This script calculates the baffle positions in a Newtonian telescope tube according to the algorithm from Nils Olof Carlin.
A typical Newtonian telescope is shown in Figure 1. Light from the object enters the telescope tube, reflecting from a paraboloidal primary mirror. The light is then reflected from an elliptical mirror that is tilted at 45 degrees. The light proceeds through a hole in the side of the tube to a focus, the image, where it may be used for viewing or imaging.
In order to be used as such, light must enter the tube from multiple angles, to provide a non-zero sized image field. Three of these angle are depicted in that figure, using a different color for each field angle. For any single field angle the light from a distant object that strikes the primary mirror is a cylinder, slightly elliptical in cross-section when the object point for that field is not directly ahead. However, the union of all such cylinders is cone shaped. For an otherwise unrestricted field of a given diameter, the minimal size of that cone is at the primary mirror.
In a refractor telescope, baffling is used to remove unwanted reflections from the tube walls. A Newtonian telescope has different problems: direct access to the eyepiece or imager from outside, and unwanted reflections on the tube interior wall from the mirror. Unwanted reflections from outside to the tube walls are not usually considered, as they would be at oblique enough angles that they would be weaker from a single wall reflection and, in addition, need at least two wall reflections to get to the eyepiece or imager.
Extraneous light can sometimes be reduced by simply disallowing light from outside the field of view. This can be achieved to some extent by introducing a ring-shaped baffle at the end of the tube. This is shown in Figure 2. This baffling may also be designed to allow some modest vignetting at the field edges.
Rather than attempt to define strict dimensions for baffling, it is often useful to allow some variation in the size. This can be achieved by considering a light cone larger than the original light cone, as indicated by the dashed lines in the figure. This ends up being a tolerance on the size of the hole.
Two baffles are used to prohibit light from directly entering the focuser.
The focuser baffle is near the focal surface and can consist of a single ring. Its diameter is chosen to match the virtual size of the secondary mirror and the size of the field. It is indicated by the arrows in Figure 3, in its default position: as far away from the image as possible without cutting into the entry cone.
Some baffling may also be needed on the tube wall opposite the focuser. It is not necessary that this be a ring, but some extent around the tube is necessary. This baffling is also indicated in Figure 3. The line-of-sight line is drawn indicating the relationship of the image, the focuser baffle, and the opposite wall baffling.
A simpler approach, though, is to have the entry aperture baffling handle the baffling for the opposite wall. As long as the Newtonian Baffle Calculator indicates that the distance of the opposite wall baffle is less than the tube length, an actual baffle is redundant.
Reflection baffling is used to disallow reflections from the primary mirror, off the tube wall and on to the image. It is easier to describe it in terms of a virtual secondary mirror. This is shown in Figure 4. That diagram indicates a slice of the telescope as seen from above, assuming the secondary mirror were replaced with a clear sheet, allowing the light to travel straight through to the image. The length of the vertical line in the figure indicating the secondary is thus the minor axis of the secondary.
For light to traverse the path described above, it must first reflect from the primary mirror and hit the tube wall. There is a closest approach to the primary mirror that can occur, when light skims the entry baffle, reflects from the opposite edge of the mirror and strikes the wall. The point on the wall of this closest approach is indicated by the arrow labeled "R" in the figure.
Similarly, for light to traverse the path described above, it must reach the image from the tube wall. To do this it must pass from the wall, hit the secondary, and be within the image. There is a closest approache to the image that can occur, when light skims the secondary and hits the opposite edge of the image. The point on the wall of this closest approach is indicated by the arrow labeled "P" in the figure.
Reflection baffles, then, only need to protect the region of the tube wall between these closest approach points, assuming, that is, that there is indeed such a region -- there may be none. As indicated in the figure, with a baffle placed between "P" and "R", each baffle protects in two ways. From the mirror side, it prevents mirror light from being reflected from the tube wall. From the image side, it precludes the light reflected from the wall from being seen in the image via the secondary.
Using the Newtonian Baffle Calculator is easy.
You must first know how the telescope will be used. For example, standard 1-1/4" (31.75 mm) eyepieces typically have a largest diameter field of about 26 mm (1.024"). The larger 2" (50.8 mm) eyepieces typically have a largest diameter field of about 42 mm (1.654"). Imaging field diameters will vary, depending on the film or chip size. The final use may also be partially determined by the primary mirror diameter and focal length -- these must also be known.
The remainder of the basic information is the inner diameter and length of the tube. The tube length used here is from the nodal plane of the primary mirror and does not include any additional length needed for attachments behind the primary.
Enter your own values for the above, overriding any existing values.
If you do not enter a field size, a size of zero is assumed. This is useful when, as in some cases, vignetting is allowed to start at the field center. Recalculation with updated values is then allowed.
If an entry aperture diameter is not provided, it will be calculated based upon the field size and focal length.
A secondary size (minor axis) and distance between the image and the tube wall may be provided. If secondary size is blank, it will be calculated, given the image to tube wall, and based upon the proportional size between the image and the primary, assuming the minimal size for no vignetting. Alternatively, if the image to tube wall is blank, the image to tube wall will be calculated, given the secondary size, and also based on zero vignetting. However, if both the secondary size and tube wall distance are blank, it is assumed that no secondary is to be used -- useful in cases where an imager is placed directly in the incoming light path.
If a secondary is used, the distance from the image to the focuser baffle and its size may be provided. If the focuser baffle distance is blank, one will be calculated. Its position will be placed at the edge of the incoming light cone. If the focuser baffle diameter is blank, one will be calculated, given its distance from the image, and based upon the proportional size between the image and the secondary, assuming the minimal size for no vignetting. No hole tolerance is used in the calculation of the focuser baffle diameter.
The default values for hole tolerance and baffle shift (moving the baffles to allow favorable geometries to be considered) may be overridden.
All inputs must be in consistent units, where "units" is specified. Outputs adapt to the same set of units as provided in the input.
After entering the desired inputs, pressing the "Calculate" button checks and completes those inputs. The complete set of baffles is also calculated.
A number of baffles may be needed to fully cover the wall region described above. These are listed. If necessary, multiple pages of data will be used. The "<<" and ">>" buttons can be used to shift between pages. For each baffle, its position, minimal diameter, and maximal diameter are given. The position is the distance from the primary mirror. The total number of baffles is also given. It may certainly be the case that a subsized baffle may be used as the last baffle. The equivalent value of the hole tolerance for the last baffle is also given in the output.
If a secondary is used, both the position and diameter of the opposite wall baffle are provided. The calculation is independent of the tube length and inner diameter. It may actually be calculated to lie outside the tube in both position and diameter.
The positions "P" and "R" are provided. "P" is given as the reflectance point and "R" is given as the aperture access point, indicating their underlying nature. In addition, the exit overlap is given. It indicates the position, relative to "R", of the last calculated point of reflection before the calculations concluded. It may be positive or negative, depending upon whether the initial position of the last baffle was after or before "R". In both cases, however, that baffle is moved and resized to achieve the desired coverage.
Copyright © 2008 Richard F.L.R. Snashall