astro-legacy-archive/firas-spectral-maps

COBE/FIRAS Pass-4 destriped spectral sky maps

This dataset contains all six files in LAMBDA's COBE/FIRAS Pass-4 Destriped Sky Spectra product: LOWF, HIGH, HRES, HIF2, HIF3, and HIF4. Each row is one explicitly identified resolution-6 COBE quadrilateralized spherical-cube pixel.

python -m venv .venv && .venv/bin/pip install datasets huggingface_hub pyarrow

from datasets import load_dataset
dataset = load_dataset("astro-legacy-archive/firas-spectral-maps", "FIRAS_DESTRIPED_SKY_SPECTRA_LOWF", split="train[:1]", columns=["PIXEL", "SPECTRUM"])
row = dataset[0]
print(row["PIXEL"], len(row["SPECTRUM"]))

0 182

import json
from huggingface_hub import hf_hub_download
import pyarrow.parquet as pq
path = hf_hub_download("astro-legacy-archive/firas-spectral-maps", "FIRAS_DESTRIPED_SKY_SPECTRA_LOWF.parquet", repo_type="dataset")
print(json.loads(pq.read_schema(path).metadata[b"explicit_pixel_table"])["physics"]["active_frequency_points"]["value"])

43

All configurations retain PIXEL, GAL_LON, GAL_LAT, WEIGHT, STR_USED, and the fixed-length-182 SPECTRUM in source order. LOWF has 6,067 rows and 43 active frequency points; HIGH has 6,067 rows and 170; HRES has 5,964 rows and 182. The documented sky-cube total is 6,144. The absent identifiers are absent source rows, not filled or classified values.

HIF2 and HIF3 each have 6,067 rows and 55 active frequency points; HIF4 has 6,067 rows and 60. Their remaining length-182 elements are source zeros.

LOWF's remaining 139 spectrum elements and HIGH's remaining 12 are genuine source zeros and remain present. NU_ZERO, DELTA_NU, and NUM_FREQ are copied from each source header into the embedded metadata with their bases. Consumers can derive a frequency axis from those statements without changing the mirrored table.

Product and table structure

The six files are calibrated, destriped spectra in ecliptic sky-cube pixels. GAL_LON and GAL_LAT are stored source fields rather than archive coordinate conversions. WEIGHT is described by the source as the pixel weight. STR_USED records whether calibrated spectra in a pixel were used for destriping: 1 means used and 0 means not used. The spectrum unit is MJy/sr; the flag, weight, and pixel fields have blank source TUNIT values.

LOWF and HIGH each contain 6,067 unique, strictly increasing PIXEL values between 0 and 6,143, so 77 identifiers are absent from each table. HRES has 5,964 rows and 180 absent identifiers in the same documented range. HIF2, HIF3, and HIF4 each contain the same 6,067 unique source identifiers as HIGH. These are source table boundaries: the dataset does not insert rows for unobserved pixels or assign them synthetic values. The source sky cube divides six faces into 32 by 32 approximately equal-area pixels, equivalent to squares 2.59 degrees on a side. The FIRAS horn is a seven-degree circular top-hat, with 2.3-degree motional smearing perpendicular to the ecliptic plane.

For LOWF, NU_ZERO=68.020812 GHz, DELTA_NU=13.604162 GHz, and NUM_FREQ=43. HIGH records 612.18729 GHz, 13.604162 GHz, and 170. HRES records 23.807283 GHz, 3.4010405 GHz, and 182. These are source header values. HIF2, HIF3, and HIF4 record NUM_FREQ values 55, 55, and 60. LAMBDA's rounded product descriptions give 60–630, 600–2880, and 30–660 GHz; both source statements are retained without reconciling them.

LAMBDA separately documents the instrumental spectral resolution as 24.6 GHz for LOWF and HIGH and 6.9 GHz for HRES. These resolutions describe the spectral response and are not the FITS DELTA_NU channel intervals above. The full FIRAS Explanatory Supplement governs the instrument, calibration, processing, errors, ancillary products, and use of the Pass-4 release; Appendix A is only one component of it.

