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--- courses:ast201:7 [2023/08/22 02:31] – asad
+++ courses:ast201:7 [2023/11/26 00:02] (current) – [3.3 Pros and cons] asad
@@ Line 132: / Line 132: @@
 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
-$$ f_{d,space} = \sqrt{\frac{hc\lambda}{\lambda Q}} \sqrt{\frac{b_\lambda}{t}} \frac{2.44}{D^2} $$
+$$ f_{d,space} = \sqrt{\frac{hc\lambda}{Q}} \sqrt{\frac{b_\lambda}{t}} \frac{2.44}{D^2} $$
-which means, increasing the aperture size gives a huge advantage in case of space telescopes because $f_\lambda \propto D^{-2}$ in that case.
+which means, increasing the aperture size gives a huge advantage in case of space telescopes because $f_d \propto D^{-2}$ in that case.
-==== - Background ====
+==== - Pros and cons ====
+Advantages are as follows.
-==== - Visible sky area ====
+  - 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
-==== - Atmosphere ====
+Disadvantages are as follows.
-==== - Environment ====
+  - 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
-==== - JWST ====
+===== - 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
-===== - Ground-based telescopes =====
+{{:courses:ast201:mirror-types.png?nolink&250|}}
 ===== - Adaptive optics =====