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--- courses:ast201:7 [2023/11/25 23:53] – asad
+++ courses:ast201:7 [2023/11/26 00:02] (current) – [3.3 Pros and cons] asad
@@ Line 146: / Line 146: @@
 Disadvantages are as follows.
-  - Cost. One 8-m Gemini telescope cost $100 million, and one 2.4-m HST telescope cost $2 billion.
+  - 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 =====