View source for Physics:Aperture masking interferometry

[[Image:Speckle aperture masking.svg|thumb|400px|a) shows a simple experiment using an aperture mask in a re-imaged aperture plane. b) and c) show diagrams of aperture masks which were placed in front of the secondary mirror of the Keck telescope by Peter Tuthill and collaborators. The solid black shapes represent the subapertures (holes in the mask). A projection of the layout of the Keck primary mirror segments is overlaid.]]

'''Aperture Masking Interferometry''' is a form of [[Physics:Speckle interferometry|speckle interferometry]], that allows diffraction limited imaging from ground-based telescopes, and is a planned high contrast imaging mode on the [[Astronomy:James Webb Space Telescope|James Webb Space Telescope]]. This technique allows ground-based telescopes to reach the maximum possible resolution, allowing ground-based telescopes with large diameters to produce far greater resolution than does the [[Astronomy:Hubble Space Telescope|Hubble Space Telescope]]. The principal limitation of the technique is that it is applicable only to relatively bright astronomical objects. A mask is placed over the telescope which only allows light through a small number of holes. This array of holes acts as a miniature [[Astronomy:Astronomical interferometer|astronomical interferometer]]. The method was developed by John E. Baldwin and collaborators in the [[Physics:Cavendish Astrophysics Group|Cavendish Astrophysics Group]].

==Description==
In the aperture masking technique, the [[Engineering:Speckle masking|bispectral analysis]] (speckle masking) method is typically applied to image data taken through masked apertures, where most of the aperture is blocked off and light can only pass through a series of small holes (subapertures). The aperture mask removes atmospheric noise from these measurements,{{how|date=June 2015}} allowing the bispectrum to be measured more quickly than for an un-masked aperture.

For simplicity the aperture masks are usually either placed in front of the [[Astronomy:Secondary mirror|secondary mirror]] (e.g. Tuthill et al.  (2000)) or placed in a re-imaged aperture plane as shown in Figure 1.a) (e.g. Haniff et al.  (1987); Young et al.  (2000); Baldwin et al.  (1986)). The masks are usually categorised either as non-redundant or partially redundant. Non-redundant masks consist of arrays of small holes where no two pairs of holes have the same separation vector (the same '''baseline''' – see [[Astronomy:Aperture synthesis|aperture synthesis]]). 

Each pair of holes provides a set of fringes at a unique spatial frequency in the image plane. Partially redundant masks are usually designed to provide a compromise between minimizing the redundancy of spacings and maximizing both the throughput and the range of spatial frequencies investigated (Haniff & Buscher, 1992; Haniff et al., 1989). Figures 1.b)  and 1.c) show examples of aperture masks used in front of the secondary at the Keck telescope by Peter Tuthill and collaborators; Figure 1.b) is a non-redundant mask while Figure 1.c) is partially redundant. 

Although the signal-to-noise of [[Engineering:Speckle masking|speckle masking]] observations at high light level can be improved with aperture masks, the faintest limiting magnitude cannot be significantly improved for photon-noise limited detectors (see Buscher & Haniff (1993)).

==See also==

* List of astronomical interferometers at visible and infrared wavelengths

== References ==
* [http://www.physics.usyd.edu.au/~gekko/papers/thesis.ps Peter Tuthill's PhD thesis on aperture masking (PostScript)] ([http://www.physics.usyd.edu.au/~gekko/papers/thesis.pdf PDF])
* [http://ukads.nottingham.ac.uk/cgi-bin/nph-bib_query?bibcode=1986Natur.320..595B&db_key=AST Baldwin et al. (1986)]
* [http://ukads.nottingham.ac.uk/cgi-bin/nph-bib_query?bibcode=1993OSAJ...10.1882B&db_key=INST Buscher & Haniff (1993)]
* [http://ukads.nottingham.ac.uk/cgi-bin/nph-bib_query?bibcode=1987Natur.328..694H&db_key=AST Haniff et al. (1987)]
* [http://ukads.nottingham.ac.uk/cgi-bin/nph-bib_query?bibcode=1989MNRAS.241P..51H&db_key=AST Haniff et al., 1989]
* [http://ukads.nottingham.ac.uk/abs/1990MNRAS.245P...7B Buscher et al. 1990]
* [http://ukads.nottingham.ac.uk/cgi-bin/nph-bib_query?bibcode=1992OSAJ....9..203H&db_key=INST Haniff & Buscher, 1992]
* [http://ukads.nottingham.ac.uk/cgi-bin/nph-bib_query?bibcode=2000PASP..112..555T&db_key=AST Tuthill et al. (2000)]
* [http://ukads.nottingham.ac.uk/cgi-bin/nph-bib_query?bibcode=2000MNRAS.315..635Y&db_key=AST Young et al. (2000)]

== Further reading ==
* [http://www.stsci.edu/jwst/instruments/niriss/ObservationModes/ami High Resolution Imaging with the NIRISS Non-Redundant Aperture Mask]

== External links ==
*[http://www.abc.net.au/science/news/stories/s1404731.htm Old method brings life to new stars] – ABC Science Online
*[http://www.physics.usyd.edu.au/~gekko/wr104.html Examples of high-resolution time-lapse movies produced with aperture masking]
*[http://www.usyd.edu.au/news/84.html?newsstoryid=634 Peter Tuthill awarded Eureka award for aperture masking work]

[[Category:Interferometry]]
[[Category:Astronomical imaging]]

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