Stopping Down: Why, When, and How
David A Harbour
Great Plains Instruments
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When I was a youngster dreaming of the moon, planets, and telescopes, a twelve and a half inch reflector in the hands of an amateur was rare; a sixteen-inch reflector was nearly unheard of and considered a giant. Nowadays reflectors ranging upwards of twenty-five and thirty inches' aperture are not too uncommon in the hands of amateurs.
There are two reasons why amateurs seek larger apertures: the deep sky enthusiasts want more light for observing their low surface brightness extra solar system objects, and the solar system observers seek the higher resolution a larger aperture yields.
The deep sky observer seldom realizes the maximum resolution of his or her mirror, typically running magnifications of only ten or fifteen power per inch of aperture, whereas the lunar and planetary observer in his or her quest for making fine detail visible must push on to higher magnifications in order to enlarge this detail above the resolution limit of the human eye. To use a large aperture telescope at low magnification prevents the human eye with its very much lower resolving power from realizing the much higher resolution of the telescope. Objects or details in the field of view of the telescope near its angular resolution limit must be enlarged to an apparent angular diameter that is larger than the eye's angular resolving power.
Nevertheless, sometimes a low power, wide-angle view of a solar system object is desired- say, for instance a dramatic, wide view of Jupiter and its four Galilean moons strung out in space to either side of the planet; or perhaps a full disc image of the moon just at or just after first quarter with the near terminator features illuminated in bold relief with the sun's low angle illumination on them. On these occasions a large reflector will overwhelm the retina of the eye with excessive brilliance in the image. When we observe bright objects at low magnifications on these occasions, we need a method to reduce the amount of light transmitted through our telescope- the direct opposite of the problem of the deep sky observer who is always seeking more light at low powers.
Many amateurs commonly use neutral density filters to accomplish a reduction in light when observing bright objects at low powers; however, stopping the aperture down offers several additional advantages. By using a diaphragm with its aperture offset between the edge of the primary mirror and the edge of the secondary mirror, we eliminate the extra diffraction caused by the edges of the secondary mirror, and realize a noticeable increase in image contrast. Also, stopping down this way improves sharpness by increasing the allowable range of wandering of the focal plane caused by turbulent or unsteady air; i.e., the focal plane may move upstream or downstream from ideal focus by a much larger amount than when not stopped down. Mirror makers will visualize this as a manifestation of a principle that they are familiar with: longer focal ratios allow a greater range of focusing error. Photographers would say that the system has more "depth of field" than when used at a faster focal ratio- thus, stopping down can improve the sharpness of the image when the seeing is very poor, although only at low magnifications.
It is surprising how many persons are unaware of yet another advantage for stopping down when using low magnifications. Those of us with astigmatism usually must wear our glasses when using low magnifications with long focus eyepieces with their large exit pupil diameters. However, stopping down the entrance pupil causes a corresponding reduction in the size of the exit pupil at the eyepiece, and since the effects of astigmatism are greatly reduced or eliminated when the pupil is contracted, an observer with astigmatism will usually notice that his or her eye is free of astigmatism at the eyepiece without glasses when the system is stopped down. This is nice: most eyeglasses wearers have a hard time getting the entire field of view visible to them with a long focus eyepiece, as the wearing of glasses often prevents one from getting close enough to see the entire field of view. When stopped down, astigmatism effects usually vanish for observers who would otherwise expect to notice their defective vision at the eyepiece without their glasses.
Many of these designs have slight aberrations peculiar to tilting their components. Most of the best designs minimize these aberrations to acceptable levels- but there is another genus of unobstructed reflector, which although not having its secondary in its light path, even so does not operate off of the optical axis. When one stops down his or her Newtonian or Cassegrain reflector with an eccentrically placed entrance pupil mask, the instrument is essentially transformed into one of this breed of telescopes.
Amateurs who are familiar with the practice of stopping down their reflectors with an eccentrically placed entrance pupil mask will often say that they stop their reflector down with an "off axis mask"; however, strictly speaking, this is not the case. The light entering and passing through the optical system when stopped down this way is still essentially parallel to the instrument's optical axis. Some illustrations help us visualize this. In figure one showing the optical system of a wide field catadioptric telescope, we see the light from a single star traversing the space between its mirrors and coming to a focus behind the primary mirror onto a little square of ground glass positioned precisely in the instrument's focal plane.
In figure two we have cut away a large section of the primary to better see what is happening behind it near the focal plane when we push the ground glass first towards, and then away from the focal plane: it shows us the little out of focus "doughnuts" we are used to seeing when we have the eyepiece of our reflector racked quite a ways inside of or outside of focus.
In figure three we have placed a cardboard diaphragm over the corrector plate of our telescope; when we examine the light again with the ground glass near the focal plane, moving it in and out to explore the cone of light inside of and outside of focus, we immediately notice something different: no more "doughnuts", but rather, a small circular patch of light that does not change its diameter nearly as quickly as the doughnuts did before when we moved the ground glass in and out by the same distance. Photographers would say that the "depth of field" of the system has been increased. This illustration also makes it clear why the exit pupil will be so much smaller when stopping down. As a matter of fact, we may accomplish an exactly identical transformation of our instrument by inserting a tiny sub-aperture mask with its exit pupil offset just in front of the eyepiece's eye lens; this used to be a well known practice.
We may even carry our illustration to a great extreme, and show what kind of an instrument we would have if we simply cut away every portion of the optical elements not actively passing or focusing light, as in figure four. It is a useful illustration, to be sure, but certainly one would not want to make an unobstructed reflector by such a drastic method (ruining forever one's former option of using it at its full aperture)! There is a wide range of designs for unobstructed reflectors that are not difficult to build available to amateurs nowadays.
Sub-aperture masks are not difficult to make; I've lost count of how many I've made for friends over the years. As can be seen in the photographs, one can make diaphragms with more than one opening in them, retaining the cutout pieces to fashion into little doors to close off the ports not in use. The large three-port diaphragm Dora is holding is for our local museum's 16" Schmidt Cassegrain reflector. The largest opening in it is not quite circular, the secondary intruding a little ways into the opening; I blended the contours of its circumference in this region of the port.
It is not necessary to buy an expensive beam compass or trammel to draw the circles for preparing a sub-aperture mask. Inexpensive trammel points are available that will affix onto an ordinary yardstick for drawing large circles.
I guarantee that observers who try a sub-aperture mask for reducing glare at low magnifications on bright solar system objects will become believers in the technique.
© 2000 David Anthony Harbour