The main lens or mirror that gathers light is called the primary objective of the telescope. Newtonians and Schmidt-Cassegrains also have a second mirror to reflect light out of the tube called a secondary.

The aperture of a telescope is one of its most important parameters. The size of the area defined by the aperture determines how much light is gathered. In this example of a Stellarvue 70ED, all of the important specifications are engraved on the lens cell. D=70 means the diameter of the objective is 70mm. F=420mm means the focal length is 420mm. f/6 means the focal ratio is f/6. The focal ratio is determined by dividing the focal length by the aperture (420/70 = 6).

Aperture - The hole in the front of the telescope that lets the light in is the aperture. It's size is determined by the size of the primary objective that gathers light in the telescope. Usually when we refer to the aperture of the telescope, we are talking about the size of the primary objective, and not just the hole, since the hole can be bigger than the main lens or mirror in some scopes. It is the primary objective that actually gathers the light. The aperture of the primary objective is measured in inches or millimeters.

The aperture is what determines how much light is collected. It is the most important attribute of a telescope because light gathering is what telescopes are all about. For astrophotography, a larger aperture means more photons get collected, and this means better pictures. If you want to view or photograph an object in the sky that is faint, it is better to use a telescope with a large aperture.

Aperture is not the only attribute of a telescope to consider when picking one for astrophotography. The optical quality is just as important. If you have a large aperture with poor optical quality, you won't get very good images.

The down side to a large aperture is that it is more costly and complex to produce a system with good optical quality. Larger apertures also mean more weight and a longer focal length and more magnification, which magnify errors, and require a larger and more expensive mount to hold them.

For a beginner astrophotographer, a large aperture is not that important. A smaller aperture can lead to more success when you are just starting out because small scopes are so much easier to use, particularly small refractors.

Focal Length - The focal length of a telescope is the distance from the objective lens or mirror to the focal plane where the light comes to focus. The longer the focal length, the larger the image is that forms at the focal plane, and the higher the magnification of the telescope. Focal length is also measured in inches or millimeters.

More magnification is a good thing for small objects like double stars and planets. But bad things also get magnified, like poor seeing (atmospheric turbulence), imperfections in the tracking, and vibrations from instability in the mount.

The beginning astrophotographer, a shorter focal length is better. There are plenty of things in the sky that you can shoot with a short focal length telescope.

Focal Ratio - The relationship between the aperture of the objective and the focal length is called the focal ratio. The focal ratio is defined as the focal length divided by the aperture.

For example, a telescope that has a focal length of 1000mm and an aperture of 125mm has a focal ratio of 1000/125 = 8. We also refer to the focal ratio as the "f-number". Our example telescope would have a focal ratio, or f-number, of f/8.

If we had a telescope with the same focal length of 1000mm but a larger aperture of 250mm, it would have a focal ratio of 1000/250 = 4, so it's f-number would be f/4.

A telescope with a large objective in relation to its focal length can gather more photons from a given area of sky and concentrate them onto the sensor of the camera at the focal plane. Because a bigger objective gathers more light than a smaller one at a given focal length, it can take a picture in a faster amount of time. A bigger objective at a given focal length is said to have a "faster" focal ratio.

In our examples above, f/4 is a faster focal ratio than f/8.

The amount of light gathered is determined by the total area of the objective. So an objective that has twice the diameter, such as 250mm vs. 125mm in our example, would really have 4 times the area to collect light.

A larger objective with a longer focal length can also resolve more fine detail in an object than a smaller telescope.

A telescope's aperture almost always stayed fixed. Sometimes, under special circumstances, they may be stopped down, but usually aren't. Their focal length, and focal ratio, however, can be changed by using a Barlow to increase the focal length, or a focal reducer / telecompressor to shorten the focal length. When the aperture stays the same size, but the focal length changes, the focal ratio changes. If the focal length gets longer with the use of a Barlow, the focal ratio increases and the telescope system speed gets slower. If a telecompressor is used, and the focal length gets shorter, the focal ratio decreases and the system speed gets faster.

For example, a 125mm aperture scope with a focal length of 1,000mm has a focal ratio of f/8. If we use a 2x Barlow, the focal length would increase to 2,000mm, and the focal ratio would increase to f/16. The aperture would stay the same.

If we used a 0.75x telecompressor with this 125mm aperture scope, the focal length would go down to 750mm, and the focal ratio would decrease to f/6. Again, the aperture would stay the same.

Point Sources and Extended Objects

A star is a point source. No matter how much you magnify it, you will not be able to see any detail in it, and it will stay basically a point. The size of this point will be larger in smaller telescopes, and smaller is larger telescopes because of diffraction. Diffraction makes a point source spread out into a disk with a measurable size.

Brighter stars are also recorded as larger disks than fainter stars in long-exposure images. This is not because they are actually bigger in the sky, or magnified more on the sensor by the scope, but because so much light comes from them it is scattered by the atmosphere and in our telescope's optics, and spills over onto adjacent pixels in the sensor.

A planet, or the Sun or Moon, or a nebula or galaxy, is an extended source. Unlike a point source, an extended source will reveal more real detail when magnified.

How faint of a point source you can record is determined by your telescope or lens' aperture alone and is independent of the focal ratio. Larger apertures will record fainter stars.

How faint of an extended source you can record is determined strictly by the focal ratio and is independent of the focal length.

This is true for both telescopes and camera lenses.

The Ideal Telescope

In a perfect world, we would want the largest aperture possible at the focal length that best frames the object we are interested in. This gives the most light gathering ability, and highest resolution. But with a larger aperture comes a higher cost. With a larger and heavier telescope, a larger mount is required to hold it rigidly, a very important consideration for astrophotography.

In the real world, we use the telescope that we have, and we make the best of it.

Aperture, Focal Length, Focal Ratio - The Bottom Line The size of the opening in a telescope or lens that lets light in is called the aperture. Larger apertures produce brighter images. The focal length is what gives an image magnification. If you want to shoot small celestial objects, you need a longer focal length. For wider fields, you need a shorter focal length. The focal ratio is the relationship between the aperture and focal length. For a given focal length, if you have a larger aperture, the focal ratio is faster and you can take an image of the same quality in less time. To get started in astrophotography, you do not need a large telescope. You can begin with a very modest sized telescope. An 80mm or 90mm refractor is a very good beginner's telescope for astrophotography. Large telescopes are difficult for beginners to use because they magnify little problems and make them big ones.