Tele Vue DeLite 4mm 62° Eyepiece - 1.25"

The DeLite Concept: 62° Field and 20mm Eye Relief

Tele Vue's DeLite manual says each DeLite has 20mm of eye relief and a 62° apparent field, and that its Delos design experience let it keep the size and weight down.

The DeLite achieves the best sharpness, linearity (freedom from pincushion distortion) and ghost suppression over the entire field for any speed (f/#) telescope. Short focal length eyepieces often have little eye relief; Tele Vue's 8mm Plössl has 6mm.

Understanding the 4.3mm Field Stop and True Field

The true field, the circle of sky you see, depends on the field stop diameter and the telescope's focal length. The DeLite manual lists 4.3mm for the 4mm.

You can calculate the true field of view (TFoV) for your telescope using Tele Vue's formula:
TFoV = (Eyepiece Field Stop ÷ Telescope Focal Length) × 57.3
For example, in a telescope with a 1000mm focal length, the DeLite 4mm yields a true field of (4.3mm ÷ 1000mm) × 57.3 = 0.25°. At 250x that frames Jupiter or Saturn closely.

Magnification and Exit Pupil from f/4.5 to f/14

The exit pupil is the beam of light leaving the eyepiece. The chart gives 0.9, 0.8, 0.6, 0.4 and 0.3mm at f/4.5, f/5.2, f/7, f/10 and f/14. Avoid exit pupils below 0.5mm in reflectors; the atmosphere usually limits planetary observing to about 300x.

• At f/10 (SCTs, Maksutovs): 0.4mm, below the 0.5mm minimum advised for reflectors; in an 8" SCT that is 508x, past the usual 300x limit.
• At f/7 (Refractors): 0.6mm. In a TV-85 (600mm) it gives 150x, the level suggested for planets and double stars.
• At f/4.5 (Fast Newtonians): 0.9mm, for globular clusters such as M13 and for planets.

20mm Eye Relief, Eyeglasses, and Dioptrx

The DeLite 4mm has 20mm of eye relief, at the top of the 15mm to 20mm recommended for observers who wear glasses to correct astigmatism. At exit pupils of 1mm or less, as this eyepiece gives in every telescope in the chart, you probably will not need glasses.

The DeLite is ideal for DIOPTRX (Y in the chart), which locks onto the top and corrects astigmatism in place of glasses. Dioptrx is always completely aligned to the eyepiece optics, eliminating aberrations from a decentered mismatch of eyeglass power and astigmatic axis.

The Adjustable, Locking Eyeguard Housing

The adjustable, locking eyeguard housing is in two pieces. In the down position it gives eyeglass wearers full eye relief; an eye too far in or out causes edge darkening and field loss.

To adjust, you grasp the upper and lower rubber grip rings and twist the lower ring to the left to loosen the housing. Without glasses, pull it fully out, then slide it down while looking through the eyepiece until you just see the full field with your peripheral vision. Note the height with the green indicator lines and lock it by twisting the lower ring to the right. Be careful not to unscrew the lower barrel section when rotating the housing, and do not unscrew the chrome barrel, which lets in dust and dirt.

Coma Correction with Paracorr Type-2 in Fast Telescopes

Newtonian and Dobsonian telescopes show coma, which makes off-center stars look like little comets; no eyepiece removes it. Tele Vue's Paracorr Type-2 eliminates it, and scopes faster than f/5.0 see the greatest results.

It goes in a 2" focuser like a 2" Barlow and has a 1.15x magnification factor, so an f/4.5 scope becomes f/5.2 and the 4mm acts like a 3.5mm.

DeLite 4mm vs. Delos 4.5mm and Nagler 3.5mm

Comparisons from Tele Vue's chart:

• vs. Delos 4.5mm: 72° apparent field and a 5.6mm field stop; it weighs 17.6 oz against the DeLite's 7.6 oz. Both have 20mm of eye relief and are Dioptrx ready.
• vs. Nagler Type 6 3.5mm: 82° apparent field, 4.8mm field stop and slightly more magnification, with 12mm of eye relief and Dioptrx with an adapter (Y*).