Hunter's Zoom Finder: This innovative design solves several problems at once.
He reports, "Telrads and other reflex finders provide wide fields of view, but in my poor skies it's often difficult to see enough stars, and looking through any glass further limits visibility. A split-pupil finder (S&T: June 2013, p. 66) eliminates the intrusion of any glass and is very wide angle, but my old bones don't like crouching to look straight through finders. Adding a large mirror to my split-pupil finder solved that problem, giving me a 50[degrees] field or more and, interestingly, the sense of slightly better star visibility, but even so I just couldn't see enough stars on many nights."
Hunter needed an optical finder that would offer some gain. The trouble is, that would lead to a narrower field of view, which would reduce the number of useful field stars and put him right back where he started. He began asking himself, "So what provides both wide-angle at about naked-eye field of view as well as some magnification when needed?" The answer: a zoom finder.
Hunter's other hobby is building stuff. With the words "zoom finder" rattling around in his head, he went into his shop and mated a Sony 20-80 mm f/2.5 TV zoom lens to an inexpensive Amici roof prism and the lenses from a 20-mm Plossl eyepiece. He couldn't use the entire eyepiece because the focal point of the zoom lens lies inside the prism, so he had to mount the eyepiece lenses so their focal point reached the same plane.
The result is a finder that zooms outward to give him a 50[degrees] field of view, which allows easy orientation even with only a few reference stars. Zooming to the narrowest field of view, 12.5[degrees], adds about 3 magnitudes of gain, bringing out more stars as the field narrows. Designed for television cameras, the lens has a smooth-acting lever for zooming, so Hunter can shift back and forth easily without jiggling the scope. At its widest, the finder's field of view includes the front of the telescope, making it difficult to judge where the center is. Even at full zoom, its 12.5[degrees] field is still a lot wider than the 1[degrees] field of a typical low-power eyepiece, and Hunter soon realized the biggest hiccup in his design: With the focal plane inside the Amici prism, he couldn't add cross-hairs to indicate the center of his finder's field. So he augmented his zoom finder with a second, conventional 5[degrees] optical finder. Together the two finders let him zero in on just about any target visible in his sky.
Then one night while zooming in on a target, Hunter had an epiphany: The zoom could function like cross-hairs.
An object that's not centered will drift to the side when zoomed, but an object in the center of the field will stay put. Hunter reports, "The long hand lever of the lens makes adjustment very easy, so I just zoom in and out, tweaking the scope position until the target star doesn't move. This is quicker than one might imagine and became second nature."
Now he says the most serious downside is that "this optics combination provides, to be kind, not the sharpest star images. But I find the images good enough, and 'good enough' seems okay for this application."
These zoom lenses are available on eBay fairly frequently and can be found with a little patience. Other zoom lenses might work as well, but make sure they come to focus far enough away from the rear element to allow room for a prism and eyepiece.
The zoom finder nicely solves the problem of star-hopping in light-polluted skies, and it would probably be equally useful in dark sky as well. Hunter's final assessment is this: "Like my scopes, the finder I use most is the best one I have ... and I use this one all the time."
For more information, contact Hunter at firstname.lastname@example.org.
* Contributing Editor JERRY OLTION festoons his telescopes with finders of all description. A zoom may soon join the others.
Caption: Left: The finder's zoom lever is easy to reach and operate in the dark. Right: Components of the zoom finder are relatively easy to assemble, and simple to use.
Caption: Hunter Davidson uses his zoom finder to locate guide stars in his light-polluted sky.
Caption: Hunder Davidson uses his zoom finder to locate guide stars in his light-polluted sky.
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|Title Annotation:||ASTRONOMER'S WORKBENCH|
|Publication:||Sky & Telescope|
|Date:||May 1, 2018|
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