Optics and Telescopes Ohio University - Lancaster Campus slide 1 of 71 Spring 2009 PSC 100 Credit: Credit:

Optics and Telescopes

Ohio University - Lancaster Campus slide 1 of 71Spring 2009 PSC 100

Credit: www.sherwoods-photo.com

Credit: www.telescopeguides.net


This evening we will investigate:

• how lenses and mirrors can be used to focus light and form an image.

• the 3 basic telescope designs and the advantages and disadvantages of each.

• some numbers that characterize a telescope: f-ratio, light gathering power, resolution, magnification

This evening we will investigate:

• recording the images produced by a telescope.

• telescopes that use the other wavelengths of light.


• Optics – The science of reflecting and/or refracting (bending) light so as to produce an image of an object. The image is usually recorded so that it can be studied more extensively.


• Regarding Mirrors– The Law of Reflection: When a ray of light

strikes a shiny or “specular” surface, the ray reflects away at the same angle at which it struck the surface. The angle of incidence equals the angle of reflection, as measured from a ‘normal’ to the surface.



i = r

a “shiny” or reflective surface

• If the reflecting

surface is curved

correctly, the light

can be focused to

a point, called the

focal point. An

image forms near

the focal point.


Credit: www.antonine-education.co.uk

• Regarding Lenses– The Law of Refraction: When light moves

from a less dense medium (empty space or air) to a denser medium (glass), the light slows down and bends INTO the denser medium.



speed of light in air =3 x 108 m/s

speed of light in glass =2 x 108 m/s

• Glass can be formed into a convex lens which will also focus light. An image forms near the focal point. The focal length is the distance from the centerline of the lens to the focal point.


focal length

• The f-ratio is a way to compare or rate convex (converging) lenses.

– The f-ratio is the focal length of the lens divided by the lens’ diameter.



Thicker lenses tend to focus closer to the lens and give brighter images. These are “fast” lenses.

Do these lenses have low or high f-ratios?

But these lenses have otherproblems.


Thinner lenses focus farther from the lens,give less-bright images, and are describedas “slow” lenses.

• When taking photographs of space objects, using a “fast” lens with a low

f-ratio means less time is needed for the photograph. This results in less blurring due to vibration of the telescope and the motion of the stars.


Credit: Gemini Observatory/AURA

• Chromatic Aberration – a problem with lenses– The edges of lenses act like prisms. They

split “white” light into all the colors of the rainbow.

– Problem: the different colors focus at different focal points. This means that if you focus the blue color of an object, the red is fuzzy, and vice versa.



Chromatic Aberration

• There’s always a trade-off in optics. The problem of chromatic aberration is worst with “fast” or low f-ratio lenses. These are the lenses we’d like to use most!

• The problem is fixed by making compound lenses out of 2 or more different kinds of glass.

• Mirror-based telescopes don’t have this problem – a definite advantage!


• 3 Types of Telescopes

• Refractors (gathers light with a lens)

• Reflectors (gathers light with a mirror)

• Mixed (uses a combination of lenses and mirrors)– Schmidt-Cassegrain Telescopes– Maksutov-Cassegrain Telescopes


• Refracting Telescopes– The “original” type, invented in the 1500’s and

first used by Galileo to explore space.– Sharpest, brightest images.– Lenses are heavy and expensive!– Prone to chromatic aberration.– Give an inverted (upside-down) image.– Can only be made up to about 40 inches in

diameter.



Credit: library.thinkquest.org

• Reflecting Telescopes…Advantages

– Mirrors are much cheaper to make than lenses, and are very light-weight, easy to carry.

– Mirrors can be VERY large. Multiple mirrors can be combined to simulate a single gigantic mirror.

– No chromatic aberration.


• Reflecting Telescopes…Disadvantages

• Not quite as sharp or bright an image as the same size refractor.

• Large scopes get currents of different temperature air inside their tubes. This can make images blurry.

• Mirrors will oxidize (corrode) over time.



• Combination ‘scopes…the Cassegrains

– Very short tube length, because the light gets “folded” back on itself twice. This makes the scope easy to handle & transport.

– Moderately expensive.– Best choice for amateur astrophotography,

because the tube doesn’t vibrate or shake very much.



The corrector plate is a type of lens. A secondary mirror is glued toits inner surface.

• The telescope mount is as important as the optics! There are two types…

• Altitude-Azimuth. Like aiming a tank. Point it in the compass direction (azimuth) you want, then point it up to the angle (altitude) you want.– Easy to use, but image rotates over time.



