From: twomack@chiark.greenend.org.uk   
      
   In article ,   
   Phil Hobbs   wrote:   
   >On 2020-03-27 06:38, Thomas Womack wrote:   
   >> Suppose you have a telescope designed for the thermal infra-red, so   
   >> surfaces machined to about 2.5um accuracy.   
   >>   
   >> If you use a dichroic mirror rather than a long-pass filter at the IR   
   >> detector, it's clearly possible to direct the visible light to a focus   
   >> on a second detector.  Do you get any form of image at that point   
   >> given that the mirror accuracy is 5-lambda?   
   >>   
   >> And, if so, assuming that everything is rigid and correctly aligned   
   >> for perfect images on the IR detector, is the _centroid_ of the image   
   >> at the visible detector going to be reasonably well-defined?  Can this   
   >> be used at least to help out the orientation-determination system?   
   >>   
   >> Thanks in advance for any help you can offer   
   >>   
   >> Tom (wishing he had checked that the university had Zemax licenses   
   >> before taking on an MSc project involving optic design ...)   
   >>   
   >   
   >Depends sensitively on the details.  If the surface is pretty smooth on   
   >a 100-nm length scale, but not shaped too accurately, you'll get some   
   >sort of visible image, as you do with a magnifying glass or a   
   >searchlight mirror, which are similarly inaccurate but smooth.   
   >   
   >If the surface errors come from coarse diamond turning, you'll get all   
   >sorts of diffraction effects, and there may not be much of an optical   
   >image at all.  The surface will consist of a series of concentric   
   >grooves that don't tilt as you go further out from the axis, so in the   
   >short-wavelength limit (ray optics) it's probably not a focusing surface.   
   >   
   >If it's random surface roughness, the reflection will be diffuse, so you   
   >won't get a visible image at all.   
      
   Thank you, that's extremely useful and clarifies things I wasn't even   
   aware I didn't know.  There's an unreasonably tight hypothetical   
   budget and I was hoping that 2.5um surface accuracy for working in the   
   thermal IR meant the optical surfaces could just be diamond-turned; I   
   hadn't thought about the tilting of the grooves.   
      
   I will tell the AOCS people that they still need a high resolution   
   star tracker ...   
      
   >(BTW Zemax has a student version.)   
      
   Unfortunately only available to explicitly participating institutions   
   of which Cranfield isn't one :(   
      
   Tom   
      
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