Question

1. Why can’t the Fermi Gamma-Ray Telescope use a mirror to focus light?

2. How do the NuStar and Chandra X-ray telescopes differ from optical telescopes?

3. Why doesn’t the Hubble Space Telescope need adaptive optics?

4. Why is the Hubble Space Telescope equipped with more infrared and ultraviolet detectors than are optical telescopes on the ground?

Answer 1

1. as fermi gamma ray telescope is meantfor sensing gamma rays, which have extremely short wavelenvgths and are transparent for passing throughh mirrors

they need diffraction gratings to be monitored

hence these diffraction gratings in the form of element crystals are used and not the conventional morrors

2. Nustar and chandra Xray telescopes are different from normal optical telescopes because

nlike visible light telescopes – which employ mirrors or lenses working with normal incidence – NuSTAR has to employ grazing incidence optics to be able to focus X-rays.

Cosmic X-ray sources are usually very weak and their detection, therefore, needs large area telescopes to gather light and sensitive detectors to enhance quantum efficiency. Conventional telescopes for visible light use refracting or reflective optics which is impractical for X-ray wavelengths because photon energies are greater than the binding energies of the typical atomic electrons leading to a refractive index for X-rays being less than unity. Thus single surface reflectivity for near-normal incidence is negligible for X-rays. However, by Snell’s Laws, total external reflection occurs and X-rays can be reflected from a surface up to a critical angle (usually about a degree for energies below 10 keV) given by cosine θ = n. This is known as the grazing angle. X-ray telescopes are made to exploit the grazing incidence from a set of co-axial and con-focal shells of paraboloidal and hyperboloidal mirrors.

this is the differnce between normal opr=tical telescopes and Nustar and Chandra X ray telescope

3. adaptive optics is needed to remove distortion due to spherical wavefronts on the telescope mirror. Now since hubble telescope is a large telescope and the wavefront is coming form a far distance, hence tghe apparent wavefront is plane rather than sphericla and hence the need of adaptive optics is not there

4. Hubble Space Telescope equipped with more infrared and ultraviolet detectors than are optical telescopes on the ground because infrared is absorbed by the atmosphere and ultraviolet is scatterd, hence the intensity of light reaching the earths surface in these spectra is very less as compared to what is recieved at the hubble space telescope. Hnece hubble has more detectors