## 9 Comments on Optical Design, Fabrication, and Potential Applications  
Thorough and complete descriptions of all the designs mentioned here are outside the scope of this work. Only some key issues and concerns in optical design, fabrication, and applications are provided in this section.  
First, and this is related to the design fundamentals, we believe that an excellent insight into the first-order geometrical optical layout is required for a good design. The Y-\(\bar{Y}\) diagram is an excellent tool for this purpose, especially when starting a new design completely from scratch. With that in mind, the key features of the Cooke triplet (or the inverse-telephoto derivative) as well as its derivatives ought to be fully understood. For example, symmetry with respect to the optical stop position can correct both coma and distortion simultaneously.  
Next, distortion is the inherited nightmare of the off-axis and decentered 3M reflective systems. However, by inserting a weak lens or a phase plate near the focal plane, distortion may be substantially reduced. Lastly, during the final optimization, one can slightly tilt and/or decenter the last imaging element to yield a performance improvement by more than 20%.  
One major drawback confronting the designs of the type RT, TMA, or WALRUS is the large volume envelope that contains the entire optical system. Space is always at a premium; the availability of such a large volume is very rare, especially in space and remote sensing applications.  
Although OR comprises just two concentric spheres, photolithography, in general, is operated in the ultraviolet (UV) or shorter spectral region. The requirements in both surface figure and surface roughness control are extremely tight. Thus, special attention should be paid to polishing, optical testing, and bidirectional reflectance distribution function (BRDF) measurements to decrease scattering.  
RT, TMA, and WALRUS have been applied mostly in the infrared (IR) spectral region where the requirement for both the surface figure and the surface roughness control are less stringent than those for the OR. Based on our experience, the misalignment in terms of the decenter and the tilt of the primary mirror in these three systems is extremely sensitive, resulting in performance degradation. A cascade of two-image forming systems in TMA and WALRUS results in their alignment being twice as challenging as for those of the other configurations.  
Recently, we are witnessing significant advances in the fabrication and optical testing of both diffractive optics and emerging freeform optics [34; 35]. The free-form optical shapes or optical surfaces are designed with little to no symmetry: they are free from any constraints of symmetry in their form and shape. In the technology advancement area, we are seeing improvements in the maturity of molding glass and non-glass materials.  
The technological advances are very encouraging, bringing about more opportunities for the applications of all 3M optical systems and their derivatives. Their utilization will depend to a large degree on the tradeoff between cost and performance. For example, as mentioned above: when the molding technique is of sufficiently high quality and the high-temperature bolometer is commercially affordable, a new generation of inexpensive wide-angle night vision commercial cameras (equipped with either an RT or a WALRUS in a scanning or a staring sensor configuration) may be employed in a terrestrial vehicle (and/or in an unstaffed autonomous vehicle (UAV)), a ship, a building, a parking lot, and an airport during any weather conditions. Those include a clear, foggy, snowy, or stormy night, or for pandemic control.  
Finally, for those who are interested in extremely wide-angle optical design, Ref. [36] is highly recommended.