Reading a space telescope picture
A Hubble or Webb picture is a set of measurements before it is an image. The telescope records how much light lands on each pixel of a detector that sees no color at all, one filter at a time. Calibration removes the fingerprints of the camera, several exposures are stacked to reject cosmic-ray hits, and only then are the separate frames given colors and combined. The articles collected here follow that process from different directions, from the physics of a single filter to the choices that make a finished picture readable.
Most pieces start from one real, published image and ask a specific question about it. Why is the hydrogen in the Eagle Nebula shown green? What are the spikes on bright stars? How can a picture of a nebula 6,500 light-years away show change over nineteen years? Answers come with the name of the camera, the filters used and the release date, so that each claim can be traced back to the data behind it and checked against the original release.
Two details on any release page help with reading the picture itself. The first is orientation: a compass arrow marks north and east, and many Hubble images are rotated so the subject fits the frame, with north pointing anywhere. The second is scale: a bar measured in arcseconds or light-years shows how large the structures really are. A nebula that fills the frame may span tens of light-years, while a planetary nebula of the same apparent size can be less than one.
How the sections fit together
How images are made covers color assignment, filters, resolution, artifacts and the craft of turning exposures into a picture. Are the Colors Real? is the place to begin, followed by the guide to image artifacts and what makes the pictures real.
The Hubble Heritage Archive is a series of object pages, one nebula, galaxy or star at a time, each with a data table listing the camera, the filters and the colors they were given. The Hubble Heritage page explains the project behind those images and lists every object.
The Solar System section collects Hubble's pictures of the planets, their moons and the asteroids that cross deep exposures by accident. It has its own overview of planetary imaging.
Hubble anniversaries are the pictures released each April to mark the telescope's launch on April 24, 1990, and Webb covers the infrared telescope that began its science observations in 2022.
Terms that come up in almost every article
Filter names follow a fixed pattern on Hubble. F502N means a filter centered near 502 nanometers, and the last letter gives its width: N for narrow, M for medium, W for wide, LP for long-pass. A narrowband filter isolates the glow of one element, such as oxygen, hydrogen or sulfur. A wideband filter collects a broad slice of starlight.
An exposure is one frame taken through one filter, often lasting many minutes. Several exposures through the same filter are combined to remove cosmic-ray hits, the bright specks left by charged particles striking the detector.
Drizzling is the method used to combine slightly offset exposures onto a finer pixel grid, which recovers detail lost to the size of the detector's pixels and fills the gaps between detector chips.
Representative color means a color stands for a wavelength the eye cannot see, or one that would otherwise be hidden by a brighter neighbor. Infrared light in a Webb picture is always shown this way.
Diffraction spikes are the bright lines crossing the brightest stars. They come from light bending around the struts that hold a telescope's secondary mirror and, on Webb, around the edges of its hexagonal mirror segments. Hubble's stars show four points; Webb's show six, with two fainter extras.
Where the pictures come from
Every image on these pages comes from NASA, ESA, the Space Telescope Science Institute or the teams that observed with Hubble and Webb, and each one carries its credit line under the picture. No picture of space is generated, retouched or redrawn. Where an article compares two versions of the same object, both are published releases, and the caption says which camera and which year each one comes from.
Dates on the articles show the day each piece was first published. The release date of an image, which can be years earlier, appears in the article text or in its data table.
Questions readers ask
Where should someone new to astronomy images start?
Start with Are the Colors Real?, which explains why space telescope pictures are made from black-and-white exposures and how color is added. The two articles under Are Hubble Images Real? then cover what in a picture comes from the camera and what comes from the sky, and A Brief History of Astrophotography shows how the methods developed.
Are the Hubble and Webb images on these pages free to use?
Images released by ESA/Hubble and ESA/Webb are published under the Creative Commons Attribution 4.0 license, and most images produced by NASA are not subject to copyright in the United States. Both require the credit line printed under each picture. Some images carry other conditions, so check the release page before reusing one.
How are the articles organized?
Every article belongs to one section: how images are made, the Hubble Heritage Archive of individual objects, the Solar System, the Hubble anniversary images, or Webb. The filter above the list shows one section at a time, and each article links to the next and previous pieces in its section.
Why do some articles carry a date years after the image was released?
The date on an article is the day it was first published. Several Heritage Archive pages describe pictures that NASA and ESA released years earlier, and those pages give the original release date in their data table, next to the camera and the filters.