High-power eyepieces used to mean a choice: pay for premium glass, or squint through a cheap 4mm Plössl with 3mm of eye relief and a field the size of a drinking straw. The Astro-Tech 5.5mm Premium Flat Field changes that equation. Six elements in four groups — the most complex design in the PF line — deliver a clean, corrected 60° field with 16mm of eye relief. That's enough room to observe comfortably, even at 218x in an 8-inch Dobsonian. It won't replace a Pentax XW or a Tele Vue Delos. But at this price, it gives you real planetary and double-star magnification without punishing you for using it.
The Optics
Six elements in four groups, fully multicoated. This is the most complex optical formula in the PF line — one more element and one more group than the 10.5mm — and it shows. A Cloudy Nights reviewer tested the 5.5mm in a 16-inch f/4.5 Dobsonian and found the field reasonably well corrected even without a coma corrector, with only mild astigmatism near the edges. With a Paracorr, the edge improved further: minimal astigmatism, no SAEP, no field curvature, no edge-of-field brightening. That's impressive behavior at f/4.5, where most budget eyepieces fall apart.
Alan Dyer's review for Cloudy Nights confirmed sharp stars across most of the field for the PF line, with softening only in the outer 10–15%. No false color on bright targets, no ghost images. At 5.5mm, you're pushing high magnification where any optical flaw gets amplified — and the 6-element design holds up.
Built for High Power
At 7.2 ounces with caps, the 5.5mm PF is the heaviest eyepiece in the line — that extra element adds glass. It's still light by any reasonable standard. The 16mm eye relief is the same as the 10.5mm and 15.5mm PFs, which means comfortable viewing even at high magnification — a dramatic improvement over short-focal-length Plössls where you practically press your eye against the lens. Rubber fold-down eyecup, standard 1.25" filter threads, tapered barrel. Nearly parfocal across the PF line.
Knowing the Limits
At 5.5mm, the exit pupil gets small. In an 8-inch f/6 Dob, you're at 218x with a 0.93mm exit pupil — a dim, tight beam that demands good collimation and rewards dark-adapted eyes. In an 8-inch SCT at 369x, you've reached the theoretical limit of the aperture on all but the steadiest nights. The 5.5mm PF is not an every-night eyepiece in large telescopes. But on a night of steady seeing, it's the eyepiece that shows you what your telescope can actually do.
Where it really earns its place is in medium-focal-length telescopes. In a 1000mm refractor at 182x, or a 1200mm Dob at 218x, or a 900mm f/10 at 164x — these are magnifications where the atmosphere cooperates more often than not, and where the 5.5mm PF delivers clean, sharp views of planets, double stars, and small planetary nebulae.
What's Included
- Astro-Tech 5.5mm PF (Premium Flat Field) 1.25" eyepiece
- Lens caps (top and bottom)
- Lens cleaning cloth
Features
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60° apparent field of view — Wider than a Plössl (50–52°). A meaningful improvement over the tunnel-vision experience of cheap short-focal-length eyepieces.
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6-element multicoated optics — The most complex design in the PF line. One more element than the 10.5mm PF. Better edge correction to handle the demands of high magnification.
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5.5mm focal length — High-power specialist. 218x in a 1200mm Dob. 164x in a 900mm refractor. 78x in the 430mm AT72EDII. Planetary and double-star territory.
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16mm eye relief — Comfortable viewing at high power. A dramatic improvement over 4mm and 6mm Plössls, which typically offer 3–4mm of eye relief.
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Nearly parfocal across the PF line — Swap from the 25mm PF to the 5.5mm PF with barely a touch of the focuser.
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7.2 oz with caps — Heavier than the other PFs (5.2 oz) due to the extra glass. Still light by any standard.
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Standard 1.25" filter threads — A planetary filter or light yellow filter can enhance contrast on Jupiter and Saturn at this magnification.
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Flat field design — Stars stay sharp toward the field edge. Tested at f/4.5 in a 16" Dob by a CN reviewer with good results.
Under the Night Sky
In an 8-inch f/6 Dobsonian at 218x on a night of good seeing, Jupiter shows the kind of detail that makes you lean forward: the North and South Equatorial Belts textured with festoons, the Great Red Spot clearly defined, a Galilean moon's shadow transiting the disk as a tiny black dot. Saturn shows the Cassini Division as a clean dark line, the Crepe Ring fading into the planet's shadow, and the globe's pastel cloud bands. This is the magnification where planets go from interesting to compelling.
