Here’s a very tight double double in Corona Borealis that requires good optics and fine seeing to pry apart.
The system, Struve 1964, WDS 15382+3615, consists of four 8th and 9th magnitude stars. Two pairs orbiting each other about 15 arc seconds apart. The AB pair is identified as HU 1167 with Struve 1964 consisting of the A,C and D components. The system can be found in Stelle Doppie here and is part of the AstroLeague Multiple Star Observing Program found here.
Sissy Haas in her book, Double Stars for Small Telescopes, lists the object as a “a pair of identical stars, gloss white in color, that are easily split by only a small gap at 25X.”, not detailing that they are both tight pairs.
The STF CD pair are the most difficult to split at 1.5” and I used averted vision to pry them apart. I confirmed the split with Chris Thuemen who sent me an image showing the pair in the correct Position Angle. I noted a few field stars confirmed by SkySafari Pro on my iPad.
I observed the system with a Burgess 127mm f8 refractor on an iOptron QZ 25 mount. The Low Power was with a Plössl 26mm EP at 36X and the High Power was with the same EP plus a 3X Barlow at 116X.
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