[OT?] Curious about Hubble Space Telescope optics, sensor, and electronics

Started Mar 20, 2014 | Discussions thread
marvin t martian Forum Member • Posts: 58
Re: [OT?] Curious about Hubble Space Telescope optics, sensor, and electronics

I'll fill in a few bits quickly so you don't have to read through the entire tech specs:

  • This will show my ignorance ... does it have lenses, mirror, or both?

Both. It uses two mirrors to gather light into an optical director system which feeds light into whichever instrument is active at the time. Each instrument will have varying optics--mirrors, lenses, or both--to direct light into its sensor(s).

  • Does it have the equivalent of a zoom lens, or is it a "prime"?

The two mirrors form a fixed focal length system. Each instrument, however, can have a different final focal length due to internal optics and sensor size. Some instruments essentially crop sensors.

  • What is its equivalent focal length? (or focal length range)

About 57.6 meters.

  • Does it have the equivalent of an aperture that can be stopped down?

IIRC, there is no aperture but there is a lens cap.

The main mirror can be blanked off with a lens cap to protect the telescope from micrometeorite damage and from excessively bright light sources.

Some instruments have disks that can be inserted into the sensor field of view to block glare from bright light sources. These disks are used to observe fainter objects near brighter ones.

  • What is its equivalent most open f-stop and most closed f-stop?

Fixed at roughly f/24.

  • Does it have the equivalent of lens sharpness rating in line-pairs per [fill-in-the-blank]?

It can resolve two point sources separated by 0.05 arc seconds. I don't know how to convert that into the usual line pairs metrics we're familiar with.

  • Does it change focus to focus on the moon vs. Jupiter vs North Star (Polaris?) vs a "nearby" galaxy vs a "distant" galaxy? Or are all or some of those considered "infinity"?
  • What is the "granularity" of its focus?
  • What is its close focus?

Astronomical telescopes are focused at infinity. Even the near planets are too far away for focusing to be an issue.

  • Can it be pointed "downwards" and focus on an object on the Earth? If so, how small of an object can it resolve?

No for two reasons. The Hubble's sensors are very vulnerable to bright light and would be damaged by pointing at the Earth. Further, the HST can't can't rotate fast enough to image objects on Earth as they move in relation to the Hubble due to orbital mechanics and the rotation of the Earth.

Theoretically, Hubble's 0.05 arcsec resolution means it could resolve--ignoring atmospheric distortion--a 140mm object on the Earth's surface as a single pixel.

  • How small of an object could it resolve on the moon?

Like Earth, the Moon is too bright for the HST to image.

Theoretically, the HST could resolve a 90 meter object on the lunar surface as a single pixel.

  • Does the HST have the equivalent of a sensor for human visible light, and, if so, what is its resolution?

Several instruments can image visible light, however they tend to use fairly narrow band color filters and can't create true color RGB images directly. Most of Hubble's famous photos are, in truth, false color images.

The resolution of the current wide angle camera is 4096 x 4096 pixels. No CFA is used; the sensor is monochromatic. Multi-spectral (color) images are created by taking multiple exposures with a different filter in the optical path for each exposure.

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