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Observatory Saltsjöbaden and its astrograph


A visit to Saltsjöbaden's Astrograph

  1. Introduction
  2. The excursion to Saltsjöbaden
  3. Historical background
  4. What is an astrograph?
  5. Optical performance
  6. Finder telescope
  7. Tracking and motor operation
  8. Object tracking, finder telescope
  9. The elevator
  10. The Dome
  11. Glass plates (film)
  12. What research was done with this astrograph?
  13. Who has worked on this astrograph?
  14. How it ended?

5: Optical performance

To get an idea of ​​the astrograph's optical performance, we can compare with my own camera, digital SLR camera manufactured by Canon, model 350D and a planned 200 mm f/5 lens. The sensor in this camera corresponding glass plate (film) has a dimensions 22x15mm comparing with this astrograph's 160x160mm. With Canon camera mounted on astrograph's lenses were each picture area just one hundredth of the visual field area astrograph takes into a single image! One can also imagine that the Canon camera mount a lens with 1/10 of the focal length of astrograph. Since the sensor in the Canon camera close to a tenth of the film (the side), the same angle obtained. A lens with a 200mm focal length and data brightness f/5 is nothing extremely objective. Why now not so, it would have been much cheaper? The answer lies in a lens theoretical resolution determined by objective lens diameter. It sets a maximum limit the resolution no matter how good the optical construction are. According to Carl Zeiss data, astrograph resolution 0.3 '' ( '' means arc seconds and 1'' is 1/3600 part of one degree), which is close to or equal to the theoretical resolution (there are many different methods to calculate this, among other things, affects the wavelength of light, commonly referred to wavelength 550 nm, yellow light). A lens at 200 mm focal length and f/5 have a lens diameter of 40 mm and is thus at best a resolution of 3''. It shall also be mentioned, the seeing of atmosphere is almost never better than 2" in longtime exposure photos.

It is not just to cover as big image area as possible as glass plates are out of this impressive size. Limitations of contemporary photographic emulsions made it advantageous to work in large format. A glass plate's resolution can be estimated at 50 lp / mm, but can be considerably higher, lp stands for line pairs, a black line followed of a white line. A primitive attempt to convert this to today's popular expressions "megapixel" is to figure as it corresponds to 100 pixels per millimeter, which gives 100x160x100x160 = 256000000, thus 256Mp, not bad, and this was 75 years ago!

My Canon's CMOS sensor has 8 million individual sensors (pixels), however, interact more individual sensors to form a "color sensor point". Feels pretty paltry in this comparison! Atmospheric seeing puts unfortunately a practical limit for how high the total angular resolution can be. My own estimate of the resolution obtained is that with the exposure times required with film in the 1930s it was not so much better than 2'' in practice, normally much worse. Later I will try to get a photographic plate taken with this astrograph. Imagine what exciting it would be to take a similar picture with current technology and compare!

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