Telescope – Beginners Guide: Reflector vs. Refractor

There are two basic types of amateur optical telescopes, reflectors and refractors. The  difference is in how they collect the light:  reflectors use a mirror, and refractors use a lens.

Even if you can’t look inside to see how the light is collected, you can usually tell them apart. In refractors, the light must travel all the way through the tube, so refractors are long for their width.  Since reflectors use a mirror, the light travels back and forth in the tub, so most reflectors are short for their width.

Basic Structure

Refractors

Refractors are generally long and narrow, with the wide objective at the front and small eyepiece at the back

Refractors always have a large lens at the front. You point this lens at the object you want to view, so it is called the objective lens. The light travels through the tube to the eyepiece, where you put your eye to view the object. If you look the wrong way through a refractor, you can still see the object you’re looking at, but minimized instead of magnified.

The refractor is the original and stereotypical telescope.  If someone wants to include a telescope in a movie (or other popular media), it is usually a refractor. Galileo and latter Kepler used refractors.  Spyglasses, prevalent in pirate movies, are refractors. However, they are usually not the best astronomical telescopes because they are limited in size and expensive to build compared to reflectors.

For more on refractors, see the entry on refractors.

Reflectors

Reflectors have a big mirror at the back, so they remain wide. The eyepiece can be located in several different places., so it is not shown here.

Issac Newton realized the limitations of refractors, and developed the first reflector.  Reflectors all have a primary mirror at the bottom of the tube. However, there are many different types of reflectors with eyepieces in back, or the side, or pure imaging ’scope with a camera mount in the middle of the tube. Some reflectors even have lenses on the front. No matter what the configuration, if you look in the wrong end you see yourself rather than the object you’re trying to look at.

Reflectors are the more common type of telescope, and all large professional telescopes are reflectors, because they are cheaper and easier to build. There are many different types, and each have their own benefits and drawbacks.

For more on reflectors, see the entry on reflectors.

Differences

Large mirrors are much lighter than large lenses, and are much easier and cheaper to make. Mirrors only require one perfect surface,  they don’t need perfect glass or supports, and they don’t need to get thicker as they get larger. Lenses must be perfect all the way through and have two perfect surfaces. Wider lenses should also be thicker, or they’ll have ridiculously long focal lengths. Mirrors can be supported all across the back surface, but lenses can only be supported around the edges so large lenses may actually sag.

There are of course plenty of other differences, and each type of ’scope has its own set of problems.  See the individual pages for each ’scope.

Holiday telescope advice from the Bad Astronomer

Phil Plait (aka the “Bad Astronomer”, through he’s really rather good it) has a blog post with holiday telescope-giving advice.
Check it out at
http://blogs.discovermagazine.com/badastronomy/2009/12/22/last-minute-holiday-gift-advice-part-1-telescopes/

And don’t forget my favorite advice: consider binoculars!  Oh, and try it before you buy it. And don’t forget the things to avoid.

Telescopes – Beginners Guide – Parts common to all types of telescopes

There are several things the all amateur optical telescopes have in common. Below are some definitions and brief explanations.

Primary or Objective

In a refractor, the big lens at the front that you point at the object is called the objective lens. In a reflector, the big mirror that gathers all the light is called the primary mirror or the primary.  Since most telescopes are reflectors, the term “primary” is sometimes used to refer to both of these, even though it is more correct to say “primary mirror or objective lens”.  If you are specifically talking refractors, you should definitely say “objective”.

The primary mirror or objective lens is responsible for most of the telescope’s properties, and especially for the light gathering power and (for smaller ’scopes) resolution.

Optical Tube Assembly

The Meade Lightbridge OTA is actually an open truss rather than a tube.

The Optical Tube assembly (OTA) is the body of the telescope.  It consists of the primary or objective and the structure that supports it and makes it possible to look through.

When the telescope is a refractor, the OTA really should actually be a tube with the objective at one end and the eyepiece at the other.  Reflector telescopes may have an open truss assembly  instead, and the location of the eyepiece can vary significantly.

Eyepieces

The eyepiece is the lens you actually look through.  Any telescope you expect to look through yourself (as opposed to attaching a camera) needs an eyepiece.  There is a nice page with lots of info and images at http://www.rocketroberts.com/astro/eyepiece_basics.htm.

barrel diameter

Eyepieces with a 2" and 1.25" barrel diameter. They are labeled with their focal lengths, 27mm and 24 mm. (on a side note, these are very nice eyepieces, but they also cost more than my telescope!)

