A Tour of

the IOTA Site

An instructive approach to touring the IR Optical Telescope Array (IOTA) is to become an incoming photon from a star, and to follow your path from star to detector. 

After your journey across the nearby Milky Way of up to 1000 parsecs, you come upon a small blue planet orbiting a run-of-the-mill G-class main sequence star. A bumpy ride through its turbulent atmosphere and you find yourself rushing up upon what appears to be twin cabana houses in southern Arizona (you are, after all, traveling at the speed of light). At the moment they are separated by about 38 meters. You note a small car between the two housings which gives you a sense of scale for this daytime image of the site (obviously, you are arriving at night). 

The first bounce during the course of your tenure as an IOTA photon is off of one of the two siderostat mirrors. These flat, octagonal mirrors have been tracking the sky, and at this particular juncture, they fortuitously just happen to be reflecting light from your star into the telescope array. 

Reflecting off of the siderostat, the next two bounces are off of the primary and secondary mirrors of a downward facing Cassegrain telescope. The 45 cm primary mirror, you note, is roughly the same size as the seeing cell you just passed through on your way down through the atmosphere (you happen to be an IR photon). 

As you pass through the hole in the primary mirror, you reflect off of the tertiary mirror located behind it, and find yourself directed downward, passing into the IOTA vacuum system. Behind the downpipe in the picture, you notice an amplifier box, which appears to be driving the tertiary mirror in tip and tilt axes at approximately 100 Hz. 

Striking that aperture's down mirror, you travel along the vacuum system into the IOTA control room, noticing stations for the down mirror (and as a consequence, the aperture housing) at every 5 and 7 meters along the way. The vacuum itself is held at approximately 2 torr. 

Entering the control room, you look back over your shoulder down the pipe of the long arm, seeing the path you traveled. The silvery track in the center of the picture is for the Anorad table, on which one of IOTA's two movable retroreflectors rides. You notice that the carriage on the Anorad table is a linear induction motor, floating on an air bearing and controlled by a closed loop laser induction servo system (dynamic positioning accuracy: 10 nm). 

Looking ahead of you, you can see the corner mirror table. This table reflects you between the retroreflecting mirrors inside the vacuum system that make up the optical delay lines at IOTA. You notice photons from the other arm of the interferometer also bouncing around in the vacuum tank, having been reflected there off of other optics on the corner mirror table. 

One final bounce, and you've been reflected out of the vacuum system, departing via one of the two aperture arm exit pipes. Looking over your left shoulder (yeah, photons have shoulders), you notice an Oriel optical table, on which the FLUOR experiement is located. Your destination, however, is elsewhere. 

From the exit pipe, you cross a few feet of open air and strike a dichroic mirror. A cruelly efficient segregrator, you watch helplessly as your visibile wavelength buddies pass through, while you get reflected over the IOTA IR table. However, your destiny is interference, being added coherently at the IR beam combiner at the center of the picture. Those visible light pansies just get used for tracking and thrown away. On your way into the IR dewar (left), you take a quick peek around your new home. 

A view of the visible light table reveals the black baffling tubes of the visible CCD star trackers (center left) from which comes the positional data for the tertiary tip-tilt tracking mirrors. In the foreground is the Quadra running the tip-tilt software. 

On a separate table are two other computers; an older machine ('Jurassic Mac') is running the siderostat tracking software, follow the sky motion of the star you just arrived from, while a newer machine (a Pentium 90) is both running the Anorad table and collecting the data generated by the dewar you just entered. 

The actual scene, is not well lit as in the previous pictures. The entire control room is pitch black, save for the dim glow of monitors that faintly illuminate the faces of two intrepid astronomers. Their exploration of the cosmos has them, at the moment, holed up in this dark, chilly room, staring intently at an oscilliscope. In a flash of coherence, you zip across the IR table and are absorbed inside the IR dewar, gone forever. However, without knowing it, you've left a signature on the oscillscope... 

This page is managed by: Gerard van Belle ( gerard@huey.jpl.nasa.gov ), The Jet Propulsion Laboratory 
Last updated: 9 Apr 1997