TeleVue How to Select Telescope Eyepieces: Plossl Radian Nagler Zoom eye pieces

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Tele Vue Eye Piece designs are configured to achieve the sharpest full field, with the number of elements and groups minimized, while yielding the largest possible true fields and eye relief. Some design types are "scaled", that is, one form is "scaled" to provide a range of focal lengths. Note that eye relief, size and weight "scale" along, but image quality and apparent field remain constant. Examples of "scaled" Tele Vue designs are Plossls, Panoptics and the Nagler type 5. Other designs set a particular characteristic, such as eye relief, and tailor each focal length in the series as individual designs. The size, weight and number of lens elements will vary little within a given series. Examples are: Radians with 20mm eye relief, Nagler type 4 models with approx. 18mm eye relief, and Nagler type 6 eyepieces with 12mm eye relief.

Design Types

• Plossl: 50° apparent field (43° in 40mm focal length). Focal lengths available: 55mm (2in.), 40mm, 32mm, 25mm, 20mm, 15mm, 11mm, and 8mm. Eye relief is approximately 70 percent of focal length. 4 elements in 2 lens groups.
• Radian: 60° apparent field, 20mm eye relief in all models. All have 1-1/4 in. barrels. 6 elements in 4 lens groups: 18mm, 14mm, 12mm, 10mm, and 8mm focal lengths. 7 elements in 5 lens groups: 6mm, 5mm, 4mm, and 3mm focal lengths.
• Panoptic: 68° apparent field, focal lengths available: 41mm (2 in.), 35mm (2 in.), 27mm (2 in.), 24mm, 22mm, 19mm, and 15mm. Eye relief approximately 70 percent of focal length. 6 elements in 4 lens groups.
• Nagler type 4: 82° apparent field, 17mm or more eye relief in all models. Focal lengths: 22mm (2 in.) and 17mm (2 in.) have 7 elements in 5 groups. 12mm has 6 elements in 4 lens groups.
• Nagler type 5: 82° apparent field, focal lengths available: 31mm (2 in.), 26mm (2 in.), 20mm (2 in.), and 16mm. Eye relief is approximately 62 percent of focal length. 6 elements in 4 lens groups.
• Nagler type 6: 82° apparent field, 12mm eye relief in all models. Focal lengths available: 13mm, 11mm, 9mm, 7mm, 5mm, 3.5mm, 2.5mm. 7 elements in 4 groups.
• Nagler Zoom: 50° apparent field 3mm--6mm and 2mm-4mm focal length ranges. 10mm eye relief. 5 elements in 3 lens groups.
• 8mm-24mm Click Stop Zoom: 40° apparent field and 10mm eye relief at 24mm, 55° apparent field and 15mm eye relief at 8mm. 7 elements in 4 lens groups.

Digiscoping and DIOPTRX

Over 20 Tele Vue eyepieces with long eye relief have eyeguard styles that accept Tele Vue digital camera adapters and Televue DIOPTRX (lenses to correct eyesight astigmatism without needing eyeglasses).

Construction

Every Tele Vue eyepiece is fully multi-coated, has blackened lens edges and anti-reflection threads for maximum contrast. Chrome plated barrels, which resist scratching and marring better than black anodized barrels, include safety undercuts and threads for filters. Upper barrels are black anodized aluminum with a rubber grip ring, engraved identification, and fold down rubber eyeguards. All Nagler type 4 and Radian models include an Insta-just eyeguard with click-stops for ideal eye positioning. Protective caps are included for top and bottom as well as the pupil guide accessory. Many of the 1-1/4 in. models across Plossl, Radian, Panoptic and Nagler models are parfocal for convenience.

Warranty

A 5-year warranty applies on all Tele Vue products for the original owner. You also have the security of knowing that every current Tele Vue eyepiece can be repaired, serviced, or cleaned at the factory. Tele Vue eyepieces are also great investments, returning high resale values, when you want to upgrade or modify your collection.

Tele Vue Eyepiece Specifications

Eyepiece Calculation Formulas

Choosing Eyepieces

by Tele Vue

Eyepieces determine your telescope's magnification. To calculate the magnification of an eyepiece in your telescope, divide the focal length of the telescope by the focal length of the eyepiece:

Magnification = telescope focal length ÷ eyepiece focal length

Eyepieces also determine the true field you see in the sky. To calculate the true field of view that you will see (in degrees), divide the eyepiece field stop diameter by the telescope's focal length and multiply the result by 57.3:

True field of view = eyepiece field stop diameter ÷ telescope focal length x 57.3

The field stop is the metal ring inside the eyepiece barrel that limits the field size. It's projected by the eyepiece so that it appears as a circle out in space when you look through the eyepiece. The angular diameter of this circle is called the apparent field of view (AFOV) and is a fixed property for each eyepiece design. For example, Plossl eyepieces have an AFOV of 50°, Radians have 60°, Panoptics have 68°, and Naglers have 82°.

For low-power viewing of large objects, or to use your telescope as a low-power finder, you will want an eyepiece that delivers close to the maximum possible true field of view (note that for 1.25 in. eyepieces, the maximum field stop diameter is 27mm; for 2 in. eyepieces, it's 46mm). Then add eyepieces covering uniform increments in smaller field stops. For example, if your widest field eyepiece has a 40mm diameter field stop and you choose a decreasing increment diameter factor of 2 (which results in a 4x decrease in area size), you'll end up with eyepieces having field stop diameters of approximately 40mm, 20mm and 10mm. To further fill in with incremental steps, add eyepieces with approximate field stop diameters of 28mm and 14mm. Of course, avoid duplicating focal lengths. For example, if you use a 31mm Nagler (with a 42mm field stop diameter), you would not need a 32mm Plossl (with a 27mm field stop diameter).

