Blog de diseño óptico
Telephoto
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Ocular 10x Apocromático.
Telescopio
Argunov-Cassegrain
In this link we can see a different design of a Maksutov-Newton, which has different characteristics. The article I write again so that the reader has all the information on this page.
In the next post we are going to review the fundamentals of a telescope and show a design of my own.
The main mission of a telescope is to allow us to see or appreciate details of very distant objects. There are many configurations of telescopes; some use mirrors, others only lenses and there are combinations of both. The type I present is a telescope made up of lenses and mirrors, that is, the catadioptric type. Specifically, it is a version of an Argunov-Cassegrein.
There is a lot of documentation about the field of telescopes. Here are some interesting links:
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http://aam.org.es/images/GCP/Fundamentos_de_telescopios_para_aficionados.pdf
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http://www.castello.es/archivos/560/Telescopios_y_your_mountings.pdf
In all telescopes, the focal point of the first group (target) coincides with the focal point of the second group. This converts the telescope into an afocal system, since the rays parallel to the axis will come out parallel as they pass through the eyepiece.
In a telescope there are several very important features.
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The lateral magnification in a telescope is calculated by dividing the focal length of the telescope by the focal length of the lens.
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The luminosity of the telescope is calculated by dividing the focal length of the telescope by the diameter of the telescope.
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Another feature is that the first group has a much larger focal length than the second. In Figure 1 we see a schematic of the telescope.
The telescope I present has a focal length of 3521 mm, an f/8, where the main mirror diameter is 406 mm. The FOV of the 2º system, a little more than the common, which is usually 1º or 1.5º.
In Figure 1 we see the characteristics of the telescope. In the previous links are explained in detail how to calculate and what are the characteristics of a telescope.
Figure 2 shows the design of the telescope.
Figure 1. Characteristics of the catadioptric telescope
Figure 2. Report of the catadioptric telescope
The system consists of a negative meniscus at the entrance to the telescope, followed by a main mirror. Next we find three lenses before the secondary mirror. For this design it has been necessary to use a "double step", since the light passes twice through the lenses located before the secondary mirror. The first time it is reflected by the primary mirror and the second time it is reflected by the secondary mirror. Finally, in the eyepiece holder we are going to find a lens and a doublet followed by a flat mirror that is going to incline the light. In this system I have placed in front of the secondary mirror an opening that blocks the rays coming from the first surface. We see that there are rays that cross the secondary mirror because I have used points from the X and Y fields.
The telescope presented is limited diffracted, as we can see from the MTF graph. In this graph we see that the theoretical MTF has a steep slope. This is due to the opening in front of the secondary mirror. This system is good as a theoretical example, but it would not be good if it were put into practice. To put it into practice I would need to reduce the diameter of the secondary mirror and the diameter of the lenses in front of it.
In the Ray Fan and OPD graphs we see that the system is well compensated, showing a little comma as we approach the end of the field. It is remarkable the scales of the graphs, 10 microns and 0.5 waves respectively. This comma affectation is also seen in the graph of the WaveFront Function, where we see a higher edge.
The Spot Size RMS is much smaller than Airy's radius in all fields, especially in semi-fields from 0º to 0.6º, as can be seen in the Spot Diagram graph. In this case the comma is very evident, but it remains totally inside the Airy disk.
The spherical aberration is introduced mainly by a group of lenses in front of the secondary mirror. Part of the spherical aberration is finally compensated by the group of lenses in front of the parallel plane mirror behind the primary mirror. Mirrors do not introduce almost aberrations because the primary mirror is elliptical and the secondary hyperbolic mirror. Although I have emphasized the defects of the system, we can see in the report that the spherical aberration is -0.143 waves and the coma 0.253 waves.
The system has a distortion less than 0.35%, the field curvature is less than 0.20 millimeters (I find this value a little high for the precision required by a system like this). The telescope is achromatic with a maximum focal range of 7.056 microns, the lateral color is less than 2 microns and the longitudinal aberration is less than 0.08 millimeters. Relative illumination is almost 100% throughout the field.
I leave images where we can see what a simulated image would look like and what the irradiance of the system is like.
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Thank you very much! See you around!