No blur

[Ben the Ignorant]

Just a quick remark on the popular 80mm FPL-53 f/6 triplet sold under many brands. After I got my 3.5mm 110° Myriad, I pushed the magnification higher than my 4mm/82° could (240x). The Myriad alone produces 137x, which I doubled with that famed and faultless Celestron/Vixen Ultima triplet barlow to obtain 274x. 204x (4.7mm Explore inserted in Ultima barlow) made no chromatic blur, and could be tested anytime.

240x (4mm/82° TS plus Ultima barlow) was not possible unless turbulence was unusually quiet, but showed no fringing either. 274x was too much to wait realistically for a super-quiet night, so I did the test by day when air motion was tempered by some cloud cover and very light rain. Despite the reduction of sunlight and transparency, these conditions make the air calmer than is customary. Well, looking at a dark electric pole in front of a pale gray cloud cover made no fringing apparent inside the dark area of the pole.

And looking at a pale gray cement chimney top against a dark foliage background didn't spur any fringing either outside the bright area of cement, despite the fact that turbulence accentuates the chromatic blur. 240x is the practical limit, very spacewalky on the Moon with no lack of brightness, but knowing the image is still free of aberration at 274x makes these triplets even more recommendable.