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Stray Light Control in Newtonian Telescopes
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Control of stray light is very important in telescope design and construction. This article looks at many of the issues involved. Preventing stray light from washing out the image formed by all those expensive optics is one of the best and least expensive ways to make you telescope perform to its utmost.
To begin, consider how light is brought to a focus in a Newtonian telescope. For the examples in this article, a 20 inch mirror with an F/4 focal ratio will be utilized. All of the figures were drawn with great accuracy (0.001"), but not with mathematical precision (errors in drawing may be cumulative).
Light enters the telescope as a parallel wavefront. This wavefront travels in straight rays orthogonal to the wavefront. Light on the optical axis will be focused on the optical axis. This entering light impinges upon a mirror with a parabolic shape and is reflected in such a way that all rays intersect exactly at the point of focus. The following figure denotes the various rays of light across the aperture and the path they take on their way to the point of focus. At all points farther from focus than the mirror, the rays are parallel and the diameter of the aperture. The focus point lies 80 inches in front of the surface of the mirror.
Light from other directions nearby the optical axis will be reflected off the mirror and brought to focus nearby the focal point on the optical axis. Thus. light from a small field of view will be brought to a focal surface and rendered as an image. The scale of this image is controlled only by the focal length of the instrument. The brightness of the image is controlled by the size of the objective1 and the reflectivity2 of the surface on that mirror.
In the following figure light rays are traced reflecting off the primary mirror and being brought to a focal surface that is one inch in diameter, centered on the optical axis. For all this light to reach that focal surface, that light must not be obstructed by any opaque surface. That is; those light beams must be unvignetted. This is then called the fully illuminated field of view.
At the height above the mirror where this image exists, the diameter of the unvignetted beams taken together is 21.00 inches in diameter. Down where the focuser will be positioned, the size of the unvignetted beams is reduced to 20.83 inches in diameter.
The astute observer will see that all these beams appear to pass through a pupil positioned at the face of the primary objective. This is called the entrance pupil.
The human eye is adept at focusing parallel rays but quite unable to focus the diverging rays directly from the telescope primary focus. Thus, an eyepiece is placed so that this image can be transferred into the human eye. Alternately, a camera can be placed at the focal surface and capture the image directly.
In the following figure on the left; a Plössl eyepiece illustrates the light path in an eyepiece. After the light cones from the primary reach focus, they diverge form focus and are intercepted by the eyepiece and are collimated back into parallel rays. As the eyepiece collimates these rays from different parts of the illuminated image, it causes all these ray beams to exit through a pupil. By placing the eye pupil at the exit pupil of the eyepiece (telescope) the eye can see a wide field of view with the property that each point in the image appears to be at infinity (focal distance-wise).
In the following figure on the right; a barlow has been inserted in the light path between the image to be formed by the primary and the image to be received by the eyepiece. The image surface for the primary mirror is marked and is in the same vertical position as the image in the eyepiece only seen to the left. The barlow takes the converging light cones from the primary and rediverts them to a new focal surface farther away. If the new focal surface is twice as far away from the point of divergence, the barlow has effectively doubled the focal length of the primary objective, and simultaneously, halved3 the focal ratio; in this case from F/4 to F/8.
The barlowed eyepiece now, effectively, operates as an eyepiece with half the focal length from before, twice the power as before, at half the focal ratio, and produce an exit pupil of half as big, but with the eye relief of the original eyepiece or slightly greater. The observer will immediately see that the barlow must remain out of the fully illuminated field of view. This will, by necessity, cause the eyepiece to be significantly farther from the unvignetted rays. Thus, when placing the secondary mirror (yet to come) consideration of the focal surface must accommodate a barlow in the path without vignetting the marginal rays. In many cases, a focal distance of 2" back from the marginal rays will accommodate the barlow in the optical path without vignetting the marginal rays.
It is the eyepiece that converts the scale of the image formed by the primary into a magnified image seen by the observer. The magnification of an eyepiece is the focal length of the objective divided by the focal length of the eyepiece. The size of the exit pupil is the size of the aperture divided by the magnification. The exit pupil size can also be calculated by dividing the focal length of the eyepiece by the focal ratio of the objective.
