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I don't believe Hershel was optical at all - pretty certain it was far infra-red

by beezle 5y ago

jacquesm 5y ago
Infra-red is optical. Just not visible light but it uses lenses and mirrors like every other visible light telescope. The main difference is that those lenses are designed to be as transparent to IR as possible instead of accidentally filtering it out. The big trick for Webb is that it will allow for extremely red shifted IR to be received, which translates into 'light from very long ago and very far away'. To be able to do that the sensors are cooled to extremely low temperatures to ensure that the instrument noise level is brought down as low as possible, allowing for extreme sensitivity.
nabla9 5y ago
Optical refers to principles that work in visible, ultraviolet, and infrared light. JWST is also primarily infrared telescope. It as some capability in red and the yellow part of the visible spectrum.
- wolverine876 5y ago
  > Optical refers to principles that work in visible, ultraviolet, and infrared light. Is there something qualitatively different about infrared and ultraviolet that they share with visible, that they are all grouped into 'optical'?
  - maxnoe 5y ago
    Yes, you can use normal mirrors and lenses for these wavelengths
    - wolverine876 5y ago
      Thanks. How is 'normal' defined?
    - nullc 5y ago
      Mirrors, mostly yes. The situation for lenses is more complicated. Fused silica is falls off in the mid IR and is opaque for long wave IR, and optimizing for UV costs you IR response. ( https://escooptics.com/pages/materials-fused-silica-quartz https://escooptics.com/pages/materials-fused-silica-quartz ) The main optical path of JWST is all reflective for this reason. There are refractive optics in some the instruments, but those are more wavelength specific. E.g. NIRcam uses refractive elements made from LiF, BaF2, and ZnSe ( http://ircamera.as.arizona.edu/nircam/pdfs/5904-9_Ryder.pdf http://ircamera.as.arizona.edu/nircam/pdfs/5904-9_Ryder.pdf ) while I don't think MIRI ( https://arxiv.org/pdf/1508.02488.pdf https://arxiv.org/pdf/1508.02488.pdf ) uses any refractive components at all except a ZnS-GE double prism ( https://orbi.uliege.be/bitstream/2268/95768/1/DPA%20v8.pdf https://orbi.uliege.be/bitstream/2268/95768/1/DPA%20v8.pdf ) which is opaque to visible light.

I don't believe Hershel was optical at all - pretty certain it was far infra-red

by beezle 5y ago

I don't believe Hershel was optical at all - pretty certain it was far infra-red

Infra-red is optical. Just not visible light but it uses lenses and mirrors like every other visible light telescope. The main difference is that those lenses are designed to be as transparent to IR as possible instead of accidentally filtering it out. The big trick for Webb is that it will allow for extremely red shifted IR to be received, which translates into 'light from very long ago and very far away'. To be able to do that the sensors are cooled to extremely low temperatures to ensure that the instrument noise level is brought down as low as possible, allowing for extreme sensitivity.

nabla9 5y ago

Optical refers to principles that work in visible, ultraviolet, and infrared light. JWST is also primarily infrared telescope. It as some capability in red and the yellow part of the visible spectrum.

wolverine876 5y ago
> Optical refers to principles that work in visible, ultraviolet, and infrared light. Is there something qualitatively different about infrared and ultraviolet that they share with visible, that they are all grouped into 'optical'?
- maxnoe 5y ago
  Yes, you can use normal mirrors and lenses for these wavelengths
  - wolverine876 5y ago
    Thanks. How is 'normal' defined?
  - nullc 5y ago
    Mirrors, mostly yes. The situation for lenses is more complicated. Fused silica is falls off in the mid IR and is opaque for long wave IR, and optimizing for UV costs you IR response. ( https://escooptics.com/pages/materials-fused-silica-quartz https://escooptics.com/pages/materials-fused-silica-quartz ) The main optical path of JWST is all reflective for this reason. There are refractive optics in some the instruments, but those are more wavelength specific. E.g. NIRcam uses refractive elements made from LiF, BaF2, and ZnSe ( http://ircamera.as.arizona.edu/nircam/pdfs/5904-9_Ryder.pdf http://ircamera.as.arizona.edu/nircam/pdfs/5904-9_Ryder.pdf ) while I don't think MIRI ( https://arxiv.org/pdf/1508.02488.pdf https://arxiv.org/pdf/1508.02488.pdf ) uses any refractive components at all except a ZnS-GE double prism ( https://orbi.uliege.be/bitstream/2268/95768/1/DPA%20v8.pdf https://orbi.uliege.be/bitstream/2268/95768/1/DPA%20v8.pdf ) which is opaque to visible light.