### Max Tegmark's cosmology library: mapmaking

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##
How to make CMB maps without losing information

### Author:

Max Tegmark
### Abstract:

The next generation of CMB experiments can measure cosmological
parameters with unprecedented accuracy - in principle.
To achieve this in practice when faced with such gigantic data sets,
elaborate data analysis methods are
needed to make it computationally feasible.
An important step in the data pipeline is to make a map,
which typically reduces the size of the data set my orders of magnitude.
We compare ten map-making methods, and find that for the
Gaussian case, both the
method used by the COBE DMR team and various variants of
Wiener filtering are optimal in the sense that the map
retains all cosmological information that was present in the
time-ordered data (TOD). Specifically, one obtains just as
small error bars on cosmological parameters when estimating
them from the map as one could have obtained by
estimating them directly from the TOD.
The method of simply averaging the observations of each pixel
(for total-power detectors), on the contrary, is found to generally
destroy information, as does the maximum entropy method and most
other non-linear map-making techniques.

Since it is also numerically feasible, the COBE method is
the natural choice for large data sets. Other lossless
(e.g. Wiener-filtered) maps can then be computed directly
from the COBE method map.
### Reference info:

*ApJ Lett*, **480**, L87-L90 (1997)
### Online references:

This site also contains the latest versions of some papers
that are referenced in the text;
Tegmark & Bunn 1995,
Tegmark et al 1996,
Tegmark & Efstathiou 1996,
Tegmark , Taylor & Heavens 1996.

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This page was last modified July 1, 1998.
max@ias.edu