SAAO - South African Astronomical Observatory

0.5-m Telescope Modular Photometer

The Modular Photometer is the dedicated 0.5-m instrument. This photometer is of a fairly conventional single-channel design except that the filter wheel and aperture wheel are plug-in modules to facilitate maintenance and repair. A Hamamatsu R943-02 GaAs tube is used to cover the UBV(RI)C passbands.

Acquisition and Apertures

The focal plane aperture wheel is manually operated and contains ten circular apertures of approximate angular diameters 10, 15, 20, 25, 30, 35, 45, 60 and 90 arcsec and 8 arcmin, the latter being used for field identification. The aperture wheel has been designed so that the centres of all the apertures locate to better than 75 ┬Ám when brought into the optical axis. The apertures are viewed directly or via an LCD display. No offset guiding is available.


The filter wheel contains ten positions for 25 mm diameter or 25 mm square filters. UBV(RI)C filters are provided as standard; observers requiring other filters should check with the Head of Telescope Operations (rrs at before applying for time.  Observers wishing to use their own filters should make arrangements beforehand. Filters and/or filter wheels must be changed by a technician and may not be changed by observers or during the night.

In addition to the colour filters, neutral density filters are provided in a four-position wheel containing: "clear" (i.e. no filter), 0.4ND, 1.0ND and 2.0ND filters, corresponding to light reductions of 0, 1, 2.5 and 5 mag, respectively.

Transformation equations for UBV(RI)C are regularly checked and updated by SAAO observers. Observers using neutral density filters must be prepared to determine their own colour equations.


A Linux PC controls filter selection and integration times according to pre-programmed instructions, and controls the instrument using the program LUCY (all-sky photometry) or MILLY (uninterrupted observations on one object with one filter with integration times in the range 2 ms-60 s). 

Observing Limits

Over-illumination of the GaAs tubes causes the tube sensitivity to be enhanced in an unpredictable way and it may take several minutes for the tube to return to normal. Serious over-illumination will destroy the tube. Consequently, the electronics are pre-programmed to move the filter wheel to a position between filters (blocking light to the photomultiplier) should a preset count rate be exceeded. At present this limit is 400000 s-1 and MUST NOT be changed by observers. The 'bright' limits are ~6.5 mag in B, V and RC , ~6 mag in U and ~5.5 in IC , with no neutral density filter. Neutral density filters must be used to observe stars brighter than these limits.

Observing limits


Further information can be found in the manuals on this page.