Hubble to use moon as mirror to see Venus transit | Hacker News Reader

Hubble to use moon as mirror to see Venus transit

sciencedaily.com

27 pointswglb9 comments

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...

(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.)

FYI, he original link has the actual images: http://www.nasa.gov/mission_pages/hubble/science/transit-mir...

> Hubble cannot look at the Sun directly.

anyone know why not?

Just like you can't look at the sun directly - because it's too bright... and Hubble is constructed to mainly look at the faint objects far-far away.

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

(http://www.stsci.edu/hst/wfpc2/software/wfpc2-help-sun_angle...)

> 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.

Ever seen an ant fried with a magnifying glass?

Well, replace the ant with a zillion dollars' worth of ultra-sensitive cryogenic CCDs and scale up the magnifying glass by a factor of a lot.

I believe it's because the optics and instrumentation would be damaged by light and heat. If I recall correctly observing the earth is also outside of its design parameters.

I don't know fully the details, but I worked on the Hubble servicing missions (I did the engineering on the boxes that transported up new cameras, gyros etc). I did not work on the actual cameras, except to package them up.

I suspect that the answer is that the cameras are designed to look into some of the deepest darkest regions of space, and are freakishly sensitive. So much so that keeping the cameras at frigid temperatures is a requirement in order to make sense of the signal they collect. Pointing it at the sun, I would assume, would be at best unusable, and at worst damaging.

For some perspective - look up the NICMOS camera:

http://en.wikipedia.org/wiki/Near_Infrared_Camera_and_Multi-...

There was a thermal short in the cooler that threatened it's life span, so they cooked up an external cooling system in order to keep the camera at some absurdly cold temperature like 60 Kelvin. (I could be wrong about the temp - but it's COLD).

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