• Equatorial. Part of the mount is aimed at the north celestial pole. The mount then swivels east-west to follow an object through the sky.– Disadvantage: a real bear to use!– Advantage: the picture in the telescope

doesn’t appear to rotate over time.


• What is the function of a telescope? It’s not just to make the image bigger!

– Gathering light– Resolving details– Magnifying the image

• A Telescope is a Light Funnel

• Gathering light from dim objects is the MOST important function of a telescope.

• Which would you rather see, a large but very dim image or a smaller, but very bright

image?


• Light-gathering power (LGP)

– How much light can the human eye gather? A “typical” human eye has a pupil that is about 0.5 cm in diameter when fully dilated at night.

– Area of the pupil = r2 = (0.25 cm)2 = about 0.2 cm2.

– The main purpose of the telescope is to take light from a much larger area and “funnel” it into your pupil.


• How much light can a telescope gather?

• A 10 inch diameter scope (25 cm diameter) gathers (12.5cm)2 = 490 cm2.

• This is 490 cm2 / 0.2cm2 = almost 2500 times more light than the naked eye.


• To compare a telescope’s LGP to that of a “typical” eye, use the formula

LGP = 4D2

where D is the telescope’s lens/mirror diameter in centimeters. (2.54 cm/inch)

• What is the LGP of a 6 inch telescope?


• Seeing Small Details – Resolution

– Resolution is defined as the minimum angle between 2 objects, that will allow you to see them as 2 separate objects and not one big blob.

– Units are arcseconds (1/3600th of a degree)– The smaller the theoretical resolution number

is, the smaller the details you can see.


• Theoretical Resolution ()=(2.1x105)(wavelength in m) (diameter of objective mirror or lens)

• The diameter is in meters, not inches!

• What is the resolution of a 10 inch scope for blue light (450 nm or 4.5 x 10-7 meters)?

• Calculate the resolution again for red light (7.0 x 10-7 meters)


• Resolution – not the same for all light!

– What color of visible light would have the poorest resolution? The best?

– What “color” of all the types of light would have the poorest resolution? How is this limitation overcome?


• There’s a practical limit to resolution for a ground-based telescope…the Atmosphere!

• Air currents in the atmosphere will make the image blurry. Think twinkling stars!

• The best time for viewing is in the hours before dawn, since the air currents are least.

• Are there any other accommodations that could be made?


• Magnification – the least important function of a telescope

• M = focal length of the objective lens or mirror

focal length of eyepiece lens

• What is the magnification factor (power) of a telescope with a 1000 mm focal length, using an eyepiece with a 2.5 cm focal length?


• My 10 inch (25 cm) Schmidt-Cassegrain telescope has a 250 cm focal length. If I use an eyepiece with a 1.25 cm focal length, what is the magnification?

• If I want to increase the magnification, should I use a 2.5 cm focal length eyepiece, or a 0.75 cm focal length eyepiece?


• A bit of review:

• If you doubled the size of a telescope’s objective mirror without making any other changes, how would the telescope’s properties change?


• Why do astronomers no longer use film in their cameras?

• Film has been replaced by CCD chips (Charge-Coupled Device).



Credit: rst.gsfc.nasa.gov/Intro/ccd.jpg


The surface of a CCDchip is divided up intorows of rectangularlight-sensitive pixels(picture elements).

Films have irregularlyshaped and distributedgrains of light-sensitivechemicals.

The pixels are usuallymuch more sensitivethan the chemical grains.Advantage???

Film Emulsions

Credit: www.imx.nl/photosite/technical/Filmbasics/grainshapes.jpg



Light-sensitive layer(gives off electrons when struck by light)

Semi-conductor layer(acts as an electron filter)

Collector layer (holds the electrons until counted)

Individual pixel

This stack of 3 layers is one pixel.


CCD Detector

• 70% efficient

• Shorter exposures

• Resolution can be higher (8 Mpixels or higher)

Film

• 5% to 10% efficient

• 7 to 14 times longer exposures

• Resolution is limited by grain size

Why use CCD’s instead of film?

• Pictures are available in seconds.

• Pictures can be digitally added together.

• Initial cost is similar to film but operating costs are much lower.

• Pictures must be developed (hours to days)

• Digital techniques are possible, but more difficult.

• Operating costs higher.



Credit: solarsystem.nasa.gov/multimedia/gallery/PIA02888.jpg

A typical,high-resimageproducedby a CCD.

• All astrophotographs are black & white.

• Photographs can be taken in color, but you lose resolution.

• 4 pixels must be “binned” or clustered for color photographs (1 B&W, 1 red, 1 green, 1 blue) This makes the overall pixel size 4 times bigger = lower resolution.



