Understanding a Premium Eyepiece Set: 82°, 68° and Nagler Eyepieces, Focal Extenders and Laser Collimators

The earlier post on this site was a used-sale listing for a set of high-end eyepieces and accessories: Explore Scientific 82° eyepieces in 4.7 mm, 6.7 mm and 30 mm, a Tele Vue Nagler Type 6 13 mm, an Explore Scientific 68° 20 mm, an Explore Scientific 2x focal extender and a laser collimator. This rewrite explains what each item is for and how they work together, and corrects an error in the original listing.

Correction: barrel sizes

The original listing described the eyepieces as having “1.5 inch” barrels. There is no standard 1.5-inch eyepiece barrel. The standard sizes are 1.25 inch and 2 inch. Tele Vue lists the Nagler 13 mm Type 6 as a 1.25-inch eyepiece. Check the barrel size of each eyepiece in a lot like this before buying, because some long focal-length wide-field eyepieces use 2-inch barrels.

Apparent field of view: 68°, 82° and beyond

Every eyepiece has an apparent field of view (AFOV), the angle the view appears to fill to your eye:

• Plössl eyepieces, common in beginner kits, are narrower. Celestron lists a 52° apparent field for the Plössls in its Eyepiece and Filter Kit.
• 68° eyepieces, such as the Explore Scientific 20 mm in this set, give a wide view.
• 82° eyepieces, such as the Nagler and the Explore Scientific 82° series, are wider still.

A wider AFOV at the same magnification shows more sky. Explore Scientific makes the point for its even wider 100° eyepieces: objects are easier to locate, and on telescopes without a motor drive they stay in the field of view for longer before you need to nudge the telescope.

The Tele Vue Nagler 13 mm Type 6

Tele Vue’s specifications:

• Model: EN6-13.0
• Focal length: 13 mm
• Barrel: 1.25 inch
• Apparent field: 82°
• Eye relief: 12 mm
• Field stop diameter: 17.6 mm
• Weight: 0.40 lb (6.4 oz)

Tele Vue describes the Type 6 as a 7-element design based on the original patented Nagler, updated to improve contrast and eye relief in a compact size. It says the Type 6 is available in 13, 9, 7, 5 and 3.5 mm.

True field from the field stop

The field stop is the ring inside the eyepiece that sets the edge of the view. A handy formula: true field (degrees) = field stop ÷ telescope focal length × 57.3. For the Nagler 13 mm (17.6 mm field stop):

• On a 1500 mm telescope such as the NexStar 6SE: about 0.67°
• On a 1200 mm telescope: about 0.84°

Magnification across the set

Magnification = telescope focal length ÷ eyepiece focal length. On a 1200 mm telescope, for example:

• 30 mm: 40x (low power, wide views)
• 20 mm: 60x
• 13 mm: about 92x
• 6.7 mm: about 179x
• 4.7 mm: about 255x (only on very steady nights)

With the 2x focal extender, each eyepiece gives double those figures.

What a focal extender does

A 2x focal extender works like a Barlow lens: it doubles the telescope’s effective focal length, and so the magnification of each eyepiece. Turning five eyepieces into ten useful magnifications is a big advantage. Watch your telescope’s highest useful magnification, though. Celestron lists 354x for the NexStar 6SE, for example. Pairing the 4.7 mm with a 2x extender on a long telescope would go well beyond that.

The laser collimator

A laser collimator is a tool for aligning the mirrors of a reflector telescope (collimation). It fits in the focuser and sends a laser beam down the tube. You adjust the mirrors until the beam returns to its source. Well-collimated mirrors give sharper stars and planets.

Treat it with care. The FDA advises never aiming a laser at a person or animal, and never at vehicles or aircraft.

Eye relief and glasses

Eye relief is how far your eye can be from the lens and still see the whole field. The Nagler 13 mm’s 12 mm eye relief suits most observers without glasses. If you need to observe with glasses on, look for longer eye relief.

Is a set like this right for you?

Premium wide-field eyepieces like these are a big upgrade over basic kit eyepieces, particularly on manual Dobsonians, where the wide field keeps objects in view longer. When buying used, check each lens for scratches, haze or fungus, confirm barrel sizes, and test the collimator’s alignment before relying on it.