When increasing the lens spacing is there any negative effect on other wavelengths?
Increasing spacing works for achromats too but improving blue-violet correction has an adverse effect on H alpha.
Raf is correct, but it might not be as bad as it sounds. I think the acid test is needed in the form of experimental real world evaluations - respacing for much improved CaK performance, while not optimal, might still prove acceptable 546 nm and 656 nm performance. While I'm certainly no expert, consider the following...
Here's the nominal spacing of 0.5 mm and the 656 spots:
Here's a respace to 0.4 mm to optimize 656 nm:
Not sure this would provide a perceptible improvement.
If we re-space for 393 nm to 0.95 m, here's the spot diagrams for 656 nm:
Compare this to the respaced/optimized 393 nm spot perfromance:
And here's 546 nm with a re-space to 0.95 mm:
Below I've plotted 656 nm for the nominal objective, and the respaced objective optimized for 393 nm. I've also increased the aperture divisions from 33 to 50 to increase the intensity of the spot diagrams.
While the spots are certainly not as good, they don't appear terrible. Indeed, the CaK respaced 656 nm plots look much better than the unoptimized nominal CaK, and quite similar to the the respaced/optimized 393 nm spots. Therefore the real world H alpha with the optimized for CaK might work pretty well under most seeing conditions:
Spots with gaussian blur applied: