The Tool Desk
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GALEX operations are over, but its processed observations remain publicly available. Coverage and depth vary strongly by position, band, and survey, so an empty search result is not automatically a scientific non-detection.
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What public GALEX ultraviolet data contain
GALEX recorded ultraviolet observations in two imaging bands: far-ultraviolet (FUV) and near-ultraviolet (NUV). It also produced low-resolution grism spectroscopy. The public archive contains processed mission products rather than a live observing service, and MAST is the NASA-sanctioned public distributor for those data. The mission archive and product descriptions are available through MAST’s GALEX GR6/GR7 Public Archive.
Your scientific question determines which archive layer matters:
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- Imaging: use FUV and/or NUV intensity maps to inspect morphology, measure fluxes, or make a finding chart.
- Catalog photometry: use source catalogs when you need positions and measured magnitudes for many objects.
- Visit-level data: use individual visits when variability, observing conditions, or visit-to-visit differences matter.
- Coadds: use combined visits when a deeper, merged image or catalog is appropriate.
- Grism spectroscopy: use extracted spectra for low-resolution ultraviolet spectral information.
GALEX’s FUV detector ceased operating in 2009, while NUV observations continued for the mission. Consequently, a target can have NUV coverage without a corresponding FUV exposure. Treat the bands as separate observation histories rather than assuming that every GALEX field contains both.
Choose the right way to search
There is no single “GALEX database” button for every task. Match the interface to the job before downloading anything.
| Route | Best for | Limitation or check |
|---|---|---|
| MAST Portal | Browser searches by resolvable target name or sky coordinates, with cross-mission context | Filter explicitly to GALEX; it is not the efficient choice for large batches of targets |
| GALEX tile forms | Survey-tile browsing, coordinate searches, previews, and selecting image or catalog files | Confirm whether the selected file is a coadd or visit-level product and understand its file type |
| CasJobs | SQL searches across GALEX source catalogs and formal mission tables | Select the GALEX_GR6Plus7 context and the table that matches your analysis level |
| Astroquery MAST | Scripted observation searches and retrieval of associated products | The GALEX workflow searches observations and products; it does not directly query GALEX catalogs |
MAST describes these alternatives and their intended uses in its GalexView end-of-life guidance.
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1. Define the target and the measurement
Write down the object name or right ascension and declination, the angular region to search, and the product you actually need. Decide whether your question requires:
- FUV, NUV, or both bands;
- a single image, a merged image, catalog photometry, spectroscopy, or time-resolved visits;
- the shallowest available survey detection or the deepest exposure available at that position.
This short specification prevents a common mistake: finding a valid GALEX file that cannot answer the intended question. A catalog row is not a substitute for an image when you need morphology, and a coadd can hide visit-level changes that matter for variability.
2. Find a few targets in the MAST Portal
- Open the MAST Portal and enter a resolvable object name or sky coordinates.
- Set the search radius to cover the target and any surrounding field you need.
- Filter the results to the GALEX mission rather than mixing in unrelated MAST missions.
- Inspect observation dates, bands, exposure information, and available products.
- Open the observation or product details before downloading so you know whether you are selecting a coadd, a visit, a catalog, or a spectrum.
The Portal is the quickest browser-first route for one or a few objects. It also reveals useful cross-mission context, but that context can make a result list look more complete than the GALEX coverage actually is; keep the GALEX filter active when making an archive decision.
3. Browse survey fields with the GALEX tile forms
Use the GALEX survey tile web forms when you want to inspect the field layout rather than search only for a named object. Enter coordinates, review the returned tile previews, and select the files associated with the relevant tile. MAST’s Getting Started with GALEX page explains the tile-oriented retrieval path.
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4. Search catalogs with CasJobs
CasJobs is the appropriate route for a formal SQL query over GALEX catalog tables. Select the GALEX_GR6Plus7 context, then choose the table that corresponds to the data level:
photoobjallcontains coadd-level source catalog data.visitphotoobjallcontains visit-level source catalog data.
A catalog query should specify a sky region and the columns needed for the decision you will make later. For example, a cone search can return source positions and ultraviolet magnitudes for a target field, while a visit-level query can preserve separate measurements that would be merged in a coadd catalog. Use the table’s documented units and flags instead of inferring meaning from a column name alone.
