40cm RC telescope

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[[Cassegrain Optics]]
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See also: [[Cassegrain Optics]]
 
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==Cassegrain Optics==
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Here are the formulas, how to calculate the basic optical parameters of a cassegrain telescope.
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First we have to declare the sign conventions we will use:
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===Sign Conventions===
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* Light entering the optical system travels from left to right.
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* Distances from left to right are signed positive; those from right to left, negative.
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* Curvatures with the convex side to the left are signed positive; otherwise they are negative.
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* Intersection points above the optical axis are positive; those below the axis are negative.
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* An angle between a ray and the optical axis is measured in the direction of the ray. When this angle is up from the axis, it is positive.
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* The sign of the refractive index is the same as the sign of the direction in which light travels in the medium.
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* In the case of reflection, therefore, the signs of the refractive indices are reversed.
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* Surfaces are numbered in the sequence that they are hit by the rays.
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===Basic parameters===
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All Cassegrain telescopes consist of a concave primary mirror with a small secondary mirror inside the focus of the primary; the secondary redirects starlight toward the primary. The image, in most cases, lies behind the primary. The convex secondary multiplies the focal length by a factor ''M''. This factor ''M'' is termed secondary magnification, and is defined:
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<div style="text-align: center;"><math>M = \frac{f}{f_{1}}</math></div>
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The focal length of the system is:
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<div style="text-align: center;"><math>f = M \cdot f_1 = \frac{f_1 f_2}{f_1 + f_2 -d}</math></div>
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We can calculate the basic parameters seen in the picture with the formulas below:
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<div style="text-align: center;"><math>bfl = d + b = \frac{(f_1 - d)\cdot f_2}{f_1 + f_2 - d}</math></div>
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<div style="text-align: center;"><math>f = d + b + M \cdot d</math></div>
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<div style="text-align: center;"><math>f_1 = d + \frac{d + b}{M}</math></div>
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<div style="text-align: center;"><math>f_2 = \frac{-(d + b)}{M - 1}</math></div>
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Where <math>f_1</math> is the focal length of the primary, <math>f_2</math> is the focal length of the secondary (it is a negative value because of the conventions), and ''d'' is the separation. ''b'' is the back focus of the system, and ''bfl'' is the back focal length.
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The focal surface has a radius of curvature:
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<div style="text-align: center;"><math>\frac{1}{R_F} = \frac{2}{r_1} - \frac{2}{r_2}</math></div>
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Where <math>R_F</math> is the radius of curvature of the focal surface, <math>r_1</math> is the radius of curvature of the primary and <math>r_2</math> is the radius of curvature of the secondary.
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The sagitta of the mirror is: (sagitta is the height of the mirror at a given radius.)
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<div style="text-align: center;"><math>z = \frac{h^2}{r(1 + \sqrt{1-(h^2/r^2)(SC+1)})}</math></div>
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where ''z'' is the sagitta, ''h'' is the radius where we calculate the height of the mirror, ''r'' is the radius of curvature of the mirror, ''SC'' is the conic constant (explained later).
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===Conic constants===
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The conic constant, or Schwarzschild constant defines the shape of a conic section.
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<div style="text-align: center;"><math>SC = -e^2</math></div>
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where ''e'' is the eccentricity of the conic section.
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====Calculating the conic constants====
