Tuesday 27 September 2011

Outer planets ablaze at 1.5 microns

Astronomer Mike Brown at Caltech hunts in the outer reaches of the solar system. He usually concentrates on what is called the Kuiper Belt which extends from the orbit of Neptune out to about twice as far as Neptune's orbit. He recently used the Keck telescope in Hawaii with the intention of imaging Neptune's moon Triton, but the views of the planet itself, and closer neighbour Uranus, were too interesting to ignore. The Keck's adaptive optics system, which compensates for turbulence in the earth's atmosphere, produces stunning results.

Neptune, glowing like a demonic cricket ball, shows the two distinct bands which are glowing at the imaging wavelength of 1.5 microns (1500 nm). Of course these are not really 'hot' since the surface of Neptune is -200 C and the bright bands will only be a few degrees warmer. These correspond to faint but visible features on Neptune's 'surface'.

With Uranus the main features are the rings, which stand out distinctly given the low level of radiation from the planet itself, and the spot to the top left which is the moon Miranda. The bright spots on the surface are clouds. This is notable because the 'surface' of Uranus, in visible light, is almost devoid of any features (although, as the 'northern' uranian hemisphere warms up some banding is appearing).

There is more at space.com.

RW Wood took infrared photographs of gas giants Jupiter and Saturn at the end of October 1915. He had been granted use of the 60 inch reflector at Mount Wilson and took photographs through infrared, yellow, violet and ultraviolet filters. His infrared filter had a band-pass of 700 nm, with its upper limit being determined by the plate sensitivity, which was probably less than 800 nm. In this case infrared photographs showed fewer features than visible light whereas belts were clearly shown on the violet and ultraviolet plates. (It is not possible to record a wavelength as long as 1.5 microns using photographic emulsion, so an electronic sensor has to be used, although this is not thermal imaging. However, this wavelength is still within an atmospheric window for infrared.)

You can read Wood's 1916 paper on the SAO/NASA Astrophysics Data System (ADS). It is entitled Monochromatic Photography of Jupiter and Saturn.

[My thanks to Mike Brown for permission to reproduce his images here.]

Tuesday 6 September 2011

Infrared invisibility cloak

RV Jones, in his excellent book on British scientific intelligence during the Second World War, called Most Secret War, tells the story of a German attempt to camouflage submarines so that they blended into the sea at both visible and far-infrared/thermal wavelengths. They developed a painting scheme which combined a black undercoat with a top layer of varnish that included powdered glass in suspension. To the naked eye the boat would look grey, presumably like the grey north Atlantic, but to an infrared viewer it would look dark, again like the sea. The properties of the undercoat, glass and varnish were such that infrared passed straight through and was absorbed by the undercoat while visible light was refracted by the glass and gave a grey appearance.

This was a neat trick, of which Jones was most complimentary in his book. Unfortunately the British were actually detecting submarines by using radar and not infrared.

This story comes to mind when seeing the news stories circulating about a thermal invisibility cloak called ADAPTIV produced by BAE systems (their press release). The plan is to enable a military object, such as a tank, to either blend into the background or to appear to be something innocuous. This is done by covering an appropriate surface or two with ingenious hexagonal panels that can be electronically set to give off suitable far-infrared radiation on demand. The trick being the ability to change temperature very rapidly and to adopt a temperature that can be lower as well as higher than ambient.

A thermal imaging camera can be used to pickup what is behind the object and use the ADAPTIV panels to show this on the opposite surface, thus rendering the object invisible. By replacing some of the output pixels with a different thermal pattern the object can be made to appear smaller and different. It's unclear whether such as system could yet be used by an individual soldier but objects larger than vehicles ... even buildings ... can be cloaked using this technology.

The promotional video [now no longer online] is rather fun, not only because of the 'so natural that you want to go to war' music but also for the shots of ADAPTIV in action. I particularly like how the panels can be used to display advertisements!