This is a precision, CNC machined 42 mm ID spacer ring that is 1 mm thick. It provides 1 mm of additional space between the field flattener and the imaging camera.
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Spacer was exactly what I was needed to reach backfocus.
Not much to be said here other than the fact that it's exactly 1mm and made nicely of anodized black aluminum. I remove one star due to the fact that 1mm of metal is the same price as the other tubes. Especially if you need multiple of them! Highly recommended though if you need to get within that critical focus zone.
backfocus now spot on (but how can something so small be so necessary)?
High quality material, fair price, made to Stellarvue's exacting standards. Highly recommended.