The Celestron Ultima 12.5 mm Eyepiece: What to Know Before Buying a Used One

The earlier post on this site was a used-sale listing for a Celestron Ultima 12.5 mm eyepiece, described by the seller as an early, high-quality “Masuyama design” eyepiece that is becoming hard to find. This rewrite turns that listing into a buyer’s guide. It explains what a 12.5 mm eyepiece does on typical telescopes and what to check when buying a vintage eyepiece.

What we could not verify

We could not find a Celestron source that loads today describing the Ultima line’s optical design, history or availability. Discussions among amateur astronomers describe it in various ways, but we have not repeated those claims here. In particular, we have removed the “Masuyama design” description from the original listing, because we could not confirm it from a manufacturer or other authoritative source. If you are buying one, judge it on its condition and the view, not on the design label.

What a 12.5 mm eyepiece does

Magnification equals the telescope’s focal length divided by the eyepiece’s focal length. Celestron’s own figures follow this rule: its NexStar 6SE (1500 mm focal length) gives 60x with a 25 mm eyepiece, and its PowerSeeker 114EQ (900 mm) gives 45x with a 20 mm eyepiece.

A 12.5 mm eyepiece gives:

• 900 mm telescope (such as a PowerSeeker 114EQ): 72x
• 1000 mm telescope: 80x
• 1250 mm telescope: 100x
• 1500 mm telescope (such as a NexStar 6SE): 120x
• 2032 mm telescope (an 8-inch Schmidt-Cassegrain): about 163x

On most telescopes that is a medium-high power, ideal for the Moon, planets, double stars and small, bright deep-sky objects like globular clusters.

Staying within useful magnification

Celestron publishes a “highest useful magnification” for each telescope. For example, 269x for the PowerSeeker 114EQ and 354x for the NexStar 6SE. A 12.5 mm eyepiece stays well within these limits on most telescopes, and with a 2x Barlow it behaves like a 6.25 mm eyepiece. Celestron’s Eyepiece and Filter Kit includes a 2x Barlow for this purpose. On a 1500 mm telescope that would mean 240x, which needs very steady air.

Barrel size

Eyepieces come in two standard barrel sizes, 1.25 inch and 2 inch. The original listing did not state the barrel size, so check it with the seller, and make sure it matches your focuser or star diagonal before buying.

How to check a used eyepiece

• Look through the lens at an angle under a bright torch. Light dust is normal; scratches, haze or fungus are not.
• Check the coatings. Coated lenses show a faint coloured reflection. Patchy or worn areas suggest heavy cleaning in the past.
• Inspect the barrel. It should be round, undented and free of burrs that could scratch your focuser.
• Check the filter thread at the bottom of the barrel if you plan to use filters.
• Look at the box and caps. Original packaging, as in the listing, helps protect the glass in storage and shipping.
• Test it if you can. Focus on a distant object in daylight. The image should be sharp across most of the field.

Caring for eyepieces

• Keep the dust caps on when not in use.
• Blow off dust before wiping anything.
• Use lens tissue or a microfibre cloth with a little lens cleaner, wiping gently.
• Store eyepieces in a dry place to discourage fungus.

Is a vintage eyepiece worth it?

Older eyepieces can give excellent views and may cost less than new premium models. But age makes condition the deciding factor. Buy from a seller who shows clear photos of the glass, and favour sellers who allow returns. If you simply need a medium-power eyepiece, a modern 12 mm or 13 mm Plössl, like those in Celestron’s kit, is a reliable alternative.

Related guides on this site

• Buying used telescope eyepieces
• Telescope eyepiece and filter kits explained
• Understanding a premium eyepiece set
• Celestron PowerSeeker 114EQ beginner’s guide

The bottom line

A 12.5 mm eyepiece is a versatile medium-to-high power for most telescopes. A clean, well-kept Celestron Ultima can be a good buy, but judge it by its condition and how it performs, not by claims about its design that are difficult to verify.

Quick summary

• A 12.5 mm eyepiece gives 72x on a 900 mm telescope and 120x on a 1500 mm telescope.
• With a 2x Barlow it behaves like a 6.25 mm eyepiece.
• We removed the “Masuyama design” claim because we could not verify it.
• Judge a vintage eyepiece by the condition of its glass, coatings and barrel.

Sources: Celestron NexStar 6SE specifications; Celestron PowerSeeker 114EQ specifications; Celestron Eyepiece and Filter Kit.