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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?

4: What is an astrograph?

Yes, one could answer simply, it's a camera. More than that it is not, but it's a camera for astronomical use and with their specific requirements. Astronomers used glass plates instead of film when they photographed not too long ago. Dimensions of a glass plate was big, often 160 x 160 mm but also even larger size could occur, this astrograph used several different sizes depending on what was to be photographed.

Field of view:

An astrograph is different from a telescope, one thing is that it has a larger corrected field of view. While Saltsjöbaden's double refractor with a focal length of 8 meters and mirror telescope of 18 meters compare to the astrograph's only 2 meters focal length. With the size of the glass plates used is obtained a field of view of about 4.5 x 4.5 degrees, this is counted as wide angle in the astronomy world, while in the normal photography world would be considered a telephoto (with respect to angle). As comparison, the moon takes up an angle of about 0.5 degrees and the Andromeda Galaxy about 3 degrees.

Optical construction

A typical simpler refractor telescope has a lens composed of only two lens elements, such lenses are often called doublet. An advantage of few lens elements is in addition to the economic light loss is low and the requirement antiglare lower due to fewer transitions between air / glass. Anti reflection treatment was not so well developed 75 years ago. The disadvantage of the simple two lens elements construction is that control of the lens refraction of light rays becomes low. The lens works only good when the lens f number is high. F-number represents the ratio of the focal length and lens diameter, are also known as opening ratio or brightness here.

Observatory Saltsjöbaden and its astrograph

The astrograph's glass plate holder, the square hole we see here is approximately 160 x 160 mm.

Double lens construction's limit for acceptable quality usually go at a ratio 1:12 or f/12 for the glass qualities that were to available at that time. For example, the double refractor's (as consists of two telescopes) which each have a double lens, where the relationship is 8000/600 = f/13. For an astrograph aims as mentioned earlier after a large field of view, the lens must have a flat focus field over the entire image area. In addition, they have low f number to reduce the exposure time and then have the ability to detect faint objects.

Observatory Saltsjöbaden and its astrograph

Here is the complicated construction with counterweights clear and the time scale to R.A. axis and again, admire the dome!

Some optical data Carl Zeiss have set for Saltsjöbaden's astrograph.

Limit Magnitude: 16  
Focal Length: 2000 mm  
Lens Diameter: 400 mm  

With this data, the opening ratio (brightness) is calculated to 2000 / 400 returns f/5. If the lens diameter "only" had been 200 mm it had gave f/10. The requirement exposure time increases with the square of opening ratio, the difference will be that a 30 minutes exposure at f/5 must be extended to 2 hours at f/10, in reality much worse. The difference between being viable or not.

To build an astrograph with a f/5 ratio in this dimension is very complicated and extremely expensive. This astrograph is a refractor construction, i.e. the use of lenses to refract the light. The lens is basically a triplet structure but where the outermost lens is split into two positive lens elements. The reason is that a single lens had given very sharp radii of curvature of the optical surfaces, with two lenses are the respective radii significantly less. As a bonus the optical-designer gets an another lens elements to use to the correction of light refraction through the lens system. A refractor in this the size of four lens elements is impressive. Now someone might say that his / her camera lens indeed has 12 lens elements, but it is quite another thing to mass produce small lenses to ordinary cameras, compared with this handmade mammoth. Modern cameras also usually have a zoom lens requiring more lens elements to handle the more complex optical design.

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