On wednesday evening, September 20, 2023, among other binocular astronomy adventures I observed a hexagonal asterism pointed out by fellow Cloudy Nights member aznuge with an APM 20×80 binocular I had been using to log Cygni 100 list doubles (back left in image). After enjoying the view, I wandered north to have a peek at f1 Cygni, aka STF 2743, which is a challenging Cygni 100 double star. What makes it difficult is that the primary is almost five magnitudes brighter than the secondary component, which is only 21 arc-seconds from the primary (4.74/9.43 sep 21″). Somewhat to my surprise, I could not resolve the secondary with the APM 20×80.
The Oberwerk 20×70 ED Ultra (center front in image) had been waiting impatiently on the sideline (a shelf on one of my rolling light shields) for a chance to show it could go the distance (which according to stelledoppie is 1,418.25 light years in the case of STF 2743). And sure enough, the secondary component was cleanly resolved with the 20×70 EDU. It was also resolved with another recent darling, the Nikon 18×70 IF WP (back right in image), but not quite so well as with the Oberwerk, thanks to the slightly smaller exit pupil and the additional 2x of magnification. Larger exit pupils tend to accentuate aberrations ahem in our own eyes and can cause problems resolving challenging double stars.
The APM does have its strengths. The additional aperture gives the views a bit more zing and it goes a bit deeper. It also has better edge performance, though the difference is not consequential for normal viewing because the 20X70 EDU edge performance is still quite good and not distracting. The difference between the two instruments in this regard is only apparent when one is staring at the edge of the field fretting about edge performance. 😉
BUT, the APM 20×80 star shapes are not ideal and have a tendency to spikiness. It doesn’t spoil the views, but it is problematic for observing double stars like STF 2743. The 20×70 ED Ultra star shapes are cleaner with no or minimal spikes on brighter stars.
Owning four 20x binoculars (OB 20×80 Deluxe III, 20×65 ED, 20×70 ED Ultra, and APM 20×80 ED MS) not to mention the Nikon 18×70 IF WP, enables me to evaluate them over time, in many observing sessions on a variety of objects to better understand the strengths of each instrument. And share points of comparison like this observation.
The above image of the f1 and f2 Cygni starfield is from SIMBAD (DSS colored). The bright orange star on the left (west) is f2 Cygni and the bright bluish white star on the right (east) is f1 Cygni. A dark nebula can be seen, west and south (below) of f1 Cygni and to the east of the variable star V2140 Cygni (the bright star on the far right) the “bright” reflection nebula IC 5076 is obvious. Neither can be observed visually from my urban yard. And while IC 5076 is commonly included in star atlases ( for example in the Sky Atlas 2000, the Uranometria 2000, even the Sky and Telescope Pocket Sky Atlas) it is rarely mentioned by visual observers — nothing, for example, in the comprehensive Night Sky Observers Guide. It is more frequently a subject for astro-imaging, any number of fine images of it can be found with a quick web search. The color contrast between f2 and f1 Cygni is readily seen and enjoyable with 10x binoculars. To the east of f2 Cygni is the double star STTA 215 (far left in image). Here are my observation notes of this Cygni 100 list double from Friday, 11 August 2023.
AC 6.55/7.52 sep 136.7″ pa 189
Distance: 2407.41 / 1107.98
Spectral Type: B6IV / M5III
Color Index: -0.02 blue-white / +1.39 orange-red
Nikon 18×70 / Friday 11 August 2023 / urban sky
Light blue primary with apricot secondary at a generous distance. Both stars bright, but the primary is brighter. A pleasing spectacle. Should be easy with mounted 10×50 binoculars.
And here is the 20×70 ED Ultra, in its glory, while the APM sulks on the sideline. 😊