Half a croissant, on a plate, with a sign in front of it saying '50c'
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Pinhole Satellite

Simple satellite brings stellar images to earth
  (+5, -1)
(+5, -1)
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This idea is for a geoynchronous or geostationary satellite comprised of a simple reflective dish, plus minimal support systems such as gyrostabiliser, energy source, and communication systems.

This is not actually a pinhole camera, but a pinhead camera. The principals and math are the same though.

The key calculation for pinhole cameras is given below:

d=1.9 SQRT(focallength.wavelength)

Taking the focal length to be the altitude of geosynchronous orbit of 35,780km and the (inappropriate?) wavelength of light at 550nm, that would imply a disc of under 8m diameter would produce an image focused on the earth's surface.

In addition, cosmic phenomena could be seen from parts of the world not actually pointing at that part of space.

Phenomena not normally visible because of backlighting could also be seen if the mirror is positioned over a dark part of the sky.

Variations may include: Mirrors of different or variable reflective diameter to reflect other wavelengths.

Mirrors reflecting onto a flat bit of the dark side of the moon.

Advertising beamed images, including advertising on dark parts of the moon.

References: Wikipedia NASA New Worlds Mission (different; that's an occluder) Pinhead Mirror USPat4,948,211 Wikipedia Geosynchronous Wikipedia Pinhole_Camera Various 007 plots

FloridaManatee, Mar 27 2008

NASA found it to be a combination of management failings, disregarding written quality guidelines, and ignoring test data http://en.wikipedia.org/wiki/PerkinElmer
[normzone, Mar 27 2008]

Liquid mirror telescope https://en.wikipedi...id_mirror_telescope
Mentioned in my anno [notexactly, Apr 16 2018]


       What's the advantage of not using a lens?
ldischler, Mar 27 2008

       So, the mirror reflects an image onto Earth from 36million metres? That mirror will have to be perfect. Pretty much absolutely *perfect*.   

       Imperfections that lead to angle of reflection change in the order of 1 nano degree will ruin it...
Jinbish, Mar 27 2008


       //Imperfections that lead to angle of reflection change //
So, don't get Perkin-Elmer to build it for you.
coprocephalous, Mar 27 2008

       [FloridaManatee]... now there's a name I've not heard in a looong time.
lurch, Mar 27 2008

       Perhaps we could somehow adapt the liquid mirror telescope [link] concept to this to achieve the necessary perfection.
notexactly, Apr 16 2018


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