Re: HST flared differently

From: Ed Davies (
Date: Sat Mar 30 2002 - 05:43:21 EST

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    Ed Cannon wrote:
    > The one
    > certain thing I've read is that its front end never points
    > closer than 50 degrees to the Sun even with the aperture door
    > closed (and maybe something similar with respect to the Moon
    > and Earth).
    According to the book "The Hubble Wars" by Eric J. Chaisson
    (page 72):
    > ..., no science data of any kind are collected when the 
    > bright limb of the Earth is within 15 of Hubble's field
    > of view or its dark limb is within 5.
    > Telescope schedulers are also careful to keep Hubble 50 and
    > 15 clear of the Sun and the full Moon.  As noted, in the
    > event of human or computer error, the telescope can 
    > independently sense these bright objects and automatically
    > "safe" itself by closing the aperture door...
    > With caution, lunar occultations - ... - can be recorded.
    He then confuses me a little by writing:
    > Spacecraft motion is also restricted because Hubble 
    > repositions or "slews" very slowly about its own axis -
    > ...  Consequently, when Earth is occulting a given celestial
    > target, Hubble cannot swing rapidly in the direction of
    > another object.  It is actually more efficient to remain
    > target toward the original object and to wait half an 
    > orbit for Earth to get out of the way...
    This sort of implies that the telescope can be left pointing
    at the Earth without harm.  Presumably the aperture door is
    not closed in these cases (as it is quite a dramatic event
    to close and open it).  Maybe it's only the night side of the
    Earth that the telescope can be left pointing at.
    It doesn't sound to me like the Hubble's attitude is 
    sufficiently constrained by these considerations for them to
    help in predicting flares.
    Ed Davies.
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