
Hubble to use moon as mirror to see Venus transit - wglb
http://www.sciencedaily.com/releases/2012/05/120504135730.htm?utm_source=feedburner&utm_medium=feed&utm_campaign=Feed%3A+sciencedaily+%28ScienceDaily%3A+Latest+Science+News%29
======
gjm11
Second-order blogspam. Original source:
<http://hubblesite.org/newscenter/archive/releases/2012/22/>; ScienceDaily
text taken verbatim from [http://www.newswise.com/articles/hubble-to-use-moon-
as-mirro...](http://www.newswise.com/articles/hubble-to-use-moon-as-mirror-to-
see-venus-transit)

(All the text of the latter is actually taken from the former, but the UI is
kinda ridiculous. The first bit is from the initial "Introduction" tab; for
the rest, go to the "Release Images" tab and click on individual ones; each is
associated with some text.)

------
sigmaxipi
FYI, he original link has the actual images:
[http://www.nasa.gov/mission_pages/hubble/science/transit-
mir...](http://www.nasa.gov/mission_pages/hubble/science/transit-
mirror_prt.htm)

------
jwb119
> Hubble cannot look at the Sun directly.

anyone know why not?

~~~
ggchappell
A good question, to which I do not know the answer. I would imagine the
problem is either that there are parts that cannot be allowed to get too hot,
or that stray light might wreck optical sensors.

So I looked into it. Wikipedia[1] explains that the Sun is never allowed to
illuminate any part of the Optical Telescope Assembly. And therefore Hubble
never points within 50 degrees of the Sun. This is the "Solar Avoidance
Angle". A forum on NasaSpaceFlight.com[2] has a message that adds more detail:

    
    
      In general the following viewing constraints apply:
      - 50 degree solar avoidance angle
      - 20 degree from illuminated Earth
      -  6 degree from dark Earth limb
    

So it seems very likely that the problem would be stray light wrecking optical
sensors.

I don't _know_ this, however. Further info from someone knowledgeable, would
be nice.

[1] <https://en.wikipedia.org/wiki/Hubble_space_telescope>

[2] <http://forum.nasaspaceflight.com/index.php?topic=17256.0>

~~~
DanBC
([http://www.stsci.edu/hst/wfpc2/software/wfpc2-help-
sun_angle...](http://www.stsci.edu/hst/wfpc2/software/wfpc2-help-
sun_angle.html))

> _Most HST observations are made when the target is roughly 90 degrees from
> the sun. However, scheduling constraints can force the observation into
> other sun angles._

> _The brightest backgrounds result when sources in the ecliptic plane are
> observed close to the sun. If you are concerned about the worst-case sky
> background, you may wish to try setting "sun angle" to 50 degrees, which is
> the HST solar avoidance angle. Ecliptic longitudes less than 50 degrees are
> possible for sources far from the celestial equator, but for these sources
> the zodiaical light is already moderately low._

> _The lowest backgrounds occur when sources are farthest from the sun, i.e.,
> "sun angle" = 150 to 180 degrees._

