I’ve been using a Baader Hyperion 8-24 mm Clickstop Zoom eyepiece in Pentax PF65, 80, and 100 spotting scopes for several years now. As furnished, the eyepiece won’t reach infinity focus in any of the Pentax scopes because of the thick attachment parts at the bottom of the eyepiece. These include a milled edge piece into which the 1.248” barrel (that seems to be standard size for “1 ¼ inch” eyepieces – two thousandths undersize) attaches, a thick threaded ring that is normally not removed, and a plastic spacer ring at the bottom of the main eyepiece body. The thicknesses of these three together hold the eyepiece bottom .278 inches above the bottom of the scope’s focuser collet. (See photo) A homemade adapter turned from 2.5” diameter aluminum stock about 1 ¾” long holds the eyepiece bottom only .068” from the focuser collet bottom, so the eyepiece is closer by .21 inches (5.3 mm). That may not sound like much, but it allows infinity focus with all three PF models and even permits folks with slight myopia to focus “in” a little bit so they can use the eyepiece without their glasses.
(NOTE: I have done this modification to the one Baader Hyperion 8-24mm zoom eyepiece I own. It was purchased used and is one of the older models, not the Mk-III version now available. These techniques may or may not work on other versions of this eyepiece.)
To modify your eyepiece to work with the Pentax scopes, you first need to remove the original parts mentioned above. Keep them in a safe place, as this modification is reversible, if you so choose. The milled-edge ring that holds the 1.248” lower barrel simply unscrews, but the thicker ring above it is threaded on the outside and inside and screws into the thin fluted ring around the upper barrel. The double threaded ring has two small holes that allow it to be unscrewed from the fluted ring using a pin spanner, but a small jewelers screwdriver in one of the holes will also serve for this task with only a little extra difficulty. Hold the fluted ring with your hand as you unscrew the double threaded ring from it. (See photo)
You can likely just machine the adapter to fit the measurements of your eyepiece now, allowing a 1.248” OD lower barrel (.95” long, though that is not critical) to go into the scope. For reference, though, I’ll include the dimensions I used. The adapter has a bottom ID of 1.148” to allow clearance for the zoom extension at the bottom of the eyepiece. The upper part of the barrel was left 2.5” OD with the inside being 2.188” to allow the lower part of the big Baader Zoom cylinder to slide into it. I left the original outside fluted ring loose on the barrel, as it just slides up above the adapter, out of the way. The upper part of my adapter is about .8” long, though again, that is not critical. What IS critical is to get the bottom of the adapter’s upper cylinder that connects to the 1.248” OD part thin enough to allow the eyepiece to sit much closer to the bottom of the collet ring. As mentioned before, mine is just .068”, though a few thousandths less would be even better if you are slightly myopic. The adapter is attached to the eyepiece bottom with three small set screws in holes tapped through the upper wall of the adapter. If these are kept sufficiently low on the adapter, the now loose fluted ring will cover the set screw divots in the eyepiece body if you choose to take off the adapter and reassemble the Baader Zoom back to its original configuration. (See photos of adapter and adapter attached to eyepiece)
Since making the adapter is not a very difficult lathe project, perhaps those who do not have access to a lathe can find a local machine shop to turn the adapter for you at reasonable cost. - Bill