QUESTION:
Does the spectroscope on the KAO receive light through the telescope?  Can
it be used at the same time the infrared camera is used?

ANSWER from Dave Cole on November 20, 1995:
The spectroscopes which are used on the KAO do indeed receive their
light from the telescope.  They're designed and built to work in the
infrared, just as the cameras are.  Cameras, infrared or not, need
telescopes to magnify and brighten objects until pictures can be taken
in reasonable amounts of time.  Spectroscopes need telescopes for
exactly the same reason; without a telescope gathering light, it would
take forever to get a spectrum of even a bright object.

Spectroscopy and imaging are really just two different ways to look at
objects, in order to learn different things.  A spectrum can tell you
what elements or molecules are in an object, while an image shows you
where an object's bright and where it's not.  As a rule, though, doing
either spectroscopy or imaging is sufficiently difficult that people
specialize and do just one or the other.

There are several different groups--12, I think--which have instruments
for use on the KAO.  Some have spectroscopes, and some have cameras,
but because of this specialization, no one has both.  Since all the
groups work separately, each instrument bolts onto the telescope in a
slightly different way.  This means it's not possible to bolt two
different instruments to the telescope at the same time.  And if they
can't both be on the telescope, they can't both be used, of course.

That doesn't really answer the question, though, for it's not just the
lack of coordination between groups that keeps us from doing imaging and
spectroscopy simultaneously.  One could, after all, design an new
instrument which has both a spectroscope and a camera, and which fits
properly onto the telescope!  Very quickly, though, one runs into a
another problem: to use both the spectroscope and the camera, you have
to split up the infrared light, sending some to each instrument.  If
you send, say, half the light to each, getting a good spectrum is going
to take twice as long as it did when you sent all the light, and the
same is true for the a good picture.  You can observe with the
spectroscope for 15 minutes, then with the camera for another 15, or
you can observe with both, giving each half the light, for 30 minutes.
The result is the same.

So, since you don't gain anything by splitting the light, and since
keeping two instruments working is more complicated than just one,
people simply don't bother.  It only takes twice as long in this
example, though, because both the spectroscope and the camera need the
same light--infrared.  If two instruments want different kinds of
light, you can use them together without doubling the exposure time.
For instance, we use a system which has both a visible light camera and
an infrared camera.  A mirror reflects the infrared light off to the
side, to one camera, while letting the visible light go straight
through, to the other.  Each camera now gets all of the kind of light
it uses; no infrared light is wasted at the visible light camera, and
vice versa.  Instead, we're looking at the object in the visible using
light that the infrared camera would have just thrown away!

Our group actually does this on every flight; we guide the telescope
using the visible light, while taking pictures in the infrared.

Dave Cole
Department of Astronomy & Astrophysics
University of Chicago