Field Curvature and Scan Relays:
Constructing an x-y scan relay using one-axis galvanometer-driven mirrors involve at least 4
positive lenses. This likely to add a positive Petzval sum to the system, resulting in a scan plane
with negative curvature.
The possible solutions are:
1. Design the scan lenses as triplets, say, such that they have a negligible Petzval sum. It
might be possible to purchase commercial scan lenses with these characteristics.
2. Design the scan objective to have a negative Petzval sum to counter the curvature added
by the scan relay(s).
3. Put an afocal lens group in the system, at or near a relay point, designed to negate the
field curvature added by the scan relays:
Here is a pair of lenses which have zero power, but add positive curvature (negative
Petzval sum) to a parallel beam: