Telescope Reviews: Minimum aperture to observe M57 central star

Well, the smallest aperture I have seen the central star of M57 in was a 9.25 inch SCT at 480x on a night of very stable seeing, although it was tough and was only visible briefly and only for a few times that night. In fact, the main keys to seeing the central star are very very stable seeing and high power (more than 300x for example). A 10 to 12 inch aperture may be a good place to start as far as aperture is concerned, although again, larger apertures may make it a little easier to see when it does appear. I remember during college when we were at a public night at Behlen Observatory's 30 inch Classical Cassegrain located well north of Lincoln at UNL's Field Laboratory near Mead, Nebraska. The scope was on the Ring but the central star was not visible at any time. I asked my professor (Don Taylor) about that, and he said that he hadn't seen the central star even in Steward Observatory's 90 inch at Kitt Peak, so at the time, I concluded that it might not be possible to see in amateur instruments. A few years later, I learned the truth at one of our club star parties at our dark sky site south of Lincoln, Nebraska. A friend of mine called me to his 20 inch Obsession to "see the central star in the Ring". I kind of said, "Yea, right", so I went up the scope's big ladder and had a peek. Sure enough, at over 400x, there, sitting right in the middle of the ring was the central star, visible with direct vision! Then, abruptly, it vanished. I watched for a while and it faded in and out as the seeing varied, so it soon became obvious what was going on. Seeing was blurring the star to a point where it just blended into the glowing nebulosity in the interior of the Ring. If the seeing wasn't absolutely tack-sharp, the central star would almost never appear. Once I figured this out, I went back to my own ten inch and watched for a while, waiting for the seeing to finally settle down for at least a few brief moments. Eventually, I did manage to catch a few short periods when the seeing stabilized and the star briefly appeared in my own scope, although in the ten inch, it was still pretty faint. When visible, it sort of 'blinks' on and off as the seeing changes, so be prepared for that behavior. The main key to observing the central star is: if the seeing isn't great, the central star will not appear no matter how big the scope is. You have to watch the center of the ring at fairly high power (at least 300x to 400x) for quite a while and not just look in and just give up when you don't see it right away. Clear skies to you.