Theta CrB (COU 610) is an interesting one that has been widening since discovery in 1971. Couteau discovered it with the 20-inch refractor at Nice Observatory in France. His measures of it that year averaged 0.47". In 1990 the separation had increased to 0.75" and in 2008 to 0.81". At present I'd estimate, based on the list of measures, that it would be ~0.85".
The PA of the pair has changed very little, suggesting the possibility of seeing the orbit nearly edge-on. Photometry of the stars is where it gets more interesting. Couteau's estimates of the brightness of the companion, based on the difference between the stars, ranged from 5.5 to 7.5, over a 10 week period for the four micrometer measures. Three of the estimates were in the range 6.5 to 7.5. In his book on Observing Visual Double Stars, published a few years later, he gives in a catalogue of doubles for amateur observers the magnitude of the companion as 7, with no decimal, unlike other doubles.
This of course is different from the WDS magnitudes.
If we look at the short article online by the Spectroscopy guru Jim Kaler we find a note that
In 1970, Theta CrB did just the opposite, and dimmed by some 0.7 magnitudes (to about 50 percent normal brightness), perhaps as a result of some kind of dust ejection. At the same time it went into a series of brightness oscillations. It is now very peaceful, as it has been for well over a decade.
1970 is just before Couteau discovered it as a double. So there could be changing brightness difference for the pair in this period, depending on when it was observed. Is the secondary star variable? - I've not yet found anything definite on this. The WDS simply has a note "variable" without any more data including whether the note applies to one or both stars.
Looking at the magnitudes measured by Tycho and Hipparcos, we find Tycho giving a Delta-m of 2.02 magnitudes (as per the current WDS), and Hipparcos giving 2.31 mags. That's for a waveband in the Visually preferred part of the spectrum (centred on 530nm and 505nm respectively). Whatever the best number turns out to be, it seems very unlikely it will be less than 2 magnitudes, and possibly somewhat more; and if variability happens that will affect the degree of difficulty of separation.
Given that the brightness difference is at least two magnitudes, it can be expected that Rayleigh rather than Dawes will be a guide to the aperture needed for resolution, especially for telescopes with secondary mirrors which enhance the amount of light in the first diffraction ring.
Rayleigh for various apertures to give a rather minimal visual effect of separation is
Therefore we can see that for a separation about 0.85" at present a 5inch or 6-inch might show some elongation, but rather more aperture - as well as good seeing - will be necessary for a hope of separation.
As for comparisons with other binaries - Zeta Herculis at present is about 1.5" separation, with Delta-m of 2.45, therefore much easier. Eta CrB is from the Ephemeris (a Grade 1 orbit) at about 0.40", with Delta-m of only 0.3, which could allow slight elongation with a 6-inch. Gamma CrB is in the closest part of its orbit, at present near 0.05", therefore a 4-metre telescope with speckle interferometry is the telescope of choice. It's a 91 year binary, that back in the 1990s-beginning 2000s was splittable with amateur scopes despite a Delta-m of ~1.5 magnitudes, when around 0.6" to 0.7".
Some of the above material I used in a discussion about Theta CrB back in 2013, here on CN. The separation of the pair was not much less at that time, and an 8-inch scope was the smallest to enable a split at fairly high power at that time.
I'll add some more thoughts later. This is long enough already.
Edited by fred1871, 18 July 2021 - 09:10 AM.