Last night I had another ‘lull’ in my occultation observing and moved over my 150mm refractor to check out whether I could see ‘the pup’ with the aperture stopped down to 112mm. (The altitude was around 55 degrees.) Okay, I was a bit cocksure and this attitude (pride comes before a fall), clearly deserves what it gets. The weather has changed here and the moist south-easters are blowing. As a result, though okay for occultations, conditions are not appropriate for splitting difficult double stars. Once again I was enchanted by the exquisite diffraction disc and rings of a refractor, somewhat larger when stopped down to 112mm, but while the disc appeared to remain fairly stationary, the rings, whilst still for the most part sharp and discrete were in constant random motion. Most of the problem was the twinkling type random flashing from the central disk. I increased the magnification to X522, and knowing where to look, with the full 150mm aperture could only maybe say that I glimpsed the pup once or twice. I could not say that I really saw it. At 112mm aperture absolutely nothing. And yet on the other occasion just a few days ago with the same instrument, same eyepieces, it was really easy at high magnification! (report above.)
Now what lesson have I learned? Firstly to reiterate, but this time very strongly, that steady seeing (and a clear sky) are essential. Now obviously altitude will play a significant part in the opportunity to achieve this. Because good altitude is a ‘given’ for me, it is quite unfair to directly apply my observations to those possible from northern Europe.
From the appearance last time in the 150mm I would feel that the original claim of visibility with a 130mm instrument is ‘do-able’ under the right conditions. The difficulty is to get those conditions from England where Sirius at 52N latitude rises to a maximum altitude under 22 degrees even allowing for refraction.