Doing prime focus astrophotography with my new 6" refractor was a challenge when using my old extension tube which had barely enough length in bringing the image into focus. It was also just barely into the focuser enough so that it could be tightened. Needless to say, when you have a DSLR hanging off the end of this setup there is the big possibility of the whole thing crashing to the ground...in the dark! This short 20mm extension tube gave some much needed length by just threading it onto the old extension tube. So now, prime focus astrophotography is a lot easier with 20mm more to work with as well as peace of mind. It was also much cheaper than buying a longer extension tube.