Re: [ap-gto] My latest review - nPAE Precision Astro Engineering 1.25 Ruby Turret

Jay Freeman

I read with some interest the recent reports on eyepiece turrets, and as one of the three ap-ug users who still admits to using eyepieces, I shall now speak up.

I have tried several eyepiece turrets, both on fast telescopes (down to f/6 or so) and on slow ones (f/16), and have had a variety of problems. To understand why, consider my intended uses of these devices -- there are two:

(A) Switch from a low-magnification, "finding" eyepiece -- often with a two-inch barrel -- to a higher-magnification one, usually in a 1.25-inch or 24.5 mm barrel, for a look at details.

(B) Change from one high-magnification eyepiece to another, in pursuit of the best view.

Difficulties include:

(1) The slip-out problem. This is characterized by the audio stream "Eek-eek-eek-eek-eek-OUCH!!" which you emit as your five outrageously expensive and now out-of-production high-end planetary eyepieces drop from their sockets on the turret and smash into the cold, hard ground, followed by your very heavy two-inch-barrel "finding" eyepiece, which lands on your foot and breaks several toes. Yes, most modern eyepiece turrets have some kind of mechanism to lock the eyepieces in place, and most of us remember to do so most of the time, except early in the evening when we are in a rush to set up and start observing, and late at night when we are tired, but does anyone remember Unitron's old "Unihex" turret, with nothing but bent tabs in the turret sockets to keep eyepieces in place? I have one, and consider its "hex" name a fortuitous aid for casting curses at manufacturers of telescope accessories (Astro-Physics excepted, of course).

(2) The where-did-it-go problem. The issue here is minimizing displacement of the line of sight of the telescope optics as you rotate the turret, and there is a catch-22: If the turret rotation is stiff, it will be difficult to avoid too much displacement, and the thing you were looking at will be out of sight when you get the new eyepiece rotated into place, yet if the turret rotates easily, it will tend to rotate of its own free will from the imbalance in weight between those tiny Orthoscopics and that enormous TermiNagler, so you won't get to use any eyepiece for long.

(3) Vignetting. This is the real killer with fast telescopes. Most turrets seem to be set up for 1.25-inch diameters. At f/6 you might or might not miss being able to use a two-inch-barrel "finding" eyepiece -- a 24 mm Panoptic will give a 4 mm exit pupil and its field lens spans the inner diameter of its 1.25-inch barrel, providing a nice wide view, but the various 1.25-inch diameter restrictions in the rest of the turret will cause serious vignetting across a large portion of that field. If the optical path length from the focal plane to the sky end of the turret's own 1.25-inch barrel is too long, even the center of the field of view may be vignetted; that is, you will not be using the full aperture of the telescope for your visual observations. A turret with a two-inch barrel (where it fits into the telescope focuser) would help some, but it is important to calculate carefully just how much vignetting any proposed telescope/turret/eyepiece combination will experience.

My personal solution to eyepiece use continues to be:

(a) Use a two-inch star diagonal.
(b) Have a two-inch/1.25-inch reducer handy, for use at the eye end of the star diagonal.
(c) Have my 24.5 mm eyepieces permanently installed into 1.25-inch/24.5-mm reducers.
(d) Develop a light touch at the knurled knobs that lock the various pieces together, so I don't jiggle the OTA when I switch eyepieces.

With this approach, I rarely need a two-inch-barrel eyepiece on fast refractors (e.g., 130 mm Astro-Physics GT), and I can use my 24 mm Panoptic and enjoy a large unvignetted field.

-- Jay Reynolds Freeman
Jay_Reynolds_Freeman@... (personal web site)

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