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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?

11: Glass plates (film)

An astrograph gives no high-quality images unless the whole chain is optimized. A very important link in this chain is the glass plates. Glass plates photosensitive surface, the emulsion is about chemistry in the highest degree. To get the maximum performance astronomers had to prepare the glass plates, exactly how much of the herein disclosed technology utilized here at the observatory in Saltsjöbaden I don't know.

Kerstin Lodén tells:

"The glass plates were bought finished (Eastman Kodak or Agfa). The format was 16 x 16 cm for the so-called direct plates. It was in the 1940's and 50's in the B (blue-sensitive), or V (visual, i.e. more yellow sensitive). Of the astrograph's view of field of 4.5 degrees, we utilized a field of 1.5 x 1.5 degrees to the direct plates we used. There was also a different size, 18 x 24 cm used for spectral. At such observation was added a prism lens to the telescope in front of the lens. This way we had spectra, while not detailed but many stars at once. The prism was put on the daytime and had to sit at a time so you had to plan your observations then. In some instances we screened of the lens opening from 40 cm to 20 cm."

A very important feature of the glass plates, or rather, photo emulsion that it is coated with is its sensitivity to detect the photons (light).

Observatory Saltsjöbaden and its astrograph

Nippe explains that this certainly is not a simple lens hood, it is a dew mantle!


An English expression to make photo plates more photosensitive.

Several techniques used:
Baking, heating glass plate for a few day, usually 50 to 75oC.

Gas atmosphere, the glass plate placed in a sealed chamber and exposed for a gaseous atmosphere in few hours to days. Nitrogen and hydrogen were common gases, hydrogen is highly explosive!

Chilling, glass plate was cooled down during exposure.

After glass plates had undergone these processes could storage time may be as short as a couple of hours. Then in other words, they must be exposed directly, in some extreme cases "gassed" during ongoing exposure. The first two methods performed before the exposure while cooling was performed during actual exposure. Emulsion sensitivity could be increased by a factor of 2 to 25 times with these methods.

Reciprocity failure:

The English expression reciprocity, a very negative characteristic, namely, that the sensitivity decreases with exposure length. Variation may be so large that only a 1/20 of the sensitivity obtained at an exposure of one hour relatively one minute. One way to reduce this was too cooling the film / glass plate. This was done practically so that on the camera was placed a piece "Dry ice", dry ice is carbon dioxide in solid. When dry ice "melts" it cools the glass plate. Carbon dioxide melts at -55oC and that is the temperature of the glass plate it cooled to.

Printing and storage:

The latter developing process was obviously also a very important step in the process to develop a picture. The handling of the film and glass plates required great care, they were very sensitive to dust, scratches, damaged by moisture and inappropriate temperatures.

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