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TNVC Night Vision
TeleVue.com: Eyepiece Accessories > TNVC Night Vision > How It Works
How Afocal NVD Works
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All Night Vision Devices (NVD) consist of an objective system (like a camera lens), an image intensifying tube, and a magnifying eyepiece which looks at the output of the tube. When we speak of using a NVD in concert with a telescope, what we have is a telescope looking into another telescope with collimated light between them.

Let's start with your telescope (any telescope). The objective's focal length along with the eyepiece's focal length determines the magnification of the scene. That scene is spread across the apparent field of view (AFoV) of the eyepiece. AFoV can be thought of as the angular size of the "window" you see looking into the eyepiece. The circular exit pupil is just an image that the eyepiece forms of the telescope objective. All of the light from the objective passes through the exit pupil, so that's why when you place your eye pupil there, you see the entire AFoV. The location of the exit pupil above the surface of the eye lens is called "eye relief."

The NVD monocular also has an objective and eyepiece, both having the same focal length, thus yielding a 1x magnification. That means the true field of view is equal to the apparent field of view. Both are 40°. Because the eyepiece is looking at an electronic screen, it acts like a magnifying loupe having no exit pupil. You just need to get close enough to see the entire 40° field.

Afocal means that the NVD objective views the telescope's image. It sees the entire image from your telescope by matching the internal pupil of its objective lens with the telescope's exit pupil. Because the NVD objective pupil is about 20mm diameter, the effect is just like viewing with a normal visual telescope. The bigger the exit pupil produced by your telescope, the brighter the image entering your eye and the NVD. Of course, the opposite is also true.

The 20mm pupil diameter also means there is no problem with presenting it larger telescope exit pupils than the traditional 7mm limit for the human eye pupil. For example, a 55mm focal length Plössl in an f/3 telescope produces over an 18mm exit pupil. If used visually, the pupil would overflow the eye's pupil, effectively limiting its brightness to that of a 7mm eye pupil produced by a 21mm focal length eyepiece. The NVD, on the other hand, can accept all the brightness of the 18mm telescope exit pupil and present it to the image intensifying tube.

There is a limitation with NVDs, however. Regardless of the AFoV of the telescope's eyepiece, the NVD can only "see" 40° of it. Back to our 55mm Plössl example; the NVD can only present 40° of the Plössl's 50°. So, while the NVD can use all of the brightness, it cannot use the entire visual field. Yet, it's still a win-win for the NVD because even with this limitation it still provides the same maximum true field you can achieve visually with a 7mm exit pupil (that would be using 100° AFoV, 21mm Ethos) and it maximizes the image brightness entering the NVD.

O-ring between 55mm ... O-ring between 55mm Plössl and EBX-2120.

Getting the Most Night Vision Field with Your Telescope

Use a Tele Vue 55mm Plössl to obtain the maximum true field potential in any telescope. Since the 55mm Plössl requires some focuser "out-travel" relative to most eyepieces, a 2" barrel extender (EBX-2120) is needed for some applications. For use with a Paracorr Type 2, recommended for fast parabolic mirrors such as f/5 to f/3, use the "A" setting (lowest in-travel adapter position). With the EBX-2120 barrel extender attched to the barrel of the 55mm Plössl, slide the eyepiece into the Paracorr so that the interface between the chrome eyepiece barrel and black extender barrel line up with the top of the Paracorr tunable top, and firmly lock in place. Tele Vue can, if you desire, provide a rubber o-ring (TV-60 O-Ring) to act as a stop between the eyepiece / barrel extender connection. Call Tele Vue at 845-469-4551 (9am-5pm ET) to order.

So, what Tele Vue eyepieces have enough eye relief to allow pupil matching thus avoiding a reduction in the 40° NVD apparent field, and provide a full magnification and true field range for almost any deep sky object? Here's the list:

Table: Recommended Tele Vue eyepieces for Afocal Night Vision monocular attachments

Note, the apparent field specification of the eyepiece for normal visual-only application does not apply.

Eyepiece
FL
(mm)
Barrel
Size
TV-60
FL: 360
(mag.)
TV-76
FL: 480
(mag.)
NP101
FL: 540
(mag.)
TV-85
FL: 600
(mag.)
NP127
FL: 660
(mag.)
Plössl 55 2" 8.7 9.8 10.9 12
Plössl 40 1¼" 9 12 13.5 15 16.5
Plössl 32 1¼" 11.3 15 16.9 18.8 20.6
DeLite 18.2 1¼" 19.8 26.4 29.7 33 36.3
DeLite 15 1¼" 24 32 36 40 44
DeLite 13 1¼" 27.7 36.9 41.5 46.2 50.8
DeLite 11 1¼" 32.7 43.6 49.1 54.5 60
DeLite 9 1¼" 40 53.3 60 66.7 73.3
DeLite 7 1¼" 51.4 68.6 77.1 85.7 94.3
DeLite 5 1¼" 72 96 108 120 132
DeLite 4 1¼" 90 120 135 150 165
DeLite 3 1¼" 120 160 180 200 220
Delos 17.3 1¼" 20.8 27.7 31.2 34.7 38.2
Delos 14 1¼" 25.7 34.3 38.6 42.9 47.1
Delos 12 1¼" 30 40 45 50 55
Delos 10 1¼" 36 48 54 60 66
Delos 8 1¼" 45 60 67.5 75 82.5
Delos 6 1¼" 60 80 90 100 110
Delos 4.5 1¼" 80 106.7 120 133.3 146.7
Delos 3.5 1¼" 102.9 137.1 154.3 171.4 188.6
Panoptic 41 2" 11.7 13.2 14.6 16.1
Panoptic 35 2" 13.7 15.4 17.1 18.9
Panoptic 27 2" 17.8 20 22.2 24.4
Nagler T5 31 2" 15.5 17.4 19.4 21.3
Nagler T4 22 2" 21.8 24.5 27.3 30
Nagler T4 17 2" 28.2 31.8 35.3 38.8
Ethos 21 2" 22.9 25.7 28.6 31.4
Ethos 17 2" 28.2 31.8 35.3 38.8
Ethos 13 2" & 1¼ 27.7 36.9 41.5 46.2 50.8

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