Astronomy: Telescopes

If you�re thinking of buying a telescope, or just want to know how they work, then this thread is for you! Just pardon my poor image creation skills.

The Basics
All telescopes work in one of two systems. They are the horizon system and the equatorial system. The horizon system works in altitude and in azimuth, which constantly changes. The equatorial system works in declination and right ascension, which never changes.

Horizon System
Altitude: The position up/down with respect to the horizon.
Horizon = 0 degrees
Zenith (directly overhead) = 90 degrees
Azimuth: The position along the horizon from North
N = 0 deg. E = 90 deg. S = 180 deg. W = 270 deg.

When using the horizon system a good approximation of an object�s position in the sky is by using the width of your fist. The width should be approximately 10 degrees. Since the sky is on constant motion, all objects positions in altitude/azimuth are also constantly changing.

Equatorial System
Declination: North/South position from the Celestial Equator (line in the sky which corresponds with the Earth�s equator.) Rotates around the North and South Celestial Poles (directions in space of the Earth�s axis of rotation.)
CE = 0 deg.
NCP = +90 deg.
SCP = -90 deg.
Right Ascension: East/West time position of an object.
0 or 24 hours RA = Vernal equinox (March 21)
6 hours RA = Summer solstice (June 21)
12 hours RA = Autumnal equinox (September 21)
18 hours RA = Winter solstice (December 21)

There are 24 hours of RA, with 1 hour equaling 15 deg. of the sky.

Telescope Math and Types
First off, the main task of a telescope is to gather light, despite the popular idea that it is to magnify an image. In this section I�ll cover how telescopes work, using simple mathematical formulas (read: �don�t get scared off because of math!�), the three types of mounts, and most importantly the two types of telescopes and their differences.

Telescope Math
Light Gathering Power (LGP)
The LGP of a telescope is very simple to determine using this formula:
(Diameter of telescope)�
(Human eye aperture)�

The human eye aperture is pretty much constant at 4 inches. So, using that formula and the constant of the human eye, you can figure out how much more light any given telescope gathers than the human eye.

Example 1
A telescope with an 8 inch diameter.
8� = 64 = 4x more LGP
4� 16

Example 2
A telescope with a 20 inch diameter.
20� = 400 = 25x more LGP
4� 16

Magnifying Power (MP)
The MP of a telescope is also very easy to determine. It can be done by using this formula:
Focal length of telescope
Focal length of eyepiece

The shorter the focal length of the eyepiece the higher the magnification. For more clarity in the image, a limit of 50x MP per inch of diameter should be used. For example, a telescope with a 4 inch diameter should not have an MP greater than 200x. Also, as MP goes up, the field of view of the telescope goes down.

Example 1
A telescope with a focal length of 200 mm, using an eyepiece with a focal length of 2 mm.
200 = 100x MP
2

Example 2
A telescope with a focal length of 328 mm, using an eyepiece with a focal length of 8 mm.
328 = 41x MP
8

Telescope Types
There are two popular telescope types. They are the refractor, and the reflector. Each uses unique methods of gathering light, and each has their own distinct advantages and disadvantages.

Refractors
A refractor telescope is a basic design for a telescope, much like a spyglass you see in movies. There are two or three lenses at one end and an eyepiece at the other end. A refractor with two lenses is called achromatic, while a refractor with three lenses is called apochromatic. The difference between the two is the clarity of the image, with apochromatic being the clearest.

A diagram of a basic refractor telescope:

(1) Light enters telescope, (2) passes through set of lenses, (3) is focused down the tube, (4) focused light enters eyepiece for viewing.

The strengths of refractor telescopes are that they give highly detailed images, making them ideal to use for viewing planets and binary star systems. They also have a very long focal length, giving a higher possible magnification. On the downside they are fairly expensive and the large glass lenses are only supported on their edges. Also their large size keeps them immobile.

Reflectors
Reflector telescopes use a system of mirrors and corrector plates to focus the image and gather light. There are three main types of reflectors: Newtonian, Schmidt-Cassesgrain, and Schmidt-Maksutov.

A diagram of a Newtonian reflector:

(1) Light enters the telescope and travels to the (2) primary parabolic mirror, the light is then (3) focused and reflected towards the (4) secondary mirror, (5) the light then reflects out the side of the telescope to the (6) eyepiece.

A diagram of a Schmidt-Cassegrain reflector:

(1) Light enters the telescope, (2) passes through a thin color corrector plate and continues to the (3) primary parabolic mirror where the light is focused and (4) reflected forwards to the (5) secondary mirror which, in turn, (6) reflects the light towards the back and center of the telescope and (7) out the eyepiece.

