Telescope Optics 101-Part1

8/7/2019 Telescope Optics 101-Part1

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Part of what we need to know as as-tronomers is how our telescopes

work. This allows us to take full ad-vantage of the capabilities they pro-vide and that’s what I’m going to bediscussing in this series of articles.

This first part will deal with how ourtelescopes’ characteristics are de-fined and what they do for us.

Some Definitions

These definitions are mainly forthose of us just entering into thehobby so feel free to skim as you

please.Optical Element

Any optical component that lightpasses through or reflects fromwithin the telescope.

Optical System

All optical elements within the tele-scope.

Optical Path or Light Path

The path that light takes through theoptical system.

Optical Tube or simply, Tube

The tube to which the optical ele-ments are mounted.

Optical Tube Assembly or OTA

The optical tube with the assembledoptical elements.

Objective Lens

The first lens in the light path of arefracting telescope.

Primary Mirror or Objective Mirror

The main mirror of a reflecting tele-scope.

Secondary Mirror

The second and subsequent mirrorsin a reflecting telescope. Most haveone secondary mirror.

Corrector Plate

A lens on catadioptric telescopes

(Schmidt-Cassegrains, Maksutov-Cassegrains, etc.) that corrects the

light path prior to reflecting off theprimary mirror.

Aperture

The diameter of the objective lens orprimary mirror. Aperture determinesthe amount of light that can be gath-ered by the optical system.

Prime Focus

The point along the light path wherethe image comes to focus. More onthis later.

Focal Length

The distance traveled by the lightpath within the optical system. Focallength determines magnification.Longer lengths have a greater mag-nification than shorter lengths.

For reflectors, the focal length is thedistance light travels from the pri-mary mirror to prime focus. For re-fractors it’s the distance from the objective lens to prime focus.

Focal Ratio

The ratio of focal length versus aper-ture. The focal ratio determines thespeed of the optical system. Lowerratios are faster and higher ratios areslower . Focal ratios are designatedas f/ and the ratio number, an f/stopnumber. This is a carryover from thephotography world. Faster f/stopsexpose film faster because there’smore light being focused on the film.

My 10” Schmidt has an aperture of10 inches or 254mm (there are 25.4mm in an inch) and a focal length of2,500mm. So the focal ratio is:

2500 ÷ 254 = 9.8

Around 10, thus, my scope is desig-nated as a 2500 mm, f/10 telescope.

Aberration

Any distortion of the light path thattakes place within the optical system.There are many different kinds.

Focal Plane

The plane perpendicular to the opti-cal path that passes through primefocus.

Field of View or FOV

The diameter of the field visiblethrough the optical system. This isusually expressed in arc-degrees,minutes and seconds.

Collimation Error

The angular error between either theprimary and secondary mirrors of areflecting telescope or between the

objective and secondary lenses of arefractor telescope.

Figure 1 illustrates collimation error.The planes shown are seen edge on.

The collimated plane is the focalplane that is properly collimated.Because this plane is perpendicularto the optical axis the entire planecan be positioned equidistantly alongthe optical axis.

The collimation error plane is a grosscollimation error. Because this planeis not perpendicular to the axis, it’s

impossible to bring the entire planeto focus along the axis. The imagewill be out of focus across the entireFOV.

Diffraction

Diffraction is the bending of a waveof light when it travels around an ob-stacle.

Refraction

The bending of a wave of light in-

Optics 101 - How Our Telescopes Work Part 1: The Basics

By Dan Lessmann

Figure 1: Collimation Error



the primary to prime focus. Thisallows for a longer focal length thanNewts and refractors in a shorteroptical tube.

These designs are more expensivethan Newts. But, inch for inch, theseOTAs are the biggest focal lengthbang for the tube size.

Binoculars

Every amateur astronomer ought to

have a good pair of binoculars andthe most common is that shown orthe Porro-Prism binocular.

Light entering through the objectiveis directed through a set of prismsthat act as mirrors bending the lightpath and increasing the focal length.

Binoculars are rated by their magnifi-cation and the size of their objectivelenses. For instance, a pair of 7x35

primary mirror to a secondary ori-ented at 45° to the primary mirrorwhich redirects the light path outthrough the side of the OTA to thefocuser and eyepeice.

The greatest advantage of Newts istheir low cost. You can buy moreaperture and focal length for lessmoney in this design than any other.

Schmidt or Maksutov-Cassegrain

In these designs, light passes firstthrough a corrector plate. The onlymajor difference in these is theshape of the corrector plate. The“Mak” has a meniscus correctorplate. The Schmidt or “SCT” has aflatter shape. The corrector platecorrects spherical aberration.

