7: Coronagraph
In the image below you see the coronagraph and here you got part of the explanation of the name of the instrument, Cor of PolCor-2.
Here in center is the coronagraph's revolver with the various masks (beam blockers), you also see two of the mirrors.
Its function is to block the star's direct light, just as when the sun's corona to be studied have the blinding light from the central part blocked.
The turret contains a full set of masks to suit different object sizes. As you may recall,
I have in a previous tutorial said that all the stars are point objects in normal telescopes.
But optics, atmospheric and other things means that the observed angular diameter gets larger.
Depending on circumstances, this diameter can be unequal, that's why several sizes of mask are needed.
The mask consists of a very thin layer of metal evaporated on a glass substrate, it is like a small dot in the middle of the filter.
The sizes that can be selected on the masks is 1.5", 3" and 6" (when instrument is mounted on NOT).
3" means 3 arc second and one arc second is 1/3600 part of a degree.
The mask does not block the light completely, a very small part of the light penetrates to give the opportunity to set the camera's and telescope's, focus and tracking.
Three different magnitudes of damping for each size can be selected: 5, 8.75 and 12.5.
The blocking dot also has "softed" edges to reduce edge effects.
Notice the "pits" in the outer edge of the revolving wheel, they have the function to fix the various filters in the correct position.
A motor controlled by a computer, select the correct mask.
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