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"Safety" undercut
1) in a lot, if not most, of the focusers with simple setscrews the screw tightens on the lip of the undercut band and the tightening if the screw lifts that side of the eyepiece slightly out of the focuser, tipping the eyepiece. This can happen in some expensive focusers and is a negative to the "safety" groove.
2) Many "compression ring" focuser bands do not have "coined" edges on those compression bands, so the edges of the band catches on the eyepiece undercut as you try to remove it. Additionally, the "spring" in the compression band metal doesn't allow the band to spring back out of the way when the setscrew is loosened on many of the lower-priced compression rings, meaning the screw has to be over-loosened to allow easy removal of the eyepiece.
3) As I look at the eyepieces on the shelf at work, I notice the lengths and depths of the undercuts vary all over the place, as do the lengths of the 1.25" barrels. There is no standard, and so the compression ring doesn't fit into the undercut on the eyepiece that much of the time. Many times, the compression ring also tightens on the lip of the undercut but doesn't lift the eyepiece out of the focuser because there is no turning torque of the screw present where it presses on the eyepiece.
4) The term "compression ring" is a misnomer. It doesn't actually compress all around the perimeter of the eyepiece, but merely puts a strip of brass or bronze between the focuser setscrew and the eyepiece to prevent marring of the eyepiece barrel.
5) The marketplace needs to return to smooth barrels on all smaller eyepieces and make safety undercuts conical on the heavier ones. The grooved undercut needs to be abandoned as soon as possible. I can't tell you how many hundreds of times the compression ring caught on my eyepieces until I replaced the focuser drawtube with a standard setscrew without the compression ring. But after I started using a Paracorr the problem returned because the Paracorr has a compression ring. It is interesting to note that the Paracorr itself has a smooth side, though.
It is also interesting to note that the 1.25" barrels on the 6 and 8mm Ethos, which have to be used in a 1.25" adapter in the Paracorr (setting 4) have smooth sides. Yet, none of these has any tendency to fall out of the Paracorr when the thumbscrew is tightened. This just prooves how unnecessary undercut grooves on eyepieces really are.
6) Star diagonals used on SCTs that have insertion barrels that are threaded into the body of the star diagonal can, if tipped to the left with a heavy eyepiece on board, unscrew, allowing the star diagonal to instantly rotate until the eyepiece points at the ground. But, even then, the eyepiece won't fall out if the setscrew is simply tightened on the barrel. People with 2" star diagonals that are used rotated from vertical should always rotate them to the right for that reason (TeleVue, AstroPhysics, and Lumicon owners can ignore that warning--on them the body and insertion tube are machined in one piece)
7) So it's time for the industry that the safety undercut groove is a failed experiment and return to smooth barrels. Either that or make all the undercuts conical in shape.
IMHO, of course.
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