Blog de diseño óptico
Telephoto
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Ocular 10x Apocromático.
Telescope
Maktusov-Newton
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 a Maksutov-Newton.
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
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ttp://brilloestelar.com/articulos/manual/03_telescopes.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 900 mm, a f/4.2, where the main mirror has a diameter of 214 mm but the lenses have a diameter of almost 300 mm. The FOV of the 2º system, a little more than 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 doublet, followed by a main mirror, a secondary mirror and a lens relay in the eyepiece holder. 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 of the X and Y field.
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. At 95 cycles per mm the system has an approximate efficiency of 45%.
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 smaller than Airy's radius in all fields, as can be seen in the Spot Diagram graph. In this case, we check, as we said before, that the image in semi-field 1º has a little comma, but it is also checked there is a little spherical aberration.
The spherical aberration is introduced above all by the first system of lenses and compensated by the same system. Mirrors do not introduce almost aberrations because both mirrors are revolution ellipsoids. Although I have emphasized the defects of the system, we can see in the report that the spherical aberration is 0.425 waves and the comma -0.364 waves, small values for a system as fast as this.
The system has a distortion less than 0.35%, the field curvature is less than 10 microns, it is achromatic with a maximum focal displacement range of 8.56 microns, the lateral color is less than 1 micron and the longitudinal aberration is less than 0.04 millimeters. The relative illumination is practically 100% in the whole field.
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Thank you very much! See you around!