Near net shape parts can be produced using some very old processes (investment casting) and the relatively new direct metal laser sintering (DMLS) process. These processes have significant advantages for complex blank lightweighting and costs but are not inherently suited for producing high performance mirrors. The DMLS process can provide extremely complex lightweight structures but the high residual stresses left in the material results in unstable mirror figure retention. Although not to the extreme intricacy of DMLS, investment casting can also provide complex lightweight structures at considerably lower costs than DMLS and even conventional wrought mirror blanks but the less than 100% density for casting (and also DMLS) limits finishing quality. This paper will cover the progress that has been made to make both the DMLS and investment casting processes into viable near net shape blank options for high performance aluminum mirrors. Finish and figure results will be presented to show performance commensurate with existing conventional processes.
Due to advances in manufacturing processes, the substrate options for high performance diamond machined mirrors are
expanding. Fewer compromises have to be made to achieve the needed weight, stiffness and finish while maintaining
reasonable costs. In addition to the traditional mirror materials like aluminum and beryllium, there are some less
common materials that can now be included in the trade space that fill the cost and performance continuum between
wrought aluminum and beryllium mirrors. Aluminum and beryllium, respectively, had been the low cost/fair
performance and very high cost/very high performance bounds for substrate selection. These additional substrates
provide multiple near net shape blank options and processes, mostly within these bounds, that can be considered in a
mirror cost versus performance trade analysis.
This paper will include a summary of some advances in manufacturing processes that provide more substrate options for
diamond machined mirrors with some sample performance analysis and data. This is merged with the traditional
substrate options to illustrate the now larger mirror substrate trade space. Some benchmark structural analysis is
provided to back up a generic mirror design trade study.
An ultra-low surface finishing process for 6061 T6 type aluminum has been developed by Corning Incorporated, Specialty Materials Division, and has been successfully applied to mirrors up to 13 inches in diameter. This paper presents finish and figure data achieved from the mirror finishing process. Mirror stability is demonstrated through Pre and post thermal cycle surface figure measurements; temperature range of cycle -55°C to +70°C. As an added benefit, the process enables the use of deterministic finishing and enhances the reflective optics resistance to corrosion. Survivability of the reflective optic is evaluated through extended humidity testing.
The demand for high performance, lightweight mirrors was historically driven by aerospace and defense (A&D) but now we are also seeing similar requirements for commercial applications. These applications range from aerospace-like platforms such as small unmanned aircraft for agricultural, mineral and pollutant aerial mapping to an eye tracking gimbaled mirror for optometry offices. While aerospace and defense businesses can often justify the high cost of exotic, low density materials, commercial products rarely can. Also, to obtain high performance with low overall optical system weight, aspheric surfaces are often prescribed. This may drive the manufacturing process to diamond machining thus requiring the reflective side of the mirror to be a diamond machinable material.<p> </p> This paper summarizes the diamond machined finishing and coating of some high performance, lightweight designs using non-exotic substrates to achieve cost effective mirrors. The results indicate that these processes can meet typical aerospace and defense requirements but may also be competitive in some commercial applications.
Extreme light-weighting is important in many aerospace and defense applications but the cost associated with beryllium or other exotic materials can be prohibitive. The current standard for producing cost effective, high performance mirrors is to diamond machine mirror blanks from aluminum alloy stock. About 80% material removal is the limit for geometrical lightweighting while still retaining the structural integrity required for optical fabrication. To reduce weight further requires alternative materials. This paper summarizes the status of diamond machined finishing and coating of magnesium alloys to produce cost effective, lightweight mirrors with high, broadband reflectivity and low scatter finish.
A thermal imaging zoom system has been developed for the mid wave infrared band with greater than 30X zoom range.
The zoom system provides continuous changes in the field of view from the narrow field of view to the wide field of
view. Athermalization was also a key feature included in the design. An active thermal compensation approach is being
used to cover a broad thermal range. A preloaded rail approach is used to maintain boresight and vibration requirements.
The final optical layout and mechanical design resulted in a system suitable for tactical and other harsh environments.
The current design is very compact for the extremely large zoom range but, the lens layout also provides adequate space
for folding. In this way the zoom system can be easily configured for applications with compact space claims such as
small turrets or gimbals. The fundamental optical design has also been found to be capable of accommodating different
camera formats (focal plane array size and F number).
Over the past few decades of computer aided engineering growth there has been much more progress in increasing the
power and capability of function specific engineering tools (e.g., optical design, finite element analysis, etc.) than in the
integration of and communication between these tools. With only a few notable exceptions, such as FEA being
imbedded into solid modeling, the communication method between the function specific tools continues to be dominated
by translation to neutral data formats (e.g., IGES, STEP) and file transfer. There are a number of problems with this
approach. The translation is a serial process where an engineer has to stop at some point in the design, make the neutral
file, send that file to the next function, and wait for feedback. The translation through a neutral format is typically one
way so the whole translation process has to be repeated when changes are required. Revision tracking of multiple files
for each design iteration is both critical and a likely source of errors. Also, as with any translation, some information is
always lost or corrupted in the process.
This paper describes some progress that has been made in more tightly integrating optical design, mechanical design,
fabrication, and testing of optical systems. Tools have been developed that connect CODE V<sup>®</sup> to SolidWorks<sup>®</sup>
(bidirectional), compensation of diamond turning CNC from interferometric data, slope analysis from interferometer and
profilometer data, and other tools for wavefront error compensation, and electronic nulls. Design, machining, testing
and inspection efficiency gains are achieved through tools that consume mechanical solid models in their native format.