TeleVue Nagler Planetary Zoom 3-6mm Eyepiece with 50° Constant Apparent Field
TeleVue Nagler Planetary Zoom 3-6mm Eyepiece
with 50° Constant Apparent Field
This is the "ultimate" tool for tweaking out the last bit of power that the atmosphere will permit.
This model has click stops every mm.
"TeleVue zoom is small, lightweight, and offers the same magnifications (with click-stop settings) as four Radians while maintaining a constant focus and 50 degree apparent field. It also lets observers choose in-between focal lengths, thus giving them the ability to tune the magnification to atmospheric conditions, which is very critical for high-power viewing. Although the zoom's 10-millimeter eye relief isn't long enough for eyeglass wearers, it is enough to make this a very comfortable eyepiece to look through." - Sky & Telescope Magazine.
Concept / Background: "I did not set out to design a zoom eyepiece, per se. David and I were discussing characteristics we'd like in a superior "planetary" eyepiece, and the zoom feature answered one of the major goals; how to obtain the highest possible useful magnification on a given night considering the ever-changing atmosphere. There were other aspects of equal importance and only the combination of all would satisfy our ethos in creating a "planetary" eyepiece: full field sharpness for any speed telescope, high contrast and transmission for natural color rendition, low scatter, and comfortable eye-relief. Being a zoom, we also concerned ourselves with making sure it was parfocal through the zoom range as well maintaining a constant apparent field of view. The one aspect we were not too concerned with was giving the eyepiece a wide AFOV.
Limiting the AFOV to 50° and the eye-relief to 10mm, we were able to create a compact physical design employing only five elements to reach the optical goals. Like other "Naglers," it utilized a negative lens group ahead of the focal plane. This permitted relatively long eye-relief for very short focal lengths. By independently moving the upper and lower lens groups, we achieved constant focus (parfocality), constant apparent field of view, and essentially constant eye-relief. Just by rotating the upper barrel, you can zoom through the power range. Click stops offer known power points for reference without needing to look at the barrel and instantaneously choose the optimum magnification for the given observing conditions.
I was sufficiently confident in the design and purposely decided to attach my name to the zoom, despite the common association between "Nagler" and 82° AFOV. I felt it was more important to stake our reputation on overcoming the "degrading zoom reputation" than maintaining the 82° assumption for "Nagler" designs."
— Al Nagler
|Focal Length (mm)||6 - 3|
|Barrel Size (in.)||1¼"|
|Apparent Field of View (deg.)||50|
|Eye Relief (mm)||10|
|Weight (lb / oz)||0.33 / 5.3|
|Field Stop (mm)||5.1 - 2.6|
|Focal Ratio f/|
|Focal Length (mm)||3|
|Eye Relief (mm)||10|
|Angular Field of View (degrees)||50|