Like adjusting the spacing between the elements?
I have a super cheap (Probably not even true doublet) 70mm refractor telescope with 360mm focal length approximately.
The spacing with the original spacer causes severe blurring to the edges of the image. but if I remove the original plastic ring spacer and use 4 tiny rectangles of duct tape on 4 corners. 2 layers tall. much thinner than the original spacer
its significantly sharper from center to edge. but I'm not sure if theres any other negative effects.
its a very very cheap refractor telescope. like under 40 bucks i think. I had to do a lot of modifications to make the image brighter in it. plastic ring thing in the focus tube. as well as focus tube being absurdly too long. causing vignetting. as well as a metal ring in the tube of the telescope also causing vignetting and stopping the lens down most likely to F11 or something.
but with all those rings taken out it gives a much brighter image at night.
The diagonal it came with was terrible though. had to replace it with a cheap but still infinitely better one. giving even more image quality.