Let's talk about eyepiece today, and as a starting article, I will talk about a very low cost series of eyepiece, which is the Celestron SMA.
Celestron C90 was the first telescope which I've bought for myself, binoculars not included. And the C90 has a 0.965" visual back. By the time I bought the C90, a 25mm Celestron SMA was included in the deal. Other than that, I've also acquired a Hybrid diagonal which allowed me to use both 0.965" eyepiece and 1.25" eyepiece with the C90. And I've also bought a 12mm Celestron SMA for higher power observation. Actually, I wanted something even higher power, but the dealer told me that they had no stock by then.
C90 is a f/11 scope, so it is very forgiving in terms of eyepiece requirement. Therefore, my overall impression to the SMAs are quite good indeed. The eye-relief is good too, I could easily see the whole field of view to the field stop fairly easily. At f/11, they also maintained good sharpness around the field, except to the last outer 10%.
Price at that time was considered to be cheap, but at that price, you could buy two Plossls with the price of a single SMA. So, i suppose there're little rooms for these otherwise fine eyepiece to survive in the market any more.
SMAs are Kellner, very comparable to Plossl at higher focal ratio. But in the days when Plossls are so cheap, they're no longer popular.
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