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Gaia has a 1.0 × 0.5 m focal plane array on which light from both telescopes is projected. This in turn consists of 106 CCDs of 4500 × 1966 pixels each, for a t

by lysace 2y ago

Gaia has a 1.0 × 0.5 m focal plane array on which light from both telescopes is projected. This in turn consists of 106 CCDs of 4500 × 1966 pixels each, for a total of 937.8 megapixels. Neat.

perihelions 2y ago
The really neat part is the instrument precision. It's terrifyingly good and I have no idea how it (really) works. - "Gaia measures their positions to an accuracy of 24 microarcseconds, comparable to measuring the diameter of a human hair at a distance of 1000 km" https://www.esa.int/Science_Exploration/Space_Science/Gaia/Cosmic_distances https://www.esa.int/Science_Exploration/Space_Science/Gaia/C...
- colechristensen 2y ago
  Isn't that just the distance between pixels and the image projected onto them?
  - perihelions 2y ago
    No: it's far weirder and I'm not knowledgeable enough to explain it.
  - echoangle 2y ago
    Probably not. The accuracy with which you know the pointing of the telescope probably also plays into it (unless the FOV is large enough to have other stars as a reference?), and you can do subpixel positioning of objects to get more accuracy than full pixel steps.
- yshklarov 2y ago
  To nitpick with the grammar in the quote: It's capable of measuring to the accuracy of 120 μm at 1000 km. So it cannot accurately measure the diameter of a human hair (which ranges from around 20 to 200 μm) at that distance, but only to the accuracy of a human hair.
  - perihelions 2y ago
    You're right: this precision is hundreds of times below the diffraction limit of even the James Webb telescope. It can't possibly measure the width of an object that finely; rather, only the relative displacement of its centroid position between two points in time. (And it's a seriously confusing physics miracle that that much is possible).
    - jdhwosnhw 2y ago
      For point source astrometry, there are a few ways to beat the diffraction limit. For instance, some observers will purposely defocus their optics to spread the target photons over a larger number of pixels, which with sufficient SNR lets you gain a better lower bound (from a Cramer Rao perspective) on precision. I think Gaia actually does something similar to this but “blurs” through time, rather than across space, by purposely not perfectly tracking stars so that they drift at sub-pixel rates across the FPA.
      - UltraSane 2y ago
        Yes, Gaia slowly spins to that the stars move across its CCD array.
- IndrekR 2y ago
  It takes about 14 pictures of each star during orbit (which is quite close to Earth’s orbit around the Sun), so approximately once per month, and then compares those to calculate the star’s distance from the parallax.
IndrekR 2y ago
And Gaia also has a downlink speed of approx 3Mbps. So it will process as much as possible on board and just send down less than 20 pixels per each star imaged. That is why you can not get a direct image out of it.

by lysace 2y ago

Gaia has a 1.0 × 0.5 m focal plane array on which light from both telescopes is projected. This in turn consists of 106 CCDs of 4500 × 1966 pixels each, for a total of 937.8 megapixels. Neat.

perihelions 2y ago

The really neat part is the instrument precision. It's terrifyingly good and I have no idea how it (really) works. - "Gaia measures their positions to an accuracy of 24 microarcseconds, comparable to measuring the diameter of a human hair at a distance of 1000 km" https://www.esa.int/Science_Exploration/Space_Science/Gaia/Cosmic_distances https://www.esa.int/Science_Exploration/Space_Science/Gaia/C...

colechristensen 2y ago
Isn't that just the distance between pixels and the image projected onto them?
- perihelions 2y ago
  No: it's far weirder and I'm not knowledgeable enough to explain it.
- echoangle 2y ago
  Probably not. The accuracy with which you know the pointing of the telescope probably also plays into it (unless the FOV is large enough to have other stars as a reference?), and you can do subpixel positioning of objects to get more accuracy than full pixel steps.
yshklarov 2y ago
To nitpick with the grammar in the quote: It's capable of measuring to the accuracy of 120 μm at 1000 km. So it cannot accurately measure the diameter of a human hair (which ranges from around 20 to 200 μm) at that distance, but only to the accuracy of a human hair.
- perihelions 2y ago
  You're right: this precision is hundreds of times below the diffraction limit of even the James Webb telescope. It can't possibly measure the width of an object that finely; rather, only the relative displacement of its centroid position between two points in time. (And it's a seriously confusing physics miracle that that much is possible).
  - jdhwosnhw 2y ago
    For point source astrometry, there are a few ways to beat the diffraction limit. For instance, some observers will purposely defocus their optics to spread the target photons over a larger number of pixels, which with sufficient SNR lets you gain a better lower bound (from a Cramer Rao perspective) on precision. I think Gaia actually does something similar to this but “blurs” through time, rather than across space, by purposely not perfectly tracking stars so that they drift at sub-pixel rates across the FPA.
    - UltraSane 2y ago
      Yes, Gaia slowly spins to that the stars move across its CCD array.
IndrekR 2y ago
It takes about 14 pictures of each star during orbit (which is quite close to Earth’s orbit around the Sun), so approximately once per month, and then compares those to calculate the star’s distance from the parallax.

IndrekR 2y ago

And Gaia also has a downlink speed of approx 3Mbps. So it will process as much as possible on board and just send down less than 20 pixels per each star imaged. That is why you can not get a direct image out of it.