Eyepieces can fit directly into the OTA, into a focuser, or into a diagonal or prism. Whatever the eyepiece fits into must match the diameter of the eyepiece. In the last 30 years or so, 1.25″ has become the standard for amateur telescopes, although large telescopes may take 2″ eyepieces.  Cheap scopes may come with 0.97″ eyepieces, usually made out of plastic and apparently designed to cause eye strain and scare people away from the hobby. Earlier telescopes may have lenses made just for that telescope, so you may find eyepieces of all different sizes, especially with telescopes built before the mid-20th century.

focal length

The focal length determines the primary properties of the eyepiece (at least, when compared to eyepieces of similar configuration). Eyepieces are frequently refered to by their focal length. It determines the magnification of the telescope.  It also determines what the field of view will be.  The shorter the focal length, the bigger the magnification and the smaller the field of view.

In addition, shorter focal length eyepieces tend to have a smaller exit pupil. For the user, this means you have to have your eye really well aligned with the eyepiece to see anything.  Additionally, less light will pass through, so the image may be dimmer.

configuration

There are many many different configurations, ranging from a single bi-convex lens to large collections of 5 (or more) lenses of various shapes with different materials in between. Many of the more complex ones are designed to make the field of view wider (usually called “wide field” eyepieces) or make the image brighter. They may also be designed to improve the eye relief.  Good eye relief means you can be relatively far away and still focus on the image, very important for people who wear glasses.

One special lens is the Barlow.  It is intended to be used with another eyepiece or a camera to improve magnification.  A good Barlow can effectively double the usefulness of your other eyepieces.

Diagonals and prisms

Most telescopes (but especially refractors) can be hard to look through when pointing at things high overhead.  A diagonal or prism redirects the light to make it easier to look through.

Diagonals work either by using a plane mirror to reflect the light at a right angle, or a prism.  A prism can either reflect the light 90º or 45º.

Mirrors are generally prefered for astronomical use since they will reduce the light less (you lose a little bit of light every time the light has to pass through or reflect off  a surface.) However for small telescopes a “correct image” prism may be prefered, because you can use it for terrestrial viewing as well.

Focuser

The focuser is the part that lets you bring an image into focus.  There are several different types of focusers, ranging from simply moving the eyepiece  by hand to motorized systems that move one of the mirrors.

Mount

You may be able to hold a small, low power telescope  steady enough by hand, but chances are you will need some sort of mount for your telescope.

Small telescopes may sit very nicely on a table (especially if the alternative is a tripod bigger than the ’scope). However, most users prefer a tripod.

There are two basic types of mounts, called equatorial and alt-az. Equatorial mounts are aligned with Earth’s axis of rotation, which makes it easy to “drive” the telescope.  Alt-az mounts move in altitude (horizon to zenith) and azimuth (north, east, south…, usually measured in degrees away from due north.)  This is a fairly complex subject, so it gets its own entry.

Drive or Motor

The Earth is constantly turning.  If your telescope doesn’t move, the thing you are trying to look at will pass out of your ’scopes field of view.  To counter that, many scopes have a drive.

Drives work best with equatorial mounts.  The Earth is rotating eastward along it’s axis.  With an equatorial mount, a motor can counteract that motion by moving the telescope westward (same direction as the sky) on it’s axis at the same rate.  You can also drive an alt-az mounted ’scope, but in that case each axis has to move at a different rate and direction depending on where the ’scope is pointed.  This is only really possible if the mount is computer controlled.

Finder

Telescopes severely restrict how much you can see.  With nothing in the way (like glasses) most people can see about 160º side to side. It’s pretty easy to spot the star you want that way. Most telescopes have a field of view that’s less than a degree.  And if you boost the magnification, it’s even smaller.  If you want to find anything with your telescope, you’ll need a finder.

Using the site on the Galileoscope: Align the rear post with the front notch and the target.

There are several different types of finders. The simplest finders are just a couple of raised bumps for a sight (see image), or a hollow tube aligned with the OTA.

