I posted these questions in another thread but it occurred to me that this thread is probably the correct one to seek insight from senior members.
1. Is it normal to observe a (very distinct) yellow ring around the near-full moon, as I did last night via the APM 100-90 and 18mm EPs?
2. Should stars be precisely pinpoint in a Semi-APO or pretty close to it but not 100 percent? And to get 100 percent one has to buy a true APO? Or is getting pinpoint stars a function of matching the "correct" EP to the BT?
1) I have this yellow ring also with my Televue Delites eyepieces + APM 100 ED Apo which are very good for 'planetary viewing' and the best wide-angle eyepieces for the moon.
I'm not 100% sure this is called 'Lateral colour aberration'
This type of aberration is from the eyepiece itself not the telescope, it should not be visible in the centre of the field, but most pronounced at the edge.
2) With refractors and ep-binoculars two major aberrations must be as low as possible to get pinpoint stars (in the centre); colour aberration and spherical aberration of the refractor lens. (plus a number of other minor aberrations or defects)
See this website for explanation of several optical aberrations: https://www.handprin.../ASTRO/ae4.html
The APM 100-APO, 120 and the big 150mm binoculars have very low colour aberration (the prism inside helps to lower this aberration) and from medium to low spherical aberration, about 1/3 to 1/4 wave of yellow/green light, they perform well at high magnification.
About eyepieces: many eyepieces perform very good or better in the centre, the difference between them is how they perform to the edge of the field.
Especial how a ep perform with telescopes with low F# , In a slow F#10 telescope nearly every ep perform very good at the edge, but with a fast F5.5 (for the APM binoculars) the sharpness at the edge could be very poor, or still good but with huge Lateral colour aberration or with Distortion...and so on...
Edge performance of eyepieces with field over 70 degree field is always a compromise between sharpness and distortion... see link above.