At a Glance
4mm
Focal Length
52°
Apparent FOV
3mm
Eye Relief
1.25"
Barrel Size
Score Breakdown
Overview
The Celestron Omni 4mm is a short focal length eyepiece designed primarily for high-magnification work on the Moon, planets, and tighter double stars. At 4mm, the magnification you get depends entirely on your telescope's focal length, so a 1000mm refractor would yield 250x, while a longer SCT at 2000mm would push 500x. Keep in mind that atmospheric seeing conditions, not the eyepiece, will often be the limiting factor at those power levels. What sets the Omni apart at this price point is its fully multi-coated, four-element Plossl design.
Those coatings matter at high magnification, where light throughput is reduced and any ghosting or internal scatter becomes more noticeable. Blackened lens edges help control contrast. The 50-degree apparent field of view is standard for a Plossl and perfectly adequate for planetary work, though it feels narrow compared to wider-field designs. The aluminum barrel and rubber eyecup construction feel solid for the $32 price.
Eye relief at 4mm is short, roughly 3 to 5mm, which is a real limitation for eyeglass wearers. If you observe without glasses, this is a capable and honest performer for its cost.
Key Features
- Four-element Plossl design with fully multi-coated optics improves light transmission and reduces ghosting at high power
- Blackened lens edges and matte aluminum barrel minimize internal reflections and improve contrast
- At 4mm focal length, delivers high magnification suited to planetary, lunar, and double star viewing
- 50-degree apparent field of view is conventional for a Plossl and works well for focused, detailed observation
- 1.25-inch barrel is filter-threaded, allowing use of lunar or color planetary filters
- Very short eye relief of approximately 3 to 5mm makes this a poor choice for eyeglass wearers
- Solid build quality and Celestron's two-year warranty make it a low-risk entry into high-magnification viewing
Specifications
| Focal Length | 4mm |
| Apparent FOV | 52° |
| Eye Relief | 3mm |
| Barrel Size | 1.25" |
| Design | Plossl |
| Elements/Groups | 4/2 |
| Series | Omni |
| Coating | Fully Multi-Coated |
| Waterproof | No |
Popular Telescopes for This Eyepiece
ZWO Seestar S50
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Celestron NexStar 130SLT
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Unistellar Odyssey
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Amazon Reviews
I got the 4mm eyepiece to use with a 114mm reflector with a FL of 500. At first I was skeptical, I read reviews and had many "experts" tell me it would provide too much magnification. I assume they saw 114mm and assumed the worst. This thing provides excellent magnification and produces a detailed i...
This eye piece hit it out of the park. I was able to get super detail pictures of the moon with it on my telescope and my cellphone camera.
As far as the 4 mm goes, it's a nice eyepiece; seems to be well built. The eye relief is not so good (i.e. you have to put your eye very close to the piece) but it is a very clear image. I also have the 15 mm, and it is fantastic. Good eye relief as well.
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