In general, for each field stop size, choosing eyepieces with shorter focal lengths and larger apparent fields of view will allow you to see more detail and fainter stars. In addition, you'll have a smaller exit pupil to better match your eyesight.

If you do not need eyeglasses to correct astigmatism, don't use them when observing. If you wear glasses to correct astigmatism, make sure they're multi-coated, and try to choose eyepieces that have at least 15mm to 20mm of eye relief, to minimize any field reduction (vignetting). However, you will find that with small exit pupils such as 1mm or less, you probably will not need eyeglasses, and can therefore use eyepieces with less eye relief.

Televue DIOPTRX Astigmatism Correction Telescope Lenses compliment your telescope ?s focuser used for near or far sight conditions by compensating for your astigmatism utilizing Tele Vue DIOPTRX Technology (lenses to correct eyesight astigmatism without needing eyeglasses).

The exit pupil is the image of the objective that is formed by the eyepiece. It's where you place your eye to see the full field of view. You can calculate the diameter of the exit pupil by dividing the focal length of the eyepiece by your scope's focal ratio:

Exit pupil = eyepiece focal length ÷ telescope f/#

For reflector telescopes, it's best to avoid exit pupils larger than 7mm or smaller than 0.5mm. Refracting telescopes have no upper limits on exit pupil sizes.

You can also choose a long focal length eyepiece with comfortable eye relief and use image amplifiers to increase power. TeleVue makes Barlows and Powermates (an improvement to the Barlow-type design) in magnifications factors of 2x, 2.5x, 3x, 4x and 5x.

If you have a Newtonian or Dobsonian reflector that's f/5 or faster, you should seriously consider using the Paracorr to eliminate coma, so your full field eyepiece sharpness is not compromised. Paracorr also acts like a 1.15x Barlow, so for example, a 1000mm f/4.5 scope becomes 1150mm f/5.2.

Once you've selected an eyepiece set based on field stop sizes, calculate the magnifications produced with your scope. For planetary or double star observing, you'll want an eyepiece in at least the 150x range. For determining maximum power, a good rule of thumb is to use no more than 60x per inch of aperture for scopes with apertures up to 6 inches. Higher magnifications may still be pleasing but will not likely reveal any additional detail. Realistically, the atmosphere will usually limit your planetary observing to a maximum magnification of about 300x, no matter how large your telescope aperture.

Basically, you'll be choosing low and medium power eyepieces by field stop increments to "frame" the subject, and high power eyepieces by magnification increments (based on your scope's aperture), to reach the optimum contrast and resolution for viewing planets and double stars.

Every TeleVue eyepiece is Q.C. tested at f/4.0, and can be serviced or repaired by Tele Vue.

TeleVue Eyepiece Specifications

by Tele Vue

1FL: Focal Length 2E / G: # Lens Elements / # Groups 3AF: Apparent Field of View 4G: Parfocal Group, eyepieces with the same letter are approximately parfocal ? require little (if any) refocusing when used together. 5ER: Eye Relief 6Eyepiece Dimensions: see diagram right

A. Height of barrel above reference surface B. Height of 1¼ in. barrel C. Height of 2 in. barrel D. Diameter of black barrel E. Depth of 1¼ in. barrel F. Approx. location of field stop (negative number is above reference surface)

• All TeleVue eyepieces listed have full multi-coatings, blackened lens edges, high index glasses, fold-down eyeguards, and filter threads. They are all based on patented designs by Al Nagler and are individually tested to be diffraction-limited on-axis. All Nagler and Radian eyepieces have internal focal planes. Field stop diameters and locations are effective. Radian and Nagler type-4 eyepieces have Instajust Eyeguards, and pupil guide accessory.
• The 55mm Plossl and 41mm Panoptic eyepieces have an adjustable height (removable) eyeguard.
• Also available: 2X and 3X parfocal 1-1/4 in. Barlows; parfocal 2.5x and 5x 1-1/4 in. Powermates; 2X Big Barlow, 2x and 4x Powermates for 2 in. focusers. Use Paracorr (1.15X) to correct fast parabolic Dob/Newts, dramatically sharpens field to take full advantage of Tele Vue eyepiece capabilities.
• All dual barrel eyepieces work equally well in 1-1/4 in. or 2 in. focusers.

Eyepiece Calculation Formulas

by Tele Vue

The eyepiece calculation formulas reveal the observing potential of your telescope with TeleVue Optics eyepieces. It accepts basic information about your telescope and give you results for all current Tele Vue eyepieces. Refer to the diagram and tables here for the definition of the input and out parameters.

• apparent field: perceived span of sky seen through eyepiece (without telescope). Not used in true field calculation.
• exit pupil: image of objective formed by eyepiece. Location where full apparent field is seen.
• f/#: a ratio that describes the relation between the aperture and focal length of the telescope - important for photography
• field stop: ring inside the eyepiece barrel that limits true and apparentfield size
• focal length: effective distance from entrance of an optical system to focal point
• magnification: relative change in angular size of object
• true field: span of sky seen through telescope/eyepiece combination

magnification =	telescope focal length	=	telescope apeture
	eyepiece focal length		exit pupil

telescope f/# =	telescope focal length
	telescope apeture

exit pupil =	telescope apeture	=	eyepiece focal length
	magnification		telescope f/#

true field =	eyepiece field stop	x 57.3°
	telescope focal length