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Starts With A Bang

Why modern astronomy needs photometry, not just more light

The photometric filters for the Vera Rubin Observatory are complete and showcase why they are indispensable for astronomy.
Astronomers have used this set of single-colour images, shown around the edge, to construct the colour picture (centre) of a ring of star clusters surrounding the core of the galaxy NGC 1512. By adding together a series of images taken with different photometric filters, a rich color image, with essential details about temperature, dust, and more, can be produced. (Credit: NASA, ESA, Dan Maoz (Tel-Aviv University, Israel, and Columbia University, USA))
Key Takeaways
  • When we turn our eyes toward the skies, we take in all the different wavelengths of light simultaneously.
  • In our eyes, the various cones respond to different colors, giving us a myriad of multiwavelength information.
  • In telescopes, we reproduce and extend that idea through the use of filters, revealing details about the Universe that would otherwise be completely invisible.

In principle, astronomy is as simple as it gets: collect all the light that arrives.

If all you do is collect light from the cosmos and view it, either with photography or by viewing with your eyes, you’ll only get a cumulative view of all the wavelengths combined. Without separating light out by wavelength, you lose out on vital information. (Credit: NPS/M.Quinn)

Taking all the light together, indiscriminately, averages over all wavelengths.

This 1888 image of the Andromeda Galaxy, by Isaac Roberts, is the first astronomical photograph ever taken of another galaxy. It was taken without any photometric filters, and hence all the light is summed together. (Credit: Isaac Roberts)

This “bolometric” approach erases color-dependent details.

This set of professional photometric filters help ensure that only the relevant wavelengths make it into the telescope’s optics, allowing us to separate out one set of wavelengths from all the others. (Credit: Travis Lange/SLAC National Accelerator Laboratory)

Instead, a key advance is the development and application of photometric filters.

A series of filters designed to only allow a certain wavelength-range through at a time allows us to collect and separate the light signals from objects in the Universe into different bands. By compositing the data from different bands together, we can construct far more scientifically informative, and aesthetically pleasing, images than otherwise. This is the r-band filter, as only red light gets transmitted through; all other wavelengths are reflected. (Credit: T. Lange/SLAC National Accelerator Laboratory)

When the incident light comes in, it gets passed through a filter.

Different photometric filters, which have been relatively standardized over the past ~100 years or so, are sensitive to a variety of wavelengths. By combining the data from multiple wavelength bands together, a more accurate, comprehensive “picture” of what’s actually out there can be put together. (Credit: Michael Richmond/RIT)

Only a specific narrow-to-broad range of wavelengths makes it through.

This image from NASA’s Chandra X-ray Observatory shows the location of different elements in the Cassiopeia A supernova remnant including silicon (red), sulfur (yellow), calcium (green) and iron (purple), as well as the overlay of all such elements (top). Each of these elements produces X-rays within narrow energy ranges, allowing maps of their location to be created once the appropriate filters are applied. (Credit: NASA/CXC/SAO)

A variety of filters allows for focusing on one specific wavelength range at a time.

photometric filter
This photograph of one of the LSST filters showcases how light is both transmitted from behind the filter in a set of wavelengths, and also how light outside of those wavelengths is reflected away, allowing us to see assembly technician Frank Arredondo in the foreground. (Credit: LLNL/G. McLeod)

Each astronomical objects emits different intensities of light across each wavelength range.

The Andromeda galaxy, the closest large galaxy to Earth, displays a tremendous variety of details depending on which wavelength or set of wavelengths of light it’s viewed in. Even the optical view, at top left, is a composite of numerous different filters. Shown together, they reveal an incredible set of phenomena present in this spiral galaxy. (Credit: infrared: ESA/Herschel/PACS/SPIRE/J. Fritz, U. Gent; X-ray: ESA/XMM-Newton/EPIC/W. Pietsch, MPE; optical: R. Gendler)

The process of building a color image works identically to our eyes: with additive mixing.