A diagram of a Schmidt-Maksutov reflector:

(1) Light enters the telescope, (2) passes through a thick, parabolic color corrector plate and continues to the (3) primary parabolic mirror where the light is focused and (4) reflected forwards to the (5) secondary mirror which, in turn, (6) reflects the light towards the back and center of the telescope and (7) out the eyepiece.

The strengths of the reflector telescope are that it is compact (fitting more focal length in less space) and that their smaller size allows them to be transported from location to location with little effort. The main weakness is that the images are not as detailed as in a refractor.

Telescope Mounts
So what do these telescopes sit on? Well more than likely it�s one of three different types of mounts. These are the Dobsonian, German Equatorial, and the Yoke (or fork).

Dobsonian

This mount is fairly simple, only works with the horizon system, and is used mainly for Newtonian reflectors. The (1) telescope has a (2) joint that it is connected to. This joint allows the telescope to move in altitude. That whole assembly then sits on the (3) box-like mount. This mount swivels on its (4) base, allowing the telescope to move in azimuth.

German Equatorial

This mount is fairly simple to use and understand, and can be used on any kind of telescope. It is most commonly used for refractor telescopes, which keeps these types of mounts, for the most part, immobile. (1) The telescope sits on one end of a (2) long rod. On the other end of the rod is (3) a counterweight, used to balance the telescope. This entire assembly sits on top of (4) a tripod (for mobile use) or central pier (immobile.) Also, if you hadn�t guessed by the name, this type of mount utilizes the equatorial system.

Yoke

This mount can be used in both equatorial and horizon systems. (1) The telescope, like the Dobsonian, has (2) a connected joint. This joint allows the telescope to move in declination and in altitude. From this joint the telescope sits on (3) a pier, which connects to (4) the wedge. The wedge is the most important part of the telescope when using the equatorial system. This is because it is what you use to align the telescope. The wedges should be set to what degree of latitude you live at. For example, I live near the 42nd N, so the wedge of my telescope would be set at 42 degrees. This whole assembly then also sits upon (5) a tripod.

So Now What?
Well now that you know just about everything you would need to know in order to operate a telescope, you probably want to get out there and use it! Now once you have it set up you should start off by looking at the brighter objects in the sky. Why�s this? Because they�re the easiest to find! A good investment would be to purchase some simple star charts, a red-lensed flashlight (red lights don�t ruin night vision as badly as white lights), or even some software for a computer such as Starry Night. Also, don�t expect to see images like those taken by giant telescopes or the Hubble.

Observing the Moon
Despite popular belief, the best time to view the full moon is not while it is full. On the contrary the best time is actually while the moon is in a crescent stage. This is because the other stages are too bright to view finer details on the lunar surface. While looking at the moon it�s best to observe craters and canyons. Also, one of the neater things about viewing a crescent moon is that the crater walls and cliffs cast intricate shadows onto the moon�s surface.

Observe Saturn and Jupiter
When looking at Jupiter through even a small telescope the largest moons can be seen. They appear as bright stars to either side of the planet. Also, some bands of gas can be seen. Using a higher magnification more bands can be seen.

When looking at Saturn through a small telescope the rings can be seen. On lower magnifications the rings look like something sticking off of the planet itself. On higher magnifications the rings will separate from the planet, and on even higher magnifications divides in the rings can be seen.

Observe the Messier Objects
The Messier Objects are a collection of 110 star clusters, nebulas, and galaxies. A great list of them, including images and information can be found here: www.seds.org...

So now that you know what to do, go enjoy your night sky!

Recommended Reading
Using a Telescope
The Munich Archive Astromaps

[edit on 8/19/2004 by cmdrkeenkid]

Thanks again. I find it fun to browse the amateur telescope making websites. There is an amazing variety of Dobsonian designs alone. The open frame designs are particularly interesting to me. I am scrounging parts for a copyscope as my first project.

COPYSCOPE 1

COPYSCOPE 2

those copyscopes are really neat. if i had the time i would without a doubt make one. i'm not a big fan of the open frame dobs though... i donno why. i do have an 18 inch dob. it's a beauty!

Originally posted by cmdrkeenkid
those copyscopes are really neat. if i had the time i would without a doubt make one. i'm not a big fan of the open frame dobs though... i donno why. i do have an 18 inch dob. it's a beauty!

I am in awe of your monster dob!!

Originally posted by cmdrkeenkid
The human eye aperture is pretty much constant at 4 inches...

Big eyes... I have to admit.

Well, about those coordinate systems, it's best to forget them at first and learn to identify constellations/star groups and use them to find objects.
On normal telesopes coordinate scales are useless because size of them would have to be very big for enough accurasy.


