Air Force lab reflects mirror technology (astronomy, spy sats)

Space Flight Now ^ | Posted: August 25, 2003 | U.S. AIR FORCE NEWS RELEASE

[alnitak]

A milestone in telescope-mirror technology, completed recently by Air Force scientists here, is leading to lightweight, space-based telescopes much larger than NASA's Hubble Space Telescope.

Dr. Richard Carreras aligns, pressurizes and cleans a prototype thin-film membrane mirror. Reflected in the mirror are co-workers 2nd Lt. Ethan Holt, the film mirror project officer, and Nima Jamshidi, a Purdue University student employee. Credit: U.S. Air Force photo by Deb Mercurio
 

Rather than use a heavy, glass mirror, researchers at the Air Force Research Laboratory's directed energy directorate were able to produce a 1-meter-diameter (about 3.25-feet) mirror, made of a thin-film membrane material. This optical-quality polyimide mirror, about the thickness and flexibility of kitchen plastic wrap, was more than three times larger than the biggest membrane mirror previously possible.

The optical quality of the membrane material was exceptional, as judged by the uniformity of its thickness, which did not vary by more than an average of about one-millionth of an inch, according to officials.

Shifting from glass mirrors was necessary because of the limited cargo capacity of space shuttles and other rocket boosters. Replacing glass with thin-film meant that a mirror could be brought to space in a folded or rolled configuration and then unrolled or expanded like an umbrella once in space, officials said.

"Our goal is to produce a telescope mirror with a diameter of 10 meters, or nearly 33 feet," said 2nd Lt. Ethan Holt, the film mirror project officer in the directorate's surveillance technologies branch. "A surveillance telescope that size in orbit 124 miles over the earth would really improve our ability to image enemy and friendly assets and capabilities."

An artist concept shows a thin-film membrane mirror, at right, in a folded configuration so it can fit aboard a rocket and then opened in space as shown at left. Researchers at the Air Force Research Laboratory's directed energy directorate here have produced a 1-meter-diameter (about 3.25-feet), optical-quality membrane mirror. Their goal is to produce a lightweight 10-meter membrane mirror, which could be used as part of a space-based telescope. Credit: U.S. Air Force
 

"The larger the mirror, the greater its ability to see, or resolve, objects on the ground," said Dr. Richard Carreras, the branch's technical adviser. "For example, a 10-meter telescope in Los Angeles would be able to tell the difference between a basketball and a volleyball as far away as Washington, D.C."

Large space-based telescopes could also be used to focus the energy from lasers, another potential application for this technology, officials said.

[alnitak]

Our goal is to produce a telescope mirror with a diameter of 10 meters, or nearly 33 feet," said 2nd Lt. Ethan Holt

Hubble, by contrast, has a 2.5m mirror. The James Webb Space Telescope will have a 6m mirror.

This is interesting stuff, but the article contains no word on how smooth the surface of the mirror was. Also, telescope mirrors need to be curved in order to produce an image, and I am not sure how they will do that.

Exciting stuff to watch for the future.

[prisoner6]

telescope mirrors need to be curved in order to produce an image, and I am not sure how they will do that.

I would imaginea combination of vacum and pressure with the film between. Kinda like air over oil suspensions. Vary the pressure and you can change the focus of the mirror. Tricky, but no t impossible.

prisoner6

[chainsaw]

Or two membranes sealed together at it's edges and inflated with a small amount of nitrogen. Presto a bicurved optical mirror.

[Libertarianize the GOP]

http://www.freerepublic.com/perl/bump-list

[alnitak]

Isn't that going to be convex though?

[Doctor Stochastic]

Make the inner circle smaller than the outer by enough and the inner version will be concave. The joins are tricky.

[Doctor Stochastic]

There also have to be braces like a circus tent has.

[alnitak]

You know Doc, I'm having difficulty visualising this. Got a picture/diagram?

[aruanan]

This is interesting stuff, but the article contains no word on how smooth the surface of the mirror was.

...have produced a 1-meter-diameter (about 3.25-feet), optical-quality membrane mirror.

[Doctor Stochastic]

More reflection makes me think that one would need too many braces. You'll have to check with chainsaw on this. You're probably right that any inflated object would try to be convex.

[alnitak]

Doing a google search for membrane mirrors (both on the web and news groups) returns some interesting links. Seems the favoured way to distort the mirror is to give it an electrostatic charge so that the back of the mirror can be attracted by an opposite charge.

[alnitak]

More reflection

Groan!

[MikeD]

1. The mirror is curved. Check out the image of the grad student and the LT in the mirror. Their heads are larger than the professor near the mirror. Therefore, the image is magnified and the mirror must be curved. Second, pay attention to the doorframe (or whatever it is) behind the LT. There is a slight curvature in it.

2. The article mentions that the surface is smooth to a millionth of an inch. That works out to roughly lambda/4, which is optical quality for most applications (but not good enough for really good astronomical optics).

3. Pardon me for shouting, but WHAT THE )(*&%^_()&&*%_) IS HE DOING WITHOUT GLOVES ON!!!! You don't mess with high quality optics without gloves, masks, smocks, and a clean environment. We treat our sounding rocket optics better than that...

My job involves designing space optics, so I'll be following this news closely. Heck, I'm working on a 1-meter telescope design right now...

MD

[MikeD]

Ping in case you missed this one.

MD

[chainsaw]

Not if one side is mirrored on the outside and clear on the other film ()->