How does Hubble work?

[tballou]

Could some please explain how Hubble works? Specifically, how does it track objects for long exposures? Earth based telescopes have a little motor thing that keeps them pointed at the same point for hours on end for long exposures. How does Hubble deal with this whilst it is flying around in orbit going thousands of miles an hour, meanwhile taking very long exposures to produce the deep field shots and others? Thanks in advance!

[apodman]

Could some please explain how Hubble works? Specifically, how does it track objects for long exposures? Earth based telescopes have a little motor thing that keeps them pointed at the same point for hours on end for long exposures. How does Hubble deal with this whilst it is flying around in orbit going thousands of miles an hour, meanwhile taking very long exposures to produce the deep field shots and others? Thanks in advance!

The Hubble has little motor things as well as a bunch of gyroscopes. On a ship, a plane, or anything not nailed down, the trusty gyroscope is your standard method of maintaining attitude. I suppose the Hubble's orientation is computer controlled based on a guide star.

One significant problem they noticed early in the Hubble's career is what happens when it passes from darkness into sunlight. The sudden force of the sunlight on the solar collector panels sends a vibration through the telescope (which is actually a kluge of different instruments), and you can't take any pictures until it settles down.

[tballou]

Ok, but how long can Hubble stare in one spot before it orbits too far along and is on the wrong side of earth? Is this a matter of minutes or hours? And for very long exposures, does it just go back to the same spot and looks some more?

[apodman]

NASA can tell you more than I can ...

Try these links to start ...

Hubble Pointing Control System

NASA Hubble Index Page

A google search will return many more links.

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Hubble, astronauts, shuttle, earth (NASA)

[BMAONE23]

I can tell you that the Hubble orbits roughly 1 time every 1hr 36min meaning that if it is imaging anything very deep in space it is likely doing this in hourly incriments and spending approx 1/2 hour per orbit waiting for the return of the vantage point and repositioning prior to continuing the imaging process.

[tballou]

Thanks to everyone that replied, but I am still having a little trouble getting my head around this. One of the Hubble links has this little tidbit:

The PCS is designed for stable pointing to within .01 arcsec and is capable of holding a target for up to 24 hours while Hubble continues to orbit the Earth at 17,500 mph.

How could this be true (ie, how could it hold a target for 24 hours) it it orbits once every 1.5 hours, taking what one responder suggested were actually repeated one hour exposures?

[bystander]

If the target is outside Hubble's orbital plane, so that the target is not occulted by the earth, I see no reason Hubble could not maintain a lock on its target.

[tballou]

Excellent point! So if Hubble is looking north or south, it should be able to pivot around as it orbits the earth and maintain that view indefinitely!

[geomouse]

Hi there...new to the board and have a q about the Hubble as well... Does anyone know the optical range in the relative short distance scale of this unit? After seeing the Deep Field, I was wondering why it's not used to survey the planets such as Mars. It would seem that an instument of this refinement would be able to peer into the smallest of spaces on any intersolar planet and save a lot of expense in sending global sureyors to each. Maybe I'm asking a previous or simplistic question. I have considered that those targets are just too 'close' to be imaged but then again, isn't the Hubble credited with some of the finest images of our closest neighbours?

geomouse

[Chris Peterson]

Hi there...new to the board and have a q about the Hubble as well... Does anyone know the optical range in the relative short distance scale of this unit? After seeing the Deep Field, I was wondering why it's not used to survey the planets such as Mars. It would seem that an instument of this refinement would be able to peer into the smallest of spaces on any intersolar planet and save a lot of expense in sending global sureyors to each. Maybe I'm asking a previous or simplistic question. I have considered that those targets are just too 'close' to be imaged but then again, isn't the Hubble credited with some of the finest images of our closest neighbours?

Hubble is used to image all the planets outside Earth's orbit. It has also imaged the Moon. From a design standpoint it is very similar to spy satellites that monitor the Earth. It hasn't been directed towards Venus or Mercury because that would require pointing too near the Sun, but optically there's no reason it couldn't image either of them.

The resolution of a telescope is determined from its aperture, and the Hubble's isn't very large by modern standards: 2.4 meters. We can get much higher resolution images from large ground-based telescopes like the Kecks. There are different ways of specifying resolution, but a common one is called the Dawes limit, and is defined as A = 116/D, where A is the angular resolution in arcseconds and D is the aperture in millimeters. For the Hubble, that gives a resolution of 0.05 arcseconds. The optical resolution may actually be a bit better than that, especially for short wavelengths, but the camera pixels are too large to take advantage of this, so most images are actually lower resolution.

Here's an example of a Hubble image of Mars. Note that this is described as the best image made from Earth. But the resolution is still nowhere near as good as what's possible from an orbiter, and that's just with the closest planet Hubble can image. So while it is occasionally used for some planetary studies, it isn't a substitute for the probes we send out.