Here's also very good page about telescopes and how they work.
science.howstuffworks.com...

Originally posted by E_T
Well, about those coordinate systems, it's best to forget them at first and learn to identify constellations/star groups and use them to find objects.

As usual you're on the ball with things and managed to figure out what the next installment into the series will be. I'm doing all 88 constellations plus the a great deal of the asterisms.

Hey, this is a great post! I've had a 4 inch refractor for a few years and did some amateur stargazing with it but never been able to really get the technical expertise I wanted. I look forward to your next installment and thanks to all who have shared their expertise as well.

For those who want to practise in advance, here's one free star chart program and some links.

www.stargazing.net...

www.geocities.com...
www.allthesky.com...

Way to go CKKid.!

Excellent, informative post..
It's really good to drill into first-time telescope buyers, that
It's not a MAG, It's the LGP!

So many folks are disappointed when they get the 350 power Sears telescope home...And they can't see crap.

Thanks for the link to the Sky Charts software E_T.

Originally posted by spacedoubt
Way to go CKKid.!

Excellent, informative post..
It's really good to drill into first-time telescope buyers, that
It's not a MAG, It's the LGP!

So many folks are disappointed when they get the 350 power Sears telescope home...And they can't see crap.

The fastest way for most folks to increase their Light Gathering Power is to quit smoking..........YOU WILL BE AMAZED!! Also don't skimp on the eyepieces.

groingrinder,

Really?
Smoking? Ironic for me, I smoke, but I have to do it outside.
Thats when I get my best observing time..!
I believe it though, it has been linked to retinal dengeneration.

a good thing is to not smoke around the scopes... the smoke gets on the mirrors or lenses of the scopes or eyepieces and creates a thin film. that film can really ruin images.

Hey Cmdr good thread so what Telescopes would you recomand to people starting out and are binocs any good?, Price wise from budget to affordible to top spec any tips would be good cheers Just like to know Im not getting ripped off here in the Uk if I wanted to buy one.

i would recommend going to an actual telescope or camera store. over here the two usually go hand in hand. going to stores like k-mart, walmart, or even run of the mill hobby shops the telescopes will be crap. also, don't let ANYONE sell you a telescope off the basis of how much magnification you get from it. ask about Light Gathering Power. that's the critical part.

as for starting out? something cheap, but still good. also, something that you ca move around easily and is low maintenance. a good pair of binoculars or perhaps a 10 in (254 mm) newtonian reflector on a dobsonian mount would be great.

if you jump right in and buy a telescope with all sorts of gizmos it takes a big bite out of the learning process, and also is a lot more tedious to set up and tear down.

so the for starting out more light power the better? and start out simple and build up from there.

Originally posted by SE7EN
so the for starting out more light power the better? and start out simple and build up from there.

for any telescope it's the LGP that matters. the more, the better. and defintely start out simple.

I have a 10 inch dob, a 16 inch dob, and a 6 inch refractor. As far as a good beginner scope, a 6 inch or 8 inch dobsonian reflector is the way to go in my opinion. The price is low for that big of a scope (250-400), the mount couldn't be easier, and with either one of those you will see many times more than with a refractor of equal cost. My refractor shows great detail, but doesnt pick up nearly the light of the 16 in dob. Both have their strengths, refractors are incredible for astrophotography, but that can cost a fortune.

The computer go-to systems are nice, but unless its GPS enabled you need to know the constellations and specific stars to line up the scope before it will take you to anything at all. My 6 inch reflector is a Meade LX-55, the computer is nice because it follows objects while taking pics and at astronomy club nights. Its nice not to adjust the scope every 5 seconds, especially at higher magnifications.

just my opinion based on my experience.

cmdrkeenkid- a 10 inch dob is pretty heavy and bulky, mine wieghs in at about 60 lbs. Although I dont mind it many people will end up leaving something 5 feet long plus the base to collect dust. The best size scope is the one you will use the most.

[edit on 23-8-2004 by who]

Originally posted by who
cmdrkeenkid- a 10 inch dob is pretty heavy and bulky, mine wieghs in at about 60 lbs. Although I dont mind it many people will end up leaving something 5 feet long plus the base to collect dust. The best size scope is the one you will use the most.

the 10 in dobs that my school uses are only about 30 pounds, plus the base... the newer scopes that don't use all the cardboard are a lot lighter.

my 18 inch dob weighs in at about 100 lbs, but it's still the one i use the most.

great thread cmdr!I have two questions for you.

1. have you ever seen an UFO through a telescope?
2. through a university grade telescope is it possible to see sattelites?