Imaging with an Explore Scientific ED 80mm Apo refractor telescope

[Johnp1951]

Hello, I am looking to find some one whom as done imaging with a Explore Scientific ED 80mm Apo refractor telescope, with Extension rings and a ccd camera and filter wheel, and weather they had problems with focusing, because I have found that I cannot reach focus, its fractionally out, and the focuser is at its end of travel 

[TiffsAndAstro]

Not used this gear but that looks like a LOT of extension.

55mm ish back focus (last glass element to sensor)  seems most common.  

[PhilB61]

I don't have this telescope but I do have an 80ED triplet, so similar. A simple check you can do is measure from the front lens assembly to the actual position of your camera sensor. This should be equal to your telescopes focal length, ideally with the focus tube in roughly mid position. If not you will need to add or remove extension tubes until it is, you should then be able to achieve focus. It does seem these scopes come with the extension tubes specifically for astrophotography so not sure why its not working for you, but possibly the filter wheel has pushed you too far out and you actually need to remove one extension?

[Elp]

If you're not using a flattener or reducer you don't need extension tubes usually. Think about how far an eyepiece is including a diagonal, it reaches focus easily, the camera sensor needs to be a similar distance.