astro-legacy-archive/iras-issa-maps

IRAS Sky Survey Atlas HEALPix Maps

This dataset contains NASA LAMBDA's four HEALPix maps made from the coadded HCON-0 IRAS Sky Survey Atlas (ISSA) plates. The configurations contain the 12, 25, 60 and 100 micron bands under their source filename stems.

Source survey	IRAS Sky Survey Atlas
Bands	12, 25, 60 and 100 micron
Pixelisation	HEALPix NSIDE 1024; 12,582,912 pixels
Ordering	NESTED
Coordinate frame	Galactic (SKYCOORD = 'Galactic')
Source fields	Intensity, N_PIXELS
Source units	MJy sr-1, ISSA pixel count

Every configuration has one source row per HEALPix pixel and two float32 columns in source order. There is no pixel-index or validity column.

How to use

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

from datasets import load_dataset
dataset = load_dataset("astro-legacy-archive/iras-issa-maps", "ISSA_B4H0_healpix_ns1024", split="train[:1]", columns=["Intensity", "N_PIXELS"])
row = dataset[0]
print(row["Intensity"], row["N_PIXELS"])

479.0029296875 4.0

import json
from huggingface_hub import hf_hub_download
import pyarrow.parquet as pq
path = hf_hub_download("astro-legacy-archive/iras-issa-maps", "ISSA_B4H0_healpix_ns1024.parquet", repo_type="dataset")
print(json.loads(pq.read_schema(path).metadata[b"healpix_map"])["schema"])

astro-legacy-archive/healpix-map

The embedded document records geometry, both field units, and all-column coverage. Its documented Intensity=-32768 no-data code and measured N_PIXELS=0 population are kept as separate per-column records.

Source structure

LAMBDA made these maps by averaging the coadded ISSA data over HEALPix pixels. It excluded overlapping data between neighbouring ISSA fields and used every field, including the Reject fields at low ecliptic latitudes. The source notes that zodiacal-model residuals are significant in those Reject fields and that minor discontinuities occur at some field boundaries.

Each FITS bintable contains 12,582,912 scalar rows with the following fields:

source field	source format	source unit	description from the source header
Intensity	1E	MJy sr-1	Average band intensity over the HEALPix pixel
N_PIXELS	1E	ISSA pixel count	Number of ISSA pixels contributing to the HEALPix pixel

The four source files differ only in band and values. Their source FREQ values are 12 micron band, 25 micron band, 60 micron band, and 100 micron band; the leading spaces in the first three are retained in the embedded physics metadata.

configuration	usable intensity range (MJy sr-1)	pixels without ISSA data
ISSA_B1H0_healpix_ns1024	-1.460486650466919 to 6280.716796875	244,294
ISSA_B2H0_healpix_ns1024	-1.4746202230453491 to 14841.6728515625	241,766
ISSA_B3H0_healpix_ns1024	-3.6768555641174316 to 18864.146484375	247,052
ISSA_B4H0_healpix_ns1024	-0.9458389282226562 to 23451.69921875	241,084

LAMBDA states that Intensity is set to -32768 for “HEALPix pixels that contain no ISSA data.” In each configuration, N_PIXELS is float32 zero at exactly those positions. The embedded metadata records -32768 with LAMBDA's documented meaning and records N_PIXELS zero only as a measured code. The source fields contain no NaNs or infinities.

Resolution and interpretation caveats

The ISSA Explanatory Supplement gives native in-scan FWHM values of 3.5, 3.5, 3.6, and 4.7 arcmin at 12, 25, 60, and 100 micron respectively. Each value and its document basis are recorded in the embedded structured physics.

ISSA was designed for relative photometry of objects outside the Solar system; it does not establish the absolute surface brightness of the sky. Quantitative surface-brightness use requires the IRAS–DIRBE calibration and zero-level context. Photometry in the low-ecliptic-latitude Reject set requires particular care because the zodiacal-emission subtraction has significant residuals. The source documentation also identifies a small amount of Galactic-plane data with saturated 60 and 100 micron detectors and general low-level artifacts from imperfections in the zodiacal-intensity subtraction model.

Projected siblings

LAMBDA separately serves Mollweide and Zenith Equal Area projected images for each band:

• ISSA_mollweide_B1H0_healpix_ns1024.fits
• ISSA_zea_B1H0_healpix_ns1024.fits
• ISSA_mollweide_B2H0_healpix_ns1024.fits
• ISSA_zea_B2H0_healpix_ns1024.fits
• ISSA_mollweide_B3H0_healpix_ns1024.fits
• ISSA_zea_B3H0_healpix_ns1024.fits
• ISSA_mollweide_B4H0_healpix_ns1024.fits
• ISSA_zea_B4H0_healpix_ns1024.fits

Those images oversample and replicate HEALPix values. They are distinct source products and are not configurations in this holding.

Relationship to IRIS

This holding is distinct from iris-infrared-maps. Both trace the IRAS survey, but these files are LAMBDA's NSIDE-1024 averages of the coadded ISSA HCON-0 plates. iris-infrared-maps contains the later Improved Reprocessing of the IRAS Survey at NSIDE 2048 and uses DIRBE information where IRAS had no observation. The two releases have different processing, grids, coverage values, and source filenames.

Provenance

The source files and their pins are:

source file	SHA-256
ISSA_B1H0_healpix_ns1024.fits	3b3cbc6eb2ef53c28dfff64cc477ad699ef352d4a05e8ba742f2b6f68d54189c
ISSA_B2H0_healpix_ns1024.fits	96e590945d5e4a1c10f8029e63aa20dd1813351230aad26a201c111ae6495b2e
ISSA_B3H0_healpix_ns1024.fits	9ec10a249d70bd5a4e471adb6742932e0c371805c20b025876351743cfab218f
ISSA_B4H0_healpix_ns1024.fits	6610949fa61a6bad3b1d4b1b331f35f72b8dc2cd74f661fe5ca357b7a705a1b0

The complete source headers, URLs, hashes, retrieval timestamps, measured codes, documented meanings, ISSA supplement pin, and regeneration command are embedded in each Parquet file. Rebuild with python tools/make_iras_issa.py data/iras-issa-maps.

Licensing, credit and citation

The product pages do not publish a named licence. HEASARC's data-use page states: “HEASARC materials are all available freely for your use.”

LAMBDA requests:

We acknowledge the use of the Legacy Archive for Microwave Background Data Analysis (LAMBDA), part of the High Energy Astrophysics Science Archive Center (HEASARC). HEASARC/LAMBDA is a service of the Astrophysics Science Division at the NASA Goddard Space Flight Center.

Please cite Wheelock, S. L. et al. (1994), IRAS Sky Survey Atlas Explanatory Supplement, JPL Publication 94-11, Pasadena: Jet Propulsion Laboratory.

The product description and downloads are available from NASA LAMBDA.

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.