You've been observing with the stock 25mm eyepiece for a few months. You know the Orion Nebula by heart at low power. But you've noticed that when you try to push in on a globular cluster or split a double star, you need more magnification — and you don't have it. The 15.5mm Astro-Tech Premium Flat Field bridges that gap. It nearly doubles the magnification of a stock 25mm, keeps the field wide at 65°, and at 5.2 ounces it won't upset the balance on any telescope. Five elements in three groups keep the field flat where a standard Plössl would start to fall apart. It's not trying to compete with premium eyepieces. It's trying to give you a legitimate medium-power view for the price of dinner out.
The Optics
Five elements in three groups, fully multicoated — the same element count as the 19mm PF in the same line. That fifth element, absent in the simpler 4-element 25mm PF, provides meaningfully better correction at the field edge, especially in faster telescopes. Alan Dyer's review for Cloudy Nights found the PF line sharp across most of the field, with softening only in the outer 10–15%. No false color on bright targets. No ghost images. The kind of clean, quiet performance you stop noticing because nothing distracts from the view.
The stated 65° apparent field of view is wider than any Plössl (50–52°) and wide enough to feel the difference the moment you look through it. In a 1200mm Dobsonian at 77x, it yields a true field of about 0.84° — wide enough to frame the Orion Nebula with dark sky around it. In an 8-inch SCT at 131x, the true field narrows to about half a degree, but at that magnification you're after detail, not context.
Built for the Job
The 15.5mm PF weighs 5.2 ounces with caps — negligible on any mount. Rubber fold-down eyecup for positioning. Standard 1.25" filter threads accept any nebula, light pollution, or planetary filter. The 16mm of eye relief is comfortable for most observers, though glasses wearers will find the 19mm PF's 20mm eye relief slightly more forgiving. Nearly parfocal across the entire PF line, so building a set means minimal refocusing when swapping eyepieces.
Binoviewing
Lightweight, flat-field, and affordable enough to buy in pairs — the PF series was practically designed for binoviewers. At 15.5mm, you get a medium-power binoview that pushes into the magnification range where deep-sky objects start showing structure: resolved stars in globulars, dust lanes in bright galaxies, detail in planetary nebulae.
What's Included
- Astro-Tech 15.5mm PF (Premium Flat Field) 1.25" eyepiece
- Lens caps (top and bottom)
- Lens cleaning cloth
Features
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65° apparent field of view — Wider than a Plössl (50–52°), narrower than an 82° UWA. A genuine step up in field width without premium cost.
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5-element multicoated optics — Same element count as the 19mm PF. Better edge correction than the 4-element 25mm PF, especially at faster focal ratios.
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15.5mm focal length — Medium power in most telescopes. 77x in a 1200mm Dob. 131x in an 8" SCT. 58x in a 90mm refractor. The step between low-power survey and high-power detail.
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16mm eye relief — Comfortable for most observers. Glasses wearers can use it, though the 19mm PF's 20mm eye relief is slightly roomier.
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Nearly parfocal across the PF line — Build a set and barely touch the focuser between swaps.
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Lightweight — 5.2 oz with caps — No balance concerns. Excellent for binoviewing in pairs.
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Standard 1.25" filter threads — Accepts any standard 1.25" astronomy filter.
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Flat field design — Stars stay sharp further toward the field edge than in a standard Plössl. Best results at f/7 and slower; still good at f/5 with some edge softening.
Under the Night Sky
In an 8-inch f/10 SCT at 131x, Jupiter shows cloud belt detail — the North and South Equatorial Belts are obvious, and on a steady night the Great Red Spot is visible as a notch in the southern belt. Saturn's Cassini Division is clean, and the shadow of the globe on the rings gives the planet its three-dimensional character. This is the magnification range where planets stop being points of light and start being places.
In an 8-inch f/6 Dobsonian at 77x, the Hercules Cluster (M13) starts resolving into individual stars at the edges — the characteristic graininess that separates a globular from a fuzzy blob. The Ring Nebula (M57) is a clean, distinct smoke ring. The Orion Nebula fills the field with sculpted nebulosity, the Trapezium stars cleanly split, and the dark lane that cuts across the bright core is visible on a decent night.
In a short refractor like the AT72EDII at 28x, this is still a low-power eyepiece — but with a true field over 2° and sharp stars across 65°, it frames the Double Cluster with room to spare and shows the Pleiades as a scattering of blue-white diamonds against dark sky.
Observing Tip
If you're building a PF set, the 15.5mm pairs naturally with the 25mm PF for a clean low-to-medium magnification range — roughly 2x between them. Add the 5.5mm PF and you have a three-eyepiece kit that covers low, medium, and high power with nearly parfocal swapping and consistent edge quality. That's real range for the price of a single premium eyepiece.
FAQ
How does this compare to the 19mm PF?
Same optical formula — 5 elements in 3 groups. The difference is focal length: the 15.5mm gives about 25% more magnification. Eye relief drops from 20mm (19mm PF) to 16mm (15.5mm PF) — still comfortable for most observers, but glasses wearers may notice the difference. Choose the 19mm if eye relief is your priority; choose the 15.5mm if you want a bit more magnification.
Will this work in my f/5 telescope?
Yes, with the same caveat that applies to any budget wide-field design: the center will be sharp, and toward the edges you'll see some softening from residual astigmatism. At f/7 and slower, the field is flat and sharp nearly all the way to the edge. The 5-element design handles faster focal ratios better than the 4-element 25mm PF.
Is 16mm of eye relief enough for glasses?
For most eyeglass wearers, yes — 16mm is adequate. If you find it tight, the 19mm PF offers 20mm of eye relief with the same optical quality. The fold-down rubber eyecup helps all observers find the right eye position.
Can I use this for binoviewing?
The PF line is excellent for binoviewing. At 15.5mm in a pair, you get medium-power binoviewing with flat fields, comfortable eye relief, and lightweight bodies that won't strain the binoviewer or your balance. Several owners bought PFs specifically for binoviewing.
What magnification will I get?
Divide your telescope's focal length by 15.5. In a 1200mm Dobsonian: 77x. In a 2032mm SCT: 131x. In the 430mm AT72EDII: 28x. In a 900mm refractor: 58x.
Accessories
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Astro-Tech PF 25mm 1.25" eyepiece — Low-power companion. Together with the 15.5mm, you have a 2x magnification range that covers wide-field survey to medium-power detail.
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Astro-Tech PF 5.5mm 1.25" eyepiece — High-power end of the line. 6-element design. Add it to the 15.5mm for planetary and double-star work.
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Astro-Tech PF 10.5mm 1.25" eyepiece — The medium-high step between 15.5mm and 5.5mm. 5 elements / 4 groups for tighter correction at higher magnification.
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Any standard 1.25" nebula or light pollution filter — Threads directly onto the eyepiece barrel.
Final Thoughts
The 15.5mm PF lives in the part of your eyepiece case that gets the most use — high enough magnification to resolve detail in deep-sky objects and show structure on planets, low enough that seeing conditions rarely shut you down. If you already own the 25mm PF and you're looking for a meaningful step up in magnification without a meaningful step up in price, this is it. Same flat-field design, same light weight, same filter threads, and nearly parfocal with the rest of the line.
