Design considerations
The design of the optics was influenced by the following technical considerations:
The collimator had to transmit well in the u’ band.
It had to be reasonably well corrected for chromatic aberrations between 320 and 900nm.
It has to correct the aberrations of the telescope (coma on the WHT) without help from the cameras.
These considerations meant that
There are very restricted glasses available, almost all of low refractive index and dispersion.
The elements have to be as thin as possible within the bounds of function and manufacturability.
Chromatic correction become difficult.
Correction for the telescope aberrations become a matter of compromise.