How images are made
Lining Up the Lagoon: How Gaia Star Positions Align Hubble Mosaics
A phone stitches a panorama from matching buildings. A Hubble mosaic of a dusty nebula needs a map of the stars, and Gaia supplied one.
Hold a grain of sand at arm's length and look at how little of the sky it covers. Hubble's cameras see about half of that. Wide views from the telescope, including many of its best-known pictures, are therefore mosaics, assembled from tiles the way a phone builds a panorama. The 2018 Lagoon Nebula images were stitched from four pointings, and getting those tiles to meet without a visible seam depended on a different spacecraft altogether.
Stitching by matching features
A phone panorama works by finding the same building or tree in two neighboring frames and sliding the frames until those features coincide. Hubble mosaics can be joined the same way, and the most reliable features to match are stars. A star near the right edge of one tile reappears near the left edge of the next; line up enough of those shared stars and the two tiles lock together.
The tolerance is far tighter than on a phone. For a Hubble mosaic to stay sharp, tiles have to be aligned to within a few milliarcseconds, thousandths of an arcsecond. At arm's length, that is like resolving one-thousandth of the width of a human hair. A small error does not just leave a seam; it doubles stars and smears fine structure along the join.
Where the method runs out
Matching works well in crowded star fields. It runs into two problems in a nebula like the Lagoon.
The first is the dust. In visible light, many of the stars are hidden by the very clouds in which they are forming, and the narrowband filters used to record the glowing gas pass little starlight. A section of the oxygen mosaic shows only a sprinkling of stars, too few in places to tie tiles together with confidence. The same part of the nebula in infrared light is full of them.
The second problem is overlap. Matching needs the same stars in two tiles, so the tiles must overlap. Every bit of overlap, though, is sky that is photographed twice instead of new sky added to the mosaic. When each tile is already a tiny patch, there is a strong reason to keep overlaps small, which leaves even fewer shared stars to work with. A dusty target observed with thin overlaps is close to the worst case for matching tiles to one another.
Gaia's map of the sky
ESA launched the Gaia spacecraft in December 2013 to measure the positions of more than a billion stars and other sources across the entire sky. It does not take pictures in the way Hubble does. It scans slowly across strips of sky, again and again, and builds up the largest and most precise star catalog ever made. The positions are good to about 2 milliarcseconds for many stars, roughly the apparent size of a car on the Moon seen from Earth.
With that catalog, the alignment problem changes shape. Instead of tying each tile to its neighbor, each tile can be tied directly to Gaia. The stars Hubble sees in a tile are matched to Gaia positions, the tile is shifted and rotated until they agree, and the next tile is treated the same way on its own.
In the Lagoon's dusty core, Gaia stars mark the anchor points across the frame, while many of the stars Hubble sees in infrared do not appear in Gaia's visible-light catalog at all, because the dust hides them from Gaia too. A handful of well-measured stars per tile is still enough.
Assembling a puzzle from the picture on the box
The difference resembles two ways of doing a jigsaw puzzle. One way is to inspect each piece and hunt for the neighbors whose edges fit. The other is to keep the picture on the box in view and drop each piece where it belongs. Gaia is the picture on the box. A tile can be placed correctly even if it shares no stars with any other tile, which frees observers to spread the tiles out and cover more sky with the same orbits.
The Lagoon mosaics were aligned this way before being combined into single images with the drizzling software described in Imaging the Lagoon Nebula with Hubble. Gaia is the first mission whose positions are precise enough to be used with many Hubble images, and the benefit reaches well beyond picture-making: accurate alignment matters just as much for measurements made from the data. How the size of a camera's field shapes a mosaic plan is the subject of Fields of View.
Questions readers ask
Why are so many Hubble images mosaics?
Hubble's cameras cover a tiny patch of sky, smaller than half a grain of sand held at arm's length. Anything larger, such as most nebulae and nearby galaxies, has to be photographed in several pointings and stitched together. The 2018 Lagoon images were made from four pointings.
What is a milliarcsecond?
It is one-thousandth of an arcsecond, and an arcsecond is 1/3,600 of a degree. Hubble mosaic tiles must line up to within a few milliarcseconds, comparable to picking out one-thousandth of the width of a human hair held at arm's length.
Why not align the tiles using the nebula's clouds?
Clouds of gas and dust have soft, low-contrast edges and can look different in each filter, so they make poor reference points. Stars are sharp points with well-defined positions, so they serve as the reference, either matched between tiles or matched to a catalog like Gaia's.
Does Gaia take pictures like Hubble?
No. Gaia scans the sky in strips to measure star positions very precisely, building a catalog of more than a billion sources. Its purpose is measurement, and that catalog now serves as a map for aligning images from Hubble and other observatories.