This is a 48 mm extension tube that adds 20 mm of length. The outside diameter is 2.0". The male thread is just under 3 mm long and the female thread is about 6 mm deep.
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The GSO 2"/1.25" 0.5X focal reducers can be converted to 0.33X focal reducers by adding an 18 mm spacer ring. I had a 1.25" 18 mm ring, but could not find a 2" ring. I called Stellarvue about my problem and Alex offered to cut down a 20 mm spacer to 18 mm. Soon thereafter I was pleased to receive a 2" 18 mm ring with great threads. I'm very pleased with the quality and price, typical with Stellarvue.
I purchased every M48 extension tube Stellarvue has to offer in order to get the spacing just right for my EdgeHD SCT and I was amazed at the build quality. They are light weight and solid. Combining them allows me to get perfect images without vignetting on a full frame DSLR.
I recently purchased 4 different extensions from Vic at Stellarvue. I've come to expect nothing but high quality coming out of Auburn--- which is what I got. I'll be giving this review for the other 3 extensions.