If some eyepieces do not come into focus because the extension tube is already fully extended, you will be short of focusing range. With this 16.5 mm (or approximately 0.65 inch) focal length extender from Starlight Instruments, a 2 inch (or approximately 0.5 inch) focal length will be added to the tube.
The increase (riser) comes between the focus base and the focus, so the entire focusing range extends outwards. You often need this when you have replaced the original focus with a focus that has a lower build-up. You can find more loose parts in the telescope parts.
At about 16.5 mm, this is a small correction, just enough if you are just a few millimeters off. First, focus properly, measure how much range you are missing, and choose the zoom that fits that just right. Don't go higher than necessary, because you lose the same amount of range inwards.
If you fall short of that amount, there is The increase of about 1 inch or The 1.4 inch increase.
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