First of all, I suggest manual exposure. Forget the meter and just play with the exposure until it is what you want. Sounds like you're over exposing. I would also suggest, just to make sure the issue isn't tripod jiggle, that you set the iso to 1600 and get a really high shutter speed. If that is sharp, experiement with lower iso. When it starts to get blurry, it means your shutter speed is too low.


You should carefully manual focus with live view (zoom in to 10x to get it exactly right).


I would also stop down as much as you can and still get sufficently low iso and high enough shutter speed (but not beyond f/10 or so because then diffraction is an issue).


I also agree with the suggestion that you first try without the extenders. There doesn't seem much point in using them unless you get something that looks sharp viewed 1-1 without them.


I don't know how blurry you mean by blurry, but the atmosphere is often the limiting factor. The picture below, taken with an 800mm f/8 telescope, is blurry. Not because of bad focus or shake (it was on a sturdy mount), but just because of the atmosphere. And this is nowhere near a 1-1 crop. So with a 500mm, extenders may not be needed unless you have steady air. (The exposure for this picture was 1/10 sec @ f/8 and iso 100 was used)


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