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IOTA - the Infrared-Optical Telescope Array

A Bit of History
The Instrument

A Bit of History

IOTA began with an agreement in 1988 among five Institutions, the Smithsonian Astrophysical Observatory, Harvard University, the University of Massachusetts, the University of Wyoming, and MIT/Lincoln Laboratory, to build a two-telescope stellar interferometer for the purpose of making fundamental astrophysical observations, and also as a prototype instrument on which we could perfect techniques which could later lead to the development of a larger, more powerful array. On site construction went on for all 1993 and 1994, with first fringes in December 1993.

A rather entertaining tour of the IOTA site (as of spring 1997) at this external site.


Some of the most important milestones in the technology development of the IOTA stellar interferometer and the growing scientific capabilities that this has implied are presented in the following table.

Year  Technology Characteristics Science examples
1995 original IR configuration InSb discrete detector
slow scan (0.03 Hz)
Mira diameters (10%, K=3) 
effective temperature of 
K,M giants
1995 original visible configuration 4 parallel channels.
VRI bands 
512x512 SITe  CCD
binary separation (V=7)
1995 original FLUOR fiber combiner single-mode image fiber beam-combination 
discrete detector 
Mira diameters (1%, K=1)
Teffto M8 (+/-50K) 
L-band (2%, L=-2)
M-band tests
1997 NICMOS3 camera with classical beam combination low read noise 
co-added reads 
2 pixels, 2 beams 
JHK bands 
 HAeBe star disks (+/-5%, J,H=7, K=6)
Miras (+/-5%, J,H=7, K=6)
copy to CHARA 
1998 rapid scan, 
classical beam combination
fast scan (10 Hz) 
stroke 60 micron 
VRI, JHK bands 
CI Cam diameters (+/-4%, H,K=4)
M dwarf masses (+/- 1%)
1999 NICMOS3 camera 
and FLUOR combination
low read noise 
4 pixels, 4 beams 
slow (0.2 Hz) scan 
SW Vir, asym. diameters (+/-2%)
zetaGem, Cepheid (phi = 2 +/- 0.1 mas)
R Leo diameters (+/- 0.2%) 
2000 3rd telescope 3 simultaneous baselines, phase-closure .......

In the past recent years the membership has evolved as the location of participant scientists have changed, so that currently SAO, Harvard, and UMass are active from the original five, and six new institutional affiliation have begun: Instituto Nacional de Astrofisica, Optica y Electronica (INAOE), Observatoire de Grenoble, Observatoire de Paris- Meudon, NASA Ames Research Center,  ESO (Garching) and MPIA (Heidelberg), the last two participating via individual scientists.

The Instrument

The IOTA, is a Michelson stellar interferometer located on Mt. Hopkins in southern Arizona. It operates with three 45 cm collectors that can be located at different stations on each arm of an L-shaped array (15 m X 35 m) and reaches a maximum baseline of 38 m. Each light collector consists of a siderostat feeding a stationary afocal Cassegrain telescope which produces a 10X reduced parallel beam. After compression each beam is directed vertically downward by a piezo-driven active mirror that corrects for tip-tilt motion introduced by atmospheric turbulence. The beams then enter an evacuated envelope and proceed to the corner of the array, where they are turned back along one arm for path compensation, which occurs by means of the delay lines. After being delayed the beams exit from the vacuum and are directed into the designated combination and detection area.

There are three combination tables at the IOTA, and all of them implement pupil-plane beam combination. In two cases, at visible and near-IR wavelengths,  the combination occurs at a beam splitter. The third table houses the FLUOR experiment, in which beam combination, also at near-IR wavelengths, occurs in single-mode fibers. For near-IR operation, a pair of dichroic mirrors at 45° transmit wavelengths less than 1 micron toward the CCD based tip-tilt servo system, and reflect the near-IR light toward the beam combining optics and science detector. The near-IR detector is based on a 256x256 HgCdTe NICMOS3 array. For visible operation only part of the visible light is directed towards the star tracker CCD, and most of the light is directed towards the visible table for combination. The IOTA visible light detector is a thinned back-side-illuminated SITe 512x512 CCD.

To IOTA Home Page