TeleVue Plössl 40mm 43° Eyepiece - 1.25"

The 27mm Field Stop: Maximum True Field in 1.25"

The true field of view you see is determined not by the eyepiece's apparent field, but by its internal field stop diameter and your telescope's focal length. The TeleVue 40mm Plössl's 27mm field stop is the 1¼" maximum in Al Nagler's guide, so it suits low-power finding.

To calculate the true field, use the formula: True Field = (Eyepiece Field Stop ÷ Telescope Focal Length) x 57.3. For a telescope with a 2000mm focal length, the 40mm Plössl delivers a true field of (27mm / 2000mm) x 57.3 = 0.77 degrees. The 32mm Plössl and 24mm Panoptic share the same 27.0mm field stop, so they show the same sky at more magnification.

Exit Pupils from 2.9mm to 8.9mm: A Bright View for f/10 Scopes

The exit pupil, or the beam of light exiting the eyepiece, dictates image brightness. Tele Vue's chart gives 8.9, 7.7, 5.7, 4.0 and 2.9mm at f/4.5, f/5.2, f/7, f/10 and f/14:

• f/10 SCT or Maksutov: 4.0mm exit pupil and, at 2000mm, a 0.77° true field: room for the Orion Nebula's core or the Hercules Cluster, though not for the Andromeda Galaxy (M31) or the North America Nebula, which span 2° to 3°.
• f/7 Refractor: 5.7mm exit pupil, for open clusters and the Milky Way.
• f/4.5 Newtonian: 8.9mm exit pupil, over the 7mm that should not be exceeded in reflectors; light that misses your pupil is lost and the secondary's shadow may show, so it suits slower telescopes.

A 43° Apparent Field: The Trade-Off for a 40mm Focal Length

Al Nagler's guide gives Plössls 50°, and Tele Vue's chart lists the 40mm at 43°. The 40mm and 32mm both reach the 27.0mm field stop, so they show the same true field; at the 40mm's lower magnification that field looks smaller, 43° against 50°.

What it adds over the 32mm is lower power, a larger exit pupil and 28mm of eye relief against 22mm.

28mm of Eye Relief: Unmatched Comfort for Eyeglass Wearers

The 40mm Plössl has 28mm of eye relief, more than the 15mm to 20mm recommended for observers who wear glasses to correct astigmatism.

It accepts Tele Vue's Eyeguard Extender (EGE-0020), which raises the eyeguard by 8mm and includes a rubber eyeguard and eyepiece cap. Units stack: two units add 16mm and make a 40mm Plössl very comfortable for solar viewing, blocking stray daylight in white light or hydrogen-alpha viewing.

System Integration: Optimizing the 40mm Plössl with Paracorr and Dioptrx

These accessories are available for it:

• Dioptrx-Ready: Its manual says it accepts DIOPTRX (Y in the chart), which locks onto the top and corrects astigmatism in place of glasses, in ¼ to 3½ diopters.
• Paracorr for Fast Reflectors: Tele Vue's Paracorr Type-2 eliminates the coma of Newtonians and Dobsonians, and those faster than f/5.0 see the greatest results. It has a 1.15x magnification factor.
• Barlows and Powermates: Tele Vue makes them in 2x, 2.5x, 3x, 4x and 5x; a 2x makes the 40mm act like a 20mm.

40mm Plössl vs. 32mm Plössl: Exit Pupil vs. Apparent Field

Since both the TeleVue 40mm and 32mm Plössls share the maximum 27mm field stop for a 1.25" barrel, they show the exact same true field of view. From Tele Vue's chart:

• TeleVue 40mm Plössl: With its 40mm focal length, 43° AFOV, and 28mm of eye relief, it provides lower magnification and a larger exit pupil. That suits long focal ratio telescopes, and the eye relief suits eyeglass wearers.
• TeleVue 32mm Plössl: This eyepiece has a 32mm focal length, a wider 50° AFOV, and 22mm of eye relief. More magnification and a smaller exit pupil give a darker sky background. Both are Dioptrx ready (Y).