2.3 Vibrating Flat Membranes
23
Fig. 2.6 In the Zöllner-photometer the light of a flame in the Bunsen burner on the
right is projected into the field of view to act as a comparison star. Between the flame
and the telescope focus a rotatable polarizer is used, which can adjust the light of the
comparison star. The structure on the left is a counterweight. See also Fig. 10.11). From:
Efron Encyclopedic Dictionary [29]
images of stars. For this he used a heliometer, that is a telescope in which the
objective lens is cut in half and both halves can move independently. In this way
he captured two random stars at the same time, which he de-focused, and then
compared the brightness of the two blurred images. Such a telescope is called
a heliometer, because it was originally developed to measure the diameter of
the Sun by making the two opposite edges coincide in the image. The Zöllner
photometer was a very significant improvement on previous methods and revolutionized the measurement of the brightness of stars. Zöllner photometers
have been in use for decades.
There is no doubt that Kapteyn has used such an instrument. Indeed, the
Utrecht Observatory had such a photometer, although preserved catalogs of
the available instruments do not go back long enough to check that it had one
in Kapteyn’s time. But most likely it did. This also means that Kapteyn must
have spent a lot of time on practical astronomy.
23
Fig. 2.6 In the Zöllner-photometer the light of a flame in the Bunsen burner on the
right is projected into the field of view to act as a comparison star. Between the flame
and the telescope focus a rotatable polarizer is used, which can adjust the light of the
comparison star. The structure on the left is a counterweight. See also Fig. 10.11). From:
Efron Encyclopedic Dictionary [29]
images of stars. For this he used a heliometer, that is a telescope in which the
objective lens is cut in half and both halves can move independently. In this way
he captured two random stars at the same time, which he de-focused, and then
compared the brightness of the two blurred images. Such a telescope is called
a heliometer, because it was originally developed to measure the diameter of
the Sun by making the two opposite edges coincide in the image. The Zöllner
photometer was a very significant improvement on previous methods and revolutionized the measurement of the brightness of stars. Zöllner photometers
have been in use for decades.
There is no doubt that Kapteyn has used such an instrument. Indeed, the
Utrecht Observatory had such a photometer, although preserved catalogs of
the available instruments do not go back long enough to check that it had one
in Kapteyn’s time. But most likely it did. This also means that Kapteyn must
have spent a lot of time on practical astronomy.