The Explore Scientific 100° Series 9 mm is a hyper-wide-field eyepiece with a 2-inch barrel. The earlier post on this site was a used-sale listing that repeated the manufacturer’s description. This rewrite explains, in plain English, what the eyepiece does and how to use it, with specifications checked against Explore Scientific’s product page.
Verified specifications
From Explore Scientific’s product page:
- Apparent field of view: 100°
- Focal length: 9 mm
- Barrel size: 2 inch
- Eye relief: 12.5 mm
- Field stop diameter: 15.7 mm
- Optical elements: 9
- Size: 145 mm long, 60 mm wide
- Weight: 602 g (21.2 oz)
- Argon-purged and waterproof; Explore Scientific says each 100° eyepiece is tested submerged to 1 metre for 30 minutes
- Fully multi-coated and edge-blackened for contrast
- Item number: EPWP1009-01
What “100 degrees” means
The apparent field of view is how wide the view looks to your eye. Many basic eyepieces are much narrower. For example, Celestron lists a 52° apparent field for the Plössls in its Eyepiece and Filter Kit. At 100°, the edge of the view is far out to the side of your vision. Explore Scientific describes it as close to the maximum for hyper-wide eyepiece design.
Explore Scientific highlights two practical benefits:
- Easier finding and tracking: objects are easier to locate, and on telescopes without a motor drive they stay in view for a long time before you need to move the telescope.
- Averted vision: the wide field makes it easy to look slightly to one side of a faint object. This uses a more light-sensitive part of the eye, helping you see fainter detail.
Magnification on common telescopes
Magnification equals the telescope’s focal length divided by 9 mm:
- 600 mm refractor: about 67x
- 1200 mm Dobsonian: about 133x
- 1500 mm Schmidt-Cassegrain: about 167x
- 2032 mm Schmidt-Cassegrain: about 226x
True field: how much sky you see
True field can be estimated from the field stop: field stop ÷ telescope focal length × 57.3. With a 15.7 mm field stop:
- 600 mm: about 1.5°
- 1200 mm: about 0.75°
- 1500 mm: about 0.6°
That is a lot of sky for the magnification. On a 1200 mm telescope you get 133x with a field of about three-quarters of a degree.
Is it right for your telescope?
- Barrel: it needs a 2-inch focuser or diagonal. It will not fit a 1.25-inch-only focuser.
- Weight: at 602 g it is heavy. On a Dobsonian you may need to adjust the tension or balance. On small mounts, check that the extra weight does not unbalance the tube.
- Eye relief: 12.5 mm suits most observers without glasses. Glasses wearers may not see the whole field.
- Telescope type: hyper-wide eyepieces show the most benefit on manual telescopes such as Dobsonians, where the wide field keeps objects in view longer.
Buying used: what to check
- Glass: look at the top and bottom lenses under a torch for scratches, haze or marks.
- Barrel: check it is round and free of burrs.
- Waterproofing: argon-purged eyepieces should show no internal fogging or moisture.
- Caps and case: these protect the large top lens.
Using it well
- Relax your eye and move it slightly to take in the whole field.
- Sweep slowly across star clusters and the Milky Way.
- Try averted vision on faint galaxies.
- Use it on the Moon and planets at medium power when the air is steady.
The bottom line
The Explore Scientific 100° 9 mm is a heavy, 2-inch, hyper-wide eyepiece that makes star fields, clusters and nebulae feel immersive, and keeps objects in view longer on manual telescopes. Check your focuser size and balance before buying, and inspect the glass carefully if buying second-hand.
Quick summary
- The Explore Scientific 100° 9 mm has a 100° apparent field, a 2-inch barrel, 12.5 mm eye relief and a 15.7 mm field stop.
- It uses 9 optical elements, weighs 602 g and measures 145 mm by 60 mm.
- It is argon-purged and waterproof, and Explore Scientific tests each one submerged to 1 metre for 30 minutes.
- On a 1200 mm telescope it gives about 133x with roughly a 0.75° true field.
- It is most useful on manual telescopes, where the wide field keeps objects in view longer, and it helps with averted vision on faint objects.
- Check that your focuser takes 2-inch eyepieces, and that your mount or Dobsonian can handle the extra weight.
Jargon buster
- Apparent field of view: how wide the view looks to your eye, in degrees.
- True field: how much actual sky you see, which depends on the telescope as well as the eyepiece.
- Field stop: the ring inside the eyepiece that sets the edge of the view.
- Eye relief: how far from the lens your eye can be while still seeing the whole field.
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