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COBE

Cosmic Background Explorer/FIRAS

Product Name
FIRAS Data In HEALPIX FORMAT
Mission
COBE/FIRAS

Description

The FIRAS data that formed the final COBE Project data release have been partially reprocessed to create sky products in the HEALPix format for compatibility with more recent CMB data. The reprocessing was performed by former COBE team members Dale Fixsen and Nils Odegard in 2006.

The FIRAS data come from 4 detectors (LL, RL, LH, RH) and the FIRAS can be operated in four scan modes (SS, SF, LF, LS). However in flight the long slow (LS) mode was only used for a negligible time so these data were not calibrated. The long fast (LF) data can be used for high spectral resolution, but these were not recalibrated for this data release.

The data were further divided into 11 time ranges, depending on how the instrument was operated. For each of the 12 data types (LLSS etc.) the data in each time range were combined by QUAD-CUBE pixel number. When less than 8 interferograms (IFGs), were available, neighboring pixels were used to add to the data set to get 8 IFGs. These sets were then deglitched and only the IFGs in the pixel were averaged to get a coadd average. These coadds remain unchanged from the Pass 4 FIRAS data release. The calibration coadds were treated in a similar fashion, except of course there are no pixels to separate the data. See the FIRAS calibration paper (Fixsen, D.J. et al. 1994, ApJ, 420, 457) and the FIRAS Explanatory Supplement.

A temperature correction was applied to the calibration coadds. There is a temperature bias in the high current readout presumably due to self heating of the thermometer. The temperature was corrected using the mean difference between the high current readings (biased) and the low current readings (noisy). This will lead to a different CMB monopole temperature (about 5 mK lower than for the Pass 4 data release) but otherwise little difference in the results.

The corrected coadds are then destriped and combined into a full sky HEALPix map, by minimizing the χ2 defined as:

χ2 = ∑i [Siν - Zip Spν- Jik Bkν]2 Wi +∑j [Cjν- Pν(Txj) - Jjk Bkν]2 Wj


Variables and indices are as follows.

Indices
i    Sky Coadd index (1 - ~135000)

j    Cal Coadd index (1 - ~1500)

ν    frequency index (1-43 for LOWF, 1-55 for HI2, 1-55 for HI3, 1-60 for HI4)

p    HEALPix pixel index (0-3071)
Variables
Si     Sky coadds

Sp    Final sky solution

C     Cal coadds

Tx    Corrected Xcal temperature

J      Stripe templates

P     Planck function

Z     Sorting of coadds into pixels

B     Stripe solutions

W    Weights

β     Orthogonal stripes

γ     Orthogonal stripe solutions

Destriping improves the data, but it also introduces correlated errors into the final map. These can be characterized by Jik Wi Jil + Jik Wi Jil, but it is more convenient to rotate the coordinates so that this matrix is diagonal. In this new coordinate system J -->β and B --> γ

Note that destriping is done independently at each frequency. Thus the noise characteristics as a function of frequency remain unaffected. These are characterized by an autocorrelation function, Α and the diagonal of the correlation matrix, C. There remains a normalization issue. We can multiply the weights by an arbitrary factor as long as the same factor is applied to C. To fix the scale we arbitrarily choose this factor such that ∑i Wi=∑p 1=3072.

HEALPix Format Destriped Sky Spectra (Spectral Sky Maps)

There are two HEALPix format Spectral Sky Maps: firas_hpx_destriped_sky_spectra_lowf.fits, and firas_hpx_destriped_sky_spectra_high.fits. Together these cover the full FIRAS frequency range at 13.6041 GHz resolution. These files contain a spectrum at each pixel and were created by coadding spectra obtained from various FIRAS channel and scan mode combinations. Prior to coaddition, the observations made in different modes were calibrated to remove the instrument signature. "Destriping" is a secondary calibration step applied to compensate for unidentified instrumental offsets. The existence of such offsets is suggested by stripes that appear in an untreated sky map at a particular frequency.

C-Vector (HEALPix Format)

C-Vectors are used in the FIRAS destriping process, as discussed in the FIRAS Explanatory Supplement (especially sections 6.2 and 7.1 ). The values (in MJy/sr) represent the variance (random noise) derived from multiple observations within the same pixel. Two FITS files are available, corresponding to the FIRAS modes HIGH and LOWF. The full variance includes both the weight and the C-Vector. The data have been renormalized from pass 4 so the C-Vectors have also been renormalized.

Stripes (HEALPix Format)

There are 4 files containing orthogonal stripe information and 4 files containing physical stripe information. For the physical stripes, spectra and covariance values are given, while stripe gammas and betas are given for the orthogonal stripes. For a full discussion, see the FIRAS Explanatory Supplement (especially section 6).

Stripe Templates (HEALPix Format)

These files contain the stripe templates used for destriping the LOWF, HI2, HI3, and HI4 data in production of the HEALPix format data products. The stripe templates are referred to as destriper model functions in the FIRAs Explanatory Supplement. Each template contains the adopted form of a residual instrumental effect in the calibrated coadd spectra.

Uncombined Destriped Spectra (HEALPix Format)

These files contain destriped calibrated spectra for individual sky coadds and calibration coadds.

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