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FOCAL LENGTH AND WHAT IT DOES

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FOCAL LENGTH AND WHAT IT DOES

Telescopes come with short, medium, and long focal lengths. Why isn't there a standard? What does focal length mean to the observer?

Every telescope takes the light from the sky and focuses the light on a final focal plane: the longer the focal length of the telescope, the larger the size of the final focal plane images. In essence, a longer focal length telescope produces larger images!higher magnifications. Thus, a particular eyepiece will produce a higher magnification in a longer focal length telescope than in a short focal length telescope.

However, when magnification goes up, the true field of view goes down. Double the magnification and the true field of view is ? as wide, or ? as large in area.

It's a compromise between our desire for those large, expansive, fields of view, and our desire to see everything large!up close and personal, so to speak. It also explains why medium focal length telescopes (f/8 to f/11) are so popular!they offer a reasonable compromise between magnification and field of view.

EXIT PUPIL EXPLAINED

So the eyepiece is a simple magnifier of the image the telescope produces on its final focal plane. An eyepiece, though, reduces the size of the image to a particular sized image of its own. That image, which forms behind the eyepiece, right where we put our eye, is called the Exit Pupil. It's not exactly a focal plane, because it stays in focus as we move our eye toward or away from the eyepiece, but there is one place where the entire image from the eyepiece, all the way out to the edge of its apparent field, is visible.

The image of the telescope is to the left, and the eye is positioned to the right, at the EXIT PUPIL of the eyepiece.

The Exit Pupil is calculated with a simple formula: EP (Exit Pupil) = Eyepiece Focal length/ Telescope f/Ratio

Exit pupil is relevant to the observer because of how our eyes work. When we are young, our eyes' pupils open to as large as 7mm, but as we age the maximum pupil size shrinks!it is rare to see pupil diameters larger than 5mm in 70 year olds. This maximum size of our eyes' pupils will determine the longest focal length eyepiece we can use in the telescope.

Additionally, studies show that best visual acuity occurs with pupil diameters around 2mm. This can help us determine what eyepiece will produce the best overall image quality in our scopes.

There is a minimum exit pupil that an eyepiece can produce that our eyes are capable of handling. It is that point where the image formed by the eyepiece is so small that blood vessels in the retina, or small pieces of debris in the eye (called floaters) can intercept the image. This is with exit pupils around 0.5mm

Using exit pupils, we can see that, with all telescopes, the power range will be determined by exit pupil!7mm on the low end to 0.5mm on the high end. With a typical f/10 telescope, that means (using our formula), eyepieces from as long as 70mm to as short as 5mm!quite a range.

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TELEVUE TELESCOPE EYEPIECES

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Televue Ethos SX Eyepiece 4.7mm 1.25 inch 110 Degree
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Televue Ethos SX Eyepiece 4.7mm 1.25" 110 Degree
Item No. 704-11008
Manuf. No. Televue ETH-04.7

Televue ETH-04.7 Eyepiece, Ethos SX, 4.7mm 1.25" 110 Degree
MSRP: $780.00
Our Price: $650.00
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TeleVue 10mm 1.25 inch 72 degree field Delos Eyepiece
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TeleVue 10mm 1.25 inch 72 degree field Delos Eyepiece
Item No. 704-09010
Manuf. No. Televue EDL-10.0

TeleVue EDL-10.0 Eyepiece, 10mm 1.25 inch 72 degree field Delos
MSRP: $445.00
Our Price: $370.00
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TeleVue 6mm 1.25 inch 72 degree field Delos Eyepiece
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TeleVue 6mm 1.25 inch 72 degree field Delos Eyepiece
Item No. 704-09006
Manuf. No. Televue EDL-06.0

TeleVue Eyepiece 6mm 1.25 inch 72 degree field Delos
MSRP: $445.00
Our Price: $370.00
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Tele Vue 3.7mm Ethos-SX Eyepiece  with 110 degree Apparent Field
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Tele Vue 3.7mm Ethos-SX Eyepiece with 110 degree Apparent Field
Item No. 704-11010
Manuf. No. Televue ETH-03.7

With 110 degree, the Ethos-SX has 21% more AFOV area than the 100 degree Ethos design. This permits a nice 62% power increase from a 6mm Ethos, while retaining 68% of its TFOV
MSRP: $780.00
Our Price: $650.00
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Televue 21mm Ethos Eyepiece - 2 inch
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Televue 21mm Ethos Eyepiece - 2 inch
Item No. 704-10092
Manuf. No. Televue ETH-21.0

