TeleVue Plössl 20mm 50° Eyepiece - 1.25"

The 17.1mm Field Stop: Framing Your View of the Sky

The true field, how much sky you see, depends on the field stop diameter and the telescope's focal length. Tele Vue's formula: True Field = (Eyepiece Field Stop ÷ Telescope Focal Length) × 57.3.

For example, in a telescope with a 1000mm focal length, the 20mm Plössl yields a true field of (17.1mm ÷ 1000mm) × 57.3 = 0.98°. That holds the full Moon with sky around it, or the Orion Nebula's bright core. The Pleiades (M45, about 2°) and the Andromeda Galaxy (M31, about 3°) need a far wider field.

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

The exit pupil is the beam of light that exits the eyepiece and enters your eye; its size determines the image brightness. Smaller exit pupils darken the sky background.

Divide the eyepiece focal length by the telescope's focal ratio. Tele Vue's chart gives:

• f/4.5 Newtonian: 4.4mm exit pupil.
• f/5.2: 3.8mm exit pupil.
• f/7 Refractor: 2.9mm exit pupil.
• f/10 SCT: 2.0mm exit pupil.
• f/14 Maksutov: 1.4mm exit pupil.

14mm Eye Relief: Observing Comfort for Eyeglass Wearers

The TeleVue 20mm Plössl has 14mm of eye relief, just under the 15mm to 20mm recommended for observers who wear glasses to correct astigmatism.

If you do not need glasses to correct astigmatism, don't use them when observing. The chart marks this eyepiece not Dioptrx ready (N).

High-Power System: Pairing the 20mm Plössl with Barlows and Powermates

Al Nagler's guide suggests choosing a long focal length eyepiece with comfortable eye relief and adding image amplifiers for power. Tele Vue makes Barlows and Powermates in 2x, 2.5x, 3x, 4x and 5x. A 2x makes the 20mm act like a 10mm.

A 3x makes it act like a 6.7mm: 179x in a 1200mm telescope, past the 150x suggested for planets and double stars.

Fast Newtonians and the Paracorr Type-2

Fast Newtonian and Dobsonian telescopes (f/5 or faster) inherently suffer from an aberration called coma, which makes stars at the edge of the field appear as small, comet-like shapes. No eyepiece removes it. Owners of f/5.0 or faster reflectors should seriously consider the Paracorr Type-2.

The Paracorr Type-2 allows an f/3 parabolic mirror to perform like an f/12 without it. It goes in a 2" focuser and acts like a 1.15x image amplifier, so a 1000mm f/4.5 scope becomes an 1150mm f/5.2, and it keeps faint stars from disappearing at the edge of the field.

20mm Plössl vs. Panoptic and DeLite: Building Your Eyepiece Set

Choose low and medium power eyepieces in field stop ratios such as 1.4 or 2.0. The 20mm's 17.1mm sits between the 25mm Plössl (21.2mm) and the 15mm Plössl (12.6mm) in the chart.

Other Tele Vue lines, from the chart:

• vs. 24mm Panoptic: 27.0mm field stop, the 1¼" maximum, and a 68° apparent field. The 20mm Plössl gives more magnification.
• vs. 19mm Panoptic: The 19mm Panoptic offers a wider 68° apparent field and a larger 21.3mm field stop, while the 20mm Plössl has 14mm of eye relief against 13mm and weighs 3.0 oz against 6.6 oz.
• vs. 18.2mm DeLite: The 18.2mm DeLite offers a wider 62° apparent field and a larger 19.1mm field stop. It has 20mm of eye relief and is Dioptrx ready (Y), so it suits eyeglass wearers.