In the last year, I have built a few such constructs in plumbing pipe. I determined spacing by trial and error using the projected image on a wall. I wanted to be a little more analytical and so purchased a cheapo double-rod optical bench from Surplus Shed.
I have found that the optimal spacing in a duplet is not as narrow as I’d thought. For example [using a collimated source image] I find that a pair of diam/FL 59/350 mm meniscus elements can start producing a sharp image when 31 mm apart. But this distance can be increased to 69 mm with no obvious deterioration in image quality.
My question is which distance do I chose? Am I correct in presuming that the smaller the distance the better? With little knowledge of optics I can convince myself that the smaller the spacing the less chance there is of overall aberration produced by marginal rays passing close to the central axis.
Any help is appreciated … thanks … John
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