TeleVue Ethos 21mm 100° Eyepiece - 2"

The 100° Apparent Field "Spacewalk"

The defining feature of the Ethos 21mm is its 100° apparent field of view (AFOV), which gives an immersive view of the sky. Tele Vue's Ethos and Ethos-SX are the widest apparent fields in its range (Plössl 50°, Panoptic 68°, Delos 72°, Nagler 82°, Ethos 100°, Ethos-SX 110°).

In practice, this immense AFOV allows the Ethos 21mm to function as both a deep-sky and a finder eyepiece. You can frame the entirety of large objects like the Andromeda Galaxy (M31) or the North American Nebula (NGC 7000) while operating at a magnification high enough to darken the sky background and enhance contrast, revealing faint structures that would be lost in a lower-power, narrower-field eyepiece.

Calculating Your View: The 36.2mm Field Stop and True Field

While the apparent field defines the immersive experience, the 36.2mm field stop determines the actual patch of sky you see. The true field of view (TFoV) is calculated with the formula: TFoV = (Eyepiece Field Stop ÷ Telescope Focal Length) x 57.3. For the Ethos 21mm, this large field stop yields exceptionally wide true fields for its focal length.

• In an 11-inch SCT of 2800mm focal length: The Ethos 21mm produces a true field of (36.2mm / 2800mm) x 57.3 = 0.74°. That is enough to frame the bright core of the Orion Nebula (M42) around the Trapezium.
• In a Tele Vue NP101is (540mm FL): The true field expands to (36.2mm / 540mm) x 57.3 = 3.84°, the figure Tele Vue prints for this pairing (25.7x, 3.84°, 3.9mm exit pupil). This turns the telescope into a rich-field instrument capable of sweeping the star clouds of Sagittarius.

Power and Brightness: Exit Pupils from 4.7mm to 1.5mm

The 21mm focal length is a versatile choice, producing a range of useful exit pupils across different telescope f-ratios. The exit pupil (the image of the objective formed by the eyepiece, where you place your eye) governs image brightness; Tele Vue calculates it as eyepiece focal length divided by the telescope's f-number.

In fast Dobsonians, the Ethos 21mm excels. An f/4.5 scope yields a 4.7mm exit pupil, ideal for maximizing the brightness of faint nebulae. For f/10 Schmidt-Cassegrains, the 2.1mm exit pupil darkens the sky background significantly, increasing contrast on galaxies and resolving the cores of globular clusters. At f/14 the exit pupil is 1.5mm (Tele Vue chart: 4.7, 4.0, 3.0, 2.1 and 1.5mm at f/4.5, f/5.2, f/7, f/10 and f/14).

Observing Comfort: 15mm Eye Relief and Handling a 36 oz Eyepiece

The Ethos 21mm provides 15mm of eye relief, a comfortable distance for most observers who do not wear glasses. This allows you to relax your eye and take in the full 100° field without straining. For observers with astigmatism, you can add the Tele Vue Dioptrx astigmatism corrector to the Ethos instead of using eyeglasses, to get the sharpest view possible (the 21mm Ethos is Dioptrx ready).

At 36 oz (1020 g), this is a substantial optic requiring proper consideration for telescope balance. For large Dobsonians, you may need to add counterweights to the mirror box to prevent the tube from tipping forward. On German equatorial mounts, ensure your payload capacity is sufficient and that your diagonal's compression ring can securely hold the eyepiece.

Optimizing for f/5.5 or Faster Scopes with Paracorr

Fast Newtonian and Dobsonian reflectors (f/5.5 or faster) naturally exhibit coma, an aberration that distorts off-axis stars into comet-like shapes. The eyepiece's vast 100° field will make this aberration apparent at the edges. To achieve pinpoint stars across the entire field, pair the Ethos 21mm with a Paracorr Type-2 coma corrector.

The Paracorr goes into your 2" focuser like a 2" Barlow and has a 15% (1.15x) magnification factor, so a 1000mm f/4.5 scope becomes an 1150mm f/5.2 scope. It permits the same kind of full-field sharpness routinely seen in Tele Vue NP-Series refractors, and the Paracorr Type-2 works with scopes as fast as f/3.0.

Ethos 21mm vs. Nagler 31mm: Magnification vs. Field Area

The most common comparison for a flagship deep-sky eyepiece is between the Ethos 21mm and the Nagler 31mm Type 5. While both are premier 2" eyepieces, they serve different primary purposes.

• Nagler 31mm (42mm Field Stop): Delivers a wider true field of view and a larger exit pupil at any given f-ratio. It suits low-power scanning of the Milky Way under dark skies.
• Ethos 21mm (36.2mm Field Stop): Provides 1.5x more magnification than the Nagler 31mm. This results in a smaller exit pupil, which darkens the sky background and increases contrast, a help for observers under light-polluted skies.

The choice depends on your goal. For the widest true field, choose the Nagler 31mm. For more magnification and contrast on large objects, choose the Ethos 21mm. Tele Vue's own advice for building a set is to choose low and medium powers by field stop steps (factors of about 1.4 or 2.0) and to avoid duplicating focal lengths.