How to Properly Center Short Focus Eyepieces

How to properly center short focus eyepieces

Short focused eyepieces are often dismissed by their relentless owners for alleged lack of performance due to disbelief in their sometimes ambitious optical designs, suspected limits of the telescopic system and of course the ever almighty seeing that rules all out. This amounts to these optical gems ending up on the shelve as collectors items rather than in enthusiastic use in high performance observation. There is of course truth in all of these factors mentioned that all do take their share but - ever thought about how we clamp our eyepieces into the rear ends of our telescopes ?

Let´s see what we have:

There is the common brass marrer, the single screw slot-in eyepiece holder that will indiscriminately take up everything that is fed into its upper end. Mine allows for an extra space of 0,5 mm on either side, so there is ample space for a rich selection of eyepieces stemming from various provenances. Think of 0,5 mm extra space with a 2,5 mm focus eyepiece! The eyepiece is distracted from the optical axis of the system by a 1/5 of its focal length! Still any doubts this will end up in nothing more than a decent performance ?

The more advanced system is a three screw eyepiece holder with an interior brass ring intended to smoothly embrace the eyepiece socket. Ever try to bring a short focused eyepiece into center with these three screws ? Honestly, it takes a micro gauge and a lot of lasting affection to work out even length of every single screw and even if you found your performance to improve you would never be certain for lack of a truly centered alignment. If not all screws are at even length, there is also the risk of considerable tilt as you would only by chance end up by truly aligning the focus plain of the eyepiece and the telescope´s optic.

There is something called „Click Lock“ with mechanics derived from fixing gear with power drills. This brings us closer to the experience of well centered eyepieces that vague contours will snap in to clear and sharp details once they are in focus. Nevertheless I found mine to be fiddly in practical usage as you have to turn the focusing collar about a 1Ú4 of a full turn more to the left than would be required just to compensate for the final right turn the eyepiece takes once you fix the eyepiece by the main compression ring. Further I discovered some eyepieces still to be slightly tilted by their own head weight if the main compression ring happens not to be vigorously closed. The threaded focusing collar only allows for moving the eyepiece in the „out“ mode, thus requiring to open the compression ring and start all over again from a lower position of the focusing collar, if focus has again eluded your efforts. This is not as bad as it may sound but it is quite peculiar and getting increasingly tedious with any sort of short focused eyepiece.

Why not keep it small and simple? There is a filter thread in most, if not in all eyepieces. Take a male 1 1Ú4´´ thread on one side of a metal tube and a female T2 thread on the opposite side, if this suits the requirements of your diagonal, and off you go ! Just screw your favorite eyepiece on top, forget about any centering sorrows, focus by minute turns of the eyepiece on the socket of your new focusing adapter and enjoy.

The more ambitious will even add a 1 1/4 ´´ threaded locking ring below the eyepiece to allow for tightening the eyepiece, once the position of true focus has been found. The reward is crystal clear razor sharp, detailed views in the previously ugly ducklings of the lower end of the range. This device will of course not make up for any deficiencies in the eyepiece´s design but you will realize there is much more in many of them than they could thus far show.It is pretty clear that any Angstrom we place an eyepiece off the optical axis of the optical system will be hazardous to the overall performance. Using short focus eyepieces in a decidedly centered way brings out the true limits of your scope.

Of course, seeing is still an issue, but you will discover the overall distortions of the picture you see to be considerably diminished. If seeing is not at all bad, the moments of true, pristine views will significantly increase. This may be due to the minor atmospheric turbulences to be less detrimental once they are seen close to the optical axis of the entire system, including the eyepiece. We may thereby pick up some amount of the useful information they may still contain, thus adding to the improved performance.

Ragold Wenz
Germany