Light enters through the correctorplate, reflects off a primary and thena secondary mirror through a hole in

ward towards the optical axis. Thisis done by any boundary the lightpasses through that is angled to theaxis. This angle is the angle of inci- dence .

Point Source of Light

A light source that is so small, or sofar away that the visible size of the

light source is an infinitesimally smallpoint. Light waves from such objectsare parallel as they enter our tele-scopes.

Types of Telescopes

Most telescopes fall into one of twocategories, refractors and reflectors.I’ll describe the more common oneshere. Please refer to the diagrams inFigure 2.

Refractors

Refractors have a straight-throughlight path. Light passes through anobjective lens where it is refractedtowards a focal point, noted as F.

At the same focal length, refractortubes are longer than reflectors asthe light path is not reflected withinthe tube so high power refractors arenot terribly portable.

However, most spotting scopes and just about all view finders are refrac-tors and medium sized refractors areoften mounted on larger scopes forastrophotography saving the cost ofa second mount.

Medium to large refractors are someof the most expensive telescopesbecause of the number of opticalelements required but they providesome of the highest quality images.Apochromatic designs are especiallypopular for astrophotography.

Reflectors

All of the other telescopes in the fig-ure are reflectors. Their greatestadvantage is that they reflect thelight within the tube resulting in alonger focal length than from a re-fractor of the same tube size.

Newtonian Reflector

These are by far the most common.In a “Newt”, light is reflected off a

Figure 2: Primary Telescope Types

Refractor

Newtonian Reflector

Schmidt-Cassegrain

Maksutov-Cassegrain

Porro-Prism Binoculars



sport glasses has 7X magnificationwith a 35mm objective lens size.

Keep in mind that there are two objective lenses and we gather lightthrough both eyes with binoculars.Thus the resolving power of 7x35binoculars is far better than that ob-tained from a 35mm refractor at thesame focal length.

How Telescopes Focus

All optical systems are made up of aprimary and secondary system. Theprimary system is all elements thatare forward of the primary focus andthe secondary system is all elementsbehind the primary focus. The pri-mary and secondary can be thoughtof as two separate OTAs.

Figure 3 shows an optical systemthat is out of focus.

The OTA prime focus is that of thescope itself and the secondary primefocus is for the eyepiece and otherelements of the secondary system.

We can see that the focal planes ofthese two systems are not aligned sothe resulting image is out of focus.

Figure 4 shows the correction neces-sary to bring the image into focus.

The focuser moved the secondaryforward some distance to align theprimary and secondary focal planes.

Note that these figures are simplified.In reality, prime focus is not locatedat the point where the light pathscross the optical axis. This point iscalled the locus and the image at thispoint would be a single point of light.

There are actually two locationsalong the axis where prime focus canbe achieved; one in front of the lo-cus, one behind and both equidistantfrom the locus. The one used bymost telescopes is the one behind.Incidentally, this is why our tele-scopes flip the image.

Prime focus is also not located at adiscrete point along the axis. It’sactually a range of points known asthe critical focus zone or CFZ .

Focusing mechanisms vary in de-sign. Most Newts and refractorsmove the secondary system with a

rack and pinion focuser. Most cata-dioptrics move the primary mirrorforward and back within the opticaltube. This moves the primary sys-tem’s prime focus. Regardless, bothmethods do the same thing. Theyalign the focal planes of the primaryand secondary systems.

Typically photography is done atprime focus meaning that the onlyelement in the secondary system isthe film or CCD chip itself. So thegoal is to position the plane of the

film or chip at the prime focus (withinthe CFZ) of the primary system.

Changing the Aperture

An increase in aperture increaseslight gathering exponentially. This isbecause we’re dealing with the areaof the circle defined by the aperture.

The area of a circle is defined as:

A = pr 2

Okay, back to my 10” scope. It’s

area is:A = 3.14 * (10÷2)2

Or around 78.5 in2

What’s the area of a 12” scope?

A = 3.14 * (12÷2)2

Or around 113 in2

That’s an increase of 44% for anincrease in aperture of only 20%.

So, barring practical matters ofportability and price, aperture is kingwhen selecting a scope.

Let’s see what the change in speedwould be. Assuming the same focallength of 2,500mm, the speed of this12” scope would be…

2500 ÷ (12 * 25.4) = f/8.2

About 20% faster than my f/10 scopewhich means brighter, higher resolu-tion images for our peepers to see.

Now suppose I cut a 2” hole in card-board and tape it to the front of my10” telescope. What have I done?I’ve reduced the aperture from 10” to2”. Changing aperture means I’vechanged the focal ratio from f/10 tof/49 or slowed the scope way down.

