Introduction to Astronomy · Star Ware: The Amateur Astronomer’s Guide to Choosing, Buying, and...

Introduction to Astronomy

Part 1 — Eyes on the Sky

Dr Adrian Jannetta FRAS

Northumberland Astronomical Society

Dr Adrian Jannetta FRAS Introduction to Astronomy

Objectives

The three main tasks of any telescope.

Telescope types

Telescope mounts

Eyepieces, focal ratio and magnification.

Filters and other accessories.

Considerations when buying...

Binoculars for astronomy


Light collection

One of the primary functions of a telescope is to collect light

from faint sources. Larger apertures collect more light. This

means fainter stars can be seen with bigger telescopes!


Magnification

Telescopes are designed to magnify the apparent size of distant

objects.

The magnification of a telescope can be varied by changing the

eyepiece used.


Resolution

For small objects such as planets, or small craters on the moon

then the resolving power of a telescope is just as important as

the magnification.

Larger aperture telescopes are able to resolve finer detail on

astronomical objects.


Refracting telescope

Refractors use an objective lens to collect and focus light.

Single lenses suffer severe optical distortions and so astronomicalrefractors employ two or more lenses to improve the quality of theimage.

Glass lenses are expensive — especially for larger apertures. Largelenses are also heavy and difficult to support so most refractorsavailable to amateur astronomers have lens diameters of between 2inches (50mm) and 5 inches (130mm).


Chromatic aberration

White light is a mixture of colours. Lenses bend the colours bydifferent amounts, bringing them to focus at different places.

This chromatic (colour)aberration can create colourfulfringes around bright objects.

It is usually minimised byconstructing the objective lensfrom two or more components.


Achromatic and Apochromatic

An achromatic doublet focuses blue and red light to the same place.This is accomplished by using component glass with differentrefractive properties.

An apochromatic triplet focuses red, green and blue colours to thesame place. Three different glass lenses with varying refractiveproperties achieve this.A telescope with this lens system is referred toas an “apo”.


Reflecting telescope

Reflectors use a primary mirror to gather and focus light. The secondarymirror directs the light out of the side of the tube near the top.

Mirrors are cheaper than lenses of the same size and large mirrors are easilyconstructed and supported, but there is some loss in contrast because thesecondary casts a shadow on the primary.

This telescope was designed by Sir Isaac Newton and they are commonlycalled Newtonians.


Aligning the optics: collimation

The primary and secondary mirrors must be precisely aligned withina reflecting telescope. The process of making adjustments is calledcollimation.

Collimating a reflecting telescope is necessary to obtain the bestpossible view through it.

Laser collimators are useful tools for quickly aligning the primary andsecondary mirrors.


Catadioptric (compound) telescope

This design uses lenses and mirrors to collect and focus the light. Thecorrector plate is a lens which compensates for distortions introduced bythe primary mirror. The tube is short because the light path is folded by asecondary mirror on the correcting plate.

The diagram shows a design called Schmidt-Cassegrain. Another commonvariation on this theme is the Maksutov-Cassegrain.

Collimation may sometimes be necessary for this type of telescope.


Comparison of telescope designs

Refractor

Bright, high contrast images (so great views of moon andplanets).Few collimation issues (fixed components).Small and portable.Large apertures are expensive.

Reflector

Large apertures are relatively cheapGood views of faint objects (nebulas, galaxies, deepskyobjects)May require frequent collimation if transported.

Catadioptric

Short tubes because of folded light pathSome collimation issues if portable.Closed tube reduces air currents (improves seeing).


Telescope mounts: alt-az

Alt-az mounts are the simplest telescope mounts.

Advantages:

Cheap and simple to operate.

Telescope moves up/down(altitude) and left/right(azimuth).

Reflecting telescopes using thismount are known as Dobsonians

Disadvantages:

Unable to follow the thenatural motion of the stars asthe Earth turns.

Imaging is restricted to shortexposures; the telescope doesnot rotate to compensate forthe motion of objects in thesky.


Telescope mounts: GoTo

GoTo mounts are a type of motorised Alt-az mount controlled by anonboard computer.

Advantages:

Intitial alignment procedure issimple (1,2 or 3 stars).

Some models use GPS forlocation.

Huge database of celestialobjects to choose from.

More time looking - less timesearching.

Object tracking. Imaging maybe possible (but field rotationis a problem like with Alt-az).

Disadvantages:

Less motivation to learn howto find objects unassisted.


Telescope mounts: Equatorial

Equatorial mounts come in two basic types: German and Fork designs.They both work on the same principles.

Advantages of an equatorial mount:

Follows the natural movementof stars.

Easy to motorise for imaging.

Setting circles allow thepositions of objects to be foundvia their Right Ascension andDeclination coordinates.

Disadvantages:

Good quality equatorialmounts can be expensive.

Takes longer to set up — mustbe polar-aligned before use.


Finderscopes and red-dot finders

Small telescope.

Image is upside down andleft-right inverted.

