Chiff and Fipple Forums

trill wrote:

Chas wrote:orbit around a Lagrange point

What's ZZ-Top got to do with it !?

Chas wrote:usually have many sectors, each with a different coating

"sectors" ? Like, pie-shaped ? How are the different wavelengths de-scrambled ?


Inquiring minds want to know !

trill

No comment on ZZ Top (not my cuppa).

But in terms of the sectors, the slice of pie is the correct description. Typically the shorter wavelengths will have bigger sectors because the sun's output goes down at shorter wavelengths. On one telescope I remember, the three shortest wavelengths were quadrants, then there were maybe four or five in the last quadrant.

The wavelengths are determined by the coatings. You can think of the multilayer coatings as either a large-spacing synthetic crystal or a very compressed laser (dielectric) mirror. The period (spacing, layer thickness) of the coating is half an optical wavelength, so a shorter-spaced multilayer reflects a shorter wavelength. There's an aperture in front of the entrance of the telescope that selects which sector is illuminated, so this is how they select the wavelength.

Chas wrote:period (spacing, layer thickness) of the coating

Is that a rugate filter ?

trill wrote:

Chas wrote:period (spacing, layer thickness) of the coating

Is that a rugate filter ?

My understanding is that a Rugate filter is a notch filter -- it rejects a narrow band.

These coatings reflect a narrow band. They're like a laser mirror (half-wave plate) but are made from absorbing materials, so the reflectivity peaks at around 70% with a band of about 4%.