Catalog lookup and observation lookup answer different questions. A successful SQL query tells you what catalog entries meet your selection; it does not prove that an unlisted object was never observed. For that determination, inspect the observation footprint and product metadata as well.
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5. Automate observation discovery and downloads with Python
Astroquery’s MAST Observations interface is useful when you need a repeatable search, a batch of targets, or a scripted product manifest. The documented workflow is described in Astroquery’s MAST observation-query documentation.
A minimal observation-and-product pattern is:
from astroquery.mast import Observations
observations = Observations.query_object("M31", radius="0.02 deg")
galex_observations = observations[observations["obs_collection"] == "GALEX"]
products = Observations.get_product_list(galex_observations)
manifest = Observations.download_products(products)Use a coordinate or a resolvable name that matches your target, then inspect the returned observation table before downloading every associated file. In a real batch job, add your own filters for band, observation type, date, or product category and save the manifest so the download is reproducible.
There is an important limitation: Astroquery’s MAST observation workflow does not directly query the GALEX source catalogs. It can retrieve tile catalog FITS files when those catalogs are associated products, but a catalog-wide SQL question still belongs in CasJobs.
Understand the product levels before analysis
Coadd images and catalogs
A coadd combines multiple visits into a merged representation. It is usually the right starting point when you want the combined signal from repeated coverage, a single field image, or a coadd-level source catalog. The archive describes coadd intensity maps and merged source catalogs as distinct products, so do not assume that a coadd image and a coadd catalog contain identical measurements or masks.
Visit-level images and catalogs
Visit-level products preserve each observation separately. They are the better choice when you need to examine changing brightness, reject a problematic visit, compare observing conditions, or reproduce how a source appeared at a particular epoch. The visitphotoobjall table is the visit-level catalog route in CasJobs.
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Grism spectra
GALEX grism observations provide low-resolution spectroscopy rather than a conventional broadband image. Extracted grism spectra are a separate product class. Confirm that the target’s observation is a grism observation and read the associated product description before treating a downloaded FITS file as a spectrum.
Read metadata and FITS headers
File names alone do not reliably tell you the units, calibration state, band, or processing level. Open the archive description and inspect the FITS header for each product type. Record the observation identifier, band, exposure information, product level, units, and any quality or masking information alongside your science results.
Know what AIS, MIS, DIS, and NGS mean
GALEX’s major imaging programs differ mainly in survey strategy and depth. The names are useful shorthand, but they are not interchangeable measurements of the same sky.
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AIS: All-sky Imaging Survey
AIS provides broad, relatively shallow coverage. It is useful for finding ultraviolet-bright sources across large areas, but “all-sky” should not be read as literally every position: historical avoidance zones and later coverage changes produced gaps and uneven depth.
MIS: Medium Imaging Survey
MIS covers selected fields more deeply than AIS. A target in an MIS field can therefore have a substantially more sensitive measurement than a nearby target observed only by AIS, even when both have GALEX data.
DIS and NGS
The Deep Imaging Survey (DIS) targets still deeper fields, while the Nearby Galaxy Survey (NGS) focuses on nearby-galaxy observations. Use the archive’s observation metadata to determine which program supplied a particular file rather than inferring the program from depth alone.
| Survey sample described by Bianchi et al. (2014) | Fields | Area | Depth or catalog scope |
|---|---|---|---|
| AIS selected two-band sample | 28,707 fields | 22,080 square degrees | Typical depth about 20 AB mag in FUV and 21 AB mag in NUV; the paper’s figure uses selected two-band fields and a central one-degree field restriction |
| MIS sample | 3,008 fields | 2,251 square degrees | Depth about 22.7 AB mag |
| AIS catalog described in the paper | About 71 million sources | Count for the catalog discussed by Bianchi et al. (2014), not a guaranteed count of every possible GALEX detection | |
| MIS catalog described in the paper | About 16.6 million sources | Count for the catalog discussed by Bianchi et al. (2014), not a guaranteed count of every possible GALEX detection | |
These figures come from Bianchi et al.’s 2014 overview, The Ultraviolet Sky: An Overview from the GALEX Surveys. They describe the paper’s selected samples and catalog scope, not every file currently discoverable in the archive.
