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Scope Types And Contrast--long Post

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Posted by Daniel Johnson on June 24, 2003 03:21:44 UTC

Here's the promised long analysis of scope types and contrast:

One often hears the claim that refractors deliver higher contrast than scopes with central obstructions. Of the scopes with central obstructions, lovers of Maksutov-Cassegrains hold a strong belief that their scopes give better contrast than do Schmidt-Cassegrains or perhaps even Newtonians.
There is a grain of truth and a lot of pure myth in these beliefs.
First, ask: what does high contrast mean? It means getting all of the light from any point in an image concentrated into the smallest possible area, with no stray light from any other source contaminating the point in question, and no light from the point straying into any adjacent point.
Contrast can be reduced by many factors. These include:
1) Stray light.
a) Reflected improperly from lens surfaces—this can give ghost images or diffuse stray light, and is reduced by modern anti-reflection coatings. Refractors are most likely to suffer from this, though it is less a problem than in past centuries. (Eyepieces also contribute. Before modern anti-reflection coatings were available, the now-popular Plossls were not used because they gave ghosting on bright objects. Now that this one flaw has been overcome, they are fine eyepieces.)
b) Light bypassing the intended pathway and entering the image by bouncing off the tube walls or other non-optical surfaces. Baffling and flat black internal surfaces reduce this.
c) Light entering into the image without first encountering all the optical surfaces (for example, imagine a Cassegrain design with no baffling—light from in front of the scope could reach the eyepiece but hit no mirror. In a Newtonian, light from the side of the scope may enter the eyepiece tube directly). Under very dark skies, views of a bright planet are only minimally affected by light bypassing the optics—there just isn’t much light available to harm the image in this way. However, under city lights or bright moonlight, SCT’s, Mak-Cassegrains, and Newtonians all suffer from this, but the amount of harm depends on the baffling system chosen by the manufacturer. Refractors can be completely free of this defect, since there is no way for light to reach the image without first encountering every optical surface. Maks often have tighter baffling than SCT’s, because their optical flaw is also their strength: they have rather poor off-axis image quality, but excellent on-axis quality. Heavy baffling in a Mak to reduce stray light may also give vignetting, but who cares? Maks are made to optimize the central spot to highest quality, not to give broad views.
2) Light scattered by diffraction from the optics themselves despite perfect construction.
a) Diffraction spikes from secondary mirror holders (a downfall of Newtonians).
b) Diffraction from a central obstruction. Refractor owners often have the misconception than their 5-inch instrument gives better contrast than a 12-inch SCT because of this. Far from it. Reason 1-c above is probably a much bigger factor. Consider the mathematics: the central obstruction does not change the size of the central circle of the diffraction pattern for a star (or for a point on a planet). Rather, it scatters more of the light into the surrounding rings, especially the first ring (envision a star’s diffraction pattern—a bright central spot with a few faint surrounding rings of light, with those rings seen only at high magnification). But this redistribution of light into the rings is only a small effect. The great majority of the light still ends up in the central circle—perhaps 75% instead of 85%. But more important, in the larger Schmidt-Cassegrain, THE DIFFRACTION PATTERN IS PHYSICALLY MUCH SMALLER. A 12-inch SCT has a diffraction pattern only 42% the diameter of the 5-inch refractor’s, which means it is only 17% the AREA of the refractor’s diffraction pattern. So you can scatter a huge amount of light into those outer rings of the diffraction pattern and still have an image of much higher contrast, because you are scattering the light into a much, much smaller area. Aperture is king. If this were not true, interferometers could not work. After all, they have central obstructions of 99.99999%.
There are other considerations: refractors tend to remain in perfect collimation, and a small tube reaches thermal equilibrium quickly. So they may outperform a poorly collimated SCT or Newtonian, or they may give a stabler image before the SCT or Newt reaches thermal equilibrium. These considerations add to the misconception that the central obstruction is the key.
In sum, yes, perfectly made refractors have better contrast than perfectly made SCTs OF EQUAL APERTURE. However, under dark skies, with all scopes in good collimation and at thermal stability, refractors cannot come close to the contrast delivered by an SCT 2 or 3 times larger in aperture. It is just no contest. Still, under bright skies, reason 1-c above is a very real advantage for refractors, and for city dwellers it is one to consider. At the same aperture—assuming that poor color correction does not spoil the image—refractors are magnificent. And yes, I own one.

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