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We will use the dimensionless quantities defined by Schwarzschild:
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<div style="text-align: center;"><math>S = \frac{f_2}{f_1} = \frac{r_2}{r_1}</math></div>
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<div style="text-align: center;"><math>T = \frac{D_2}{D_1}</math></div>
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<div style="text-align: center;"><math>R = \frac{d}{f_1}</math></div>
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<div style="text-align: center;"><math>E = \frac{b}{f_1}</math></div>
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<div style="text-align: center;"><math>M = \frac{f}{f_1}</math></div>
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<div style="text-align: center;"><math>B = \frac{b}{f}</math></div>
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The '''third-order Seidel Coefficients''' for a two-mirror Cassegrain are:
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'''Spherical aberration:
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'''<div style="text-align: center;"><math>A_{cass} = 1 + SC_1 - \Bigg[SC_2 + \Bigg(\frac{M + 1}{M - 1}\Bigg)^2\Bigg]\frac{(M - 1)^3\cdot(1 - R)}{M^3}</math></div>
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'''Coma:'''
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<div style="text-align: center;"><math>B_{cass} = \frac{2}{M^2} + \Bigg[SC_2 + \Bigg(\frac{M + 1}{M - 1}\Bigg)^2\Bigg]\frac{(M - 1)^3\cdot R}{M^3}</math></div>
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'''Astigmatism:'''
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<div style="text-align: center;"><math>C_{cass} = \frac{4(M - R)}{M^2(1 - R)} - \Bigg[SC_2 + \Bigg(\frac{M + 1}{M - 1}\Bigg)^2\Bigg]\frac{(M - 1)^3\cdot R^2}{M^3(1 - R)}</math></div>
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To simplify the formulas above, we will use additional quantities:
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<div style="text-align: center;"><math>\alpha = \Bigg(\frac{M + 1}{M - 1}\Bigg)^2</math></div>
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<div style="text-align: center;"><math>\beta = \frac{(M - 1)^3\cdot(1 - R)}{M^3}</math></div>
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<div style="text-align: center;"><math>\gamma = \frac{(M - 1)^3\cdot R}{M^3}</math></div>
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<div style="text-align: center;"><math>\delta = \frac{2}{M^2}</math></div>
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<div style="text-align: center;"><math>\epsilon = \frac{4(M - R)}{M^2(1 - R)}</math></div>
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<div style="text-align: center;"><math>\vartheta = \frac{(M - 1)^3\cdot R^2}{M^3(1 - R)}</math></div>
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Since the spherical aberration is zero; <math>A_{cass} = 0</math>, we can calculate the different conic constants of the primary and secondary mirrors.
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For the '''classical Cassegrain''', the primary is parabolic <math>SC_1 = - 1</math>, so the secondary is:
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<div style="text-align: center;"><math>SC_2 = - \alpha</math></div>
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For the '''Ritchey-Chrétien''', both spherical aberration and coma are eliminated, so <div style="text-align: center;"><math>A_{cass} = 0</math> and <math>B_{cass} = 0</math>. The conic constants of the mirrors are:
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<div style="text-align: center;"><math>SC_1 = - \Bigg( 1 + \frac{\beta\cdot\delta}{\gamma}\Bigg)</math></div>
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<div style="text-align: center;"><math>SC_2 = - \Bigg(\alpha + \frac{\delta}{\gamma}\Bigg)</math></div>
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==Gallery==
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{{The RCC and its subsystems}}
 
{{The RCC and its subsystems}}
  
 
{{Piszkéstető}}
 
{{Piszkéstető}}

Latest revision as of 08:56, 14 January 2015

The 40cm Ritchey-Chrétien telescope is located at Piszkéstető Mountain Station with a primary mirror of 40cm and has an effective focal ratio of f/6. The telescope was manufactured by Gemini Telescope Design. Currently the telescope is under developement and not working.

[edit] Parameters

[edit] Optical parameters

Primary mirror diameter 400 mm
Primary mirror focus 1405 ± 30 mm
Secondary mirror diameter 160 mm
Secondary mirror focus -977 ± 22 mm
Separation 907 ± 5 mm
Focal length of the system 2460 mm (2840 mm without the focus reductor)
Field of view 25.1 x 18.9 arcmin
Pixel scale 0.453 arcsec/pixel

[edit] Detector

FLI ML8300

Camera Chip Model Kodak KAF 8300 chip
CCD type Front illuminated
Maximum Cooling 60°C Below Ambient
Full well capacity 25500 e
chip size 17,96 x 13,52 mm
chip size 3326 x 2504 pixel
pixel size 5.4 μm

See also: Cassegrain Optics

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