Reflective microscopes

There is a limit to the effectiveness of telescopes with glass lenses; refractive distortions blur the edges and tinges objects with a rainbow effect.

Reflective telescopes, using highly polished concave mirrors, eliminates these distortions.

Is it possible to construct a super-crisp microscope using reflective optics?

IIRC, the problems with lenses pop up with increasing size, so with small sizes, it’s no big deal. That’s not to say reflective microscopes are worse, though.

Dispersion problems are harder to manage with fast optics (short focal length, large diameter) than slow optics. Due to the fact that many astronomical objects of interest are very dim, fast optics are very desireable in a telescope.

Microscopes are typically quite slow by comparison. Since the illumination source can be controlled, this is typically not an issue, unless the object being studied can’t tolerate the high light level.

Large lelescopes have a number of features that complicate refractive objective lenses:

-You need a piece of glass that big with no defects, and uniform optical properties.
-You need a second piece of glass (at least) with same criteria, but different dispersion.
-You may need a third piece of glass, and here there is a problem, because the third element in a three-lense objective (so called apochromats) tend to have poor weather resistance, and telescopes need to be used outdoors.

  • All these lenses must be self supporting, ulike a mirror that can be cast with support ribs on the back, and have elaborate “mirror cell” structures attached to provide even and stress-free support.

None of this matters with small, slow lenses. At most an acromat is needed (two lenses) and they are so small that uniformity of the glass is a non issue.

So there is not a strongly compelling reason to make a reflective microscope, and reflective optics have thier downsides:

-Any physical surface errors on a mirror are doubled in terms of optical path. Contrast this with lenses, where physical surface error only changes optical path length by ~40% of the physical error. (Nglass-Nair) Mirrors need to be made about 5X more accuratly than lenses to obtain the same optical performance.

-Similarly, surface finish on a mirror, notably minute scratches known as “sleeks” can contribute to diffration and loss of contrast much more than similar defects on a lens surface would.

-The most common reflective telescope design (a newtonian reflector, parabolic primary, flat sedondary diagonal mirror) has severe off-axis aberations. (strong astigmatism, and horrible coma) More advanced designs avoid this, at the expense of additional mirrors and/or lenses. The Hubble ST was supposed to be a Ritchie-Creten (sp?) design. In many ways, this is a result of the the desireability of fast telescope optics. All these off-axis aberations are greatly reduced, if not eliminated, in slow designs.

-On axis reflective systems require a turning mirror, or at least a film holder or CCD at the focus. This obstructs the aperture and causes diffration. It also causes a hole in the center of the light bundle reaching the eye. This could be a serious problem with a microscope, as the light bundle is small, and in room light, the viewers pupil will be constricted, possibly smaller than the hole in the light bundle, causing the image to black out when the eye is centered in the eyepiece.

-Off-axis reflective telescopes do exist, and they are thier own can of worms.

Lastly there is the issue of physical construction. As working distance increases, the amount of light the objective intercepts falls off with the square of distance. Thus, unless required, long working length is seldom an advantage. A short working length would leave little space to locate the secondary optics.

Reflective microscope objectives do exist, and have the advantage of having extremely wide wavelength range (visible through UV) without chromatic aberration.

They are uncommon because for most purposes, you can combine several lenses to correct for aberration. You need to combine lenses with different curvatures and materials, but it’s easy to do this for small optical systems like microscopes and camera lenses. Even chromatic aberration can be cancelled this way - not perfectly, but good enough for most applications. But as the optical system becomes larger, it becomes prohibitively expensive to do this. An 8" diameter block of artificial fluorite crystal (one of the best for suppressing chromatic aberration) can set you back thousands of dollars.

Another reason is that reflective systems either have obstruction or off-axis elements. A typical reflective telescope has a secondary mirror supported in the middle of the light path. On a large telescope, it’s quite easy to use thin metal blades (“spiders”) to support this secondary mirror, and this would have minimal effect on image quality. But it’s difficult (=expensive) to scale this down to a 1/2-inch diameter microscope lens. You can avoid this by using an off-axis design, but that usually requires asymmetric (aspherical) mirrors, which are very difficult and expensive to make.