From: pcdhSpamMeSenseless@electrooptical.net   
      
   On 2020-03-28 09:47, Thomas Womack wrote:   
   > 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.   
      
   Folks often do a quick polish of a diamond-turned surface to reduce the   
   scatter and other artifacts.  If that's been done, the surface will   
   focus visible light at some level.   
      
   > 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   
   >   
      
   There are also some very decent free packages.  Beam Four is now free at   
   , and our erstwhile regular poster Jim   
   Klein has a pretty powerful lens design program KDP-2, which you can get   
   at  (password is avon1).  He worked on it   
   for a good 20 years, and it can apparently do just about anything.   
      
   KDP2 is actually open source, but (a) it's in Fortran, and (b) it relies   
   on some proprietary graphics framework whose name I forget.   
      
   Cheers   
      
   Phil Hobbs   
      
   --   
   Dr Philip C D Hobbs   
   Principal Consultant   
   ElectroOptical Innovations LLC / Hobbs ElectroOptics   
   Optics, Electro-optics, Photonics, Analog Electronics   
   Briarcliff Manor NY 10510   
      
   http://electrooptical.net   
   http://hobbs-eo.com   
      
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