Out-Of-Focus beammaps

The "old" holography system scans the Fressnel diffraction amplitude pattern of a nearby 94 GHz source with 2 or more different focus settings. The reduction software then "recovers" the phase information (which is not directly measured) from these amplitude maps using a technique known as "Fresnel-region phase-retrieval holography". This technique can also be applied for celestial sources. If a strong source such as a planet is mapped with different focus offsets, the same reduction software can be used to obtain the dish shape. Calculating the dish shape from Out-Of-Focus (OOF) beammaps is a very useful addition to the standard holography, because it allows the shape of the surface to be measured at different Elevation angles and gravitational effects on the surface can so be quantified.

OOF maps are obtained with SCUBA. The size of the OOF map is limited to about 100 arcsec by SCUBA's field-of-view and by the chop throw. Better resolution of the surface shape can be obtained from larger maps but observing time and S/N then soon become a limiting factor. Two OOF map examples are shown below, one for a focus offset of +1.0mm, one for a -1.0mm offset. The picture below the OOF maps shows the surface profile for these OOF images.

The software to calculate the surface profile from a set of OOF maps is currently being rewritten at MRAO (Nikolic, Hills and Richer 2001, in preparation) to handle SCUBA maps and to provide a better interface with the holography software, but is not yet finished. The OOF maps and surface plot below are images obtained from simulations with the new OOF software.

Some new results from OOF SCUBA observations can be found here .

Last modified: 8 January 2002
Jan Wouterloot