I am in no way an expert on eyepieces. I am a casual observer with the same interests as many others.
I bought a used 21mm Baader Hyperion a while back. I had two extra 14mm Baader fine tuning rings in my eyepiece case from even further back. I added one 14mm fine tuning ring to the 21mm Hyperion as I wanted just a bit more magnification. It needed a bit more "in focus"since adding the fine tuning ring, so I added an adapter which makes it parfocal to the rest of my eyepieces, which was a bonus, and there is no longer a need for "in focus" and re-focusing the focuser.
The 21mm by itself is fairly nice, but once I added the fine tuning ring, it made the field stop look sharper and it cleaned up the "soft look" of the 21mm when used alone w/o the fine tuning ring, (it had slight edge of field brightening).
I then bought a 10mm Baader Hyperion, and when using it alone, there was a substantial amount of edge of field brightening, and the field stop was not in focus, so I tried an experiment.
I added a 14mm fine tuning ring, and an empty filter cell and now the edge of field brightening is gone, and the field stop now looks to be in focus. Eye placement has also become a bit easier on both the 21mm and the 10mm Hyperions.
I just wanted to share my experience and little "experiment" I tried with both the 21mm and the 10mm Baader Hyperions, which worked out really good. If any of you have those two eyepieces, you could try this to see if you like them better. Once I added both the 14mm fine tuning rings to both, and a empty filter cell to the 10mm, I have just left them that way, and don't plan on changing them back.
Make sure to do the switch in a dust-free environment, as you don't want any dust or debris to get in between the Smythe lens and the body of the eyepieces. Do it as fast as possible also, to avoid dust or debris as well.
I made sure to use a air blower bulb on the inside of the Smythe lens and bodies of both eyepieces to make sure there is no dust, etc in between the lenses.
I have since tested both eyepieces on star clusters. The 21mm is sharp out to about the 80% point with mild astigmatism near the field stop. the 10mm was pretty much sharp out to the edges. I tested both using my 10" Orion XTi without using a coma corrector and both were perfect in the Orion ED 100.
Edited by Miranda2525, 26 February 2020 - 07:56 AM.