Stray Light Estimates

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Stray Light Estimates

• M.Lampton
• U.C.Berkeley Space Sciences Lab
• Jan 2002

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Unwanted photons at a pixel

• Directly-viewed Zodiacal light
• StarlightZodi scattered off Primary
• StarlightZodi scattered off spider
• Sunlight scattered into telescope
• Earthlight scattered into telescope
• Internal glints and leaks

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Diffuse NIR foreground
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Directly-viewed Zodiacal light
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StarlightZodi scattered off Primary
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StarlightZodi scattered off spider

• Spider legs are obstructions that block pupil
• obstructions cause small angle scattering
• therefore broaden point spread function
• this effect is considered part of PSF calc
• Reflections from spider leg structure
• these are NOT small angle, nor diffraction
• these would be significant stray light sources
• can be eliminated by tapering or shielding

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Sunlight scattered into telescope
sunlight
A B
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Earthlight scattered into telescope
Earthshine
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Internal glints leaks

• Stovepipe will surely require outer and inner
  baffle blades
  
• Bottom-stovepipe inner baffle ring serves as a
  field stop at the cassegrain quasifocus
  
• This field stop keeps all sources beyond nominal
  field from illuminating the coffin
  
• This number is very encouraging! These photons
  are our science!
  
• Game plan this limited FOV limits stray light
  within coffin. This field stop, being in nearly
  complete darkness, is far more effective in
  killing hatstovepipe edge rays than any
  forward baffle blocking of those same rays.

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Use stovepipe inner baffles and cassegrain
quasifocus stop to block off-axis rays dont
rely solely on the forward outer baffle (its
bright!)
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Reverse Ray TracingViewpoint is point near edge
of GigaCamgoal is to identify quantify all
irradiances visible to pixel
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(No Transcript)
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Explanation

• Coordinate angles (v,w) are direction cosines
  measured from viewpoint approximately same as
  radians.
  
• Outermost blue region is the cryostat,
  contributes negligibly to our noise level
  
• Red zone within that is the interior of the
  coffin and the rear of the primary -- warm!
  Causes noise!
  
• Green zone within that is a reflected scene,
  might be partly warm (needs separate ray trace)
  
• Blue zone within that is the sky!
• Central tiny red zone with four red spider arms
  is the sky blocked by the secondary mirror
  assembly and hat.