In a 90mm f/10 refractor at 164x, Epsilon Lyrae — the Double-Double — splits into all four components on a steady night. Albireo shows its gold-and-blue color contrast strikingly. And small planetary nebulae like the Blue Snowball (NGC 7662) or the Cat's Eye (NGC 6543) show their disks instead of looking stellar.
One CN observer pushed the 5.5mm PF through a 2x Barlow in a 16-inch Dobsonian — 530x — and reported M13 appeared like a river of stars. That's well beyond the magnification this eyepiece was designed for, and the fact that it held up speaks to the quality of the optical design.
Community Says
"I'll cut to the chase: nice eyepiece!" This owner tested the 5.5mm PF in a 16-inch f/4.5 Dobsonian — one of the most demanding fast scopes you can put an eyepiece in. Without a coma corrector, the field was "reasonably well corrected with some mild coma/stig near the edges but nothing to complain about." With a Paracorr: "minimal stig at the edge" and no field curvature, no brightening, no SAEP. The same observer later barlowed it to 530x on M13 and found the eyepiece handled it cleanly. — Cloudy Nights 5.5mm PF Review. This owner observes with a 16-inch f/4.5 Dob under moderately light-polluted skies.
Observing Tip
High magnification demands good collimation. If you're using the 5.5mm PF in a Dobsonian or Newtonian, make sure your collimation is dialed in before blaming the eyepiece for a soft view. A miscollimated telescope at 200x+ will show comatic star images that have nothing to do with the eyepiece. Collimate first, then evaluate. On a night of steady seeing with a well-collimated scope, the 5.5mm PF will show you more than you expect.
FAQ
Is 5.5mm too much magnification for my telescope?
Divide your telescope's focal length by 5.5 to find out. As a rough guide, useful magnification tops out around 50x per inch of aperture on a good night — so an 8-inch telescope can handle about 400x in theory, though 200–250x is more realistic most nights. If the 5.5mm PF puts you above that range, wait for the best seeing nights or use a Barlow with a longer PF eyepiece for a more moderate boost.
How is this different from the 10.5mm PF?
Roughly double the magnification, one more optical element (6 vs. 5), one more group (4 vs. 4 — but a different configuration). The 5.5mm is a dedicated high-power eyepiece; the 10.5mm is a versatile medium-high. Both have 60° AFOV and 16mm eye relief. The 5.5mm weighs 7.2 oz vs. the 10.5mm's 5.2 oz due to the additional glass.
Will this work at f/5?
Yes. A CN reviewer tested it at f/4.5 in a 16-inch Dob and found the field well corrected with only mild edge astigmatism — and that's without a coma corrector. At f/5 and slower, performance improves further. The 6-element design handles fast focal ratios better than simpler eyepiece designs.
Is 16mm of eye relief really 16mm at this focal length?
Yes — that's the advantage of the multi-element design. Traditional 5–6mm Plössls offer about 3–4mm of eye relief, which means pressing your eye against the lens. The PF's 16mm is comfortable and workable, including for many glasses wearers.
Can I Barlow this eyepiece?
A CN reviewer barlowed the 5.5mm PF with a 2x Barlow in a 16-inch Dob (530x) and was impressed with the results. That's extreme magnification, useful only on the steadiest nights, but it confirms the optics hold up under a Barlow. A more practical use is a 2x Barlow in a medium-aperture scope — giving you an effective 2.75mm eyepiece at 16mm eye relief.
Accessories
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Astro-Tech PF 10.5mm 1.25" eyepiece — The medium-high power companion. Together with the 5.5mm and 25mm PF, you complete the three-eyepiece starter set Alan Dyer recommended in his Cloudy Nights review.
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Astro-Tech PF 25mm 1.25" eyepiece — Low-power wide-field. The bookend to the 5.5mm — from survey to detail in two eyepieces.
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Astro-Tech 2x Telecentric Barlow (AT2XTB) — Doubles the magnification of any PF eyepiece while maintaining eye relief. Turns the 5.5mm into an effective 2.75mm.
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Any standard 1.25" planetary or color filter — A light yellow (#8) or blue (#80A) filter can enhance planetary contrast at high magnification.
Final Thoughts
Short-focal-length eyepieces have always been where budget designs hit a wall — too few elements to correct the field, too little eye relief to observe comfortably, too narrow a view to enjoy the magnification. The 5.5mm PF pushes through that wall with six elements, 16mm of eye relief, and a 60° field that actually lets you use the magnification it delivers. It was tested at f/4.5 in a 16-inch Dob and held its own. If you observe planets, double stars, or small planetary nebulae and you've been putting off buying a high-power eyepiece because the good ones cost ten times this much — stop putting it off.
