Without seeing what you're describing, it sounds to me like you're describing the inner ring of a T-mount. If you have a T-mount adapter handy, take a look at it. I'm pretty sure they're all made similarly, although there may be an exception, like M42. Anyway, take a look at your T-mount adapter. face on. You should see that it is two pieces, and these two pieces are held in place by three set screws located along the edge of the adapter. The reason for them is so you can center the T-mount on your camera so the lens will be in a face-up orientation. If you remove these three set screws, and then separate the two rings, you'll see that the inner ring has a pronounced groove that the screws fit down into. So, if I'm right about this, what you have on your lens is just 1/2 of a T-mount adapter. Unscrew it and mount a complete one and you should be on your way.
If this isn't the case, then please let us know.
---------- Post added 01-12-17 at 05:02 AM ----------
Originally posted by mlynch53 After having read through this forum over the last few weeks, I decided to start adding photos I've taken with a recently acquired Sigma 600mm f/8 mirror lens for my Pentax K-r. I bought the lens with the aim of using it for the August total eclipse of the Sun.
I've spent some time taking experimental shots, including (of course!) some shots of the Moon. I'm getting more and more comfortable with the lens and taking better shots with it. I really like the lens and I've come to terms with its limitations; I think it should work well for eclipse shot as well as some more static terrestrial shots, as well, perhaps including bird shots in the fields behind our house, which have a variety of wild birds....
The Sigma 600mm f/8 can be an incredibly sharp optic or just an okay one. I know because I've had both. Back in about 1984 I bought one in Canon FD mount. It was so long that it took me a while before I was any good shooting with it, but as I became better with my technique, I got some awesome pics with it. I foolishly sold it when I temporarily changed systems to Nikon. Back then, my best shots came from using a stout tripod and cameras with mirror lock-up.
Well, I never forgot how good that lens was and then a couple years ago, I came across a great deal on one in EOS mount, which is fairly uncommon. I just assumed that Sigma quality control was such that all 600/8 mirrors were the same optically. Unfortunately, I was in for a big disappointment. The one I currently own is "just okay." Judging from your second shot, I'd say you got a good one. It's hard to see much detail in your moon shot.
A word about the solar eclipse -- I've been through one that was almost total -- back in about 1991, I guess it was. And I was savvy enough about astronomy and astrophotography to know what should and shouldn't be done when taking pics of a solar eclipse.
First, keep in mind you're taking pics of the sun, and even when total the coronosphere is still very bright. So what you will need to get for your lens is a solar filter that you can fit over it. Now, there are expensive solar filters and there are cheap ones, but they all do basically the same job -- filtering out enough of the sun's rays so your eyes won't be damaged. I bought a solar filter for the eclipse I witnessed, but I decided just to buy a cheap one. I think I may have paid about $20 for it. What it ended up being was just a mirrored mylar disk in a round plastic frame. Mine was about 5 inches in diameter. When it came time to observe the eclipse, I just taped it to the front of a small 4 inch (about 600mm) scope I had, and this was perfect for viewing the eclipse. I suspect a cheap filter like this would even work OK for photography. Or you can go the expensive route and pay a couple hundred bucks if you feel so inclined.
I just checked eBay -- it might be a good source for you, whether you're looking for a cheap one or an expensive one.
Of course, if you already know all this, well then, never mind.