All columns are finite. LOWF's active spectrum values range from -112.1513367 to 399.4904785 MJy/sr; HIGH ranges from -7536.6450195 to 8111.7973633 MJy/sr; HRES ranges from -1095.1163330 to 2163.3710938 MJy/sr. The complete fixed-size spectrum is always delivered, so model code can see the source representation directly.

configuration	rows	active points	source SHA-256
FIRAS_DESTRIPED_SKY_SPECTRA_LOWF	6,067	43	42747c37c11451aee67cca1debe009a74337ac2b225874886bbe3a255c0fa304
FIRAS_DESTRIPED_SKY_SPECTRA_HIGH	6,067	170	0ca29551c03ffeda2efc5adb6d109947955e52e4d31e4a40fa74a1c5115b05ce
FIRAS_DESTRIPED_SKY_SPECTRA_HRES	5,964	182	cb41ef57c736ab0410d9bb80f90b2a597fdd0db77595e3c0836193025a5e454c
FIRAS_DESTRIPED_SKY_SPECTRA_HIF2	6,067	55	997c0e9f7f7582e758adadbfab1891725f3233b15154539f2233c1995a4d63ad
FIRAS_DESTRIPED_SKY_SPECTRA_HIF3	6,067	55	4634023cbbbf3767cc9539d5ca088efd42a6b73504a7329fa51f3c4a8d1552ca
FIRAS_DESTRIPED_SKY_SPECTRA_HIF4	6,067	60	85153b457a2e0997b9b439e1f145714eef7ed302ae149a89234c25e6d27d1a2a

LOWF contains 88 STR_USED=0 rows and 5,979 STR_USED=1 rows. HIGH contains 475 and 5,592 respectively; HRES contains 81 and 5,883. HIF2, HIF3, and HIF4 each contain 475 and 5,592 respectively. These flags state use in destriping, while every released row and its other source fields remain in the table. The measured Galactic longitudes span nearly the full 0–360-degree range, and latitudes reach about ±88 degrees in each product.

Provenance and reconstruction

The source files and descriptions are published by NASA LAMBDA, whose Pass-4 product description documents the spectral resolutions. Their combined size is 27,155,520 bytes; individual SHA-256 pins travel in the files and facts.yaml. Rebuild with python tools/make_firas_spectral_maps.py --out data/firas-spectral-maps --source /var/tmp/will/firas-spectral-maps-source --provenance-commit <commit>.

The embedded document records the complete primary and table headers, the pixelisation statements and their source bases, field descriptions and units, frequency statements, source pin, credit, and structured regeneration record.

Considerations for use

These are all six files in the Pass-4 Destriped Sky Spectra product. They do not combine C-vectors, A-vectors, covariance matrices, stripes, uncombined spectra, interferograms, or time-ordered data. The angular response is much broader than the pixel spacing. STR_USED=0 is a documented processing flag, not a blank-spectrum instruction: the corresponding source rows and spectra remain measurements in this mirror.

The LOWF and HIGH zero tails are especially important for shape-sensitive training or reconstruction. NUM_FREQ describes the active prefix but does not redefine TFORM6 or TDIM6. A reader that wants only active frequencies can slice using the embedded header value; the archived logical field remains the source length 182.

No explicit data licence was found on the checked LAMBDA pages. LAMBDA's COBE credit is: “The COBE datasets were developed by the NASA Goddard Space Flight Center under the guidance of the COBE Science Working Group.” Cite Fixsen et al. (1994), doi:10.1086/173577; Fixsen et al. (1996), doi:10.1086/178173; and Fixsen, Bennett & Mather (1999), doi:10.1086/307962.

Fixsen et al. (1994) describe FIRAS calibration, Fixsen et al. (1996) present the full-data-set CMB spectrum, and Fixsen, Bennett & Mather (1999) discuss Galactic spectral-line observations. The LAMBDA Pass-4 release supersedes products released before 1998-01-01.

Licence

HEASARC states verbatim:

HEASARC materials are all available freely for your use.

This is a free-use statement rather than a named formal licence.

Errata and requests

To report an erratum, propose a correction, or ask a question about this dataset, open a Discussion or Pull Request in its Community tab; posting requires a free Hugging Face account. Dataset requests and archive-wide questions may be posted in the README space's Community tab or the Community tab of any dataset. Confirmed corrections are made in the source conversion tools and published by rebuilding the dataset.