Explore Scientific 92° 17mm Long Eye Relief Waterproof Eyepiece - 2"

The 92° Apparent Field at 17mm

You may be startled at the wide expanse of inky black sky the first time you look through a 92° Series eyepiece, and may even forget there is an eyepiece between you and the universe. With an apparent field close to the maximum for hyper-wide designs, objects are easier to locate.

In a non-driven telescope, an object stays in the field a very long time before you need to adjust. Hyper-wide eyepieces also make averted vision easy: looking slightly to the side of a faint object puts its light on a part of your eye that is more sensitive to light, allowing you to see fainter images.

20.02mm of Eye Relief

The 92° 17mm has 20.02mm of eye relief; the 92° Series is an extra long eye relief design. This measurement refers to the fixed distance from the eyepiece's final lens to the point where your eye must be positioned to see the full field of view. How much of the field an eyeglass wearer sees depends on the glasses.

Long eye relief also means you do not have to crowd the eyepiece.

An 8-Element, 6-Group Design

The 17mm uses 8 elements in 6 groups. It has an optimized optical design, exotic glass combinations and durable coatings.

The optics are fully multi-coated and edge blackened, which produces maximum contrast.

Argon-Purged Waterproofing for All-Weather Reliability

Each eyepiece is O-ring sealed and purged with an inert gas to create a waterproof environment, with a waterproof, fog-proof housing.

The 2-Inch Barrel and 1302g Build: Balancing the Load

The eyepiece needs a 2-inch focuser or diagonal. A tapered 2" barrel adds extra security. It measures 152mm long and 69.8mm wide.

With a weight of 1302 g (2.87 lbs), the 92° 17mm is a substantial piece of optical equipment. Make sure the focuser holds it firmly, and expect to rebalance the telescope when inserting or removing it.

Calculating True Field of View from the 27.46mm Field Stop

The true field of view (TFOV), how much actual sky the eyepiece shows, is determined by its field stop diameter and the focal length of your telescope. The Explore Scientific 92° 17mm has a field stop of 27.46mm. To calculate the TFOV in degrees for your specific telescope, use the following formula:

TFOV = (Field Stop Diameter / Telescope Focal Length) x 57.3

• 2000mm focal length: (27.46 / 2000) x 57.3 ≈ 0.79 degrees.
• 1200mm focal length: (27.46 / 1200) x 57.3 ≈ 1.31 degrees.

17mm in Practice: Magnification Examples for Your Telescope

The magnification an eyepiece provides is calculated by dividing your telescope's focal length by the eyepiece's focal length.

Magnification = Telescope Focal Length / 17mm

• 600mm focal length: 600mm / 17mm = 35x magnification.
• 1200mm focal length: 1200mm / 17mm = 71x magnification.
• 2800mm focal length: 2800mm / 17mm = 165x magnification.

Expanding to 2x or 3x Magnification with Explore Scientific Focal Extenders

You can increase the versatility of the 92° 17mm eyepiece by pairing it with an Explore Scientific Focal Extender. Optional focal extenders are available for the 92° Series, and they increase the magnification of any eyepiece with the same barrel size.

• With a 2x Focal Extender (FE02-020): The 17mm eyepiece will perform like an 8.5mm eyepiece, doubling the magnification.
• With a 3x Focal Extender (FE03-020): The 17mm eyepiece will perform like a 5.7mm eyepiece.

In Explore Scientific's words, 3 eyepieces offer 6 magnification choices with a focal extender.

Handling and Care for Your 92° Series Eyepiece

At 1302 g, grip the body firmly when inserting or removing it, and tighten the focuser's lock screws.

The sealed housing keeps the internal optics out of reach of dew and dust; clean only the outer lenses, and only when needed.