Sketch of the intended modifications.
Using the lathe to remove 13 mm from the top of the barrel.
Reinstalling the optics after cutting the barrel on the lathe.
Full Plössl line-up L-R: original Meade 26 mm, Star 25 mm, modified Meade 26 mm (x2).
Orwell Astronomical Society (Ipswich)
Surgery On A Meade Plössl Eyepiece
The Meade Series 4000 (S4K) Plössl eyepieces are well-established and feature four lens elements arranged in two symmetrical groups. Having looked through several examples, I've concluded that the eye lens of the 26 mm in the series is far too deeply recessed into the barrel, making it impossible to see the full field of view when wearing glasses. The reduction in usable field of view is significant at around 40%. Some benefits of this configuration include:
At Orwell Park Observatory, we have a 26 mm S4K stamped "Japan" on the barrel. In the used market, Japanese-manufactured Meade eyepieces are more desirable and fetch significantly higher prices than later Chinese-manufactured equivalents. Chinese-manufactured Meade S4K eyepieces have no barrel stamp. At Newbourne we have three such 26 mm examples, one in good condition and two in a poorer condition. I decided to do some surgery on the poorer examples to make them usable with glasses.
My inspiration came from the 25 mm in my set of Star Plössl eyepieces. This is known in the trade as a "volcano top" design due to the bevelled edges at the top. The eye lens is recessed near the top at a depth of 2 mm making it easy to view the full field comfortably with glasses. In a symmetrical Plössl configuration, this is feasible at focal lengths of 25 mm and greater but, at focal lengths below 20 mm, it becomes increasingly difficult due to the optical design. If you wear glasses and are in the market for Plössls, look out for less recessed designs and don't go much below 20 mm!
The first step involved disassembling and cleaning/storing the optics allowing the eyepiece barrels to be measured. I created a sketch of the eyepiece, in its original form and how I intended the final version to appear. Removal of the top 13 mm of an original anodised aluminium barrel should result in the eye lens being recessed 2 mm below the metalwork. This would leave an exposed silver ring. The ring would be bevelled downwards at an angle of 20 degrees from the inner edges then finished.
Martin Cook kindly agreed to perform the required metalwork on his lathe at home and I arranged to visit. The work was successful, resulting in the eye lenses being recessed by 3 mm, slightly more than expected. I should have measured the barrel one more time! Eye relief with and without glasses was comfortable when viewing the Sun through Martin's Lunt LS60 Hα telescope. The field stop was visible in the daytime when wearing glasses. We agreed that no further changes were needed. Later that day, I tested both eyepieces at home using a Tasco 60 mm refractor. The Moon gave a sharp image and views in the modified Meade compared very well with the Star Plössl with a slightly wider field of view. The exposed aluminium ring did not cause any unwanted reflections and helped with eye positioning.
If the lack of an eyeguard is of concern, a useful tip from Starman1, a member of the CloudyNights forum, is to cut a segment from a bicycle tyre inner tube and attach it to the top of the eyepiece, doubling it over from the top to form a usable eyeguard.
Here are photographs showing various stages of the work.
Sketch of the intended modifications.
Using the lathe to remove 13 mm from the top of the barrel.
Reinstalling the optics after cutting the barrel on the lathe.
Full Plössl line-up L-R: original Meade 26 mm, Star 25 mm, modified Meade 26 mm (x2).
Neil Morley