Tele Vue Delos 12mm 72° Eyepiece - 1.25"

The Delos 12mm: A 72° Field with 20mm of Eye Relief

The Delos line was developed from Tele Vue's experience with the 100° Ethos series, which let Tele Vue keep Delos size and weight down. Each Delos in the instruction sheet has 20mm of eye relief with a 72° apparent field.

The Delos achieves sharpness, linearity (freedom from pincushion distortion) and ghost suppression over the entire field for any speed (f/#) telescope.

Calculating True Field from the 15.0mm Field Stop

An eyepiece's true field of view (TFoV), how much actual sky it shows, is determined by its field stop diameter and the telescope's focal length. The Delos 12mm has a 15.0mm field stop. You can calculate the true field for your specific telescope using Tele Vue's formula:

TFoV = (Eyepiece Field Stop ÷ Telescope Focal Length) x 57.3°

For example, in a telescope with a 1200mm focal length, the Delos 12mm would deliver a true field of 0.72°: (15.0mm ÷ 1200mm) x 57.3° = 0.72°. That frames the bright core of the Orion Nebula (M42), or Jupiter and its moons.

Performance Metrics from f/4.5 to f/14: Magnification and Exit Pupil

The 12mm focal length produces useful magnifications for a wide range of targets. In a popular 8" f/10 Schmidt-Cassegrain (2032mm focal length), it yields 169x, a useful power for planets and double stars. In a short, 600mm f/5 refractor, it provides 50x, for sweeping the star fields of Sagittarius.

The exit pupil, which controls image brightness, varies with the telescope's f-ratio. Tele Vue's chart gives 2.7, 2.3, 1.7, 1.2 and 0.9mm at f/4.5, f/5.2, f/7, f/10 and f/14:

• At f/4.5: 2.7mm exit pupil (nebulae and galaxies in a Dobsonian)
• At f/7: 1.7mm exit pupil (clusters and lunar detail)
• At f/10: 1.2mm exit pupil (planetary viewing)
• At f/14: 0.9mm exit pupil (bright planets in long-focus scopes)

20mm Eye Relief: A Comfortable View with or without Eyeglasses

Every 1¼" Delos has 20mm of eye relief. The Delos is designed for optimum viewing whether or not you wear eyeglasses, and its eyeguard housing in the down position gives eyeglass wearers the full eye relief. The suggested range for eyeglass wearers is 15mm to 20mm.

Tele Vue adds that the generous eye relief minimizes eye lens fogging from eye moisture in cold weather. Without glasses, the adjustable eyeguard sets your eye at the right height, since an eye too far in or out causes edge darkening and field loss.

The Adjustable, Locking Eyeguard Housing

To manage the long 20mm eye relief, the Delos has a two-piece, adjustable eyeguard housing. To set its position, you grasp the upper and lower rubber grip rings and twist the lower ring to the left to loosen the assembly. This allows the eyeguard to slide up and down freely.

While looking through the eyepiece, start with it fully extended and slide it down until you just see the full field with your peripheral vision. Green indicator lines on the inner barrel help you note your preferred position. To lock it in place, simply twist the lower grip ring back to the right. This system also minimizes extraneous outside light, for the highest contrast on faint objects.

Sharpness and Linearity at Any f-Ratio

The Delos keeps sharpness, linearity and ghost suppression over the entire field for any speed (f/#) telescope. In a fast Newtonian, the telescope's own coma still needs a corrector (see below).

Freedom from pincushion distortion means straight lines stay straight across the field, and objects keep their shape as they drift toward the edge.

Optimizing for Fast Newtonians with Paracorr Type-2

Newtonian and Dobsonian reflectors with fast focal ratios (f/5 or faster) suffer from inherent coma, an aberration that makes stars at the edge of the field look like small comets. Tele Vue's Paracorr Type-2 eliminates it, with the greatest results in scopes faster than f/5.0.

The Paracorr inserts into your 2" focuser and has a 1.15x magnification factor, like a mild Barlow, so a 1000mm f/4.5 scope becomes an 1150mm f/5.2 scope.

Delos 12mm vs. Nagler and DeLite: Choosing Your Mid-Power Eyepiece

Comparisons from Tele Vue's chart:

• vs. Nagler Type 6 (13mm or 9mm): The Naglers have an 82° apparent field and 12mm of eye relief, and need an adapter for Dioptrx (Y*). The Delos 12mm has 20mm of eye relief and is Dioptrx ready (Y).
• vs. DeLite 13mm: The DeLite 13mm also has 20mm of eye relief and is Dioptrx ready, with a 62° field and a 13.8mm field stop; the Delos 12mm has the wider 72° field and a 15.0mm field stop.

Accessory Ecosystem: Dioptrx, FoneMate, and BandMate Filters

The Delos manual lists these Tele Vue accessories:

• DIOPTRX™ Ready: The Delos is ideal for Tele Vue's Dioptrx astigmatism corrector, which locks onto the top of the eyepiece and is always aligned to the eyepiece optics.
• FoneMate™ Compatible: The FoneMate smartphone adapter clamps to compatible Tele Vue eyepieces for still, video or timelapse images.
• BandMate™ Filters: The 1¼" barrel takes 1¼" filters (M28.5 x 0.6). BandMate filters make nebulae "pop out" by passing select wavelengths and darkening the sky.

Build, Weight (408g), and In the Box

The Delos 12mm is a substantial eyepiece, weighing 14.4 oz (408 g). The chrome 1¼" barrel is threaded for 1¼" filters.

Tele Vue cautions against unscrewing the chrome barrel from the main body to avoid getting dust and dirt into the eyepiece. The eyepiece comes with top and bottom dust caps for protection during storage. It is covered by Tele Vue's Lifetime Limited Warranty.