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) passbands.

 

The control electronics, data acquisition and reduction systems are essentially the same as those for the St. Andrews Photometer on the 1.0-m telescope (Section 3.3).

 

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 when brought into the optical axis. The apertures are viewed directly or through a red-sensitive image intensifier. No offset guiding is available.

 

Filters

The filter wheel contains ten positions for 25 mm diameter or 25 mm square filters. UBV(RI) not be changed by observers or during the night. Observers wishing to use their own filters should make arrangements beforehand. filters are provided as standard; observers requiring other filters should check with Dr. Dave Kilkenny (dmk@saao.ac.za) before applying for time. Filters and/or filter wheels must be changed by a technician and may

 

Neutral Density Filters

A four position filter wheel contains "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.

 

Photomultipliers

A Hamamatsu R943-02 GaAs tube is provided as standard, although it is possible that S11 or extended S20 tubes can be used for specific purposes. Observers who believe they will need tubes other than the standard GaAs should notify SAAO well in advance of their observing run.

 

Electronics

A PC controls filter selection and integration times according to pre-programmed instructions. Data acquisition is by pulse-counting and on-line reductions are standard. An oscilloscope allows monitoring of the signal by sampling every second (sample rate can be changed).

 

Data

Data acquisition, storage and transfer to Cape Town are as for the 1.0-m telescope and St. Andrews photometer (Section 3.3). Transformation equations for UBV(RI) are regularly checked and updated by SAAO observers. Observers using neutral density filters must be prepared to determine their own colour equations.

 

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 400 000 s and MUST NOT be changed by observers. The 'bright' limits are 6.5 mag in B, V and R , 6 mag in U and , with no neutral density filter. Neutral density filters must be used to observe stars brighter than these limits. 5.5 in I

 

Performance

Performance will be similar to that of the St. Andrews Photometer as used on the 1.0-m telescope, but for stars 1.5 magnitudes brighter.

 

Manuals

• 20" Photometer Operating Manual (Mechanical)
• LUCY: A program for the acquisition of data and on-line reduction of photometry
• MILLY: A high-speed photometry program