Observatories that I have visited:
the Astrograph at
Stockholm's "new" Observatory, Sweden

Content:

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?

7: Tracking and motor operation

When you doing astrophotography and takes images of faint objects (not the moon, the sun and planets), it requires very long exposure time particular in that days insensitive glass plates.

Kerstin Lodén says:

"Exposure times due course, on the bright stars the program was. To get down to B = 14 m, which was about as far down to the faint stars that telescope allowed, exposed at most 20 minutes. Though must also exposing the default stars in each panel (more on this later). For spectral plates was the longest exposure time I used 60 minutes. It could therefore be quite a few plates in one night."

To make long term exposures possible the camera (astrograph) have to be mounted on a swivel device so the rotation of the earth can be counteracted.

It is this small motor that driving the telescope of several tons! To function correctly it requires that all equipment are carefully balanced. The force transmitted from motor to telescope is through the black vertical axis in the background.

Here is where the driving motor shaft come up, underneath and in the middle of the image. It drives the worm gear (you see part of it) to R.A. (Right Ascension) axis. It does one revolution in 23 hours and 56 minutes (but the scale is 24h) to counter compensate the Earth's rotation. It is a "star day" taking this time, 4 minutes shorter than our normal "sun day". Has to do with the Earth's motion around the sun.

The counterweight on Dec. axis. All these counterweights must be perfect balanced if the motor drive control shall have its precision maintained.

This precision mechanics have also been designed and built by Carl Zeiss. The engine propelling the assembly can be synchronized by a central clock. Such construction is at a constant rate, however, if you seeks high resolution images is not even this enough. Metal construction (assembly), which holding the astrograph is sensitive to temperature variations, they deform and thus give "pointing" error when the night's chill affect the assembly. Yet another thing complicates the whole, the atmosphere refracts light different for different elevation angles. Taking exposures of several hours (which is not made here) it will let the object to be photographed go from low to high elevation angles and down again and thus affect the engine speed, it's necessary to have a motor speed compensation.

Even the second axis, Dec. (Declination), need to be fine-tuned during the exposure. This shaft has no motor, the adjustment is made entirely by hand. Dec. axis is perpendicular to R.A. axis. Here is the exchange and lever of this riveting, it is mounted on the tube.