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Thread: Hubble telescope and earth axis in precession

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    Hubble telescope and earth axis in precession

    On page 55 of "Bad Astronomy" Mr. Plait says that it is important for the Hubble Space Telescope to figure in the precession of the earth axis. Why is this if the telescope is out in space and not on the earth?

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    Quote Originally Posted by DaveH View Post
    On page 55 of "Bad Astronomy" Mr. Plait says that it is important for the Hubble Space Telescope to figure in the precession of the earth axis. Why is this if the telescope is out in space and not on the earth?
    The coordinate system against which celestial objects are measured (right ascension/declination) is anchored to the Earth's rotation, and therefore the direction of the celestial poles change with the Earth's precession. At the very least, any accurately pointed observation (on Earth or off) has to take into account the epoch of the coordinate system used as input (typically these days, 2000.0) and the precession from then to the observed time. In principle one could work straight from some well-defined inertial coordinate system, but systems anchored to the Earths axis remain the most accurately defined (AFAIK).

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    I understand the need for precession to be accounted for when observing from the earth. Your statement does not explain why this is necessary in space. I would think that if you are orbiting Jupiter you do not need to take earth's precession into consideration when locating an object. I would think a co-ordinate system separate from the one used for ground based telescopes would be better. The only reason I can think of to use the ground based system would be that the Hubble Telescope's orbit is somehow linked to the earth's axis and through (gravity?) is preturbed to mimic the precession of earth's axis.

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    Precession?

    Quote Originally Posted by DaveH View Post
    On page 55 of "Bad Astronomy" Mr. Plait says that it is important for the Hubble Space Telescope to figure in the precession of the earth axis. Why is this if the telescope is out in space and not on the earth?
    I hope I can possibly answer your question---although it has been several years since I read the book-- there are probably a number of reasons---

    the first of which may be: we have (for the most part) been building fewer ground-based optical-type telescopes for some time now ... because they eventually are rendered too costly because of light and particle pollution. The most notable exceptions are those that on the Hawaiian islands and in a few countries in South America. Also the atmosphere of the earth is impenetrable to many types of electromagnetic radiation--so it seemed logical to put telescopes into orbit

    the second of which may be: Servicing of the telescopes might be easier if they were put into predictable orbits.

    the third of which may be: the rendering of post-image processing might serve to cancel any propagation of error that may occur in the production of the image(?)--that is my logical guess?

    I hope someone else can lend a hand here----

    Cheers and Welcome to the forum...

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    Wink

    Quote Originally Posted by DaveH View Post
    On page 55 of "Bad Astronomy" Mr. Plait says that it is important for the Hubble Space Telescope to figure in the precession of the earth axis. Why is this if the telescope is out in space and not on the earth?

    The recorded positions of celestial objects are given in a coordinate system that has a relation to the Earth. The positions of stars change in this system due to (among other things) precession of the equinoxes. Stored in the HST’s computer’s memory are the positions of stars at 2000.0 in that Earth based coordinate system. On other dates, the predicted position of the stars must be adjusted to account for (among other things) precession of the equinoxes. Indeed, if the positions of the stars had been initially recorded in an inertial coordinate system, and if the HST used that system, then an adjustment for precession would not have been necessary.
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    one more try

    i don't think the posts have quite got to dave h's query, which has three layers.

    the celestial coordinate system, right ascension and declination, is the dominant standard system for identifying a specific location in the sky. it can be determined highly accurately and standardized at any given time, and as a general framework it gets printed up as the coordinate system in star atlases, standardized as an "epoch" that is updated every 5 decades or so.

    the precession of the earth's axis causes this coordinate system literally to shift across the celestial sphere, literally week by week for extremely precise locations, so to pinpoint locations for hubble it must be adjusted by an orthogonal rotation, a simple calculation, as necessary.

    that's the first part of the question. the second part might be, why not just orient hubble to three standard stars, say to vega, rigel and canopus, and ignore the earth system? then the problem is that hubble might "find" something that needs to be communicated back to earth observatories, which will still be using the earth based system, so the correction has to be made there anyway!

    the third part: why doesn't astronomy just get on board a fixed, firm, nonarbitrary system, say the galactic coordinate system, for all observatories everywhere? that's possible and entirely convenient with modern computers, but the problem is anchoring it among wandering stars. i think there's currently a program to create a system based on a few hundred quasars as the coordinate anchors, but that is very far from being a standard system yet.

    between the fact of precession, and the practicalities of communication, making adjustments for precession to the celestial coordinate system is the currently optimal solution.

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    Touching on the celestial coordinates issue, here's a wiki article on coordinates as a starting point. A work related issue made me want to read up a little on the different coordinate systems in use, old and newer. Here's a document I enjoyed going through for a bit on the Dutch systems (it's both in Dutch and English).

    It's a fascinating field, never thought there would be so much behind it. Also interesting to see how GPS turned it upside down. Even the few centimeter per year tectonic plate movements have to be taken into account, as well as some plates that are still slowly recovering from the tilting due to ice age glaciers.
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    Hi Dave,

    I think what you're not getting, which several people have made a superb effort in trying to explain, is that space-based telescopes use exactly the same coordinate system of Declination and Right Ascension as ground-based telescopes. However the coordinates of any given object in the sky changes over time as the equinox moves slightly each year as a result of the precession of the Earth's rotational axis. About every 50 years the stellar cartographers have to change the star Atlases to reflect the different coordinates of every star and deep-sky object out there.

    Space telescopes such as Hubble, use the very same coordinate system as ground-based telescopes. So if the coordinates are not accurate, then it will not be pointing at the right region of space when it tries to observe an object. I can't put it any simpler than that.

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