Tele Vue Delos 17.3mm 72° Eyepiece - 1.25"

The Delos Design: A 72° Field Corrected for Any Focal Ratio

The Delos achieves sharpness, linearity (freedom from pincushion distortion) and ghost suppression over the entire field for any speed (f/#) telescope. The 72° apparent field is wide enough to frame large open clusters and nebulae.

• Sharpness: Based on Tele Vue's Ethos design experience, held over the entire field.
• Linearity: The view is free from pincushion distortion, so straight lines stay straight across the field.
• Ghost Suppression: Ghost suppression over the entire field is one of the Delos design goals.

Field Geometry: The 21.2mm Field Stop, True Field, and Exit Pupil

The true field of view you see is determined by the eyepiece's 21.2mm field stop and your telescope's focal length. You can calculate it with the formula: True Field = (Eyepiece Field Stop ÷ Telescope Focal Length) x 57.3. For example, in a telescope with a 1200mm focal length, the Delos 17.3mm yields a true field of (21.2mm ÷ 1200mm) x 57.3 = 1.01°.

The exit pupil, which dictates image brightness, is determined by your telescope's f/ratio. Tele Vue's chart gives 3.8, 3.3, 2.5, 1.7 and 1.2mm at f/4.5, f/5.2, f/7, f/10 and f/14. In a fast f/4.5 Newtonian, it produces a bright 3.8mm exit pupil for nebulae. In a typical f/10 Schmidt-Cassegrain, it yields a 1.7mm exit pupil for globular clusters.

Viewing Comfort: 20mm Eye Relief and a Locking Eyeguard

Every 1¼" Delos, including the 17.3mm, has 20mm of eye relief. The Delos is designed for optimum viewing whether or not you wear eyeglasses, and the generous eye relief also minimizes eye lens fogging in cold weather.

To ensure perfect eye placement, the Delos has an adjustable, locking eyeguard housing. By twisting the lower grip ring, you can slide the eyeguard to the ideal height and lock it in place. For non-eyeglass wearers, start with the housing fully out and slide it down until you just see the full field with your peripheral vision; this also minimizes extraneous outside light. For eyeglass wearers, the eyeguard retracts fully to provide maximum clearance.

Fast Newtonians, Barlows and Powermates

Fast Newtonians and Dobsonians show mirror coma. Tele Vue's Paracorr Type-2 removes it and works with scopes as fast as f/3.0, with the greatest results in scopes faster than f/5.0. The Paracorr acts as a 1.15x amplifier, effectively turning an f/4.5 scope into an f/5.2, for which the Delos 17.3mm provides a 3.3mm exit pupil.

Choose a long focal length eyepiece with comfortable eye relief and add a Barlow or Powermate to increase power. A 2x Barlow makes the 17.3mm act as an 8.65mm eyepiece for planets and double stars.

Delos 17.3mm vs. Nagler & Panoptic: Choosing Your Mid-Power Eyepiece

The Delos 17.3mm compares naturally to eyepieces with similar field stops, such as the 19mm Panoptic and 16mm Nagler Type 5.

• vs. Panoptic 19mm: The Panoptic has a 21.3mm field stop, nearly identical to the Delos's 21.2mm, meaning they show almost the same true field of view. The Panoptic offers a 68° apparent field with 13mm of eye relief. The Delos 17.3mm provides a 72° apparent field and 20mm of eye relief.
• vs. Nagler 16mm Type 5: The Nagler offers an 82° apparent field with a larger 22.1mm field stop, showing slightly more sky. Its eye relief is 10mm against the Delos's 20mm, and the chart marks it not Dioptrx ready (N).