TeleVue Ethos 13mm 100° Eyepiece - 1.25"/2"

The 100° Spacewalk: Defining the Ethos Experience

Only the 110° Ethos-SX is wider in Tele Vue's range (Plössl 50°, Panoptic 68°, Delos 72°, Nagler 82°, Ethos 100°). The 13mm Ethos gives enough magnification to show detail in galaxies and nebulae while the 100° field keeps a wide view of the sky around them.

The 13mm focal length provides a significant step up in power from low-magnification finder eyepieces, darkening the sky background and increasing contrast on faint nebulae and galaxies. The 100° field ensures you see not just the object itself, but its full context within the surrounding star field, useful on large objects such as the Veil Nebula or the Andromeda Galaxy (M31).

Framing the Cosmos: The 22.3mm Field Stop and True Field Calculation

While the 100° apparent field of view (AFOV) defines the immersive experience, the 22.3mm physical field stop inside the eyepiece determines the actual patch of sky you see, known as the true field of view (TFoV). The AFOV is a fixed property of the eyepiece design, but the TFoV changes depending on the focal length of your telescope.

You can calculate the true field of view for the 13mm Ethos in your telescope using Tele Vue's formula:

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

• In a Tele Vue NP101is (540mm FL): (22.3mm ÷ 540mm) x 57.3 = 2.37° true field at 41.5x, the figures Tele Vue prints for this pairing.
• In a 10" f/10 SCT (2540mm FL): (22.3mm ÷ 2540mm) x 57.3 = 0.50° true field, about the width of the full Moon.

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

The exit pupil (the image of the objective formed by the eyepiece, where you place your eye) is a key factor for image brightness and matching the eyepiece to your eye. The exit pupil is calculated by dividing the eyepiece focal length by the telescope's focal ratio (f/#).

Tele Vue's chart gives these exit pupils for the 13mm Ethos (2.5mm at f/5.2 as well):

• f/4.5 Telescope: 2.9mm exit pupil, for detailed views of galaxies and nebulae under dark skies.
• f/7 Telescope: 1.9mm exit pupil, which provides a darker sky background and enhances contrast on planetary nebulae.
• f/10 Telescope: 1.3mm exit pupil, delivering higher magnification for resolving globular clusters and planetary detail.
• f/14 Telescope: 0.9mm exit pupil, for splitting tight double stars and observing fine lunar features.

Observing Comfort: 15mm Eye Relief and Dioptrx Astigmatism Correction

The 13mm Ethos features 15mm of eye relief, which is the distance you must place your eye from the top lens to see the full field of view. This distance is comfortable for most observers who do not wear glasses.

If you require eyeglasses to correct for astigmatism, the 15mm eye relief may not be enough to see the entire 100° field at once. The 13mm Ethos is Dioptrx ready: you can add the Tele Vue Dioptrx astigmatism corrector to the Ethos instead of using eyeglasses, to get the sharpest view possible. Dioptrx corrects astigmatism; your focuser still corrects near- or far-sightedness.

The Dual 2" / 1¼" Barrel and Focus Position

The 13mm Ethos has a dual barrel that allows it to be used in both 1.25" and 2" focusers and diagonals. The eyepiece is optically designed as a 1.25" eyepiece, and the 2" barrel is included just for convenience. It is important to note there is no optical benefit to using it in a 2" holder.

• Parfocal with 10mm Ethos: The 13mm and 10mm Ethos eyepieces are parfocal, Tele Vue adds that in 2" mode they focus out about 3/8" further than a 26mm Nagler Type 5, and in 1¼" mode in about ½" further than a 25mm Plössl.
• Secure Mounting: When used in a 1.25" holder, a slot in the 2" barrel section accommodates the holder's lock screw, which clamps to the 1¼" barrel, while the eyepiece's own lock screw can clamp to the outside of the holder. The eyepiece is heavy and can fall out of slip-fit holders unless the lock screw is used. In 2" holders, remove the eyepiece lock screw and do not let the holder screw slip into the slot.
• Filter Use: The 1.25" barrel is threaded for standard 1.25" filters. To use 2" (48mm) filters, the optional EBX-2120 2" extension tube can be threaded onto the 2" barrel.
• In-Travel Limitation: The optical path of some diagonals, like the Tele Vue 60° Everbrite, is unusually long and may prevent the 13mm Ethos from reaching focus in some refractors.

Paracorr, Barlows and Powermates

A 100° field shows the telescope's own off-axis aberrations, most notably coma in fast Newtonian reflectors.

For this reason, use the Paracorr Type-2 coma corrector in Newtonian or Dobsonian telescopes with focal ratios of f/5.5 or faster. The Paracorr eliminates the mirror coma, giving the same kind of full-field sharpness Tele Vue's NP-Series refractors show. Do not set the Paracorr's tunable top to its lowest setting with a 1¼" filter in the Ethos, as the filter could touch the Paracorr's upper lens; a 2" filter on the Paracorr or the EBX-2120 is the better route. When using Powermates for higher magnification, the Ethos 13mm should be inserted using its 2" barrel. Be aware that using a 1.25" filter on the eyepiece can risk contact with the upper lens of a 2x Powermate; it is safer to thread a 2" filter onto the front of the Powermate itself.

Building a Flagship Set: Ethos 13mm vs. Nagler and Delos Eyepieces

The 13mm Ethos occupies a key position in the Tele Vue lineup, offering a different balance of field of view and eye relief compared to its siblings. When building an eyepiece set, choose eyepieces based on their field stop diameters to ensure uniform steps in true field of view.

• vs. 16mm Nagler Type 5: The 16mm Nagler has a nearly identical field stop (22.1mm) but a narrower 82° apparent field. The 13mm Ethos provides the same true field of view at a higher magnification, resulting in a darker sky background.
• vs. 12mm Delos: The 12mm Delos offers 20mm of eye relief and a 15.0mm field stop with a 72° field; the 13mm Ethos has 15mm of eye relief and a larger 22.3mm field stop with a 100° field.
• vs. 10mm Ethos: The 10mm Ethos is the next logical step up in power. With its 17.7mm field stop, it offers a smaller true field but higher magnification. Since they are parfocal, they pair naturally.