Hack 36. Eliminate
Spikes and Increase Contrast
Build a $0 aperture mask to
your $500 Dob into a $2,000 apo refractor.
Well, not really. But a simple, cheap hack can let your inexpensive Dob provide most of the benefits of an expensive apo refractor. Read on.
There's an old saying among astronomers: Aperture Rules. And it's true. A larger aperture gathers more light and provides higher resolution of fine detail. Assuming the larger scope has at least decent optics, you can simply see more with a larger scope than with a smaller one, period. That's the reason Newtonian reflectors, particularly Dobsonians, are so popular. They provide a lot of decent-quality aperture for not much money.
But if you
a large star party, you'll see something puzzling. There'll be a lot of people using Dobs up to 20" or larger. There'll also be a lot of people using 8", 10", and 12" SCTs. But mixed in with these mid-
and large scopes, you'll see a fair number of people using 3" to 5" premium apochromatic refractors, such as those made by Tele Vue, Takahashi, and TMB. And those refractors, small as they are, aren't cheap. Even a small apochromatic refractor can easily cost $2,000 or more without a mount. The 5" models sell for $4,500 and up. So why would
pay so much for such a small scope? In a word, image quality. (Well, OK, two words.)
The big Newtonian reflectors and SCTs provide a lot of aperture, but they are obstructed scopes. They have a secondary mirror in the light
and, in the case of Newts, a spider assembly and secondary holder as well. Figure 3-9 shows a view looking down the tube of our Newtonian reflector. The 10" primary mirror is visible at the back of the tube. The secondary holder, held by its three spider vanes, is visible at the front of the tube.
Figure 3-9. The secondary holder and spider vanes of a Newtonian reflector
All of that gubbage in the light path has two undesirable effects:
central obstruction (CO)
image contrast relative to an unobstructed scope like a refractor. The amount of contrast reduction depends on the percentage of central obstruction. Typical Newts have a CO
from 15% to 25% linearly (2.25% to 6.25% by area). Slower Newts, such as f/8
models, tend to have central obstructions in the lower part of that range. Faster Newts, such as f/4.5 and f/5 models, typically have central obstructions at or near the top of that range. Typical SCTs and MCTs have a CO ranging from 33% to 40% linearly (10.9% to 16% areally).
At 4% or less areal CO, the effect of the central obstruction on contrast is unnoticeable. At 6% to 8%, the reduction in contrast becomes noticeable, particularly on low-contrast targets, such as planetary surface detail. At 8% to 10% areal CO, contrast reduction becomes severe; at 12% areal CO, many observers consider the contrast reduction unacceptable. (This is why SCTs, despite their flexibility and popularity, are not the best choice for planetary observing.)
Diffraction from the circular secondary mirror in Newts and SCTs causes overall contrast reduction, but Newts have a further problem. Those straight spider vanes also cause diffraction, but rather than scattering the
light over the whole image, they concentrate it into sharp spikes that are visible around bright objects, such as planets and bright stars. Many observers find diffraction spikes distracting, and they can actually interfere with some observing activities, such as splitting double stars.
Refractors have no central obstruction, and so do not suffer from these diffraction effects. The upshot is that refractors provide sharp, high-contrast views that are more esthetically pleasing to many observers than the views through a Newt or SCT. But the superiority of a refractor view is an illusion. A larger obstructed scope provides more image detail than a small refractor, but the image is not as pleasing. Still, a lot of astronomers are willing to pay large sums for that pleasing image.
Steve Childers, Paul Jones, and Robert did a field test one night in Steve's front yard. Steve set up his 10" Dob, Paul his 8" SCT, and Robert his 90mm refractor. We then turned all three scopes toward Jupiter. The planet was well placed, and the atmospheric stability (seeing) was
. We wanted to see how these three scopes, of widely differing types and apertures, performed on Jupiter. To keep the comparison fair, we ran similar magnificationsabout 225X for the Dob, 230X for the SCT, and 210X for the refractor.
We won't keep you in
. We all agreed that the 90mm refractor provided the most esthetically pleasing views. They were sharp, crisp, and of exceedingly high contrast. But in terms of actual detail visible, the 10" Dob beat the 8" SCT and simply blew away the 90mm refractor. Aperture Rules.
