1.5 - telescopes and technology

what is collecting power?

• a measure of the amount of light energy (incident EM radiation) a telescope collects per second

• i.e., power per unit area

• i.e., intensity of the incident radiation collected

what exactly is the light energy the collecting power measures?

incident EM radiation

what is collecting power equivalent to?

• power per unit area

• intensity of the incident radiation collected

why is collecting power equivalent to the power per unit area?

do later

why is collecting power equivalent to the intensity of the incident radiation collected?

do later

collecting power and luminosity

do later

what is the amount of light energy a telescope collects per second known as?

the collecting power

what is collecting power directly proportional to?

collecting power ∝ D² of aperture on objective lens

collecting power is proportional to the diameter (squared) of what?

the aperture on the objective lens

if collecting power ∝ D², then what else is it proportional to?

surface area of objective lens, πD² / 4

why is collecting power ∝ πD² / 4 ?

• collecting power ∝ D²

• collecting power is a measure of the amount of light energy a telescope collects per second, i.e., luminosity

• luminosity ∝ surface area

• in surface area equation, πD² / 4, D² is the only variable value as π and 4 are constants (so they can be ignored)

• therefore, collecting power ∝ πD² / 4

total power of detected light = ?

total power of detected light = intensity from source x gathering area what do that mean

what does collecting power determine?

the brightness of the image

why does collecting power determine the brightness of an image?

because collecting power is a measure of the amount of light energy a telescope collects per second

what does collecting power increase with?

• increased diameter (squared) of the objective lens

• which comes from a larger telescope

what does brightness increase with?

• increased collecting power

• which comes from an increased diameter (squared) of the objective lens

• which comes from a larger telescope

how do we get a brighter image?

• increase the size of the telescope

• this will increase the diameter of the objective lens

• which will therefore increase the D²

• collecting power ∝ D²

• therefore collecting power increases

• collecting power = amount of light received by telescope,

• therefore an increased collecting power means a brighter image

why does a large telescope mean the image is brighter?

a larger telescope means the diameter of the objective lens is larger. as collecting power ∝ D², and collecting power determines brightness, a larger objective lens diameter means a larger value for collecting power, meaning the image is brighter

why does a larger diameter (of the objective lens) mean the image is brighter?

because collecting power ∝ D² of the objective lens

what does resolving power determine?

the resolution of an image (i.e., how clear it is)

what is the difference between resolving power and collecting power?

• resolving power determines how clear an image is

• collecting power determines how bright an image is

which one determines how clear an image is - resolving or collecting power?

resolving power

which one determines how bright an image is - resolving or collecting power?

collecting power

why does a larger telescope produce an overall better image?

resolving power and collecting power both increase with increased D of the objective lens (as minimum angle separation = λ / D, and collecting power ∝ D²). this means a larger telescope produces a clearer and brighter image

what is the resolving power?

the telescope’s ability to produce separate images of near objects

what must happen for the image to be resolved?

the angles between straight ray lines must be, at least, the minimum angular resolution (θ)

what is minimum angular resolution (θ) measured in?

radians (rad)

what does resolving power depend on?

the minimum angular separation, θ = λ / D of the straight light rays of the object. the smaller the minimum angular separation, the greater the resolving power

why does resolving power increase with decreasing angular separation?

• the resolving power is the telescope’s ability to produce separate images of near objects

• a smaller minimum angular separation means the smaller the minimum distance between two lines or points can be and still be distinguished

• therefore, the smaller the minimum angular separation the greater the resolving power

why does resolving power increase with a greater diameter of the objective lens?

• resolving power depends on minimum angular separation; the smaller the angular separation, the greater the resolving power

• θ = λ / D

• when the diameter of the object lens, the minimum angular separation gets smaller, because when dividing by a larger denominator the result gets smaller

• therefore minimum angular separation decreases with increasing diameter of the objective lens, therefore resolving power increases with increasing objective lens diameter

what is an optical telescope?

a telescope that detects wavelengths of light from the visible part of the EM spectrum (i.e., a telescope that detects visible light)

what does an optical telescope detect?

visible light

how does an optical telescope ensure it only detects visible light?

by only detecting wavelengths of light in the visible light part of the EM spectrum

what is a non-optical telescope?

what does a non-optical telescope detect?

how does a non-optical telescope ensure it only detects light that isn’t visible?

DO THESE OPTICAL VS NON OPTICAL LATER

when comparing optical and non-optical telescopes, which factors can we consider?

• structure

• position

• uses

what is the location of infrared telescopes?

• mostly space

• sometimes ground-based

what is the wavelength range of infrared telescopes?

700nm - 1nm

what is the resolution of infrared telescopes?

• ground-based = 10^-6 rad

• space = 10^-7 rad

what is the resolution of a ground-based infrared telescope?

10^-6 rad

what is the resolution of a space-based infrared telescope?

10^-7 rad

what are the structural similarities between infrared and optical telescopes?

they both have a primary concave mirror and a secondary convex mirrors

DIAGRAM

what are the structural differences between infrared and optical telescopes?

the mirrors in an infrared telescope must be kept very cold to avoid interferences from the surrounding heat

what are the positional differences between infrared and optical telescopes?

which telescope must have it’s mirrors kept very cold?

infrared telescopes

why must infrared telescopes have their mirrors kept very cold?

to avoid interferences from the surrounding heat

why would the surrounding heat interfere with an infrared telescope?

what is the structural composition of an optical telescope?

primary concave mirror and secondary convex mirror

DIAGRAM

what is the structural composition of an infrared telescope?

primary concave mirror and secondary convex mirror

DIAGRAM

in an optical telescope, is the primary mirror convex or concave?

concave

in an optical telescope, is the secondary mirror convex or concave?

convex

in an infrared telescope, is the primary mirror convex or concave?

concave

in an infrared telescope, is the secondary mirror convex or concave?

convex

which non-optical telescope has the same structural composition as an optical telescope?

infrared telescope - with a primary concave mirror and a secondary convex mirror, except mirrors in an infrared telescope must be kept at much lower temperatures

which telescope type has a wavelength range of 700nm - 1nm?

infrared telescopes

which telescope type has a resolution of 10^-6 rad?

ground-based infrared telescopes

which telescope has a resolution of 10^-7 rad?

space-based infrared telescopes

resolutions of telescopes;

TABLE HERE

wavelength ranges of telescopes;

TABLE HERE

which telescopes are based in space and which are ground-based?

what does ground-based mean?

what is resolution measured in?

radians (rad)