Series Overview:
This series will be a write up of “Source book in Astronomy 1900-1950”. The book is split up into 13 sections with each one covering a different topic in astronomy. My plan is that as I read through each topic I write up the important points and then condense all of my notes into a blog post here. My hope for this series is to walk within a few feet of modern astrophysics. The sections are
- Instrumentation
- The Sun
- The Planetary System
- The Position and Motion of the Stars
- The Spectra of Stars and Nebulae
- Variable Stars
- Stellar Structures
- Spectrum-Luminosity Relationships
- Interstellar Phenomena
- Stellar Evolution
- Galaxies
- Relativity and Cosmogony
- Survey of Astrophysical Progress
I completely expect most of the information here to be outdated and not close to the cutting edge but hopefully will provide a historical foundation.
Instrumentation:
The 200-inch Reflector on Palomar – George Ellery Hale
This entry described a new 200 inch main mirror diameter telescope being built on mount Palomar. The telescope was funded by the Rockefellers and specified that a condition of the funds was that Cal Tech would create an astrophysics program. The new telescope was designed to supplement an existing observatory on mount Wilson. The reason behind its large size was not just to create an additional large telescope but to spur the development of auxiliary equipment for use by large telescopes. One of the main developments was a new material for the main mirror disk. Unlike prior telescopes this was made out of Pyrex by Corning due to its low coefficient of thermal expansion. Originally they had planned to cast the mirror out of fused silica but it was unable to be cast in large enough disks.
The reasoning behind why it was built in the northern hemisphere was because as a large telescope it had a small field of view. A small field of view means it isn’t very good at scouting for new stars, its only good for observations. Most known stars are mapped well in the northern hemisphere making it the more desirable hemisphere for an observatory.
A New Type of Mirror System – Bernhard Schmidt
Telescopes with large aperture ratios suffered from large spherical aberrations. This paper talks about the placement of a corrective lens in front (or behind) the primary lens in a telescope to correct for spherical aberrations allowing for large aperture ratios. The correcter plate would in essence be have the curvature of the main mirror but in reverse. Now both the correction plate and the main reflector have aberrations but they are opposite and equal causing them to be cancelled out and resulting in a new aberration free image. The corrector plate also corrected for comma and astigmatism. This corrective plate is called a Schmidt corrector plate and while it does decrease the overall efficiency of the telescope it is extremely helpful when building large aperture ratio telescopes and led to the creation of Schmidt Cassegrain telescopes. This paper also describes a small amount of how you would form this corrector plate.
Invention of the Coronograph – Bernard Lyot
Historically, the only time the corona has been able to be studied has been during solar eclipses. This was because the sun is so much brighter than its corona. Someone was smart enough to realize, hey lets make an eclipse inside the telescope.
Coronograph
They placed a blackened copper disk at B to create an artificial eclipse of the sun allowing only the light from the corona to pass further through the telescope. This telescope arrangement was quite sensitiveness to dust and was placed in the Pyrenees at an altitude of 2870 m to help alleviate this dust problems. Coronographs became popular tools at most solar observatories around the world.
A Stratospheric Solar Observatory – Meghnad Saha
Ozone stops most ultraviolet and larger wavelength light from reaching sea level. This makes astronomical observations of these bands extremely difficult if not impossible. Cairo expected that the poles would not have any ozone durring their winters due to the lack of sunlight but due to the mechanics of ozone destruction this turned out not to be the case. Regner has floated balloons into the upper atmosphere to take automated readings and has had success with imaging ultraviolet light there. At 40 Km a spectrogam of the sun will extend the spectrum to 2000 nanometers. That additional spectrum would provide information about
- L_alpha of hydrogen
- The lithium continuum
- About resonance lines of elements from Be to O
- Resonance lines of magnesium and aluminium to silicon
- Information regarding transition elements, mainly Fe+
The Beginning of Radio Astronomy – Grote Reber
There’s an electromagnetic disturbance in the microwave range arriving from the milky way. This is not caused by interstellar dust particles. Different stars, planets, and nebula were investigated and were found not to be the cause.
Note: this would later be known as cosmic microwave background radiation and would earn a Nobel in 1978, three years after my copy of SBA was printed.
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