Homemade Eyepiece - soft around the edges

Sorry for the duplicate post in the "Telescope Making" forum:

I've been experimenting with making my own eyepieces from surplus optics. I've made a 23mm Orthoscopic with 16mm dia. x 40mm f/l plano convex eye lens and a 16mm dia. x 50mm triplet. All lenses are coated and I blackened the edges. The field stop is the retaining ring about 15mm. The eyepiece performs very well when the object is centered, but starts to degrade a 1/4 of the way from the edge. Eyerelief is good too. The FOV is very close to 50 deg. and has a very dark background. Actually darker than my 25mm Plossl. I was testing it in my ST80 (f/5). I know the faster the scope is, the more demanding it is on eyepieces. I will test it in my 8" (f/6) Dob tonight. Last night, with my ST80 (17x), I was able to easily resolve the cloud bands on Jupiter, Saturn's rings, individual stars in M36 & M37 and M45.

My question is the following; Is there anything I can do to help with the edge softness? Increase or decrease spacing between eye lens and field lens (currently about 2mm to 3mm). Decrease the field stop (smaller)?

Clear skies,

Tim Hornbeck

I took apart the eyepiece again. I put a 36mm achromat between the PCX and the triplet. This would give me around a 15.5mm focal length. The design is a Konig. It really expanded my FOV. I didn't test it at night, because the added element really brought out the scratches and blems in the surplus optics. I did test it during the day and it wasn't bad. It was still soft around the edges, but again it was in the ST80 at f/5. The scratches and black spots really bothered me. So, I pulled out the achromat and shortened the distance between the PCX and the triplet. Then I tested it at night in my 8" dob at f/6. It was improved. Still had some softness, but not as much. Larger FOV than my UO Ortho, but that eyepiece is sharp to the edge. If I field stop the eye lens on the homemade eyepiece this should help.

Overall not too bad. I have only $14 invested.

Clear skies,

Tim Hornbeck