There's a magnification range where deep-sky observing gets interesting — where globular clusters stop looking like fuzzy spots and start resolving into stars, where planetary nebulae show their shapes instead of just their glow, and where the planets finally show enough detail to be worth studying. In most telescopes, that range starts around 100–120x. The Astro-Tech 10.5mm Premium Flat Field puts you there. In an 8-inch Dobsonian at 114x, you're in the zone. In an 8-inch SCT at 194x, you're pushing into serious planetary territory. Five elements in four groups — an extra group over the longer PF eyepieces — keep the 60° field flat and sharp where lesser designs start to fall apart.
The Optics
Five elements in four groups, fully multicoated. The extra group compared to the 19mm and 15.5mm PF (both 5 elements / 3 groups) provides additional correction at the higher magnification this focal length demands. Alan Dyer's review for Cloudy Nights tested all five PF eyepieces and found sharp stars across most of the field, with softening only in the outer 10–15%. No false color on bright targets like Jupiter or Saturn. No ghost images. The kind of clean performance that lets you focus on what you're looking at instead of what the eyepiece is doing wrong.
The 60° apparent field of view is 5° narrower than the longer PF eyepieces (25mm, 19mm, 15.5mm are all 65°). That's a design trade-off: at shorter focal lengths, tightening the field slightly lets the optics maintain edge quality without adding more glass. It's still meaningfully wider than a Plössl's 50–52° and more than enough to frame any deep-sky object you'll be observing at this magnification.
Built for the Job
At 5.2 ounces with caps, the 10.5mm PF adds nothing to your balance equation. Rubber fold-down eyecup, 16mm of eye relief — comfortable for most observers including glasses wearers. Standard 1.25" filter threads for nebula or planetary filters. Nearly parfocal across the entire PF line, so swapping between the 10.5mm and any other PF eyepiece is close to seamless. Dyer specifically noted the near-parfocal behavior in his review — a genuine convenience when observing.
Binoviewing
The PF line excels in binoviewers, and the 10.5mm is arguably the most useful focal length for binoviewing deep-sky objects. At this magnification, you're resolving real detail — but the flat field means both eyes see the same sharp view. Lightweight bodies, comfortable eye relief, and a price that makes buying two a reasonable proposition.
What's Included
- Astro-Tech 10.5mm PF (Premium Flat Field) 1.25" eyepiece
- Lens caps (top and bottom)
- Lens cleaning cloth
Features
-
60° apparent field of view — Wider than a Plössl (50–52°). The slightly tighter field compared to the longer PFs (65°) is a deliberate design choice that helps maintain edge sharpness at higher magnification.
-
5-element / 4-group multicoated optics — One more group than the 15.5mm and 19mm PFs. The additional grouping provides better correction at the higher magnifications this focal length delivers.
-
10.5mm focal length — Medium-high power workhorse. 114x in a 1200mm Dob. 194x in a 2032mm SCT. 86x in a 900mm refractor. The magnification range where deep-sky objects start showing real structure.
-
16mm eye relief — Comfortable for most observers, including many glasses wearers. Fold-down rubber eyecup for positioning.
-
Nearly parfocal across the PF line — Build a set and barely touch the focuser between swaps.
-
Lightweight — 5.2 oz with caps — No balance concerns. Excellent for binoviewing in pairs.
-
Standard 1.25" filter threads — An OIII or UHC filter at 100–120x on emission nebulae can reveal structure invisible without filtration.
-
Flat field design — Stars stay sharp toward the field edge. 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/6 Dobsonian at 114x, M13 (the Hercules Cluster) resolves. Not just the outer fringe — individual stars appear across the cluster face, and the characteristic three dark lanes radiating from the core begin to emerge. This is the magnification where globulars transform from fuzzy objects into three-dimensional stellar cities. M92, the often-overlooked second globular in Hercules, shows its tight, blazing core. M3 in Canes Venatici resolves beautifully.
In an 8-inch f/10 SCT at 194x, you're in planetary territory. Jupiter shows cloud belt detail — festoons, ovals, and the shadow of a Galilean moon transiting the disk on a good night. Saturn shows the Cassini Division cleanly, the Crepe Ring, and the shadow of the planet on the rings. Mars near opposition shows the dark surface markings and a polar cap. This is serious planetary magnification, and the 10.5mm PF delivers a clean, sharp view to support it.
In a 90mm refractor at 86x, the Ring Nebula (M57) is a clean oval ring with dark center. The Dumbbell Nebula (M27) shows its distinctive shape. And double stars like Albireo and Epsilon Lyrae are cleanly split — the latter revealing all four components on a steady night.
Observing Tip
Alan Dyer's review for Cloudy Nights recommended the 25mm, 10.5mm, and 5.5mm as a three-eyepiece starter set from the PF line. Those three give you roughly 2.5x magnification steps between them — enough to cover low-power survey, medium-high detail, and high-power planetary work. Because the PF line is nearly parfocal, you spend your time observing instead of refocusing.
FAQ
How is this different from the 15.5mm PF?
More magnification (about 50% higher in any given telescope) and a slightly different optical formula — 5 elements in 4 groups instead of 3 groups. The apparent field is 60° instead of 65°, which is a design trade-off to maintain edge quality at the higher magnification. Eye relief is the same 16mm. Choose the 15.5mm for wide-field deep-sky; choose the 10.5mm when you need to push in for detail.
Will this work in my f/5 telescope?
Yes. At f/5, the center will be sharp and clean. Some edge softening is visible, as with any budget wide-field design at fast focal ratios. The 4-group configuration helps manage this better than a simpler design would. At f/7 and slower, the field is flat and sharp nearly to the edge.
Is 194x too much for an 8-inch SCT?
Not at all — 194x is well within the useful range of an 8-inch telescope. The practical limit for most 8-inch scopes is around 300–400x on the best nights, though atmospheric seeing usually limits you to 200–250x. At 194x you'll be seeing conditions-limited on some nights, but on a steady night, the view is excellent.
Can I use this for binoviewing?
The PF line is excellent for binoviewing, and the 10.5mm is one of the most useful focal lengths for binoview deep-sky observing. Flat field, lightweight, comfortable eye relief, and affordable enough to buy in pairs.
What magnification will I get?
Divide your telescope's focal length by 10.5. In a 1200mm Dobsonian: 114x. In a 2032mm SCT: 194x. In the 430mm AT72EDII: 41x. In a 900mm refractor: 86x.
Accessories
-
Astro-Tech PF 25mm 1.25" eyepiece — Low-power companion. The 25mm + 10.5mm pair covers survey to detail in two eyepieces.
-
Astro-Tech PF 5.5mm 1.25" eyepiece — High-power end of the line. 6-element design for maximum correction. Together with the 10.5mm and 25mm, it completes the three-eyepiece set that Alan Dyer recommended.
-
Astro-Tech PF 19mm 1.25" eyepiece — Fill the gap between the 25mm and 10.5mm with a medium-power step.
-
Any standard 1.25" nebula or planetary filter — An OIII filter at 100–120x on emission nebulae is one of the most dramatic improvements you can make to a deep-sky view.
Final Thoughts
The 10.5mm PF hits the magnification range that most observers reach for most often — high enough to resolve real detail in deep-sky objects and show cloud belts on planets, low enough that atmospheric seeing doesn't shut you down on an average night. Its 4-group optical design is the most refined in the PF line apart from the 6-element 5.5mm, and at this price, it's hard to argue with. If you're building a PF set, this is the one Alan Dyer put in his recommended starter kit. There's a reason for that.
