Tonight I got my 5" newt (F/5) on a bench pointing at an artificial star and stuck a 4mm eyepiece in it. I got a Baader FFC last month so I wanted to test it with at higher magnificaton. I previously did with a GSO x2 barlow and also a GSO x3 barlow.
x2 would give me very good performance on axis but light scattering was a real shocker off axis. x3 would be hopeless. Chromatic aberration even on axis: 3 stars (RGB) clearly separated in one line at the center of the field.
I stuck the FCC and started at x3 with the same 4mm PLOSSL eyepiece. Really clean picture. Extremely sharp and contrast was excellent. Then I kept on stacking tube spacers until I run out of it getting closer and closer to a point where light started scattering around the spider vanes and around the star, not unlike diffraction spikes on long exposures but I guess by then I must have been close to x8 or x7.
The scope is really well collimated so I could see the very first ring around the airy disc. I've seen airy discs on a real star on and off with the seeing but never steady like that before with a uninterrupted ring around the central dot.
Off axis it still looked good so I think it's going to work well comes first light. Very impressed with it. It's like my first very nice piece of glass with a telescope (consumables) around it.
