Radar CCD's?

[Andry the Stargazer]

Does anyone know if a camera that picks up on radio waves exists for amateur telescopes? If not, why?

[Dr_Ju_ju]

What do you mean by "radio waves", what wavelength, ultra low to past the visible spectrum ??  

[michael.h.f.wilkinson]

4 hours ago, Andry the Stargazer said:

Does anyone know if a camera that picks up on radio waves exists for amateur telescopes? If not, why?

Radio waves require very different apertures and detectors compared to visible light. In the visible spectrum, individual photons have enough energy to general electron-hole pairs in semiconductors, which then can be accumulated and detected (this is how CCDs and the like work). In radio frequencies, each photon carries far too little energy for this process, so no "radio CCD"could be built. Furthermore, to resolve anything, the aperture of the telescope needs to be much larger, as the resolution depends on the ratio of wavelength to aperture

[gajjer]

The wavelength of a 2.4GHz WiFi signal is 12.5cm. The wavelength of green light is around 550nm. The pixel size for a digital camera will be around 5um. The sensor size has to be bigger than the wavelength you are trying to detect. So it works for light but not for radio waves and WiFi is at the top end of the radio frequency spectrum. You also have to consider that focusing radio waves is done differently to focusing light.

Hope that helps.

gaj

[michael.h.f.wilkinson]

Actually, an antenna can be shorter than the wavelength it detects, but it has to be the same order of magnitude. Focusing of radio waves can be done in the same way as light (look at all the parabolic dishes used), but they can be full of holes (which must be significantly smaller than the wavelength you want to detect). However, to get sufficient resolution, it would require HUGE dishes, so instead we use multiple smaller antennas, and combine the signals, and digitally recreating an image with a resolution equal to a dish as large as the widest separation between antennas. Examples are the Westerbork Radio Synthesis Telescope, in the Netherlands, the Very Large Array in the USA, and more recently LOFAR (all over Europe), and SKA in South Africa and Australia.

[Andry the Stargazer]

On 07/07/2023 at 15:56, michael.h.f.wilkinson said:

Radio waves require very different apertures and detectors compared to visible light. In the visible spectrum, individual photons have enough energy to general electron-hole pairs in semiconductors, which then can be accumulated and detected (this is how CCDs and the like work). In radio frequencies, each photon carries far too little energy for this process, so no "radio CCD"could be built. Furthermore, to resolve anything, the aperture of the telescope needs to be much larger, as the resolution depends on the ratio of wavelength to aperture

Thanks, that helps explain it. Also, you'd probably end up with a similar problem to Infrared: Too much outside exposure! Wifi towers are everywhere now, and a mobile device or TV in your neighbours house will blot out everything anyway right? Infrared cameras have to see through the heat of everything radiating. Not easy either. That's why those are in space.

[Andry the Stargazer]

On 07/07/2023 at 16:00, gajjer said:

The wavelength of a 2.4GHz WiFi signal is 12.5cm. The wavelength of green light is around 550nm. The pixel size for a digital camera will be around 5um. The sensor size has to be bigger than the wavelength you are trying to detect. So it works for light but not for radio waves and WiFi is at the top end of the radio frequency spectrum. You also have to consider that focusing radio waves is done differently to focusing light.

Hope that helps.

gaj

Thanks!

[michael.h.f.wilkinson]

On 08/07/2023 at 12:56, Andry the Stargazer said:

Thanks, that helps explain it. Also, you'd probably end up with a similar problem to Infrared: Too much outside exposure! Wifi towers are everywhere now, and a mobile device or TV in your neighbours house will blot out everything anyway right? Infrared cameras have to see through the heat of everything radiating. Not easy either. That's why those are in space.

Near infrared is possible however, and the atmosphere is fairly transparent in a number of bands (e.g., the one around 10 micron)

[Andry the Stargazer]

23 hours ago, michael.h.f.wilkinson said:

Near infrared is possible however, and the atmosphere is fairly transparent in a number of bands (e.g., the one around 10 micron)

Good to know, that's nice! I'm actually thinking of maybe trying to make a cloud filter, by identifying what bands clouds absorb less or just don't, and extracting those to give at least some vision through the cloud. Might not work but who knows?

[michael.h.f.wilkinson]

8 hours ago, Andry the Stargazer said:

Good to know, that's nice! I'm actually thinking of maybe trying to make a cloud filter, by identifying what bands clouds absorb less or just don't, and extracting those to give at least some vision through the cloud. Might not work but who knows?

Clouds block all wavelengths except the really long ones (i.e. radio waves)

[Andry the Stargazer]

On 11/07/2023 at 05:57, michael.h.f.wilkinson said:

Clouds block all wavelengths except the really long ones (i.e. radio waves)

Oh. Well then, good thing I haven't started trying yet!