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Optical Long Baseline Interferometry News

Desktop Interferometer for Aperture Synthesis Imaging

Donald's Demo | Example fringes | Parts List | SPIE Paper 4838-99

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Please read the SPIE paper from conference 4838, Desktop Interferometer for Synthesis Imaging (PDF), by Peter Lawson, Donald Wilson, and John Baldwin. It gives a more coherent description of the interferometer than you will find here. The following is mostly a set of notes I wrote up as I was building the JPL demo. Please contact me if there is anything further you wish to be included here. I will continue to improve these notes as time permits. (Peter Lawson: September 5, 2002.)

I have tried to use fast optics where possible, as the CCD camera I'm using is probably not as sensitive as Donald's Baxall camera.

Aperture Mask

An aperture size of 1/32 inch is 0.8 mm, and close to what Donald used.
A hole that size resolves objects larger than 0.8 mm at 1 meter, 1.6 mm at 2 meters, and 2.4 mm at 3 meters; assuming a central wavelength of 0.65 microns (red filter). So the total structure in your artificial source must be smaller than the above. This is roughly consistent with Donald's dimensions.
A 3-hole, or at most a 4-hole mask is best for the demo - otherwise there is too much instantaneous (u,v) coverage.
What should the hole spacings be?
The finest fringes shouldn't resolve the "points" in the source.
A hole separation of 2D, where D is the hole diameter, is still so close that the spatial frequency coverage of neighboring holes overlap. The separation should be larger, say 3D, which yields 7.3 fringes across the Airy disk.
If a 4-hole Golay mask is used, with a nearest hole separation of 3D, the highest spatial frequency which should be two or three times 7.3 fringes, or about 15 or 20 fringes across the source.
The holes should be arranged such that if a circle were drawn passing the two furtherst holes in the array, its center would coincide with the center of rotation of the disk. (This will help keep the illumination uniform when using a laser beam for alignment.)
Rough sketch of mask: PostScript program, PDF format. Here is also Jim's drawing (PDF)
Note that Jim's mask has the right hole size, but doesn't have the right long baseline.

Artificial Star

The angular size of the artifical star should be no larger, but comparable to, the Airy disk diameter of the holes at HeNe. [2.4 lambda/D]. The physical diameter of the source is then [2.4 lambda (x/D)], where x is the distance to the source. If D is 1/32 inches, and x is 39 inches (1 meter), then the size of the source is 1.9 mm at HeNe. The hole diameters in the source should be about one twentieth of that, or about 100 microns.
Rolf's interferometry demo cards have two holes of 0.1 mm diameter, spaced 0.3 mm apart.
If the hole sizes in the artificial star are to be 1/64 inch, the total dimension of the source would be twenty hole diameters, or about 0.31 inches (8 mm). The artificial source would then need to be placed at a distance of at least 10 meters to be unresolved by a hole 0.8 mm in diameter.
I intend to use a Mini-Maglite as the source with a f=17 mm condenser lens.
As per Donald's advice, the condenser lens should be adjusted to focus an image of the Mini-Maglite filament on the hole array when the star (mask) is removed.

A 1/32 inch hole resolves the following as a function of distance at HeNe (632 nm). Resolved size is [1.2 lambda (x/D)] and the Airy Disk diameter is twice that. Source extent is half of the resolved size.

Distance	Objects > (mm) resolved	Airy Disk Diameter (mm)	Source Extent (mm)
5 in (12.7 cm)	0.121	0.243	0.061
10 in (25.4 cm)	0.243	0.485	0.121
20 in (50.8 cm)	0.485	0.971	0.243
40 in (101.6 cm)	0.971	1.942	0.485
80 in (203.2 cm)	1.942	3.883	0.971
160 in (406.4 cm)	3.883	7.766	1.942

Rolf Danner's interferometry demo cards have two holes of 0.1 mm diameter, spaced 0.3 mm apart, and would be about right at a distance of 30".

For that hole size (0.1 mm), if we wanted the source to have 20 fringes across it, it should be 2 mm across and be about 4 meters away. If we only need 10 fringes across it, the source moves in to 1 meter, which is reasonable.

Random Notes on Software and Hardware

Robot Store and its Gear Box kits: Planetary Gear Box.
Capture-to-Go
System Compatibility
176x144 and 320x240 both work
Margi Driver Updates
VAIO F590
Zoomed Video Port
Linux PCMCIA DVD Card
SAOimage
AIPS++
FITS
Image Processing
Carry on must be 22" x 14 x 9" and less than 50 lbs on United Flights, such as the case sold by Jensen
Platt Cases
BMP file format
Java Advanced Imaging API
Java Advanced Imaging API Documentation

Updated 5 September 2002

Maintained by Peter Lawson
MS 301-451, Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, CA 91109

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