Maximize your "spacewalk" experience with the 21mm Ethos. Transform your passage through the Milky Way with the 21mm's nexus of contrast, power and field
MSRP: $1,075.00
Our Price: $895.00
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Televue 10mm Ethos Eyepiece 2 inch / 1.25 inch
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Televue 10mm Ethos Eyepiece 2 inch / 1.25 inch
Item No. 704-11003
Manuf. No. Televue ETH-10.0

The Televue 10mm Ethos is the perfect fit between the 8mm & 13mm! The TeleVue 10mm Ethos Eyepiece employs flat finished baffles and specialty coatings that are tuned to the composition of each element for maximum contrast.
MSRP: $780.00
Our Price: $650.00
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Tele Vue 6mm Ethos 1.25/2 inch Eyepiece 100 degree
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Tele Vue 6mm Ethos 1.25/2 inch Eyepiece 100 degree
Item No. 704-11007
Manuf. No. Televue ETH-06.0

The Televue 6mm Ethos delivers: 100 degree apparent field (50% larger in area than 82 degree Naglers), comfortable eye-relief, accepts DIOPTRX eyesight astigmatism correctors, distortion correction, high contrast and on-axis sharpness for planetary viewin
MSRP: $740.00
Our Price: $615.00
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Tele Vue 17mm Ethos 2 inch Eyepiece 100 degree
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Tele Vue 17mm Ethos 2 inch Eyepiece 100 degree
Item No. 704-10096
Manuf. No. Televue ETH-17.0

The Tele Vue 17mm Ethos is the latest in the Ethos family of eyepieces, providing an even larger field of view than the original 13mm.
MSRP: $950.00
Our Price: $790.00
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Televue ETHOS 100 degree 8MM Eyepiece
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Televue ETHOS 100 degree 8MM Eyepiece
Item No. 704-11005
Manuf. No. Televue ETH-08.0

Televue ETHOS 8MM Eyepiece for Televue Scopes TV-60, TV-60is, TV-76, NP-101, NP-101is, TV-85, NP-127is, TV-102, TV-102iis
MSRP: $740.00
Our Price: $615.00
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TeleVue Eyepiece Barrel Extender
TeleVue Eyepiece Barrel Extender
Item No. 704-17000
Manuf. No. Televue EBX-2120

Tele Vue Eyepiece Barrel Extender EBX-2120 reduces focuser travel with TeleVue Plossl 55.0mm Eyepiece.
MSRP: $40.00
Our Price: $30.00
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Tele Vue Ethos 13mm 1.25 inch Eyepiece - 100 degree Apparent Field of View
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Tele Vue Ethos 13mm 1.25 inch Eyepiece - 100 degree Apparent Field of View
Item No. 704-11000
Manuf. No. Televue ETH-13.0

Tele Vue Ethos 13mm 1.25" Eyepiece - 100 degree Apparent Field of View - ETH130
MSRP: $780.00
Our Price: $650.00
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TeleVue Eyepiece Carry Bag
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TeleVue Eyepiece Carry Bag
Item No. 704-90017
Manuf. No. Televue ECB-0010

The Tele Vue Eyepiece Carry Bag is a wonderful accessory for carrying your Tele Vue accessories and eyepieces!
MSRP: $170.00
Our Price: $150.00
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TeleVue Plossl 55mm
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TeleVue Plossl 55mm
Item No. 704-16000
Manuf. No. Televue EPL-55.0

The 55mm focal length has the largest true field possible for 2" eyepiece holders -3 times the true field area of any 1-1/4" eyepiece
MSRP: $305.00
Our Price: $255.00
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TeleVue Panoptic 41mm
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TeleVue Panoptic 41mm
Item No. 704-15080
Manuf. No. Televue EPO-41.0

The 41mm Panoptic eyepiece is a big brother to the 24mm Panoptic (largest 2" field).
MSRP: $660.00
Our Price: $550.00
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TeleVue Panoptic 35mm
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TeleVue Panoptic 35mm
Item No. 704-15075
Manuf. No. Televue EPO-35.0

The 35mm has a large true field with plenty of eye relief for eyeglass wearers.
MSRP: $500.00
Our Price: $415.00
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