Direct Stray Light
There are tow kinds of stray light; direct and indirect. All direct stray light should be eliminated. As much indirect stray light as practicable should be eliminated. Observations from very dark sights can ignore most indirect stray light. Observations from light polluted areas should prevent as much indirect stray light as practicable.
Consider placing an eyepiece in such a manner that the eyepiece collimates the illuminated focal surface so that an eye can be placed to observe the illuminated field of view. All of the light reflected from those beams of light and illuminating the focal surface contribute to the brightness and contrast of the image. Any other light which can reach that focal surface will also illuminate the image and degrade the contrast of that image. Thus, this article is about where stray light comes from and how to manage the stray light, so as to achieve the maximum contrast of the illuminated field of view.
In the following figure the eyepiece focal surface is made coincident with the focal surface of the telescope. This is the optimal position for an eyepiece to transfer the formed image into a human eye that happens to have no optical aberration--that is perfect vision. That eye placed at the exit pupil would see the illuminated image--except that the body of the observer would be blocking most of the light! But, I digress-- this article is about stray light.
All the light coming from the primary is light that will form the image, and all the light coming from everywhere else is light that will degrade that image. In use, the head of the observer will block stray light from the human side the eyepiece. This still leaves a lot of solid angle where stray light can illuminate the image surface from the telescope side. Thus, stray light that must be addressed if the image is to be both bright and display high contrast. The following figure illustrates the size of the volume--it is considerably bigger than the mirror itself!
One of the most powerful tool in blocking stray light is a focuser baffle. This is a opening placed in front of where the image will be formed that lets in all of the light from the primary and blocks a large portion of light from elsewhere. Using knowledge of the depth needed for a barlow to be inserted in the optical path, a focuser baffle is positioned just far enough into the light from the primary so that the barlow can fit. Later on, we will position this focuser baffle just beyond the marginal rays where it will not vignette any part of the image rays. In the following image, we see that this focuser baffle has eliminated roughly 2/3rds of the stray light in a linear sense, getting rid of almost 90% of the stray light in a solid angle sense.
The diameter of the opening is calculated to be 1.50 inches to allow an F/4 scope to fully illuminate our focal surface 1.00 inches in diameter where the distance from the focuser baffle to the image surface is 2.00 inches.
The focuser baffle can be positioned deeper into the light path and block more stray light. This will end up requiring a larger secondary mirror to fully illuminate that field. This will require that the eyepiece also be positioned farther from the marginal rays to avoid having this focuser baffle vignette the marginal rays. Taken to extreme, especially with large heavy eyepieces, this can also stress the focuser and the superstructure holding the focuser.
The size of the hole in the focuser baffle can also be reduced in diameter to block more stray light. This can be used to shrink the size of the secondary mirror but also shrinks the size of the fully illuminated field. The smaller secondary might require placement farther forward into the light path. But: Big telescopes are all about big bright wide fields of view4.
Stray light control in Truss Newtonian Telescopes
So, the eyepiece is positioned with adequate depth from the marginal rays and then the light path is diverted towards the side of the telescope with a secondary mirror. The following figure illustrates the insertion of the secondary mirror into the light path. Thus, finally, a Newtonian telescope has been implemented.
A 4.00 inch secondary mirror5 can fully illuminate the 1.00 inch diameter focal surface 12.41 inches from the optical axis. A barlow and eyepiece combination will be right at the edge of the marginal rays in this figure. Notice that the center of the focal surface can be fully illuminated with a secondary mirror only 3.20 inches in diameter4.
Any superstructure than can hold the eyepiece, secondary mirror surface, and primary mirror surface in perfect collimation will allow these components to function as a Newtonian telescope. Thus, the glass of the primary and secondary mirrors, the mirror cell, the spider, and focuser exist simply to hold the reflecting surface in shape and allow those surfaces to be held in position.
