What Is a Zygo Test Report (and How Do You Read One Without Falling Asleep)?
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If you’ve ever shopped for a telescope, you’ve probably seen phrases like “premium optics,” “high precision,” or “diffraction-limited.”
But what do those phrases actually mean?
At Starfield Optics, we believe in showing real proof, not just using big words. That’s why we test our lenses with something called a Zygo interferometer—and we share the results.
Let’s break it down in a way that’s easy to understand.
Think of Light Like Ripples on Water
Imagine tossing a pebble into a calm pond. The ripples move outward in smooth, perfect circles.
Light behaves like waves too. When light passes through a telescope lens, it should travel in smooth, even waves and meet at one sharp focus point.
If the lens isn’t shaped correctly (even by a tiny amount), those waves get bent or warped—and that can make stars look a little soft, bloated, or less “pinpoint.”
A Zygo test checks how smooth and accurate those light waves are after they pass through the lens.
So… What Is a Zygo Test?
A Zygo interferometer is a precision testing tool used in serious places like research labs and high-end manufacturing.
It works by:
- Sending a very precise “reference” wave of laser light through the lens
- Measuring how the lens changes that wave
- Turning those changes into pictures and numbers
In simple terms:
A Zygo test is like a microscope for how your lens shapes light.
It can reveal tiny errors—often far smaller than a human hair.
One important detail (still simple, still true): this kind of testing is typically done with one laser color (one wavelength) in the test setup. That means it’s a super-strong measurement of lens figure quality under that test condition.
Why Are Zygo Reports So Colorful?
Because they’re basically “weather maps” for your lens.
Zygo reports often show a colorful circle (and a 3D “landscape” version of it).
Here’s how to read the colors:
- Green = close to perfect
- Red/blue = tiny high/low areas
- Smooth changes = good
- Sharp, messy changes = not so good
If the map looks like a calm island with gentle hills, that’s great.
If it looks like a jagged mountain range, that’s when you start worrying.
The Two-Minute “How to Read a Zygo Report” Guide
We’re going to read a real Starfield report now:
Starfield Optics Triplet APO, 80mm f/480, FPL53, Serial 0026
If you’re new to all this, here’s the secret:
If you understand RMS and Strehl, you can read 90% of the report.
Step 1: Find the “Report Card Numbers”
On Serial 0026, the report shows:
- PV = 0.133 wave
- RMS = 0.019 wave
- Strehl = 0.981
Now let’s translate those into normal human language.
Step 2: PV (Peak-to-Valley) — The “Worst-Case Stretch”
PV = 0.133 wave
PV stands for Peak-to-Valley. It means:
“What is the difference between the highest error point and the lowest error point?”
Real-life comparison:
Think of PV like measuring a road by looking for the deepest pothole and the highest bump, then reporting the difference.
That’s useful—but it can be a little dramatic if there’s one tiny pothole.
A PV around 0.133 is excellent for a production refractor lens.
Step 3: RMS — The “Overall Smoothness Score”
RMS = 0.019 wave (about 1/53 wave)
RMS is the best “big picture” number on most Zygo reports.
Real-life comparison:
If PV is “the worst pothole,” RMS is “how bumpy the whole road is on average.”
So if you’re deciding whether a road is comfortable to drive on, you trust RMS more than PV.
An RMS of 0.019 is excellent. It means the lens is very smooth and very well figured overall.
Step 4: Strehl Ratio — The “How Close to Perfect?” Score
Strehl = 0.981
If there’s one number customers should care about, it’s Strehl.
Strehl is a score from 0 to 1:
- 1.000 would be perfect under the test conditions
- Closer to 1 is better
Rule of thumb:
- 0.80+ = good (diffraction-limited)
- 0.90+ = excellent
- 0.95+ = premium
- 0.98–1.00 = exceptional
A Strehl of 0.981 means this optic is premium and very close to exceptional.
Real-life comparison:
Imagine throwing basketballs at a hoop.
- A “good” lens gets most shots in
- A “premium” lens gets almost all shots in
- A 0.981 Strehl lens is basically hitting swishes all day
Important accuracy note: This Strehl value is typically calculated for the test wavelength and on-axis performance in that setup. It doesn’t automatically describe edge-of-field performance on a large camera sensor, and it doesn’t fully describe all colors of light for refractors. But it is an excellent measure of optical figure quality when tests are done consistently.
Now Let’s Read the Pictures (The Fun Part)
The Color Map: “Where the Lens Is Slightly High or Low”
The colorful circle shows the wavefront error across the lens.
What you want:
- smooth gradients
- no harsh, broken patterns
Serial 0026 shows smooth transitions, which matches the strong RMS and Strehl numbers.
Real-life comparison:
It’s like looking at a topographic map.
Gentle hills = smooth lens.
Sudden cliffs = something weird.
The 3D Plot: “The Lens as a Tiny Landscape”
This is the same info as the color map, just shown in 3D.
What you want:
- gentle slopes
- no sharp spikes
Serial 0026 looks smooth overall—another sign of good figure quality.
The PSF: “What Will a Star Look Like?”
PSF stands for Point Spread Function.
This is basically the report saying:
“If a star is a perfect point of light, what shape will it turn into through this lens?”
What you want:
- a bright, tight center
- round rings around it
- symmetry
Serial 0026 shows a tight, clean PSF—exactly what you’d hope for with a Strehl of 0.981.
Real-life comparison:
Think of a flashlight shining through a stencil.
A great lens makes a neat circle.
A poor lens makes a messy blob.
The Fringe/Intensity Map: “The Test Pattern”
This shows the interference fringes used during testing.
What you want:
- stable-looking fringes
- no chaos
Serial 0026 shows stable fringes, consistent with a good measurement.
“Bonus Section”: The Seidel Coefficients
This report also lists Seidel terms (types of aberrations). You might see things like:
- astigmatism
- coma
- spherical aberration
These are real lens behaviors, but here’s the easiest way to think about them:
Most people don’t need to judge the lens by these numbers.
The RMS and Strehl already summarize the total effect.
And since Serial 0026 has RMS 0.019 and Strehl 0.981, it tells us those leftover aberrations are well controlled.
What This Specific Report Means in Plain English
Here’s the “translation” of Serial 0026:
- RMS 0.019 = the lens is very smooth overall
- Strehl 0.981 = premium performance (near exceptional)
- The maps and PSF match the numbers
This is clearly above-average optical quality for an 80mm triplet objective.
What You’ll Notice Under the Night Sky
For astrophotography
A lens with this quality usually means:
- tighter stars near the center of the image
- cleaner, more precise focus
- better contrast in fine details
Real-life comparison:
It’s like upgrading from a slightly smudged camera lens to a clean one.
The scene doesn’t change—but the crispness does.
One honest note: stars in the corners of a photo depend heavily on the rest of the system too (field flatteners, spacing, camera tilt, seeing, guiding). A Zygo report is mainly proving the lens figure quality.
For visual observing
Premium optics usually give:
- sharper lunar and planetary details
- that satisfying “snap” when focus hits perfectly
- strong contrast
Also true: the atmosphere can limit you more than the lens most nights. The good news is that at 0.981 Strehl, the lens is rarely the weak link.
Closing Thoughts
At the end of the day, a Zygo report is just a fancy way of answering a simple question: How well does this lens shape light? With Serial 0026, the answer is: really well. The numbers (especially the low RMS and the high Strehl) match what you want in real life—sharp focus, strong contrast, and tight stars.
We share these reports because we think buying a telescope should feel exciting, not like a gamble. When you choose a Starfield optic, you’re not just hoping it’s good—you’re seeing the proof.