Interpret coverage and depth correctly
MAST’s mission summary says GALEX covered 77% of the sky at varying depths in at least one band. That percentage is a broad mission-level statement, accessed on October 13, 2026, and is not equivalent to the restricted two-band AIS and MIS samples in the 2014 paper.
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Several historical factors explain why a coordinate search can produce an uneven result:
- Original safety exclusions affected parts of the Galactic plane, the Magellanic Clouds, and regions around very bright stars.
- Later releases, including GR7, added some low-latitude, Magellanic Cloud, and other coverage.
- FUV and NUV coverage diverged after the FUV detector ceased in 2009.
- Exposure depth varies by survey program and by position within a field.
- Masking, field edges, and catalog detection thresholds can remove a source from a catalog even when imaging exists.
Therefore, a blank catalog result can mean no observation, no coverage in the requested band, a masked or edge location, or a source below the catalog threshold. Check the observation footprint, band, exposure, masks, and product metadata before describing the target as a non-detection.
Use scan-mode data with extra caution
MAST warns that the standard calibration pipeline was not optimized for scan-mode observations. Scan-mode files may require additional analysis or a user-written reduction before they are suitable for quantitative science. Do not treat every archive product as equally analysis-ready simply because it has passed through a pipeline.
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Is GalexView still available?
No. GalexView is a retired interface. Its former browser experience depended on Flash, which browsers disabled in 2021. Do not plan a workflow around screenshots or instructions that require the old GalexView application.
Use the current alternatives instead:
- MAST Portal for target and coordinate searches;
- GALEX tile forms for field previews and downloads;
- CasJobs for SQL catalog work;
- Astroquery for scripted observation and product retrieval.
The retirement notice and replacement routes are documented by MAST at GalexView End of Life.
A download-and-analysis checklist
- Record the target definition. Save the object name, coordinates, search radius, and coordinate epoch or catalog convention used by your project.
- Separate bands. Note whether the result contains FUV, NUV, or both; never infer one band from the presence of the other.
- Identify the program. Record AIS, MIS, DIS, NGS, or another observing program shown by the archive.
- Choose the processing level. Mark each file as coadd, visit-level, catalog, intensity map, or grism spectrum.
- Inspect the footprint. Check field edges, masks, effective exposure, and whether the target falls inside the useful area.
- Read the headers. Confirm units, calibration information, band, observation identifier, and quality flags from the FITS metadata.
- Preserve provenance. Keep the archive URLs, query parameters, downloaded filenames, and product manifest with your analysis.
- Document caveats. Flag scan-mode processing, band-specific absence, shallow depth, or catalog-threshold effects in your notes and final interpretation.
Which route should you use?
- One named galaxy or star: begin with the MAST Portal, then open the associated GALEX products.
- A field around known coordinates: use the GALEX tile forms to inspect coverage and neighboring sky.
- Thousands of catalog sources: use CasJobs with the
GALEX_GR6Plus7context and selectphotoobjallorvisitphotoobjalldeliberately. - A reproducible batch pipeline: use Astroquery for observation discovery and product retrieval, while keeping catalog SQL in CasJobs.
- A suspected non-detection: verify the footprint, band, mask, exposure, and catalog threshold before drawing a scientific conclusion.
- A scan-mode observation: expect additional calibration or custom reduction rather than assuming the standard pipeline is sufficient.
The central discipline is to distinguish an observation from a catalog row, a coadd from a visit, and archive availability from detector coverage. Once those distinctions are explicit, GALEX’s public data can support reliable ultraviolet imaging, catalog, temporal, and low-resolution spectroscopic work without relying on the retired GalexView interface.
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Quick Recap
Primary archive and documentation links
- GALEX GR6/GR7 Public Archive
- Getting Started with GALEX
- MAST GALEX mission page
- GalexView End of Life and alternatives
- Astroquery MAST observation queries
- Bianchi et al. (2014), The Ultraviolet Sky: An Overview from the GALEX Surveys
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