I've been spending the past several nights fine tuning the spacing between my QSI 683wsg-8 camera and an Explore Scientific 3" Field Flattener .7X Focal Reducer. I've been imaging with an Explore Scientific 127mm Apo Refractor. Somewhere between 10 and 15mm spacing I expect the stars in the corner of the image to be nice and circular. At 10mm they are oblong pointing towards the center, and at 15mm they are slightly oblong perpendicular to the direction to center. I may need, say, 13 or 14mm. Unfortunately my set of spacing rings doesn't allow me to fine tune any further than this.
Can somebody recommend spacers that allow for the kind finer of adjustment that I need? For example, rings of sizes not confined to 5mm steps, variable, smaller than 5mm, etc.? Thanks for any tips on how to make this happen.