M42 T-Thread Spacer Ring Kit
Set of four spacers for t-thread (M42x0.75): 5mm, 10mm, 15mm and 30mm. Total length is 60mm.
All have female thread on one end and male thread on the other.
Just about every astro-imager will need these spacers at some stage.
These are especially required if you use a field flattener or coma corrector that has a set "back focus". This is the distance required between the flattener and your camera's sensor, and is not affected by your focuser as the whole imaging train moves together with the focuser.
Another common use for 42mm spacers if if you're attaching a DSLR to a refractor using a t-adapter. If you take out the right-angle, you will need to rack the focuser out a long way, and it can begin to droop. These spacers will help avoid racking the focuser out beyond that point.
This set includes: