Detecting things in the IR

There has been a fair bit of discussion about the IR and detecting
things
on the list so I thought I would toss in my two cents (US).

It seems quite reasonable that in the near future IR telescopes will
be a really good way to find spaceships (from space).  The temperature
every talks about is around ~300 Kelvin (or ~27 C).  At those sorts
of temperatures, space ships will be radiating at a peak wavelenght
of about 10 microns (this has ramifications I will discuss later).  
For a comparison, the sun's peak wavelenth is around 0.5 microns, which
is of course where the human eyeball is the most sensitive.

I did a rough back of the envelope calculation (assuming perfectly
spherical spaceships and all of that).	It works out that at reasonable
ranges, a spaceship will be quite bright.  Even at around a light second
the flux from a moderate sized ship (say a 100 meters in radius) would
be about that of a bright star.  Beyond that, say 10 light seconds, and
now you have to worry about the ship dissapearing into the background of
nearby stars.  At 10 light seconds, you will not be able to resolve ship
with any reasonable sized telescope (a few meters or so) and the number
of stars at the same brightness as the ship appear is pretty high (a few
per square degree, or, say, one per full moon sized area).  

Nonetheless, it is still pretty easy to find things.  Why?  No star has
a 
temperature of 300 Kelvin.  So you can use the color to determine
if something is a ship or not (ie. if something looks really really red
in the Infra-red, odds are it is a ship).  

The other advantage you will have is that ships will be moving rather
differently then anything else in a solar system.  They will appear to
move much faster than background objects (distant stars) unless they are

moving on an orbit that will send them straight at the observer.  

So, how do you hide?  The temperature of a ship I assume is the same
as the earth, so a low earth orbit will make you hard to find.
Interestingly, at 10 microns the earth's atmospher glows.  So a ship
in space will be very hard to detect from the ground.

The other trick to hide yourself is to reduce your surface
temperature.  The luminosity of a blackbody (a blackbody is a
technical term, the common household oven, however, makes a pretty
good black body, an incandescent lightbulb also makes a good black
body) depends on the temperature to the fourth power.  So, if you
reduce the hull temperature by a factor of two, your brightness to an
observer is reduce by a factor of 16.  Now, you still have to get rid
of the waste heat your ship generates but I can imagine a "stealth
mode" where ships deployed radiators (big hunks of superconductor or
something) that spread out the area over which the waste heat is
dumped and thus decreases the effective temperature of the ship.  Of
course this puts a gap in your armor.

In terms of other objects confusing detectors, I don't think that is
an issue in most places like the solar system.	There are not many
rocks at the sizes of a ship, and almost none of them have
temperatures of 300 Kelvin.  It possible in other solar systems such
places exists (in fact I can think of one place where this is
definitely true) but they are unusual places.

Someone mentioned that optical instruments have certain ranges in which
something is in focus.	This is true when the object you are taking a 
picture of is close, where close means the distance between you and
the object is a few times the focal length of the camera.  The focal
length
of a telescope is usually a few meters, so this will not be a problem
for
ships in space.

Finally, in terms of the data rate problem (can one ship process all the
data covering the whole sky?)  Well, there a couple of projects to do
all sky monitoring now.  Heck, there is one that is to be done in
semi-real time and is done by only amatures (well, kinda), check out:

http://www.tass-survey.org/tass/tass.shtml

Another example is ROTSE, which is not all sky yet (though I know
people are building up an all-sky follow up as I write this)

http://www.umich.edu/~rotse/

I hope what I wrote above is not too muddled.

cheers
Bradford Holden

"It's not a pleasant situation, but even members of Congress, some of
us, have principles." - Rep. Sonny Callahan (R-Ala.)