Description
Both numbers appear because they measure different things. The physical eye relief distance from the eye lens to the eye point is about 23mm. But the eye lens sits roughly 10mm below the top of the eyecup housing, so the effective eye relief from the eyecup is about 15mm. For most observers without glasses, 15mm is comfortable. Eyeglass wearers may find it tight one CN reviewer specifically noted it wasn’t enough for his glasses.
Is this good for binoviewing?
Yes this may be the 25mm PF’s single best use case. Multiple owners specifically bought these for binoviewing. They’re lightweight (2 oz each), inexpensive enough to buy in pairs, and at 25mm provide a comfortable low-power binoview. In an f/10 SCT the most common binoviewer host the PF’s off-axis correction is at its best.
How does the 25mm compare to the other PF focal lengths?
The 25mm is the simplest design in the line 4 elements versus 5 in the middle trio (19mm, 15.5mm, 10.5mm) and 6 in the 5.5mm. Those extra elements buy you better edge correction. If you try the 25mm and like it, the 10.5mm and 5.5mm are where the PF design really shines Alan Dyer’s review for Cloudy Nights called the set superb performance for the price, and found the shorter focal lengths well-corrected across the entire field. Nearly parfocal across the line.
Accessories
- Astro-Tech PF 10.5mm 1.25″ eyepiece The medium-power companion. 5-element design with better edge correction than the 25mm. Pair them for a two-eyepiece starter kit covering low and medium power.
- Astro-Tech PF 5.5mm 1.25″ eyepiece The high-power end of the PF line. 6-element design. Alan Dyer’s review called the 25mm + 10.5mm + 5.5mm a good three-eyepiece beginner set.
- Any standard 1.25″ nebula or light pollution filter Threads directly onto the eyepiece barrel. An OIII or UHC filter at 4080x on emission nebulae transforms the view.
Final Thoughts
The 25mm PF is a telescope upgrade for people who aren’t sure they want to spend money on telescope upgrades yet. It’s the answer to “what’s the least I can spend and still see a real difference?” and in an f/8 or slower telescope, the difference is real. Wider field than the stock Plssl. Comfortable eye relief. Light enough to forget it’s there. It won’t replace a premium eyepiece, and it doesn’t need to. It just needs to show a beginner or a binoviewer builder, or someone who wants a grab-and-go low-power eyepiece they won’t cry over if it gets scratched that better views don’t always require a bigger investment. At this price, with this performance in the right telescope, it does that job.
Tech Details:
| Focal Length | 25mm |
| Apparent Field of View | 65 (stated) / ~5860 (measured) |
| Field Stop Diameter | 25.9mm |
| Optical Elements | 4 elements / 3 groups, multicoated |
| Eye Relief | 15mm effective (23mm physical) |
| Barrel Size | 1.25″ tapered |
| Filter Threads | Yes standard 1.25″ |
| Weight | 5.2 oz (with caps) |
| Eyecup | Rubber fold-down |
| Warranty | 1 year |
Reviews
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