This is what most large scope solarfilters do. When viewing the sun,

brightness is not a problem. No, it’sa huge problem. The sun is too bright. By reducing the aperture andslowing down the scope, and using a solar filter over the smaller aperture ,I can safely view the sun.

Please! If you choose to view thesun, you MUST use a high quality solar filter designed for your scope! Also remove any finder scopesmounted to your scope or you mayburn a hole through your foreheadwhile enjoying your view of the sun.

Remember! Only you can preventforehead fires! (Sorry Smokey)

When changing the aperture, I canonly go smaller unless I get a biggerscope and when I go smaller, I’ll slowdown the optical system if the focallength remains the same.

Changing the Focal Length

Focal length controls power and theratio of this and aperture is the focalratio. Does this mean two tele-

scopes that have the same focal ra-tio will have the same performance?

Nope. A scope with a 1” apertureand a 250mm focal length has thesame f/10 focal ratio as my Schmidt.But we won’t be able to see thesame things with both scopes.

It’s important to remember that focalratio is merely a ratio of two charac-teristics. Its these characteristics

Figure 3: Out of Focus Optical System

Figure 4: In Focus Optical System



that determine performance, not theratio between them. What we’re in-terested in is how we can change thecharacteristics to produce differentresults.

Okay. What about changing focallength? Though you may not knowit, you adjust focal length all the time.Any change of the magnifying opticalelements in the system will changethe focal length. When you use ahigher power eyepiece, you increasethe focal length and slow down thefocal ratio. That is, you increasemagnification and decrease resolu-tion. When you use a lower powereyepiece, you speed up the focalratio or, you decrease magnificationand increase resolution.

Focal Ratio Controls Field of View

Here’s a simple experiment. Nab onto a paper towel roll and cut a 1” sec-tion off of one end. You now havetwo optical tubes. One is 1” long, theother about 10” long and both about1 ? ” in diameter.

Look through the 10” tube and centersome object across the room. Nowlook around the edge of the field andnote a few other objects that arenear this edge.

Now do the same thing with the 1”tube and note where the same ob-

jects are in this FOV.

The 1” tube has a much wider field ofview than the 10” tube. This is dueto the difference in focal ratios of thetwo tubes.

The 10” long tube has a focal lengthof 10” and a 1.625” aperture so thefocal ratio is:

10 ÷ 1.625 = f/6.2

For the 1” long tube:

1 ÷ 1.625 = f/0.6

What aperture would a 10” long tubeneed to have for the same FOV asthe 1” long tube? The focal ratio ofthis tube has to be f/0.6 so work itbackward:

10 ÷ ? = f/0.6

-or-

? = 10 ÷ 0.6 = 16.6”

A 10” long, 16.6” diameter tube willhave the same FOV as the 1” longtube. Optical systems of the samefocal ratio will have exactly the samefield of view at prime focus.

So, while we can’t see the samethings with that little 1” aperture,250mm long scope in the previousexample, it would have the sameFOV as my 10” Schmidt when atprime focus.

I should also note that other thingscan reduce the FOV. The field ofview allowed by the focal ratio atprime focus is the greatest field of view I can achieve with this tube.

So Ends Part One

I think that’s enough for this monthbut let’s review a few key points.

We’ve leaned a bunch of terms usedto describe an optical system and itscomponents.

We’ve reviewed the major amateurtelescope types and their character-istics.

We’ve learned how a telescope isfocused and that focus is achievedwhen the focal planes, or the CFZs,of the primary and secondary sys-tems are aligned.

We’ve leaned that all telescopes

have some common characteristics:Aperture

The diameter of the objective lens orprimary mirror. Aperture determineslight gathering capacity. We’velearned there’s not much we can doto increase aperture but we can de-crease it with a simple mask.

We’ve learned that, because wecan’t increase aperture, this charac-teristic is probably the most impor-tant one when choosing a scope as

long as we can afford it and can stilltote the thing around.

Focal Length

The distance light travels from theobjective element to prime focus.Focal length determines magnifica-tion power. We’ve learned that wechange focal length with any changein the optical elements that affectmagnification.

Focal Ratio

The ratio of focal length versus aper-ture. Focal ratio is the speed of theoptical system. Larger focal ratiosare slower speed and smaller focalratios are faster speed and they aredesignated by an f/stop number likef/10.

We’ve learned that faster focal ratiosare at lower power and higher reso-lution and brightness and that slowerfocal ratios are at higher power andlower resolutions and brightness withthe same OTA.

We’ve learned that focal ratio deter-mines the maximum field of view forthe system.

Next month we’re going to look at thedifferent methods and equipment wecan use to alter these characteristicsand what capabilities can be gainedby doing so.

Until then… Clear Skies!

Telescope Optics 101-Part1

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