So, consideration continues with the issue of stray light. The concept of the focuser baffle is accepted and added to this Newtonian telescope. Light from the opposite side of the optical path can now illuminate the focal surface unless a stray light shield is also added. The size of this shield is calculated directly from the size of the fully illuminated field of view, the size and position of the focuser baffle and where the stray light shield is placed. This is shown in the following figure.
The rays diverging from the focuser baffle cross the rays of light entering the telescope. A stray light shield opposite the focuser can prevent most of the remaining stray light from illuminating the focal surface. The stray light shield can be placed just outside of the marginal rays as is done with Dobsonian telescope using an upper cage assembly. Alternately, Dobsonian telescopes of the ultralight variety generally place the stray light shield outside the upper ring assembly where the stray light shield is out of the busy secondary support structures. As we shall see later, the ultimate stray light baffle is a complete tube.
Zooming in at the eyepiece area, we can see that stray light from above and below the stray light shield can only arrive outside of the fully illuminated filed of view. Thus, the stray light shield prevents stray light from washing out the image formed by the optics.
Lest anyone think that we are done with stray light, they should be reminded that optics is a difficult business.
The addition of the secondary mirror in the light path provides another path for stray light to arrive at the focal surface and degrade contrast. The only way to avoid this source of stray light is to use a secondary mirror that only illuminates the central point at focus. This is not in line with the principles of the Dobsonian telescope to provide big bright images.
This path has been overlooked in the classical Dobsonian architecture. The following figure shows how light from the near the primary mirror can reflect off of the secondary mirror and illuminate the focal surface. The large scale optical path is shown in the following figure.
Some may consider this a minor issue, but it is often the case that a tarp be placed on the bare ground just under the rocker box, some observe on concrete surfaces. These surfaces may seem dark, but the sky illuminates these surfaces, and they emit significant amounts of light. Thus it is best to shield the telescope optics from being able to see these areas. In addition, some telescopes have electronics mounted on the top just outside of the mirror box. These electronics gadgets create their own stray light and need to be shielded.
Notice, that the secondary mirror (in all these figures) is properly offset in the optical path. Notice, also, that the shape of the area to be shielded is not concentric with the optical axis. A secondary that is not offset6 will have an even larger non concentric area to be shielded.
A baffle near the primary, can be used to block this stray light. Alternately a shroud of dark fabric can surround the lower parts of the optical path and eliminate this direct stray light. In either case, the top of the mirror box should be flat black.
Finally, we add the most-marginal rays to the focal surface. The most-marginal ray comes to focus at the very edge of the eyepiece inner barrel. These rays are illustrated in the following figure.
These rays enter the optical train slightly vignetted by the upper ring assembly which was placed at the edge of those rays that illuminate the fully illuminated field of view. These rays are completely vignetted by the focuser baffle at an image diameter of 2.00 inches. Were the focuser baffle not be present, stray light from the upper ring assembly could add stray light to the outer reaches of the illuminated image through a path that cannot otherwise be baffled. Thus the back side of the upper ring assembly or upper cage assembly should be flat black. Furthermore, nothing on this side of the upper assembly should emit light.
Indirect Stray Light Control
Stray Light Control in Eyepieces
Eyepieces can also be treated to stray light control to improve image contrast. The telescope has been constructed to fully illuminate a field of view and to prevent stray light from damaging the contrast in this fully illuminated field of view. This presents certain issues when attention turns to baffling eyepieces.
Up first, is a typical Plössl eyepiece scaled such that it has a 30mm focal length. This is about the longest focal length Plössl eyepiece that can be fit into a 1.25 inch barrel and still deliver a 52 degree apparent field of view. This eyepiece accepts an illuminated field almost exactly the size of our fully illuminated field of view, besting it at a mere 1.063 inches.
When baffling an eyepiece, a ray is traced from the edge of the field of view near the stop of the eyepiece across the face of the eyepiece barrel on the opposite side, continuing to the wall of the focuser tube. Were this wall of the focuser tube to be illuminated by stray light, some of that stray light might find its way to the field of view and degrade contrast. The stray light entering the focuser baffle can be prevented from illuminating this edge by the placement of another baffle. The baffle must admit the entire cone of light that will be accepted by the eyepiece. This process is carried out repeatedly until the front of the focuser tube cannot be illuminated by any stray light. The following figure illustrates this construction technique.
