10: Lyot Stop
We follow the light path ahead and
then mover over to the second chamber of the instrument box.
At the top shows the unit that compensates away diffraction from the secondary mirror holder.
In image above, at the top is the optical device that reduces
the diffractions spikes.
The diffraction spikes we've talked about earlier causing trouble here, just like in the amateur astronomer Newton telescope.
The spikes gives disturbing light on the vague light from the dust.
It lowers the contrast and something must be done about it.
Using a technique called Lyot Stop based on Fourier transform, these spikes is compensated away or at least reduced.
One thing that complicates the matter is that NOT (the telescope normally used) does not have an equatorial mount, it has an azimuthal mount.
A good technique for larger telescopes because it simplifies the mechanics but with the disadvantage that the image field rotates.
Field of rotation compensates for the telescope itself but spiders (secondary mirror holders) are not.
It must be dealt with within this instrument, it is the computer's task to control the servo motor and the rotating filter (Lyot Stop)
so it is in the correct position relative the spiders and the spikes it caus. A disadvantage of the Lyot mask is that it screens of some light.