Hubble is essentially a Keyhole satellite so I assume if they flipped it around we’d have really clear photos of earth similar to the one trump got in trouble for showing on live tv.
this post was submitted on 06 Dec 2023
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Those spy satellites are focused at the distance of earth. Hubble is focused at infinity or near-infinity.
You'd just get some really blurry images.
Is focus to infinity that different for these types of scopes compared to a conventional camera which focuses to infinity after a couple hundred metres? I would have thought the draw back would be the focal lengths giving you a very small area in frame.
Focal length and aperture both affect how much focus matters.
Also with most regular cameras, from what I've heard they actually have their max focus slightly past infinity to account for thermal expansion so the general advice for doing astrophotography is to focus on the subject, not just whacking it to infinity.
Not really. Hubble is set up to image objects light years away and moving relatively slowly.
Keyhole stats as set up to image the earth's surface that is only hundreds/thousands km away and moving quite fast.
Two different missions would lead to two different designs.
That would lead to different focal lengths not different designs completely. Both optical systems for HST and KH were designed by Perkin Elmer so I’ll stick with my first thought that they would be very similar in capabilities.
Did you watch the video?
The tracking problems Hubble would have imaging the earth surface are a direct guide to what differences the design would be.
Did you finish the video where they essentially say what I said in my first comment? Hubble and spy satellites share a lot of the same technologies. I wasn’t aware of the speed issue but other than that they are similar like I said.
Wouldn’t tracking be software and not hardware?
they have to rotate the whole satellite to point it at something
And the angular momentum is conserved.
Your choices are basically RCS thrusters or reaction wheels. Thrusters burn limited fuel. Reaction wheels are flywheels inside the satellite that you spin in the operator opposite direction to where you want to rotate. They are limited by the mass and size of flywheel, and the maximum speed you can spin it up to.
electromagnets also work as the earth has a magnetic field. a pair of reaction wheels can be rotated (which yes, adds complexity) opposite directions along an axis perpendicular to the axles once they have reached saturation, effectively resetting the reaction wheels
Like the other satellites do as well yeah?
They have to rotate it fast enough, and hubble is not built to rotate that fast
Is it an actual limitation of the hardware or a software safety limit? The designs are similar it more has to do with limitations put on it.
They mention they can’t because the gyro vibrates, so it does sound like it’s capable of spinning faster it’s limited for specific reasons. Now are those reasons detrimental to its use or would they just have to get fancier.
These can’t be answered.
The sattelite bus for the KH-11 Spy sattelites (which hubble is based on) uses thrusters for orientation (and has a huge propellant tank) while the Hubble sattelite bus uses several gyroscopes for orientation. They are not as similar as you might think.
With reaction wheels/gyros could they not build up momentum over time to achieve the appropriate spin rate to match earths rotation?
After that it would just be timing and they could also layer multiple images together to work out details in theory.
You cannot spin up gyros indefinitely like e.g. in KSP.
Why would you need to? What would strip the momentum enough for it to constantly need to spin after it’s built up?
Nothing. It just wouldn't reach the rotation rate in the first place. (and the camera equipment would be unusable due to the vibrations of the gyros long before it reaches theoretical maximum, as already stated in the video)
How do you know it can’t? What the limitation?
Stop the gyros, take the picture activate again if needed to keep momentum.
There’s ways, you’re just not thinking creatively enough and you also aren’t providing actual explanations for why.