TeleVue Plössl 25mm 50° Eyepiece - 1.25"

Where the 25mm Plössl Fits

Al Nagler's guide gives Plössls a 50° apparent field, against 68° for Panoptics and 82° for Naglers. The 25mm has 4 elements in 2 groups.

The 32mm and 40mm Plössls reach the 27mm field stop, the 1¼" maximum; the 25mm's 21.2mm is the next step down, with more magnification for clusters and galaxies.

Calculating True Field with a 21.2mm Field Stop

The true field depends on the field stop diameter and the telescope's focal length. Tele Vue's formula:

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

For example, in a telescope with a 1200mm focal length, the 25mm Plössl yields a true field of just over one degree: (21.2mm ÷ 1200mm) x 57.3 = 1.01°. That is twice the full Moon's width. The Pleiades (M45), about 2° across, need a shorter telescope: in a TV-76 (480mm) the field is 2.5°.

2.5mm to 5.6mm Exit Pupils from f/10 to f/4.5

The exit pupil, the beam of light leaving the eyepiece, sets how bright the view looks: eyepiece focal length divided by the telescope's focal ratio.

Tele Vue's chart gives:

• 5.6mm at f/4.5
• 4.8mm at f/5.2
• 3.6mm at f/7
• 2.5mm at f/10
• 1.8mm at f/14

17mm Eye Relief for Eyeglass Compatibility

The 25mm Plössl has 17mm of eye relief, inside the 15mm to 20mm recommended for observers who wear glasses to correct astigmatism.

Tele Vue's chart marks it not Dioptrx ready (N), so observers with astigmatism keep their glasses on with this eyepiece.

Barrel and Filters

The 1¼" barrel takes 1¼" filters (M28.5x0.6). The optical length is 2.5" (64 mm).

Tele Vue's insert lists BandMate filters for it.

Framing the Sky for Medium-Power Sweeping

Build a set in uniform steps of field stop diameter. From the 32mm Plössl's 27mm, the 25mm's 21.2mm is a 1.27 step.

Switching from the 32mm to the 25mm gives 28% more magnification and a darker sky background, useful in the Milky Way's star fields or among the Virgo galaxies.

Coma Correction with Paracorr Type-2 at f/5.2

Fast Newtonian and Dobsonian reflectors suffer from an inherent optical aberration called coma, which distorts stars at the edge of the field into comet-like shapes. Tele Vue's Paracorr Type-2 eliminates it.

Owners of f/5.0 or faster reflectors should seriously consider it. It also acts like a 1.15x image amplifier. For example, with a Paracorr Type-2, a 1000mm f/4.5 telescope becomes an 1150mm f/5.2 system. The 25mm then gives 46x and a 4.8mm exit pupil.

Higher Powers 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 (which it calls an improvement to the Barlow-type design) in 2x, 2.5x, 3x, 4x and 5x.

With a 2.5x Powermate the 25mm acts like a 10mm, 203x in an 8" SCT of 2032mm.

TeleVue Plössl 25mm vs. 32mm Plössl & 24mm Panoptic

Comparisons from Tele Vue's chart:

• vs. 32mm Plössl: 27.0mm field stop, the 1¼" maximum, and 22mm of eye relief, Dioptrx ready (Y). The 25mm gives 28% more magnification and a darker sky background.
• vs. 24mm Panoptic: The 24mm Panoptic also has a 27mm field stop, giving it the same true field as the 32mm Plössl but with more magnification. It has a 68° apparent field and accepts Dioptrx with an adapter (Y*). The 25mm Plössl has more eye relief, 17mm against 15mm.

BandMate Filters

Tele Vue's insert says BandMate filters pass select wavelengths and block others to darken the sky, so nebulae "pop out".

• Bandmate Nebustar: a general-purpose nebula filter that, unlike typical UHC designs, blocks red wavelengths and eliminates star bloat.
• Bandmate OIII: enhances planetary nebulae and supernova remnants.
• Bandmate H-Beta: brings out the hydrogen-beta emission lines of hydrogen emitting nebulae.

Weight and Warranty

The 25mm Plössl weighs 4.3 oz (122 g) and has an optical length of 2.5" (64 mm).

Tele Vue eyepieces carry a Lifetime Limited warranty against defects in material or workmanship.