This photograph, from 1911, demonstrates the technique of additive color mixing as applied to photography. Three color filters, blue, yellow, and red, were applied to the subject, producing the three photographs at right. When the data from the three are added together in the proper proportions, a color image is produced. (Credit: Sergei Mikhailovich Prokudin-Gorskii)

By combining at least three different wavelength responses, a richly varied palette is created.

The same object, the Pillars of Creation in the Eagle Nebula, can have vastly different details revealed dependent on the wavelength of light used. Here, the visible light (L) and near-infrared (R) views are shown, both taken with the Hubble Space Telescope and both taken with multiple, independent filters. (Credit: NASA, ESA/Hubble and the Hubble Heritage Team)

Multiwavelength astronomy now extends far beyond optical limits.

photometric filter
LLNL engineers Justin Wolfe and Simon Cohen hold the u-band filter, which only allows near-ultraviolet light through it. As a result, the filter appears as a mirror to human eyes, as it doesn’t transmit any visible wavelengths through it. If we had ultraviolet-sensitive eyes, however, we’d see a specific amount of light transmitted through it. (Credit: Frank Arredondo)

Longer wavelengths signify intrinsically redder, cooler temperatures.

This image showcases the open star cluster NGC 290, as imaged by Hubble. These stars, imaged here, show a variety of colors because they are at different temperatures, and so the hotter stars emit more blue-than-red light while the cooler ones emit more red-than-blue. Different colors can only be revealed by imaging stars in multiple different wavelengths. (Credit: ESA & NASA; Acknowledgement: Davide de Martin (ESA/Hubble) and Edward W. Olszewski (University of Arizona))

Interstellar gas and dust more efficiently blocks shorter-wavelength light.

Visible (left) and infrared (right) views of the dust-rich Bok globule, Barnard 68. The infrared light is not blocked nearly as much, as the smaller-sized dust grains are too little to interact with the long-wavelength light. At longer wavelengths, more of the Universe beyond the light-blocking dust can be revealed. (Credit: ESO)

Meanwhile, the expansion of the Universe stretches all wavelengths equally.

expanding universe
This simplified animation shows how light redshifts and how distances between unbound objects change over time in the expanding Universe. Note that the objects start off closer than the amount of time it takes light to travel between them, the light redshifts due to the expansion of space, and the two galaxies wind up much farther apart than the light-travel path taken by the photon exchanged between them. (Credit: Rob Knop)

A variation in a single wavelength can signify an important cosmic change.

This photograph shows all six photometric filters, pre-installation, designed for the LSST camera at the Vera Rubin Observatory. They span the gamut of wavelengths from ultraviolet through the optical and into the infrared. (Credit: Travis Lange/SLAC National Accelerator Laboratory)

Vera Rubin Observatory will conduct our most sensitive rapid, large-area survey ever.

The LSST camera, designed for the Vera Rubin Observatory, is arguably the most advanced photometric system ever constructed, capable of revealing changing and varying details about the Universe that have hitherto been elusive. (Credit: Chris Smith/SLAC National Accelerator Laboratory/NSF/DOE/Rubin Observatory/AURA)

Photometric filters enable wavelength-specific sensitivity to change.

LLNL optical engineer Justin Wolfe inspects the alignment of the optic and lift fixture for one of six optical filters for the Vera C. Rubin Observatory that were examined in the LLNL National Ignition Facility optical assembly building. (Credit: Gerry McLeod)

Wavelength-dependent views are essential for monitoring how objects — and environments — change.

These four images show Betelgeuse in the infrared, all taken with the SPHERE instrument at the ESO’s Very Large Telescope. Based on the faintening observed in detail, we can reconstruct that a “burp” of dust caused the dimming. Although variability remains larger than it was previously, Betelgeuse has returned to its original, early-2019-and-before brightness. (Credit: ESO/M. Montargès et al.)

Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words. Talk less; smile more.

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