When you get straight lines in DPAC when testing an objective that means that the spherical aberration is well corrected. With fast lens like 5" f/5 or 6" f/5 and being an achromat it still has a huge amount of chromatic aberration that is worse then 1/2 wave. So the POLYchromatic wavefront is still around 1/2 wave or worse. With a lens ones has to take into account both spherical aberration and chromatic aberration to determine the total performance. A singlet lens can test perfect in monochrome light in DPAC but as we know has a huge amount of chromatic aberration. So just keep that in mind. These larger aperture fast achromats do make for nice Richest Field scope when used at low power.
This is why I made a 4" aperture stop for my 6" F5 Jeagers (yours Henry, looks a whole lot like a Jaegers, crinkle finish dew cap and all). That really cleaned up the longitudinal color differential a lot giving notably sharper images at high power (very sharp actually) despite the reduced aperture and remaining residual color differential. Mine had a nice figure in green at full aperture in DPAC, a little over corrected overall, good edge and yes, spherochromatism in red and blue. Yellow was the best correction, almost neutral. Again, stopping it down to 4" aperture cleaned up the lens to basically neutral in green/yellow but the real story was the big reduction in red and blue sphero. My sample also showed no on-axis astigmatism or coma.
BTW Henry, the lens is, I believe, if a Jaegers, a Fraunhofer aplanat design, being free of spherical and coma aberrations at one wave length, typically green.
Edited by Jeff B, 05 February 2021 - 10:25 PM.