The eyepiece baffles are not constructed from the light path in, but from the field of view outwards to the focuser baffle or, in the case where there is not focuser baffle, the edge of the focuser tube itself.
Next up is a 50mm Plössl eyepiece. This eyepiece accepts a larger filed of view than is fully illuminated. Nevertheless, the construction principles remain the same. The edge of the field of view to the wall of the focuser baffle determine the diameter of the baffles placed inside the focuser tube in order to optimally baffle this eyepiece. The following figure illustrates this construction.
It is immediately obvious that the larger the field of view accepted by the eyepiece, the more baffles are needed to eliminate all stray light.
When an eyepiece of conventional design has a very short focal length, a modification of the construction is appropriate. A ray is traced from the edge of the accepted field of view and outward to the opposite edge of the focuser baffle. Then a second ray is traced from the opposite edge of the focuser baffle to the inside of the eyepiece barrel. A baffle can be placed at the intersection of the light cone illuminating the accepted field of view. The following figure illustrates the simplicity of baffling a 6mm Plössl eyepiece. A single baffle placed somewhat inside the 1.25 inch barrel completely prevents any stray light from entering--thanks to the focuser baffle.
Our next example illustrates the technique of baffling eyepieces where the focal surface is inside the eyepiece. This example will also serve to illustrate how to baffle a barlow containing any eyepiece. The only real difference is that one constructs the cone on which baffles are placed such that it intersects the edge of where the field would have been imaged if the Smyth lens were not present. Then, one must prevent the outermost lens from being illuminated, rather than the field of view accepted by the eyepiece. The following image illustrates this construction for a 13mmNagler type 1.
It is immediately obvious that the wider the field of view accepted by the eyepiece, and the lower the first lens is to the focuser baffle, the more baffles are required. When 1.25 inch eyepieces are used in 1.25 inch focuser tubes even more baffles are required for complete control of stray light
For the big eyepieces such as the 31mm Nagler, no baffling is possible as the Smyth lens group is nearly at the focuser baffle, anyways, and because the eyepiece accepts almost the full field of view that enters the focuser baffle. In this telescope, the 31mm Nagler creates and exit pupil of 7.75mm which is larger than the dark adapted human eye.
Eyepiece manufactures cannot baffle eyepieces because the size and placement of the baffles is dependent on the fully illuminated field of view, the Focal ratio of the telescope, and whether a focuser baffle is present.
A telescope user may have more than one telescope. The telescope with the largest focuser baffle will be the telescope for which the eyepiece baffles are constructed. This prevents any one telescope from suffering some vignetting of the illuminated focal surface in any of the telescopes. Although suboptimal in a pedantic sense, this works exceptionally well.
Stray light control in Tube Newtonian Telescopes
In a very practical sense, using a tube as the superstructure of a 20 inch telescope is not very popular since the advent of truss tube Dobsonians, however in the sense of completeness we address this alternate architecture to stray light control.
A Newtonian telescope can utilize a tube to hold the optical elements in collimation. One suitable for this article is illustrated in the following figure.
Notice how long the tube extends in front of the focuser in order to block stray light. The tube is 84.00 inches long even after the focuser baffle is in place. The widest vehicle allowed on public roads is only 82 inches wide. Thus transporting this telescope to a dark site would require longitudinal placement or disassembly. Issues such as these were and are the driving force towards truss tube Dobsonians.
This particular tube is sized 1.00 inches larger than the primary mirror and has marginal rays just miss the opening at the front. It is likely that this tube would suffer from tube currents as warmer air flows off the primary mirror and into the light paths.
The length of the tube beyond the primary mirror was size only to prevent stray light from reaching the secondary mirror. In all likelihood it will have to be still longer to accommodate a mirror cell, collimation bolts, and mirror cooling fans.
