I would concur Peter. I have been observing double stars for about 30 years now using a variety of high quality, high contrast telescopes including a 7″ F8 Mak-Newt with 1/10th wave quartz optics and 13% obstruction, an 8″ F6 Mak-Newt with 18% obstruction, 1/9th wave astro sitall optics and an OMC 200 Mak-Cas, F20, 4,000mm FL with 22% obstruction. I’ve failed to see it in all of those scopes despite numerous attempts. I have seen it twice in my Takahashi Mewlon 300, a 12″ high contrast Dall-Kirkham design and once in my 18″ F4.5 Newtonian which has a superb mirror. The first time in the Mewlon was back in October 2007 when it was just below 8″ separation. The seeing was excellent, the E & F stars in the trapezium stood out like there was always 6 on show but even so, the pup was just momentary glimpses in the glare of Sirius A but consistent in it’s position. A check the next day confirmed I had the correct PA. The next time was a couple of years later in the 18″ at Kelling Heath. Skies were claggy but seeing good so I changed my targets. Again the E & F stars stood out so I gave Sirius a go but first being a Dobsonian which always has some flexure I collimated the scope for that altitude. The observation was similar to that I’d had with the Mewlon previously. Then last month in the Mewlon, again E & F stars steady and tight (I think this is a pre-requisite in my book – if you can’t see them well, don’t even bother going for Sirius B). This time of course the seperation is 11″ and I had an almost continuous view of the pup for about half an hour, a faint spec just below the diffraction spike from the spider. Even so the pup was very faint and almost averted vision in the glare of Sirius B. It’s altitude in the UK does not help, even when the seeing is very good high up, it rarely is at the altitude of Sirius. I know an occulting bar would help but for me the beauty of observing this is seeing it against the glare of the primary and not in isolation.
Andrew