# What Telescope Specifications Mean

Telescope Specifications - What do they mean?

**See also:**

Telescopes - Site reviews

How Telescopes Work

A clear guide to

how they work

New to telescope buying? If so, you might find some of the descriptions a little confusing. To help, we have taken 2 telescopes and listed the specifications for each with added notes explaining what each of the specifications mean. The telescopes are both astronomical telescopes but the same principles apply for terrestrial scopes. We hope these examples will help you to choose the right telescope for you.

**Example 1: Celestron Firstscope 80 EQ Refractor **

First off: This is a refracting telescope (it uses a lens to capture light) as opposed to a reflecting telescope (which uses a mirror to capture light). If you would like more information on how refracting and reflecting telescopes work, refer to our article How Telescopes Work. In general, refracting telescopes tend to give sharper images than reflecting telescopes the same size. However, they also tend to be bigger and more expensive for the same size!

**Specifications of the Firstscope 80 EQ Refractor:**

Aperture 80 mm

Focal Length 900 mm

Focal Ratio f/11

Computer Control - no

1.25" Eyepieces: Focal Lengths 20 mm and 10mm

Magnification Levels: 90x and 45x

Finderscope: Star Pointer

Tripod: Aluminium adjustable

Mount: equatorial

Weight: 24 lbs.

Multi coated optics

Can be used for both terrestrial and astronomical viewing

Aperture 80mm

What is meant here is that the diameter of the objective lens is 80mm, which is 3.1inches.

The objective lens is the large lens that captures light and focuses it into an image that can then be magnified. In general, the larger the objective lens, the better. You should get as big an objective lens as you can afford because the bigger the objective, the more light the telescope can gather which in turn means that fainter objects can be observed. The size of the objective determines what magnification the telescope can achieve without losing resolution. A quick rule of thumb is 50 times the diameter of the objective in inches. So, you could reasonably expect this telescope to magnify images ~ 150 times (50 x 3.1) before resolution is affected. The diameter of this particular lens is probably about as small as you would want to go, and most expert skywatchers would turn up their noses at it. But bear in mind that this is not a particularly expensive telescope, refracting telescopes always tend to be smaller in size than reflectors for the same money and it does have "firstscope" in the title!

Focal Length

The focal length stated is the focal length of the objective; that is, the distance needed by the objective to bring all the light collected to a focus. So for this telescope, an image is formed 900mm from the objective lens. You will need this number when you are working out magnification - more on this under *eyepieces *.

Focal Ratio f/11

Sometimes known as the f/number. Focal ratios are the focal length divided by the diameter, hence for this telescope: 900mm ¸ 80mm = 11, written as f/11.

A quick rule of thumb is:

f/10 and higher: smaller field of view; good for looking at the moon, planets and double stars

f/8: intermediate and good for general all round viewing

f/6 and below: wider field of view; good for deep sky objects

Computer Control - no

Rather obviously, this telescope does not have computer control. To see what a telescope that does have this feature can do, see the next example.

1.25" Eyepieces 20mm and 10mm

This particular telescope comes complete with 2 eyepieces both of which have diameters of 1.25". The focal length of the eyepieces are 20mm and 10mm.

**Lets look at the diameter first.** The larger the diameter of the eyepiece, the larger the field of view will be. There is a calculation you can perform:

Field of view in degrees = eyepiece field stop diameter divided by telescope focal length x 57.3

A 1.25" eyepiece will give you a maximum field stop diameter of 27mm.

Therefore the 1.25" eyepieces combined with the focal length of this telescope will give a maximum field of view of:

Max Field of view = 27mm divided by 900mm x 57.3 = 1.7 °

To put this in perspective , the field of view needed to see the whole of the moon is about 0.5° . (Incidentally, Dartmouth College in New Hampshire has produced a series of telescope exercises for their students. There is an interesting one called Moon Lab. It is in pdf format, so you will need Acrobat Reader installed. if you like this you can "get at" other exercises by viewing the list here, clicking on one and noting certain keywords and then using google to bring up the pdf document - I don't seem able to navigate directly to them from the website.)

**Now for the focal length of the eyepiece.** This is important when you want to find out what magnification your telescope is capable of.

Sorry about all the maths, but here goes!

Magnification = focal length of the objective divided by the focal length of the eyepiece.

So, if you use the 20mm eyepiece in this telescope, you will get a magnification of 900mm divided by 20mm = 45 times.

If you use the 10mm eyepiece, you will get a magnification of 900mm divided by 10mm = 90 times

You can see that the magnification depends crucially on the focal length of the objective and the focal length of the eyepiece you use. You can always buy extra eyepieces for your telescope but you cannot change the focal length of the objective. This is something to bear in mind when buying your telescope. In practice there is a limit to the amount of magnification you can usefully achieve without compromising resolution - see above.

Some telescopes have fittings that allow them to use larger diameter eyepieces, such as the 2inch one in our second example which has a maximum field stop diameter of 46mm. This would give you a much bigger field of view.

Magnification levels: 90x and 45x

See above

Finderscope: Starpointer

A finderscope is a smaller, secondary, telescope mounted on the main telescope and is used to locate objects in the sky. The Starpointer is made by Celestron and uses an LED (Light Emitting Diode) to provide you with a red dot of light to centre on the object you wish to view, the idea being to make object location easier. Usable in daylight.

Tripod: Aluminium adjustable

The tripod is the thing you mount the telescope on!