The tube current issues can be addressed by making the tube even larger in diameter. The following figures illustrates adding a full inch of radius so that the warm air does not interact as much with the light we want to fully illuminate our chosen field of view. The front of the tube grew another 0.6 inches in length just to prevent stray light from peeking over the front. At the back the tub grew by another 0.4 inches to prevent stray light from reaching the secondary mirror.
An alternative to making the tube this long is to add the stray light shield to a shorter tube, a combination of the tube architecture and the truss architecture.
In this case, notice that the focuser baffle is inside the optical tube and will vignette the most marginal rays even though it does not vignette any of the fully illuminated field of view. Also notice that it may be a little difficult to position a camera sufficiently near the focal surface unless the focuser itself traverses the tube body.
The tube Newtonian architecture can be baffled for optimal use in light polluted areas. The construction of baffles follows the same general principles as seen when baffling eyepieces. Stray light can enter the tube from the front. The primary mirror should not be allowed to see any of the tube wall illuminated by this stray light. However, there is a special place where one should stop baffling the tube. This is where the secondary mirror can also see the tube wall. A baffle placed deeper into the tube from this position will reflect light into the cone visible off of the secondary mirror. Previous treatments of baffling tube based Newtonian telescopes have failed to illustrate this point.
Baffles can be extended deeper into the tube if the baffles are made non concentric with the optical axis. In any event, only one more baffle can be inserted into the design.
The correct placement of baffles and the correct place to stop is illustrated in the following figure.
The placement of baffles in a tube Newtonian brings back the issues of thermal control as warm air passes over each baffle edge and impinges on the marginal rays of the aperture.
In both tube and truss architectures, the constructor should pay particularly close attention to making the visible surface of the baffles and shields have as low an emissivity as possible. That is, one wants these surfaces to be flat black at all wavelengths that can reach whatever device is placed to collect the photons from the image. Thus, even if a tube Newtonian architecture is considered, the area that need the greatest attention to darkening are exactly those area that are baffled or shielded in the optimized open truss design.
After making sure that stray light cannot enter the focal surface, then it is time to do as much as possible so that the baffles and shields are illuminated as little as possible. If one has not prevented direct stray light from reaching the focal surface, it does little good to prevent the baffles and shields from being illuminated by (say) starlight.
Such a telescope has been constructed and used for nearly a decade. The following image taken at the radius of curvature illustrates the three principle means for controlling stray light. This telescope is as it will be used later than night. Notice that there is no light shroud. With proper control over stray light such a shroud is not needed at a dark site.
For this particular telescope, the focuser baffle allows a fully illuminated field of view of 0.7 inches, while the stray light shield provides a fully dark field of view of 1.2 inches, and the secondary mirror is but 3.7 inches on the minor axis. With this slight narrowing of the fully illuminated field of view, and the slight reduction in size of the secondary mirror the primary baffle diameter need only be as wide as the upper ring diameter--making for compact storage. This fully illuminated field of view has proven to be completely invisible at the eyepiece, allowing a 31mm Nagler eyepiece to avoid showing obvious vignetting on the observed field of view of 76 arc minutes. Even though this eyepiece in this focal ratio creates an exit pupil larger than can be admitted into the dark adapted human eye.
It should be observed that the spider vanes are almost invisible. This is on purpose. They are very thin (0.006") and also spray painted flat black.
The second image shows the rest of the stray light control used on this telescope. Notice that even the truss poles are blackened where they are near the light path. Everything in or near the light path is painted flat black, everything not near the optical path is finished like fine furniture.
1 the primary mirror in this case.
2 or transmission characteristics of a refracting objective
3 Opticians speak of focal ratios in the reciprocal sense: so F/4 is twice as fast as F/8
4 David Kriege, "The Dobsonian Telescope" Berry and Kriege. page 96
5 minor axis. Thus, the minimum possible obstruction in this telescope is 16% = 3.2 / 20 * 100%
6 This is decidedly not a rational to argue that offset secondary mirrors are necessary.
- HandyAndy, Oberon, mark77 and 2 others like this