User:Jbovaird/sandbox/Membrane Mirrors

Membrane Mirrors are lightweight, reflective mirrors made of sheets of thin film materials stretched to a solid metal frame, projected to be used as replacements for plate mirror elements in spacecrafts to reduce weight. Currently, membrane mirrors are being studied in adaptive optic systems (technology to reduce wavefront distortion) for the improvement of space telescopes and visual optics. In addition, membrane mirrors are quite frequently used for laser applications such as femtosecond laser compression experiments. Popularity is increasing because they are inexpensive to launch due to their thin structures and ability to fold to reduce size. Membrane mirrors possess diverse applications with low cost.

1990s

 * 1999 The idea of an inflatable membrane structure for space telescopes that could be folded to allow for a single small booster launch was presented by Ivan Bekey.

2000s

 * 2000 The "Gossamer Spacecraft Initiative" (NASA Program) pushed for the creation of lightweight telescopes, accelerating the need for membrane mirrors.
 * 2001 Control issues such as the lack of surface precision, manufacturing errors, and distortions caused by temperature changes set back production.
 * 2002 An active inflatable structure was used with piezopolymer (a polymer that can reversibly convert pressure to electricity) sheets, which allowed for more precise control of the membrane material.
 * 2003 Microelectromechanical systems (MEMs) were posed as a solution to control flexible surfaces by S.E. Lyshevski.
 * 2004 Vibration issues of the membrane surface were reduced by the addition of piezopolymer patches. Deformations caused by gravity resulted in research approaches of shape control to study the placement of actuators (component for controlling a system) along the membrane mirror frame.
 * 2006 Wrinkle elimination and membrane shape control advances so clearer images are perceived from the membrane mirror.

Materials
Shaped either round or rectangular, membrane mirrors are made of thin metals (silver, gold, or aluminum) coated over a membrane, usually of polyamide or silicon. There are four key considerations for membrane selection: ductility and tensile strength, desired frequency response, ability to be manufactured flat, and ability to be coated with highly reflective materials.

Polyamide films are being manufactured with 10-20 μm thickness and a thickness variation of about 1 μm. Surface uniformity must increase by a factor of 10 to meet the desired surface flatness. Previously popular plastic films have poor tensile strength, or bad resistance against being pulled apart, yet the slow deformation (or creep) in the plastic film smooths out creases. Nickel as a membrane material can crease easily, yet has high strength and greater surface control than plastic films. For high frequency applications, titanium membranes are preferred.

The membrane is stretched flat to the metal frame which has attached spaced calibrators to reduce vibration across the membrane.

Laboratory Testing
Different surfaces for the membrane have been tested based on their ability to resist environmental deformation and distortion, provided they can survive the harsh conditions of their specific application. Testing is done on three different size scales to eliminate any inaccuracies due to the test environment, such as any local acoustic vibrations that cause the membrane to oscillate.

Large scale testing involves membranes over 1 meter in diameter. A reference sphere, whose diameter is larger than the membrane, is used to observe the behavior of the membrane as a laser illuminates the entire surface and a camera measures the reflected light.

Intermediate scale testing uses membranes between 6 to 12 inches. Surface errors, like variations in thickness, must be below 0.5 μm in order to test using interferometry (technique in which the interference of waves is used to gather information) accurately. With a smaller the scale, there is a desirable decrease in surface distortion as seen by the use of the interferogram.

Small scale testing of membranes considers the 1 mm size. Tests of the same type as the intermediate scale are conducted, only much more precise and concentrated.

For Space Telescopes
Large membrane mirrors can be constructed together, creating a large reflective surface. The combination of the lack of gravity and lightweight material will allow such a large film to be stretched completely flat. Multiple pieces of the mirror can be packed together and assembled after launch in space, unlike current space mirror systems. This will allow launching such material to be cheaper because of its size and weight.

Limitations
Membranes augmented with actuators are very complex, and with some too large or expensive to test before launching, accurate modeling must be absolutely precise. Membrane mirrors for space telescopes also require testing in a vacuum, a lengthy and expensive process that necessitates extreme accuracy in modeling before testing.

Plastic films, which are currently used, deform over time as force and pressure are applied, meaning the current design is not durable. There is also a problem with wrinkles in the membrane after being folded.