Just a short time after starting my journey in to astrophotography, I ran in to a snag. After purchasing the various connectors necessary, I found that I was not able to properly focus a scene with my DSLR attached to the telescope. However, I was able to focus properly when I used one of the telescope eyepieces. It turns out my problem is not unique, and it’s a matter of physics.
First, it helps to understand how a DSLR captures an image. This first diagram shows the basics of how a DSLR makes an image. The front lens element (B) bends the light reflected off the subject (A) and, combined with any other lens elements (C), refines and focuses the light to form an image on the sensor (D). The sensor and other electronic components in the camera then do their job to turn that light in to the bits and bytes that get stored on the memory card. The key point to remember, though, is that for an in-focus image, the light hitting the sensor has already been focused. If the image is not in focus by the time it hits the sensor, the sensor records a blurry image.
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Next, let’s take a look at how a standard refractor telescope works. A refractor telescope works much like a magnifying glass. The objective lens (B) at the front of the telescope is a piece of convex glass that bends the light and brings it in to focus at the focal point (C). When an eyepiece is used, the focal point falls inside the barrel of the eyepiece (D), and the eyepiece magnifies the image as presents it to the viewer. The position of that focal point in relation to the correct spot in the eyepiece is adjusted to achieve the proper focus using the focuser. Even when different eyepieces are used, that focal point (C) is always the same in the telescope, and that’s what causes the problem with the DSLR.
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The final image shows what is happening when I attach a DSLR to my telescope. Recall from the DSLR section that the image needs to be in focus before it hits the DSLR sensor. As I mentioned above, that focal point is effectively a fixed distance from the objective lens (B). In order for that to happen given the physics going on inside the telescope, that means that the DSLR sensor (D) needs to be at the focal point (C) of the telescope. With my telescope, however, I am not able to slide the focuser far enough forward to bring the sensor to the focal point, so I am not able to capture an in-focus image.
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So what now? Short of buying a new telescope geared towards DSLR astrophotography, I think I have a few options. I can experiment with one of my point & shoot cameras. Since the camera body is thinner, I believe I can get the sensor far enough forward to be able to be at the focal point. Although, I think my subject would have to be brighter objects like the moon since my control over the aperture and shutter speed are somewhat limited. I will look for another way to connect my DSLR to my telescope that doesn’t involve three feet (ok, a few inches) of adapters between the two.
If anyone else out there has had a similar experience and came up with a creative answer, please leave a comment!
