Takahashi TPL-50 50mm Plossl Eyepiece

Product Overview

The TPL-50 is the long-focal-length end of Takahashi's TPL Plossl series — a 50 mm eyepiece with a 48° apparent field and 37 mm of eye relief, from four elements in two groups. Where the rest of the TPL range fits a standard 1.25-inch (31.7 mm) barrel, the TPL-50 uses a 2-inch (50.8 mm) barrel, the widest field stop the series offers.

At 520 g it is also the heaviest eyepiece in the line, in keeping with its larger barrel and field lens.

Who It's For

This is a good match if you already own, or are adding, a 2-inch diagonal or focuser and want the lowest power and widest true field the TPL series offers — for sweeping the Milky Way, framing large open clusters, or a comfortable wide finder-style view before switching to a higher-power eyepiece. The 37 mm eye relief is generous enough for extended sessions and most eyeglass frames.

It is a visual eyepiece. Imaging trains connect a camera directly and do not use one.

Key Features & Design

• 50 mm focal length: the lowest power and widest true field in the TPL series — see Compatibility for real figures.
• 48° apparent field: the traditional Plossl field width, sharp across the center with mild softening toward the edge.
• 37 mm eye relief: generous, relaxed viewing, comfortable with most glasses.
• 2-inch (50.8 mm) barrel: larger than the rest of the TPL range, which uses a 1.25-inch (31.7 mm) barrel.
• 4 elements in 2 groups: the symmetric Plossl configuration, scaled up for the longer focal length.
• Multi-coated optics: 99% visible-light transmission with a matte barrel finish to control stray light.

Optical Design

The Plossl design pairs two achromatic doublets symmetrically around the field stop — four elements in two groups. At 50 mm focal length the field lens and barrel scale up accordingly, which is why the TPL-50 moves to a 2-inch barrel where the rest of the series stays at 1.25 inches: a 48° field at this focal length needs a wider field stop than a 1.25-inch barrel can pass without vignetting.

Takahashi describes the TPL series as sharp from the center out to roughly the halfway point of the 48° field, with mild star distortion toward the corners. Takahashi also states that pairing the series with the 2x Ortho Barlow clearly improves flatfieldness — though at 50 mm, doubling the focal length through a Barlow gives up most of the low-power, wide-field advantage that makes the TPL-50 worth owning in the first place.

Recommended Uses

• Sweeping the Milky Way and rich star fields at the lowest power your telescope can offer.
• Large open clusters and asterisms that do not fit the field of a shorter-focal-length eyepiece.
• A wide finder-style view for locating targets before switching to higher power.
• Relaxed, extended viewing sessions given the 37 mm eye relief.

Compatibility and Accessory Notes

• Magnification: divide your telescope's focal length by 50. An FS-60CB at 355 mm gives about 7x; an FC-100DZ at 800 mm gives about 16x; a Mewlon 210 at 2415 mm gives about 48x.
• Barrel: 50.8 mm (2-inch) — this eyepiece needs a 2-inch diagonal or focuser. It will not fit a 1.25-inch visual back.
• Diagonal: a 2-inch star diagonal or a focuser drawtube with a 2-inch visual back is required; Takahashi's 2-inch mirror diagonal is a direct pairing.
• 2x Ortho Barlow: Takahashi publishes that flatfieldness across the TPL series clearly improves when used through the 2x Ortho Barlow, at roughly double this eyepiece's magnification.
• Back focus: for adapter and spacing questions across your imaging or visual train, see the Takahashi Back Focus Guide.

Good to Know Before You Order

• This is the one TPL eyepiece that needs a 2-inch diagonal or focuser. The rest of the series shares a 1.25-inch barrel; the TPL-50 does not, and a 1.25-inch visual back will not accept it.
• 48° is a traditional field, not an ultra-wide one. An 82° or 100° eyepiece gives a more immersive view at the same low power; the TPL trades that width for a simpler design that still delivers a sharp center field.
• Magnification depends on your telescope. The same 50 mm gives about 7x on a short 60 mm refractor and 48x on a Mewlon 210 — send us your telescope model and we will confirm where it lands in your eyepiece set.
• At 520 g, it is worth accounting for in the balance of a lighter refractor's focuser, though most Takahashi drawtubes carry it without issue.

Frequently Asked Questions

Is it difficult to set up?

It needs a 2-inch diagonal or focuser rather than the 1.25-inch fitting most eyepieces use — once that is in place, it drops in and you focus like any other eyepiece.

What magnification will I get?

Your telescope's focal length divided by 50. Send us your telescope model and we will work out where it fits alongside the eyepieces you already own.

Why does this one need a 2-inch barrel when the rest of the TPL series doesn't?

A 48° field at 50 mm focal length needs a wider field stop than a 1.25-inch barrel can pass without vignetting, so Takahashi built the TPL-50 on a 2-inch barrel instead.

Is it good for deep-sky observing?

Yes — it gives the widest true field and lowest power in the TPL series, suited to large clusters, asterisms and Milky Way sweeping.

Can I use it with glasses?

37 mm of eye relief is comfortable for most glasses wearers.

Will it work with a Barlow?

Yes — the 2x Ortho Barlow doubles the magnification and, per Takahashi, improves the TPL series' flatfieldness at the same time, though doing so gives up most of the low-power view this eyepiece is built for.

Is it parfocal with other Takahashi eyepieces?

Swapping to a different eyepiece family means refocusing.

Bottom Line

In short: a 50 mm Plossl eyepiece with a 48° field, 37 mm of eye relief and a 2-inch barrel — the widest, lowest-power eyepiece in the TPL range, built for sweeping rich star fields once a 2-inch diagonal or focuser is in place.