If it was OVERcorrected, the spacing could be enlarged, and you could make a significant change. Sounds like a drastic action, modifying a surface is in order.
I would suggest -NOT- to mess with R3, nearest the crown---only consider R4 to be the one to get tweaked.
R2 and R3 have a very close tolerance (critical) in respect to each other. Likely varies by a set number of interference fringes.
If you enlarge R1, that could add some correction, but you'd add to the focal length fairly quickly as well.
That leaves your option you found--- by changing the conic of R3 (if it isn't coated)
If you miss, and go a bit too far, then you could use the spacing idea to tweak the correction to where you want. So there would be an option that could be called a "fudge factor".
Yes, I ran out of spacing options which produced little effect.
The reason I'd mess with R3 is because I'm familiar with working and testing concave surfaces. In OLSO it shows that just a bit of aspherization, like going from +2 to -2 makes a huge difference in the total SA. I can work in this range. At least I think I'm assessing this right.
I found this thread helpful.
I'm sure that by just tweaking a conic, there are other less than optimal effects induced, but at this point, the SA is quite bad, and getting that 5th order spherochromatism and other HSA issues handled is not the first order of the day.
Yes. My mechanical design mentor said that you should always have a 'hole card' or 'plan b'. Spacing changes can provide that fine tuning.
Edited by ccaissie, 24 November 2018 - 06:53 AM.