If you have a medium or large Dob or other obstructed scope, you can gain most of the benefits of that $2,000 apo refractor at a cost of $0 and a few minutes' work. All you need is a sheet of cardboard, a ruler, a compass, a pencil, and a sharp
. You'll use these to build an
off-axis sub-aperture mask,
usually called an
An aperture mask blocks the scope's aperture entirely except for a small off-center circular hole, cut so the secondary holder and the spider vanes do not intrude. Light enters the scope only through that off-axis hole. Because the secondary holder and spider vanes are masked off, they contribute no diffraction to the image. In effect, the large masked mirror functions like a small mirror without any obstruction.
To create an aperture mask for your scope, follow these steps:
Measure the inside diameter of the the tube and the diameter of the secondary holder. Measure the primary mirror, or simply use the size specified. In our case, the tube was 300mm inside diameter, the secondary holder 63mm, and the primary mirror 250mm.
Use your compass to draw three concentric circles of those sizes on a sheet of stiff, thin cardboard.
Use the ruler to draw a line from the center of the circle to one edge.
Measure the distance between the inner circle (the secondary edge) and the middle circle (the primary mirror edge). Call that distance X (X is half the difference between the diameter of the primary and the diameter of the secondary). In our case, X was about 93mm.
Calculate 0.5X and place a pencil mark at the mid-point of the radius line between the two inner circles.
Set your compass to 0.5X (less a couple millimeters for slop), put the center pin of the compass at the pencil mark you made in the
step, and score a circle. This circle should touch neither the inner circle (the secondary edge) nor the middle circle (the primary mirror edge). We could have drawn the aperture circle up to 93mm in diameter, but that would assume a perfect fit and no slop. Instead, we decided to use an 85mm circle to allow for some misalignment and slop while still being sure that
the secondary holder nor the edge of the primary mirror would intrude into the aperture mask hole.
Intuitively, it seems as though it should matter whether you have a three-vane or four-vane spider. In fact, it doesn't. The limiting factor on the diameter of the aperture mask circle is the distance between the edge of the primary and the edge of the secondary in either case.
Carefully cut the cardboard around the outer circle to form the body of the aperture mask. Make radial 1/4" long cuts every couple inches around the circumference of the circle to make the aperture mask easy to seat and remove.
Cut out the inner off-center small circle that will be the aperture mask hole. Try to make a smooth, regular, circular cut, but don't worry too much about minor mistakes. (Even a square or oval hole works, although a perfect circle is optimum optically.)
To install the aperture mask, simply slide it into the front of the tube, arranging it so that none of the spider vanes are visible through the aperture. As it
, we installed our aperture mask, shown in Figure 3-10, with the aperture hole near the focuser. We could have placed the mask between any two of the vanes. Unless your mirror is hideously bad, it doesn't matter which pair of vanes you choose.
Figure 3-10. The aperture mask in place on our 10" Dob
So how does it work? Pretty well, actually, if you like the idea of turning a 250mm scope into an 85mm scope. The 85mm aperture-masked scope is a lot dimmer than a 250mm scope, and it can't resolve detail as finely. But it does provide the kind of image you'd expect looking through a $2,000 85mm apochromatic refractor. Sharp, snappy, high contrast, and visually pleasing, with no false
or other aberrations.
An inexpensive Dob with an aperture mask isn't the true equivalent of the expensive apo, of course. If it were, no one would buy apo refractors. If you think of the apo as a fine Swiss watch, the Dob is more like a crude Soviet tank. If you value fine workmanship, the apo refractor is simply in a different class.
The other difference is focal length. A typical 85mm or 90mm apo refractor has a focal length of 450mm to 600mm. Our 85mm aperture-masked Dob still has the native focal length of the underlying scope, 1,255mm. At high magnification, such as for Lunar and planetary viewing, the different focal lengths don't matter. But for rich field viewing, such as browsing Milky Way star fields, the shorter focal lengths of the apo refractors give them a huge advantage. Our aperture-masked Dob has a maximum possible true field of about 2, versus twice that or more for the apo refractors.
Is it worth building an aperture mask yourself? Sure, why not? It costs
to nothing, takes only a few minutes to do, and you may fall in love with the apo-like images. But before you decide to make the aperture mask a permanent part of your observing kit, we suggest you do some side-by-side comparisons. View the Moon, planets, double stars, and other objects with and without the mask in place. We think you'll decide, as we have, that the advantages of larger aperture greatly outweigh the disadvantages.