Avionics Multiple Choice Questions on “Stellar Sensor Design”.
1. Which of the following uses lens and mirrors for focusing?
a) Refractive
b) Reflecting
c) Catadioptric
d) Orthogonal
Answer: c
Clarification: Telescopes are of three types: refractive, which uses lenses as their primary focusing elements, reflective, which uses a curved mirror for focusing, and catadioptric, which mix mirror and lens systems.
2. What is the physical diameter of the first optical element in the telescope?
a) Optical aperture
b) Field of view
c) Effective focal length
d) Light detector
Answer: a
Clarification: Each design of a telescope is characterized by an optical aperture, an effective focal length, and a field of view. The optical aperture is the physical diameter of the first element in the telescope.
3. What is the image of the star at the focal plane called as?
a) Star fixed image
b) Celestial interference image
c) Diffraction limited image
d) Spot image
Answer: c
Clarification: The image of a star at the focal plane of the telescope with perfect, unobstructed optics is a bright spot surrounded by a series of concentric dark and light rings. This image is called a diffraction limited image.
4. The physical size of the star depends upon __________
a) Angular size of image
b) Intensity of image
c) Colour of image
d) Shape of image
Answer: a
Clarification: The physical size of the star image depends on the angular size of the image and the effective focal length of the telescope: were d is the physical diameter of the rings in the diffraction image.
5. The ratio of the focal length to aperture is called the __________
a) Critical aperture
b) Effective aperture
c) Focal number
d) f/ number
Answer: d
Clarification: The ratio of focal length to aperture is called the f/ number. For example, an f/10 telescope may have a 30 in. focal length and a 3 in. aperture.
6. Star signal power is directly proportional to an effective area of a telescope.
a) True
b) False
Answer: a
Clarification: The signal power of a star, as seen by a light detector at the focal plane of the telescope, is the effective area of the telescope entrance times the irradiance of the star.
7. Anti reflection coated optics reduce __________
a) Optical transmission losses
b) Reflection angle
c) Refraction
d) Intensity of star image
Answer: a
Clarification: For anti-reflection coated optics, the optical transmission losses may be 1% or 2% at each air to glass or glass to air optical control surface within the telescope, including windows or sensor covers or surfaces. For uncoated optics, the loss per surface is about 4% to 5%.
8. The star signal power does not depend on _____________
a) Spectral irradiance of stellar light
b) Spectral response of the light detector
c) Brightness of the star
d) Size of the star
Answer: d
Clarification: Star signal power is the effective area of the telescope times the irradiance of the star. It depends on the spectral irradiance of the stellar light, the spectral response of the light detector, and the brightness of the star.
9. The sky background power does not depend on _______
a) Effective entrance area to the telescope
b) Field of view of the telescope
c) Background radiance
d) Intensity of observing star
Answer: d
Clarification: The power from the sky background at the focal plane of a telescope depends on the background radiance, the effective entrance area, and the field of view of the telescope.
10. The effect of night sky background effect is negligible with respect to star power.
a) True
b) False
Answer: a
Clarification: The night sky radiance is so low that its effect may be ignored in comparison with star power. The day light sky, however, is extremely bright in comparison with a star. Typically, the sea level sky radiance, 45° from the sun on a clear day is about 2000ft-lamberts.
11. The ratio of the photons converted to signal generating electrons to the total number of photons arriving is called as?
a) Quantum efficiency
b) Photon ratio
c) Conversion ratio
d) Photons to electron ratio
Answer: a
Clarification: Only a certain number of photons arriving at the detector are converted to signal generating electrons. The quantum efficiency of the detector is defined as the ratio of the photons converted to signal generating electrons to the total number of photons arriving.
12. The photovoltaic or photoconductive detectors are packaged as ___________
a) Star detectors
b) Star position fixer
c) Charge coupled devices
d) Photon coupled devices
Answer: c
Clarification: The photovoltaic or photoconductive detectors are packaged as charge coupled devices. These can produce an array of small cells or pixels. CCD is a closely spaced monolithic array of metal insulator semiconductor capacitors.
13. The use of ______ allows a small sky background view with a large telescopic field view?
a) CCD
b) CVF
c) VVR
d) CSK
Answer: a
Clarification: The use of CCD arrays allows a small instantaneous field of view for sky background, for improved signal to noise ratio, while maintaining a large telescopic field of view.
14. The threshold level of star detection is a function of _________
a) Star brightness
b) Star position fix
c) Star size
d) Sky view
Answer: a
Clarification: The threshold level of star detection is a function of star brightness and noise variance and is performed on subsum of pixels of a size to contain star image.