What Is a Coronagraph and How Does It Reveal Exoplanets and Faint Objects?
A coronagraph is an instrument that blocks a star's light so fainter objects near it become visible. It works by placing an occulting mask over the star's position in the telescope's focal plane, then using a Lyot stop to suppress the diffraction and scattered light that would otherwise swamp the faint signal. Coronagraphs on Hubble and Webb are used to image exoplanets, debris disks, and other faint structures close to bright stars. You can follow current coronagraph observations through Space Telescope Live, which shows what Hubble and Webb are observing in real time.
How a Coronagraph Works
The core problem is contrast: a star can be billions of times brighter than a planet or disk orbiting it. A coronagraph attacks this in stages.
The Occulting Mask
An opaque mask is placed at the position of the star in the focal plane, blocking the bulk of the starlight before it reaches the detector. The mask may be a hard-edged spot, a graded or "apodized" pattern, or a shaped mask designed to control diffraction.
The Lyot Stop
Blocking the star is not enough. Light diffracts around the mask edges and scatters through the optical system, creating a bright halo. A Lyot stop is an aperture placed at a later pupil plane that blocks this diffracted light while letting the remaining signal from the surrounding field through.
Why Both Are Needed
Without the Lyot stop, diffraction from the mask itself would leave a residual glow that hides faint companions. The mask and stop work as a pair: the mask removes the star's core, and the stop removes the diffracted light the mask creates.
What Coronagraphs Enable
- Direct imaging of exoplanets: capturing light from a planet itself rather than detecting it indirectly through its star's wobble or dimming.
- Debris disks: imaging the dust and debris rings around stars, which trace planetary systems in formation or evolution.
- Coronal and extended structures: revealing faint material close to bright sources in a range of astrophysical settings.
How Hubble and Webb Use Coronagraphs
Both telescopes carry coronagraphic capability, and both use it in real observing programs. Space Telescope Live (spacetelescopelive.org) is an official source for following current, past, and upcoming Hubble and Webb investigations. Its landing page asks "What is Hubble observing now?" and "What is Webb observing now?", giving you a direct way to see whether a coronagraph or another instrument is in use at a given time.
To follow coronagraph observations specifically:
- Go to spacetelescopelive.org.
- Check the current Hubble or Webb observing status.
- Look at the instrument and mode listed for the active observation.
- Cross-reference the observation with the investigation it belongs to.
Coronagraph vs. Starshade vs. Standard Camera
| Feature | Coronagraph | Starshade | Standard Camera |
|---|---|---|---|
| Location | Inside the telescope optics | Separate spacecraft flying in formation | Inside the telescope |
| How it blocks starlight | Occulting mask + Lyot stop | External occulter positioned between star and telescope | Does not block starlight |
| Primary use | Imaging faint objects near bright stars | Same goal, different approach | General imaging and photometry |
| Key limitation | Residual diffracted and scattered light | Requires precise formation flying | Bright star overwhelms faint nearby objects |
A standard camera alone cannot separate a faint planet from its star's glare. A coronagraph does that within the telescope. A starshade achieves similar suppression externally, avoiding some internal diffraction but requiring two spacecraft to fly in precise alignment.
Where to Follow Real Observations
Space Telescope Live is the entry point for tracking what Hubble and Webb are doing right now, including coronagraphic programs. Because the site covers current, past, and upcoming investigations, you can use it both to catch an observation in progress and to look up how a completed coronagraph study was carried out.