[tballou]

But doesn't this also get back to my very first question about how Hubble targets objects while orbiting at high speed? Since Mars and Earth are in the same plane, it would seem that any image of Mars by Hubble would require many repeated, short exposures, each taken as it orbits around Earth every few hours. Meanwhile, Mars is rotating and orbiting, so it would seem that there is a limit to the exposure times for photos of objects in our solar system.

[Chris Peterson]

But doesn't this also get back to my very first question about how Hubble targets objects while orbiting at high speed? Since Mars and Earth are in the same plane, it would seem that any image of Mars by Hubble would require many repeated, short exposures, each taken as it orbits around Earth every few hours. Meanwhile, Mars is rotating and orbiting, so it would seem that there is a limit to the exposure times for photos of objects in our solar system.

There is an exposure time limit for moving objects. But the planets are so bright that this isn't a factor. The Hubble saturates its CCD in a fraction of a second on planetary targets. So there's no need for long exposures. All exposures are planned around the telescope's orbit. Any object that is in its orbital plane will be regularly interfered with by the Earth (especially as they generally try to keep the telescope pointed quite far from the Earth, because of thermal issues). There is a complex scheduling system that lets the scope image as long as possible, move to another target, move back again... it can be juggling quite a few simultaneous imaging sessions.

The Moon is so close that motion is a problem. When the Hubble imaged the Moon, it utilized a special program that rotated the entire telescope to track the motion during the exposure (this is something that many planetary probes do, as well). But the attempt to image the Moon was really just an experiment in the device's capabilities; AFAIK no other lunar imaging has been attempted.

[BMAONE23]

here are some great hubble images of mars
A large image of the apollo 17 landing site from hubble
Or
Copernicus Crater
Another Copernicus
Though not a Hubble image of the moon, still an interesting image for size
also
from here

[astrolabe]

Hello All,

I find it remarkable that in a Deep Field image there seems to be little or no blurring or fuzziness in even the most distant objects. One would think that, in spite of corrections for drift, solar wind temperatures and pressure and perhaps other(?) outside factors, photons travelling over such distances would suffer a bit from diffraction, absorption, gravitational influences and the like and would create unique problems that, coupled with more localized anomalies, might result in a lack of clarity- but nooooooo! Why not? Anyone?

[Chris Peterson]

I find it remarkable that in a Deep Field image there seems to be little or no blurring or fuzziness in even the most distant objects. One would think that, in spite of corrections for drift, solar wind temperatures and pressure and perhaps other(?) outside factors, photons travelling over such distances would suffer a bit from diffraction, absorption, gravitational influences and the like and would create unique problems that, coupled with more localized anomalies, might result in a lack of clarity- but nooooooo! Why not? Anyone?

Diffraction isn't really an issue, regardless of distance. There might be some absorption, depending on areas of dust and gas, but this would only reduce the intensity a bit, not cause any blurring or distortion. Gravitational influences are only significant when there is a huge mass (galaxy or galaxy cluster) in front of a really distant source. Then you get gravitational lensing, which is of course a kind of distortion.

For the most part, space is very empty, so light travels huge distances with nothing more than a bit of absorption loss and some redshift. So even the deepest images Hubble or other telescopes can produce are limited in resolution by the optics (or atmosphere), not by the effects of distance.

[tballou]

Not to mention the fact that Hubble is able to maintain its laser-like focus on one spot, or return to that exact same spot for multiple exposures!

From all the preceeding discussion (which by the way is greatly appreciated!), I gather that Hubble goes through quite a lot of gyrations over the course of a single day. Telescopes on earth are from my perspective quite staid and statuesque, rotating every so slowly with the earth's rotation. Hubble, on the other hand, must be much more active and dynamic. Unless of course I am missing something about how it really works out there.

[Chris Peterson]

Not to mention the fact that Hubble is able to maintain its laser-like focus on one spot, or return to that exact same spot for multiple exposures!

That's true, but it's also true of ground-based telescopes.

From all the preceeding discussion (which by the way is greatly appreciated!), I gather that Hubble goes through quite a lot of gyrations over the course of a single day. Telescopes on earth are from my perspective quite staid and statuesque, rotating every so slowly with the earth's rotation. Hubble, on the other hand, must be much more active and dynamic. Unless of course I am missing something about how it really works out there.

I don't think you're missing much about how it works out there. But your impression of things down here isn't quite correct. Ground-based telescopes usually track multiple targets over any evening, so they do plenty of slewing around as well.

[geomouse]

Thanks for your reply Chris...That explains things for me. I am very impressed with your observatory project! What events of significance are you panning to observe, (and graciously share with us)!?! I am a very novice astronomer and keen to learn about the great beyond. Most impressive for me has to be the Hubble Deep Field images. They are truly mind numbing and awe inspiring at the same time.

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