Wavelength (CWL): The peak transmission wavelength of the etalon. For H-alpha filters, we verify alignment to the 656.28nm hydrogen-alpha emission line with ±0.02nm accuracy.
Full Width at Half Maximum (FWHM): The bandwidth of the filter measured at 50% of peak transmission. This determines how much of the solar chromosphere you see versus the photospheric continuum. Our 0.09Å instrument resolution means we measure actual filter performance, not instrument limitations.
Transmission Profile: The complete spectral response curve reveals sideband suppression, blocking effectiveness, and overall filter quality in ways that single-number specifications cannot.
Sciencetech 9150 Monochromator System
| Parameter | Specification |
|---|---|
| Model | Sciencetech 9150 |
| Focal Length | 1.5 meters |
| Grating | 1800 lines/mm, blazed @ 500nm |
| Spectral Resolution | 0.009nm (0.09Å) FWHM @ 632.8nm |
| Wavelength Accuracy | ±0.02nm (measured: 0.007nm avg) |
| Repeatability | ±0.017nm |
| Calibrated Range | 253–761nm |
| Minimum Step Size | 0.002nm (2 picometers) |
| Detector | Photomultiplier Tube (Hamamatsu R928) |
All specifications verified by Sciencetech Inc. factory testing, October 2025
Why Monochromator Focal Length and Grating Density Matter
Spectral resolution in a grating monochromator depends on focal length and grating line density. Our 1.5-meter focal length combined with an 1800 lines/mm grating achieves 0.009nm resolution—roughly 15× better than typical 0.25-meter bench spectrometers.
This matters because:
- Accurate FWHM measurement: A 0.5Å filter measured on a 0.15Å-resolution spectrometer would read as ~0.52Å due to convolution. Our 0.09Å resolution adds negligible broadening.
- True CWL verification: ±0.02nm wavelength accuracy means we can verify H-alpha alignment to better than 0.2Å—critical for filters designed to sit precisely on the 656.28nm emission line.
- Sideband detection: Higher resolution reveals spectral features that coarser instruments blur together, allowing us to verify blocking performance and detect manufacturing issues.
The difference between "good enough" QC and laboratory-grade metrology is the difference between hoping your filter performs and knowing it does.
Why We Don't Use Spectral Lamps for FWHM Measurement
Some manufacturers measure etalon bandwidth using a hydrogen spectral lamp. This method is fundamentally flawed: a hydrogen discharge lamp produces a line only ~0.1Å wide. You cannot measure a 0.5Å filter with a 0.1Å source—the lamp runs out of photons before you reach the filter's half-maximum points. It's like measuring a doorway with a laser pointer.
We illuminate our etalons with a broadband continuum source and scan with the monochromator. The etalon is the narrowest element in the optical path, so we measure the etalon—not our source. This is the only valid method for true FWHM verification.