Magnification: 6× 30, or8× 50 typically.

Faint objects are located andcentred in the finderscope.

Places a faint red marker inyour field of vision.

View of the sky is notmagnified

Not upside down, not inverted!

Point the red dot at thelocation of the object; itshould then be visible throughthe telescope.


Telescope focal ratio

The focal ratio is a useful number for thinking about the type of objects thetelescope will excel at viewing.

Larger f-numbers are more capable of producing higher magnifications;better views of small objects like planets.

Small f-numbers give bigger telescopic fields of view; ideal for large objectssuch as nebulas, star clusters and galaxies.

The focal ratio of a telescope is defined by:

Focal ratio =Focal length

Aperture

The same units must be used for the two measurements on the RHS!


Eyepieces

Change the eyepiece to adjust the magnification of a telescope.

Standard fit (1.25 inch or 2 inch barrel).

Special coatings to reduce internal reflections.

Zoom eyepieces are popular. Choice may depend f-ratio of ‘scope.

Useful magnification: about 50x per inch of aperture...

Barlow lenses work with existing eyepieces to double or treble themagnification.

There are a huge variety of different eyepiece designs! Seewww.astrosurf.com/re/eyepieces_data.jpg for a summary.


www.astrosurf.com/re/eyepieces_data.jpg

Magnification

Some formulas for calculating telescope magnification:

Magnification =Telescope focal length

Eyepiece focal length=

(f-number)× Aperture

Eyepiece focal length

For example, consider a 9mm eyepiece used with the following telescopes:

1 Celestron NexStar 102 SLT. Focal length 660mm.

Magnification =660

9≈ 73×

2 Meade Lightbridge. Aperture 16 inches (406 mm), f/4.5.

Magnification =4.5× 406

9= 203 ×

3 Meade LX200. Aperture 14 inches (356 mm), f/10

Magnification =10× 356

9≈ 284×

The magnification of a given eyepiece will vary with the telescope.


Planetary (colour) filters

Colour filters are mostly used toenhance the colour contrastdifferences between surfacefeatures on the moon or planets.The filters screw into a standardeyepiece barrel.

The contrast differences are subtle and some observers report

little or no observable differences. It may take patience and

experience to recognise the differences.


Broadband and Narrowband filters

UHC Ultra High Contrast filter. Increases the contrast ofplanetary and emission nebulas.

OIII Oxygen-III filter. Similar to UHC, but higher contrast oncertain nebulas.

Hβ Hydrogen Beta filter passes a particular colour of hydrogenlight found in certain nebulas.

Polariser Darkening the moon and enhancing contrast on the planets.

IR pass Infrared pass filter is useful for getting steady views of theplanets because IR is less disturbed by the atmosphere.

Light pollution A filter to increase contrast of objects when observing fromurban areas. Darkens the sky by blocking street-lightswhilst passing light from other colours.


Solar filters

Home made white-light solarfilters can be constructed at fairlylow cost.

The filter reduces the amount ofsunlight entering the telescope bya factor of about 100,000.

This type of filter allowssunspots, eclipses and transits ofMercury and Venus to be viewedsafely.


Spectroscopy

Spectroscopy is the analysis of light.

Light can be separated into aspectrum of colours.

The presence of bright or darkbands can provide informationabout an object - such as thecomposition - that cannot be seenthrough a telescope.

The following image shows thespectrum of the star Betelgeuse inOrion.

It was produced using a diffractiongrating (Star Analyser 100) andwebcam on a small telescope.


Binocular Astronomy

A binocular is a pair of refractor telescopes for each eye.

The light path is folded using prisms. The light is reflected andrefracted many times so glass surfaces should be fully coated toprevent internal reflections and loss of contrast.


Binocular considerations

Binoculars are usually described by a pair of numbers:

15× 50Here, the magnification is 15× and the objectives are 50mm in diameter.

Great views of the moon,Milky Way, star clusters,nebulas, galaxies and brightcomets.

Planets and bright asteroidscan be seen.

Useful aid for aligning atelescope.

Choose binoculars that arecomfortable to hold for prolongedperiods. Large binoculars may needto be tripod mounted for comfort.


Further reading

P S Harrington.

Star Ware: The Amateur Astronomer’s Guide to Choosing, Buying, and

Using Telescopes and Accessories.

Wiley, 4th edition, 2007.

C R Kitchen.

Telescopes and Techniques: an Introduction to Practical Astronomy.

Patrick Moore’s Practical Astronomy Series. Springer, 2nd edition, 2003.

P C Moore.

Exploring the Night Sky with Binoculars.

Cambridge University Press, 4th edition, 2000.

R Scagell.

Stargazing with a Telescope.

Philips, 2004.

R B Thompson and B R Thompson.

Astronomy Hacks: Tips and Tools for Observing the Night Sky.

O’Reilly, 2005.


Introduction to Astronomy · Star Ware: The Amateur Astronomer’s Guide to Choosing, Buying, and...

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