Strömgren Photometer

The Strömgren photometer is attached to the East focus of the T90 telescope. It was designed and manufactured in Denmark, the electronic control and data acquisition being supplied by the IAA. This photon counting photometer has two modes of operation allowing simultaneous observations of the bands uvby of the Strömgren system or the bands n and w Crawford Hβ photometric system.

In the uvby mode the photometer uses a diffraction grating to separate the different spectral components and four photomultiplier tubes to measure the four channels. In the Hβ mode the light beam is divided by a beam-splitter and transmitted to two photo-multipliers through one wide band and one narrow band interference filter, respectively. The filters are centred on the band and measure the continuum and the spectral line.


Simplified photometer design.


As it can be seen in the simplied design, at the entrance of the instrument (to the left in the figure) we find a Barlow lens, necessary for adapting the focal length of the photometer, f:15, to the focal length of the telescope, f:8. Behind the lens there is a shutter in the form of a wheel with three positions: open, closed and neutral filter. Behind the wheel shutter is the prism for the field view-finder and the prism for the diaphragm view-finder, both located at the fromt of the photometer. Between the two prisms is a wheel containing the six different aperture diaphragms (not shown in the figure as it is below the spectral slits). In the diaphragm view-finder there is a CCD that allows automatic centralisation of the object by the general control programme.

  • Strömgren uvby mode

    In the optical axis, after the wheel containing the diaphragms, there is an achromatic collimating object lens before the diffraction grating. The reflecting diffraction grating, in the Littrow mode, has 1200 lines per millimeter. For fine adjustment of the angle of incidence at the grating, there is a micrometer screw with a relative dial indicator located at the front of the instrument. The light reflected by the grating passes back through the refocusing object lens to a row of spectral selection silits. There are two possible spectral slit options: the first allows operation with only the sepctral slits and a second, that is actually in use, where a combination of slits and interference filters are in operation.The mirror surfaces are coated with an interfering deposit giving maximum reflection in their particular spectral band, reducing spurious light and reproducing the original Strömgren filter responses. In front of the photo-multipliers are the very high transmission (from 76% in u to 90% in y) interference filters that with the spectral slits give the responses shown in the following table.

  • Crawford Hβ mode

    After the diaphragm wheel the photometer mechanically inserts a plane mirror in the light path at an angle of 37°.5 to the optical axis to reflect light to the Hβ. A beam-splitter splits the light into two paths, 85% to the Hβn (narrow) filter and 15% is reflected to a reflecting collimator that displaces the beam vertically over the other path to the Hβw (wide) channel. The spectral characteristics are shown in the table.After passing through the slits a group of spherical mirrors reflect the different beams onto the photo-multipliers.


Band λmin (Å) λmax (Å) λcentral (Å) FWHM (Å)
u 3686 3324 3505 330
v 4222 4006 4110 170
b 4801 4572 4685 183
y 5635.5 5346 5488 235
Hβn     4862 30
Hβw     4875 150
Spectral bands defined by the slits and interference filters


Although the common use is in the uvby and Hβ modes the photometer may also be used in other configurations such as with the use of neutral filters or in the mode.

  • Neutral filters

    There are two types of filters: The neutral filter with an optical density of 10 and the vb. The first is located at the photometer entrance to protect the photo-multipliers from bright objects and to enhance the response to faint stars. The second neutral filter vb attenuates, even more, the v and b chanels up to 50% of the first filter. Both filters can be inserted manually into the optical path by a lever next to the diaphragm view-finder on the front panel.

  • Hδ mode

    There is also a calibration mode using an additional spectral slit, 200 m wide (4 Å in the spectrum) in the channel and centralising the wavelength corresponding to the Hydrogen line, Hδ. Thanks to this spectral slit very precise calibrations can be made using the absorption line Hδ of the trails of stars of low radial velocity and suitable spectral quality.

The most important advantage of this photometer is the simultaneous measurement in different bands, in both working modes, that permits rapid and precise measurement obtaining the colour indices avoiding the problems of interpolation and variations in extinction.