Does the central obstruction ruin the image

One reason apo refractors perform so well is that they have no central obstruction. The central obstruction in a reflector or catadioptric degrades the image -but not as much as some people think.

Why doesn't the obstruction simply produce a hole in the image? Because light from all parts of the image can still reach the focal plane, and no part of the image is affected more than any other.

What the obstruction does do is increase diffraction. The light is diffracted by two circular boundaries, not just one. As a result, more light is shifted into the diffraction rings. This causes some blurring of low-contrast planetary detail, but it does not harm the view of double stars or other high-contrast targets (Figures 5.6,5.7).

The obstruction is normally expressed as a fraction of the aperture. For example, an 8-inch (20-cm) SCT with a 3-inch (7.5-cm) obstruction is slightly less than 40% obstructed.

Unfortunately, misconceptions abound. "A 40% obstruction costs you 40% of the contrast" is simply not true. First of all, it costs you only 16% of the light, because 0.402 = 0.16. Second, removing light does not reduce contrast; the loss of contrast is caused by diffraction, and it only affects details whose contrast was already so low that a small amount of additional light in the diffraction rings can make a difference.

One popular rule of thumb is that if you subtract the diameter of the obstruction from the aperture of the telescope, you get the equivalent unobstructed aperture. Thus an 8-inch telescope with a 3-inch obstruction is as good as a

8-inch (20-cm) telescope No obstruction

8-inch (20-cm) telescope 40% central obstruction

5-inch (12.5-cm) telescope No obstruction

Figure 5.6. A close double star viewed in obstructed and unobstructed telescopes. The obstruction shifts more light into the diffraction rings.

8-inch (20-cm) telescope No obstruction

8-inch (20-cm) telescope 40% central obstruction

5-inch (12.5-cm) telescope No obstruction

Figure 5.6. A close double star viewed in obstructed and unobstructed telescopes. The obstruction shifts more light into the diffraction rings.

8-inch (20-cm) telescope No obstruction

8-inch (20-cm) telescope 40% central obstruction

5-inch (12.5-cm) telescope No obstruction

Figure 5.7. Simulated views of Mars in perfectly steady air, generated with Aberrator. On low-contrast planetary detail, a 40%-obstructed 8-inch (20-cm) telescope (such as the typical Schmidt-Cassegrain) is at least as good as a perfect 5-inch (12.5-cm).

8-inch (20-cm) telescope No obstruction

8-inch (20-cm) telescope 40% central obstruction

5-inch (12.5-cm) telescope No obstruction

Figure 5.7. Simulated views of Mars in perfectly steady air, generated with Aberrator. On low-contrast planetary detail, a 40%-obstructed 8-inch (20-cm) telescope (such as the typical Schmidt-Cassegrain) is at least as good as a perfect 5-inch (12.5-cm).

perfect 5-inch, even on the most subtle planetary detail. On double stars and on faint objects requiring larger light grasp, the obstructed 8-inch is of course much better.

If central obstructions degrade the image, why do we put up with them? Mainly because we choose telescopes for cost and portability, not aperture. The choice is not between obstructed and unobstructed telescopes of the same diameter. An 8-inch Schmidt-Cassegrain of average quality outperforms the finest 4-inch refractor - dramatically on many objects, only slightly on some - and is cheaper and more portable. An 8-inch refractor would hardly be portable at all.

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  • Arthur
    Why does a central obstruction reduce contrast?
    1 year ago

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