1 big pixel if the photo is taken in color

4 smaller pixels if thephoto is taken in B/W.Better resolution.

• So how can we see all those beautiful “color” photographs?


NGC 2393 – The “Eskimo” NebulaCredit: Andrew Fruchter (STScI) et al., WFPC2, HST, NASA

http://antwrp.gsfc.nasa.gov/apod/image/0607/eskimo2_hst_big.jpg

• We take 4 pictures in succession then combine them into a single image:

– one photo through a red filter.– one photo through a green filter.– one photo through a blue filter.– one photo in B/W (often called a Luminance

filter) for overall brightness levels.



M57 Ring Nebula taken through red, green, and blue filters.Notice the different details which come out.Credit: Chris Brown, University of Manitoba

The compositecolor photo.

• Telescopes which “see” at other wavelengths than visible light.

• Not all objects are visible at optical wavelengths (400 – 700 nm).

• Many hot objects are only visible at shorter wavelengths (UV, X-rays, -rays)

• Many cool objects are only visible at longer wavelengths (IR, microwaves, radio waves)



• Radio Telescopes– Detect cool gases: H+ H H2

– Can detect molecules out in space:• oxygen O2,

• carbon dioxide CO2

• hydrogen cyanide HCN

• formaldehyde H2CO

• Ethanol CH3COOH

• Advantages & Problems

• Operate night or day

• Atmosphere doesn’t absorb radio waves

• Poorest resolution of any type of light (doesn’t see details well)

• Solution is to make antennas (dishes) VERY large



The Arecibo Radio Telescope, Puerto Rico.Credit: National Radio Astronomy Observatory


The Green Bank Telescope(GBT) in Green Bank, W.Va.

The largest steerable dish inthe world. As tall as theStatue of Liberty, the dishwould hold the buildingyou’re in.

Credit: National Radio Astronomy Observatory


The Very Large Array (VLA), Socorro, N.M.Credit: National Radio Astronomy Observatory

• Infrared Telescopes– Very similar to visible wavelength telescopes,

except for the detector, called a bolometer.– IR scopes detect heat from warm gas or warm

objects. “Warm” means not hot enough to glow in visible light.

– These scopes must be kept very cold or the heat that the ‘scope itself radiates will swamp out what is being observed.


• What kinds of objects do IR telescopes observe?

• IR telescopes “see” molecules & dust. In some cases, they can look through cooler dust to see what’s inside the dust clouds!

• Since stars form where there’s lots of dust, these ‘scopes are used for for looking inside dusty nebulas where new stars form.



Star-forming regions around Orion, in visible and IRCredit: Akira Fujii / NASA


The Spitzer Space Telescope,part of the “Great Telescope” SeriesCredit: NASA/JPL-Caltech


The Sombrero Galaxy(in Leo) in IR andin visible light.Credit: JPL / NASA (top)Credit: NASA/ESA andThe Hubble Heritage Team

STScI/AURA)

• Ultraviolet Telescopes

– Look for hot, young stars.– These stars help us better define star-forming

regions, which contributes to a better understanding of the evolution of our galaxy.

– They also look for hot, distant galaxies, as they looked in the early universe.

– What famous ‘scope is also a UV telescope?



GALEX Telescope (Galaxy Evolution Explorer)Credit: JPL / NASA


Galaxy NGC 300 in Sculptor Constellation,7 million light years away Credit: NASA/JPL-Caltech/Las Campanas

• X-ray and Gamma Ray Telescopes

• “See” very hot objects:– Black Holes– Pulsars & Neutron Stars– Supernovas

• VERY good resolution – great ability to observe fine details



Core of the Elliptical Galaxy NGC 4261 (accretiondisk of a black hole.) Credit: NASA/ESA and The Hubble Heritage Team STScI/AURA)


Cassiopeia A - the remnant of a supernova whichexploded about 300 years ago.Credit: X-ray: NASA/CXC/SAO; Optical: NASA/STScI; Infrared: NASA/JPL-Caltech


The Chandra X-ray Telescope, part of the ‘Great Telescopes’series. Credit: chandra.nasa.gov (artist’s conception)


A gamma ray burst beginning.Credit: NASA (artist’s conception)

The GLAST ( Gamma-ray Large Area SpaceTelescope, renamed FERMI )Credit: General Dynamics for NASA (artist’s conception)

Documents

Optics and Telescopes Ohio University - Lancaster Campus slide 1 of 71 Spring 2009 PSC 100 Credit: Credit: