Explore Scientific 100° 25mm Waterproof Eyepiece - 2"

The 100° Apparent Field of View

You may be startled at the wide expanse of inky black sky the first time you look through a 100° Series eyepiece, and may even forget there is an eyepiece between you and the universe. Objects are easier to locate, and in a non-driven telescope an object stays in the field a very long time before any adjustment is needed.

Hyper-wide eyepieces also make averted vision easy: looking slightly to the side of a faint object puts the image on a part of your eye that is more sensitive to light, allowing you to see fainter images.

A 9-Element, Fully Multi-Coated Optical System

The 25mm uses 9 lens elements. It has an optimized optical design, exotic glass combinations for optimum results and durable coatings.

All 100° Series eyepieces are fully multi-coated and edge-blackened to produce maximum contrast.

Managing 14.5mm of Eye Relief for Viewing Comfort

The eyepiece features an eye relief of 14.5mm, which is the fixed distance from the top lens to the point where your eye must be positioned to see the full field of view.

Explore Scientific does not say how much of the field eyeglass wearers see.

Argon-Purged and Waterproof to 1 Meter

Each 100° Series eyepiece is internally sealed and purged with inert, dry argon gas, which also maximizes the life of the internal coatings, with a waterproof, fog-proof housing.

Every 100° eyepiece is tested by submerging it to 1 meter of water for 30 minutes.

The 2-Inch Barrel and 1177g Weight

This eyepiece is built with a standard 2-inch barrel, designed to fit any telescope with a 2-inch focuser or diagonal. The barrel is tapered for extra security. It measures 140mm long and 73mm wide.

At 1177 g (41.5 oz), expect to rebalance your telescope when switching from a smaller 1.25-inch eyepiece.

What the 41mm Field Stop Reveals About Your True Field

The true field of view, the actual circle of sky you see, is determined not by the apparent field, but by the eyepiece's internal field stop diameter and your telescope's focal length. The 25mm 100° eyepiece has a large 41mm field stop, which is the physical aperture inside the eyepiece that defines the edge of the view.

You can calculate the true field of view in degrees using the formula: TFOV = (Field Stop Diameter / Telescope Focal Length) * 57.3. For example, in a telescope with a 2000mm focal length, this eyepiece would yield a true field of (41 / 2000) * 57.3 ≈ 1.17 degrees. At 1200mm it is about 1.96 degrees, and at 600mm about 3.9 degrees.

Pairing the 25mm Eyepiece with 2-Inch Focal Extenders

For added versatility, the 25mm eyepiece can be paired with Explore Scientific's 2-inch Focal Extenders. Optional focal extenders are available for the 100° Series, and a focal extender increases the magnification of any eyepiece with the same barrel size.

The catalog lists two 2-inch models:

• 2x Focal Extender (FE02-020): Turns the 25mm eyepiece into a 12.5mm eyepiece, doubling the magnification.
• 3x Focal Extender (FE03-020): Converts the 25mm eyepiece into an 8.3mm eyepiece.