The nearly full Moon gets set to occult Antares. Shot with a Canon Digital Rebel XS (1000D) and a StellarVue SV70ED doublet achromatic refractor at 420mm of focal length on a fixed tripod with a 1/400th second exposure at ISO 400 at f/6. This image is cropped to make the Moon larger in the frame.

To get larger images where we can see more detail on objects such as the Moon, we need to use more focal length.

The examples in this section were all shot at prime focus. Prime focus simply means that no reduction or amplification of the native focal length was employed, such as with a focal reducer or Barlow. Prime focus can be used with camera lenses or telescopes. Although technically even a wide-angle lens is used at prime focus, the term usually refers to longer focal lengths.

Sunspot Active Regions 10486 and 10488 were shot with a 5-inch refractor at 1,000 mm of focal length at f/8 with a Nikon D1H DSLR Camera and a white-light solar filter at 1/2000th second exposure at ISO 200 on a tracking mount. False color was added later in Photoshop.

With focal lengths of 200 mm, 300 mm, or 400 mm, we can get detail, and still use exposures short enough to put the camera and lens on a fixed tripod. At longer focal lengths, a tracking mount is not absolutely required, but it certainly does make it easier to keep the subject in the frame.

Hold your mouse cursor over the image of the crescent Moon with Earthshine and Venus to see a comparison between an image that is blurred by camera motion caused by the wind and one that is not. The image was shot with a 300 mm lens on a fixed tripod. The blurred image was a 1 second exposure. The sharp image was a 1/6th second exposure during a period when the wind was not blowing as hard. We can see the effect of the wind vibrating the camera and lens, causing the image to be blurred in the longer exposure. The zig-zag line, which should be straight, is a plane that moved during the exposure. Camera vibrations caused the zig-zag.

Once we start getting into longer focal lengths, we get into more magnification. While a longer focal length magnifies details in the object we are shooting, it can also magnify things such as camera shake. We need to take precautions to guard against these kinds of problems.

A good sturdy tripod is the first line of defense against camera movement. We can also make it more sturdy by hanging a heavy camera bag on it to stabilize it.

Using a remote release and mirror lockup will also help prevent images from being blurred from mirror slap or touching the camera when opening the shutter.

The moons of Jupiter were shot with a StellarVue SV70ED doublet refractor with a Canon Digital Rebel XS (1000D) DSLR at f/6 at 1/8th of a second exposure at ISO 800.

It is even possible to shoot the moons of Jupiter with a short focal-length telescope on a fixed tripod. The above image shows the four bright Galilean moons, from left, Europa, Io, Ganymede, Callisto. Fifth magnitude Mu Capricorni is also in the frame to the upper left.

No detail is visible on Jupiter because it is over-exposed to record the fainter moons. At the time this image was taken, Jupiter shone at magnitude -2.4, while the moons shone at magnitudes 5.2 to 6.5. That makes Jupiter 7 magnitudes, or more than 600 times brighter than Ganymede, the brightest moon.

The purple fringing around Jupiter is typical of a fast focal ratio doublet refractor. It is caused by chromatic aberration.

Shooting Techniques

Almost all of the shooting techniques and camera settings that we used for wide-angle scenic shots apply to prime focus single frame images also.

The only differences are that focus is a little easier to achieve because we are at longer focal lengths, and exposures usually can't go as long on a fixed tripod before we get trailing.