Explore Scientific 100° 5.5mm Waterproof Eyepiece - 2"

High-Power Viewing for the Dedicated Observer

At 5.5mm it gives the highest magnification of the 100° Series in a given telescope.

The 100° Apparent Field

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. The huge field gives a panoramic view and relaxes the eye, so you can pay attention to subtle details for extended periods.

With an apparent field close to the maximum for hyper-wide designs, 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.

A Note on the 10.5mm Eye Relief

The Explore Scientific 100° 5.5mm eyepiece has an eye relief of 10.5mm. This is the distance from the top lens of the eyepiece to the point where your eye must be positioned to see the full field of view. A shorter eye relief is a common trade-off in high-power, wide-field eyepiece designs.

A 9-Element Design

The 5.5mm 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.

Argon-Purged and Waterproof to 1 Meter

Each 100° Series eyepiece is internally sealed and purged with inert, dry argon gas, with a waterproof, fog-proof housing.

It says the purge also maximizes the life of the internal coatings, and that every 100° eyepiece is tested by submerging it to 1 meter of water for 30 minutes.

The 2-Inch Barrel and 612 g Build

The eyepiece needs a 2" focuser or diagonal. The barrel is tapered for extra security. It measures 150mm long and 56mm wide.

At 612 g (21.6 oz), you may need to rebalance the telescope when switching from a small 1.25" eyepiece.

Calculating the 9.6mm Field Stop's True Field

The true field of view (TFoV) is the actual patch of sky you see through the eyepiece. It is determined by the eyepiece's field stop diameter and the focal length of your telescope. For the 5.5mm eyepiece, the field stop is 9.6mm. To calculate the TFoV in degrees, you use the formula: (Field Stop Diameter / Telescope Focal Length) x 57.3.

For example, in a telescope with a 2000mm focal length, the calculation would be: (9.6mm / 2000mm) x 57.3 = 0.275°. While this is a narrow patch of sky, the 100° apparent field makes this small window feel vast and immersive. At 1200mm focal length, the true field would be larger: (9.6mm / 1200mm) x 57.3 = 0.458°.

Power Potential with a 5.5mm Focal Length

Magnification is calculated by dividing your telescope's focal length by the eyepiece's focal length.

• 600mm focal length: 600 / 5.5 = 109x.
• 1200mm focal length: 1200 / 5.5 = 218x.
• 2000mm focal length: 2000 / 5.5 = 364x.

Reaching Extreme Magnification with 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. With a 5.5mm eyepiece they mainly suit short focal length telescopes.

The catalog lists two 2" models:

• 2x Focal Extender (FE02-020): This accessory effectively transforms the 5.5mm eyepiece into a 2.75mm eyepiece, doubling its magnification.
• 3x Focal Extender (FE03-020): This accessory turns the 5.5mm eyepiece into an effective 1.83mm eyepiece.

What's Included with Your Eyepiece

The package holds the eyepiece itself, for any telescope with a 2-inch focuser or diagonal.