Astrophysics

Why astronomical telescope should have objective lens of long focal length

Long focal length objects lens projects a larger real image

Why astronomical telescope should have Eyepiece with a short focal length

Magnifies image from objective lens more powerfully; overall magnification is f0/fe

Magnification for length

Fo/Fe

Magnification for angle

M= angle image / angle object

Chromatic Aberration

RI of material varies with wavelength so lens has diff focal length for light of diff wavelength. Results in blurred image

Spherical Aberration

blurring of images because light passes through diff parts of lens is brought to focus at slightly diff position

Rayleigh Criterion

2 point object can just be resolved when central max of diffraction pattern of one object coincides with first minimum of diffraction pattern of the other

Telescope 90-120nm. Why this used in space.

One advantage of this compared to one with visible light

Telescope detects UV wavelength shorter than visible light, absorbed by atmosphere so located in space.

Higher resolution as angle=wavelength/d. Smaller min angle for same distance