This can even work for a Dobsonian that is not tracking the sky.

For a really bright object like the Moon, you can hand hold your camera because the exposure will be short enough that you won't have to worry about movement. If you are trying to shoot the Moon through a Dobsonian, this is probably even the best way to do it. It will allow you to look through the eyepiece of the Dob to put the Moon at the edge of the field, and then give you time to put the camera up to the eyepiece before the Moon drifts out of the field of view.

The northern portion of the first quarter Moon is seen here in an afocal image taken with a Canon 1000D (Digital Rebel XS) DSLR camera with a 50mm lens on it at f/2.8 shot through an f/9.4 refractor with 80mm of aperture through a 14mm Meade Ultrawide eyepiece. The exposure was 1/30th of a second, hand held, at ISO 1600. Mare Serenitatis, the Sea of Serenity, is the large basin at lower right. It was formed by ancient volcanic eruptions on the Moon

Generally the afocal method is more often used with digital point and shoot cameras where you can't take the lens off. If you have a DSLR camera, you are usually better off removing the lens and shooting through your scope. If you need a lot of magnification, you can use eyepiece projection, which will be covered in the next section, or a barlow.

Afocal Procedure

• Focus the telescope on infinity. Some times this can be tricky because your eye is very accommodating and can make up for differences in focus, especially with long focal length eyepieces. To be sure your scope is focused on infinity, you can focus a pair of binoculars on a star, and then use one side of the binoculars to look through the eyepiece of your scope, and then focus the scope's eyepiece on a star.

• Focus the camera lens on infinity. Use the Live View focus to focus on a bright star or try to use autofocus on the Moon. Then turn off autofocus on the camera lens.

• Set your camera to ISO 1600 to start with, and set the len's aperture to wide open. For a bright object like the full Moon, you can start with an exposure of about 1/500th of a second. If you are shooting the Sun with a solar filter, you'll be able to use a lower ISO.

• Hold the camera up to the eyepiece of the telescope and try a test exposure. Be careful to hold the camera so the focal plane is perpendicular to the optical axis of the eyepiece. Try not to touch the eyepiece or you may shake the telescope causing a blurred image.

• Adjust the exposure on the camera by examining the image on the LCD on the back of the camera. Also check the focus when you look at the test exposure.

With Afocal imaging, you use a lens on your camera and shoot through the eyepiece of the telescope.

The equivalent focal length of the system is the telescope's magnification multiplied by the focal length of the camera lens. You can determine the scope's magnification by dividing the scope's focal length by the focal length of the eyepiece.

For example, if your scope has 500mm of focal length, and you are using a 25mm eyepiece, the magnification is 500 / 25 = 20x. Then if you are using a 50mm lens on your camera, the equivalent focal length of the afocal system is 20 x 50mm = 1,000mm.

The equivalent focal ratio is determined by dividing the diameter of your scope's objective into the equivalent focal length of your afocal system. So if your scope's objective is 80mm, in this example with an equivalent focal length of 1,000mm, your focal ratio would be 1,000 / 80 = 12.5. Your focal ratio is f/12.5.

Vignetting

With some setups, you may experience vignetting. This looks like the corners of the image are darker than the rest of the image. In severe cases, it may look like you are looking through a tunnel.

To reduce vignetting, use an eyepiece with a large lens. It helps if the lens of the eyepiece is not recessed. If you have a rubber eye shade on the eyepiece, remove it if you can.

If you are using a camera lens with a lens shade on it that prevents you from getting it close to the telescope's eyepiece, remove it.

Secondary Shadow

If you are using a Newtonian, Dobsonian, Schmidt Cassegrain, or other scope with a secondary mirror, and a long focal length eyepiece in the scope for your afocal setup, your images may show a dark shadow area in the center. This is the shadow of the secondary.

To reduce this effect, use a lens with a large aperture and do not stop it down. Also, try using a shorter focal length eyepiece.

Afocal - The Bottom Line In afocal astrophotography, you shoot with your camera and lens held up to the eyepiece in your telescope. This method can provide a bit more magnification than shooting prime focus with the camera hooked up directly to the telescope.