What is stray light?

What is Stray Light

There are two types of stray lights: ghosts and flare/veiling glare. Ghosts have more than one unwanted reflection of light from the source in the image field. The light falls on the imaging device and creates a ghost image. An example can be seen below:

These are usually due to reflections between imaging surfaces. Furthermore, this can be caused by higher or unblocked diffractive orders from a grating. Thus, a secondary image of bright scattering surfaces is created.

Flare or veiling glare is due to internal light scattering in the optical system. This can be due to imperfections in optical surfaces or mechanical system offsets. Sometimes, this can be caused by atmosphere reflection such as a haze of sky glow. The incident light on the image is from the outside of the optical system’s field which can be shown on the right side of the image example below: Flare

Why do we care?

Stray light can reduce contrast of image with the deviations of amounts of light on the image. This can reduce the sensitivity for detection systems or make poor quality images.

How do we detect stray light?

Empirically:

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What is stray light and how is it monitored

Note: All lights should be turned off (in the lab) during tests.

Part 1-Get Spectral Accuracy:

  1. Obtain a light source and send in light at the first testing wavelength.
    1. The light source will be a uniform light source that uses an integrating sphere and a Xenon lamp — see Spectrum Alignment Test
  2. Record the wavelength that the sensor reads based on the row of pixels with a peak in intensity. This is verified so that the value is within one spectral resolution step of the wavelength sent by the uniform light source.
  3. Shift the uniform light source’s signal higher by one spectral resolution step and verify that the measured wavelength has moved one row.
  4. Shift by another spectral resolution step and then also shift one and two steps below the original to record the wavelengths.