Most telescopes use a very small refractor, typically about  40 mm in diameter with a 5º field of view. The telescopic finders usually have mount with 3 screws to adjust the alignment with the OTA.

Simulated view through the window of a telrad

There are also more unusual finders, like red dot or telrad finders.  These use a red light, usually from an LED, projected onto a screen or mirror to show where the telescope is pointed.

A more recent option is a green laser pointer mount.  Green laser pointers are usually bright enough to use as a pointer at night. We can take advantage of this by putting a mount on the telescope to align the laser pointer with the OTA. Turn the laser pointer on, and you can see exactly where the telescope is pointing.

Telescopes – Beginners Guide – Basics

Basic Telescope Properties

Telescopes are basically light buckets.  Their real purpose is to gather as much light as possible to make images appear brighter and clearer. The light gathering power is a measure of the brightness, and resolution is a measure of how sharp the image is. In addition, telescopes usually magnify the image.

Light Gathering Power

A wider container can catch more rain. Similarly, a wider telescope catches more light.

The Light Gathering Power  (LGP) is just what it sounds like, a measure of how much light the telescope can collect.

The amount of light a telescope can collect depends on the area of the primary mirror or lens. Since the primary is normally round, the area is proportional to the diameter squared. So, a telescope that is 2 times wider than another ’scope will have a LGP of 2² = 4 times bigger.  If another telescope is 3 times widder, it will gather 9 times more light.

The bottom line: a little more width gets you a lot more light.

Resolution

Resolution is a measure of how sharp an image is. It is usually given and the smallest feature you can distinguish, so the smaller the number, the better.

Sizes on the sky are measured in degrees, arcminutes, and arcseconds.  Degrees you’ve probably heard of. There are 360º in a circle, or about 180º from horizon to zenith to the opposite horizon. However, most astronomical objects are too small for degrees to make sense, so we need smaller units.  Each degree is divided into 60 “minutes of arc” or arcminutes (abbreviated with a double quote “), and each arcminute is divided into 60 seconds of arc or arcseconds (abbreviated with a single quote ‘).

The full Moon is (on average) about half a degree, or 31 arcminutes. The stars Alcor and Mizar, the second star in the handle of the big dipper, are 11 arcminutes and 48 arcseconds apart, and anyone with good vision can distinguish them. A good amateur telescope will resolve Mizar into two stars,14 arcseconds apart. If this is the smallest thing the telescope can resolve, than we say the telescope’s resolution in 14 arcseconds.

Alcor and Mizar as seen through a good telescope. Mizar can be resolved as two stars, 14 arcseconds apart.

The resolution of the telescope depends on the diameter of the telescope.  Photons come in to the telescope from all directions. The photons  that strike the edge of the mirror or lens are diffracted from the edge and interfere with the other photons trying to come through.  The wider the telescope, the less diffraction there is.

There is one caveat here though.  Although the diffraction limit of a telescope may be exceptionally small, most telescopes are still on Earth, where we have the atmosphere to deal with.  The atmosphere generally limits the resolution at most viewing sites to 1 arcminute, which is about the resolution of an 8″ telescope. Professional telescopes get around this with techniques like active or adaptive optics, interferometry, or by putting telescopes in space.

The bottom line: unless you are dealing with a professional observatory, wider is better up until 8 inches. After that, it doesn’t really matter much.

Magnification

Magnification is a measure of how big something looks through the telescope.

Magnification is a comparison of how much bigger something looks through the telescope compared to the original, un-magnified view.

Magnification is rally a comparison between how it looks through the telescope to how it looks to the unaided eye. If something looks twice as big through the telescope, the magnification is 2.  If it looks 100 times bigger, the magnification is 100.

To magnify an image you need an eyepiece.  You can calculate the the theoretical magnification by dividing the focal length of the primary by the focal length of the eyepiece.To change the magnification, change the eyepiece.  Professional telescopes usually completely ignore magnification since in most cases they don’t use eyepieces.

The bottom Line: magnification doesn’t really matter.

Field of View

Related to magnification is the field of view. This is the area on the sky that you can see through the telescope.  For example, if you can just barely see all of the Moon through a telescope, it would have a field of view of about half a degree.  If you switch to an eyepiece that magnifies the moon 4 times more, you will only see 1/4 of the moon, or about 1/8º.