Mount: equatorial

The purpose of the telescope mount is to secure the actual telescope to the *tripod *. There are 2 types of telescope mounts available. First, the alt-azimuth mount where you simply set your co-ordinates and then lock them in. These cannot automatically track objects so they have to be readjusted as the object moves out of view. Equatorial mounts are more expensive and a little more complicated to set up. However, once aligned, this type of mount will track an object across the sky. If you want to do astrophotography, then you will need an equatorial mount.

Multi-coated optics

The coatings are to protect the lenses and to reduce glare

**In General **

This particular telescope is around £200 to buy. You might think that this is expensive for a first telescope, and for that matter you've just seen one down your local camera shop for around £70! The difference between a cheap telescope and a more expensive one is usually in things like objective lens size, focal length of the objective, the quality of the optics, the type of mount, the diameter of the eyepieces and the general sturdiness of the tripod and the telescope body itself. Also, if there is a feature you are keen to use and it is not clear from a description if the telescope you are thinking of buying has it, then you should ask the sop or visit the website ofthe company that makes the telescope to get more information. Do decide what you want to use the telescope for and then check out the specifications of the telescope you want to buy to see if it fits the bill. I think even £70 is a lot of money for something you might possibly be dissatisfied with in a few weeks.

# Example 2: Bushnell computerised star locator telescope 675 x 4.5

**Specifications:**

675 x 4.5" Computerised Star Locator Reflector

Quick release heavy duty aluminium tripod

Red dot LED finderscope, three point finderscope mount

Focal length: 900mm

1.25" format eyepieces, 4mm/20mm

Magnification: 45, 135, 225, 675

Barlow lens

Camera adaptable

675 x 4.5" Computerised Star Locator Reflector

This tells us that the telescope is a reflector (uses a mirror to capture light) as opposed to a refractor which uses a lens to gather light. If you would like more information on how refracting and reflecting telescopes work, refer to our article How Telescopes Work. In general, refracting telescopes tend to give sharper images than reflecting telescopes of the same size. However, they also tend to be bigger and more expensive! It is easier and cheaper to produce a mirror than a lens for the simple reason that a lens has to be ground to a high degree of accuracy on both sides, which is not true for a mirror, where only one side is needed. For this reason it is often very cost effective to buy a reflector as you can often get the same size *objective * for less money. The objective is either a lens, or in this case, a mirror, that captures light and focuses it into an image that can then be magnified. In general, the larger the objective lens, the better. You should get as big an objective lens as you can afford because the bigger the objective, the more light the telescope can gather which in turn means that fainter objects can be observed. The size of the objective determines what magnification the telescope can achieve without losing resolution. A quick rule of thumb is 50 times the diameter of the objective in inches. In this case the specifications tell us that the objective mirror has a diameter of 4.5", so we can expect this telescope to magnify up to (4.5 x 50 =) 225 times before resolution degradation starts to become an issue. However, the specification 675 tells us that the maximum magnification is 675 times! This is the *theoretical maximum * magnification this telescope can achieve with the eyepieces supplied but note the caveat above! Notice that a 4.5" mirror is a very acceptable objective size, especially for a beginner.

Computerised means the telescope has a computer which will automatically locate the object you wish to view, after you have input the co-ordinates. Neat!

Quick release heavy duty aluminium tripod

The tripod is a platform that holds the telescope steady.

Red dot LED finderscope, three point finderscope mount

A finderscope is a smaller, secondary, telescope mounted on the main telescope and is used to locate objects in the sky. This particular one has a mirror box showing the sky and the target illuminated by a red LED (light emitting diode) spot.

Focal length: 900mm

The focal length stated is the focal length of the objective: that is, the distance needed by the objective to bring all the light collected to a focus. So for this telescope, an image is formed 900mm from the objective. You will need this number when you are working out magnification - more on this under *eyepieces *.

1.25" format eyepieces, 4mm/20mm

This particular telescope comes complete with 2 eyepieces, both of which have a diameter of 1.25". The focal length of one eyepiece is 4mm and the second has a focal length of 20mm.

**Lets look at the diameter first.** The larger the diameter of the eyepiece, the larger the field of view will be. There is a calculation you can perform:

Field of view in degrees = eyepiece field stop diameter divided by telescope focal length x 57.3

A 1.25" eyepiece will give you a maximum field stop diameter of 27mm.

Therefore the 1.25" eyepieces will give a maximum field of view of:

Max Field of view = 27mm divided by 900mm x 57.3 = 1.7 °

**Now for the focal length. **This is important when you want to find out what the magnification is.

Magnification = focal length of the objective divided by the focal length of the eyepiece.

So, if you use the 4mm eyepiece in this telescope, you will get a magnification of 900mm divided by 4mm = 225 times

If you use the 20mm eyepiece the magnification will be (900/20) 45 times.

Magnification: 45, 135, 225, 675

If you look at the magnification claimed for this telescope you will see the 2 magnifications calculated above, 45 and 225 x, included in the list. If you are wondering about the *other * two magnifications mentioned, see below.

Barlow lens

The Barlow lens is used in combination with other eyepieces to increase magnification. These ones are 3x, so the magnifications provided by the 2 eyepieces, 45x and 225x can be trebled. Therefore, the total magnification with the lenses provided with this telescope is 45x, 135x, 225x and 675x.

Camera adaptable

If you fancy your hand at astrophotography, then you should be able to do this using this telescope.

Good telescope hunting!

If you have any questions or comments on this article, please
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