Part 2: Direct lens replacement of my damaged 2.5mm Nagler Type-6 with the eye lens group from a 3.5mm Nagler Type-6 Eyepiece.
Since I own various refractors with a Focal Length (FL) ranging from 360mm to 660mm used on non-tracking Alt-Az mounts, I depend on my 2.5mm and 3.5mm Naglers for high magnification views, so losing either of them is not an option for me. Unfortunately the 2.5mm Nagler T6 eyepiece was discontinued years ago and it doesn’t show up that often in the used market, the option that I’d prefer than shipping it to the manufacturer for repair.
As a general rule, I don’t disassemble an eyepiece for cleaning since I’d rather buy a replacement than risk causing irreversible damage due to inexperience or lack of the proper tools. But once I declare an item as a total loss, as is the case with my 2.5mm Nagler eyepiece, I am no longer constricted by that rule. So using a previous experience when I took apart my Meade Series 4000 6.7mm Ultra Wide Angle eyepiece for cleaning a couple of years ago, I went ahead and started disassembling the 2.5mm T6 Nagler with the intention to find the dirty lens and attempting to clean it if possible.
Unfortunately, I found that the damage of my 2.5mm Nagler was on the interior surfaces of the 2-element cemented eye lens and there was nothing that I could do to clean it –advanced ATMs with access to certain chemical substances can repair cemented lenses, but that’s beyond my abilities.
One important bit of information that I learned when I took the 2.5mm Nagler T6 eyepiece apart is that the eye lens group easily detaches by grabbing it from the eye guard and unscrewing it –no tools needed. I compared it to my old 3.5mm Nagler T6 and I noticed that both unscrew at the same place. Looking at the curvature of the lens by looking at reflections I started to wonder if they somehow shared the same lens design, after all: How many times have we read that short FL in the short FL Naglers is achieved by using a Smyth lens group at the front to produce high magnification? So I did a thorough search on the astronomy forums trying to find out if someone had attempted to swap the lenses from T6 Nagler eyepieces, but my search came up empty.
Since swapping the 2.5mm Nagler 2-element eye lens group with the 3.5mm eye lens group is as easy as removing and replacing a screw bottle cap (it doesn’t require any tools, you only need to keep the eyepieces standing up to prevent the field lens group from accidentally coming out), I saw no harm in trying. Fortunately, the bet paid off! After screwing the 3.5mm eye lens group on the 2.5mm Nagler eyepiece, the eyepiece came back performing as good as it had for most of its lifespan. I did a series of empirical tests with the eyepiece looking through a refractor on linear terrestrial structures and I couldn’t notice any aberrations or distortions after the eye lens swap. I went back and forth multiple times testing both eye lens groups on the 2.5mm Nagler and the magnification remained the same on all the visual comparisons that I could make to the best of my ability. Later at night, I tested the renewed 2.5mm Nagler on my TV-85 and NP127is refractors on DSOs and stellar objects and I didn’t notice any loss of performance of the eyepiece from my memory of its good old days. So for all practical purposes, my 2.5mm Nagler T6 eyepiece is back to its normal used condition, as clean as you could expect from a well-used, well cared 11-year old eyepiece!
My intention with this posting is that next time someone is in the same situation that I was, when they do a search for a possible solution, hopefully they won’t come out empty handed!
Here is a picture of my old 2.5mm and 3.5mm Nagler T6 eyepieces with the eye lens group partially unscrewed. Then a picture of my 2.5mm Nagler T6 eyepiece eye lens group detached showing the threads and rubber eye guard. Last, a close-up picture showing the damaged, cemented 2-element lens, just for illustration, not necessary at all to extract it from its metal casing for doing the swapping.