Afocal photography is taking a picture through your telescope with an eyepiece in the scope and a lens on your camera. You hold the camera with lens on it up to your eyepiece, and shoot through the eyepiece.
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.
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.
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.
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.
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.
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