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--- courses:ast201:7 [2023/08/20 00:49] – [2.2 Cassegrain and Georgian] asad
+++ courses:ast201:7 [2023/11/26 00:02] (current) – [3.3 Pros and cons] asad
@@ Line 59: / Line 59: @@
 The tube of the Cassegrain is much shorter than a Newtonian which means Cassegrain is much cheaper to build. A Gregorian is longer than a Cassegrain but still shorter than a Newtonian.
+The conic constant for the primary $K_1=-1$ (paraboloid) for both, and for Cassegrain the conic constant for the secondary
+$$ K_2 = -\left(\frac{m+1}{m-1}\right)^2 $$
+which is determined by the requirement that SA should be zero.
+==== - Ritchey–Chrétien (RC) ====
+Both Gregorian and Cassegrain have $K=-1$ which means they cannot avoid coma and astigmatism. But an **aplanatic** (without coma and astigmatism) two-mirror telescope called the Ritchey–Chrétien (RC) is made using a primary mirror of different. In an RC, both the primary and secondary are hyperboloid and their conic constants
+$$ K_1 = K_{1c} - \frac{2(1+\beta)}{m^2(m-\beta)} $$
+$$ K_2 = K_{2c} - \frac{2m(m+1)}{(m-\beta)(m-1)^3} $$
+where $K_{1c}$ and $K_{2c}$ are the conis constants of the Cassegrain.
+RC reduces coma and astigmatism, but the other aberrations are higher in RC than in Gregorian. But Gregorian has longer tube and greater central obstruction due to the concave paraboloid. So RC is the design of choice in many cases.
+==== - Nasmyth and coudé foci ====
+Below panel (a) shows the Nasmyth focus and (b) the coudé focus. Both are used in cases when heavier instruments need to be placed at the final focus. Nasmyth can take heavier instruments than Cassegrain and RC, but coudé can take even more load than Nasmyth.
+{{:courses:ast201:nasmyth-coude.png?nolink|}}
+In (a) a flat mirror intercepts the light from the secondary and redirects it horizontally along the altitude axis to the Nasmyth focus which remains fixed relative to the mount during tracking. Gravitational stress on the telescope and mount do not change over time for this.
+In (b) (shown in an equatorial mount here) the light from the secondary is intercepted and, first, brought along the declination axis and then to the polar axis.
+==== - Schmidt telescopes ====
+RC can give good quality images for up to 1 deg, and maybe up to 3 deg with corrector lenses. Schmidt telescopes are used for larger field of view, 6 to 8 deg. They are specially good for photographic surveys.
+{{:courses:ast201:schmidt.png?nolink|}}
+In this configuration, a spherical mirror is used as primary and an **aperture stop** is placed at the center of curvature of the primary to remove SA. A corrector plate is placed on top of the aperture stop.
+Light from different angles illuminate different parts of the primary, and the focal surface, but the chief ray from any direction always passes through $C$. So $b=0$ and all off-axis aberrations are gone.
+The corrector plate then removes SA. The marginal rays ($\rho$ large) converge more strongly than the axial rays ($\rho$ small, closer to axis) as you saw in Section 5.3 of Chapter 6. A Schmidt corrector has higher power for the axial rays than for the marginals, this bringing all rays to a common focal plane.
 ===== - Space telescopes =====
+==== - Resolution ====
+$$ \theta = 2\alpha = \frac{2.44 \lambda}{D} $$
+==== - Sensitivity ====
+The number of photons detected by a detector is the **Signal**
+$$ S = \frac{\pi D^2}{4} \frac{\lambda}{hc} f_\lambda tQ $$
+where the factor $hc/\lambda$ converts from the energy units to number of photons, $Q$ is a factor dependent on the bandwidth and efficiency of the detector, $t$ is the exposure (integration) time, and $f_\lambda$ is the actual flux from the object at a particular wavelength.
+A star is detectable if $S$ is about the same size as its uncertainty. The measurement
+$$ M = S+B $$
+where $B$ is the **background** containing everything other than the star in the image. The background can actually be both in front of the star or behind star or at the same distance as the star. The background
+$$ B = \frac{\pi D^2}{4} \frac{\lambda}{hc} \frac{\pi\theta^2}{4} b_\lambda tQ $$
+where $\theta$ is the size of the star in the image and $b_\lambda$ the actual background flux. Now according to Poisson statistics the uncertainty in counting photons, the noise,
+$$ N = \sigma(S) = \sqrt{(S+B)+B} \approx \sqrt{2B} = \frac{\pi D\theta}{4} \left(\frac{2\lambda b_\lambda tQ}{hc}\right)^{1/2} $$
+and, thereby, the signal-to-noise-ratio (SNR)
+$$ \frac{S}{N} \approx \left(\frac{\lambda Qt}{hcb_\lambda}\right) \frac{D}{\theta} f_\lambda $$
+for a star that is marginally detectable. Purring $S/N=1$ results in the flux of such a star
+$$ f_\lambda = \sqrt{\frac{hc}{\lambda Q}} \sqrt{\frac{b_\lambda}{t}} \frac{\theta}{D} $$
+for a telescope whose image results in the diameter of a star to be $\theta$. In case of ground-based telescope, the size is that of the **seeing disk**. For space telescope putting $\theta=2.44\lambda / D$ gives
+$$ f_{d,space} = \sqrt{\frac{hc\lambda}{Q}} \sqrt{\frac{b_\lambda}{t}} \frac{2.44}{D^2} $$
+which means, increasing the aperture size gives a huge advantage in case of space telescopes because $f_d \propto D^{-2}$ in that case.
+==== - Pros and cons ====
+Advantages are as follows.
+  - Background $b_\lambda$: the darkest background for HST is $23.3$ mag arcsec$^{-2}$, and the background for the darkest ground-based telescopes is $22.0$ mag arcsec$^{-2}$. For JWST, the sky at 5 $\mu$m is 12 mag darker in space than on the darkest ground.
+  - Atmospheric transmission
+  - Visible sky area
+  - Gravitational and frictional stress
+Disadvantages are as follows.
+  - Cost. One 8-m Gemini telescope cost 100 million USD, and one 2.4-m HST telescope cost 2 billion USD.
+  - Thermal stress
+  - Harmful radiation, high-energy particles
 ===== - Ground-based telescopes =====
+The [[wp>Hale telescope]] had 5-m mirror made of Pyrex glass and weighed 14 tons. The paraboloidal mirror needs a moving support structure that weighs 530 tons. Completed in 1949, it set the standard of the time. There are 3 stages of making a mirror.
+  - Casting: molten glass poured into a cylindrical mold
+  - Annealing
+  - Grinding and polishing
+{{:courses:ast201:mirror-types.png?nolink&250|}}
 ===== - Adaptive optics =====