SAAO - South African Astronomical Observatory

SAAO CCD Camera (STE3/STE4)

Introduction

The SAAO CCD camera is available for direct imaging on the 1.0-m and 1.9-m telescopes. Two SITe back-illuminated detectors are available: either a 1024x1024 pixel CCD (designated STE4) or a 512x512 pixel chip (STE3).  When mounted on the 1.9-m, the camera is controlled by a DOS PC (manual available here).  On the 1.0-m, the control software is Linux-based (manual available here). 

The detector is contained within a cryostat mounted at the Cassegrain focus, together with a remotely-operated filter box and an autoguider.  Software is available at the telescope for determining guide star positions.  This setup is describe in the TCS manual for each telescope.

Filters

Bessell UBVRI filters are available for use with the SAAO CCD and are installed at all times; there is also an empty slot in the filter wheel for white light observations.  We have one of each of the following filters, available on request:

  • Hα
  • BG38
  • Z

Please be sure to list every filter you require for your program in Section 4 of the SAAO Telescope Time Application form. 

Visitor filters up to 51 mm square and 10 mm thick can also be accommodated.  Visitors wishing to use their own filters should contact the Head of Telescope Operations (rrs at saao.ac.za) to discuss this when applying for telescope time.

Detector Properties

The table below gives the properties of STE4.  STE3 has very similar attributes.

Device Name STE4
Manufacturer SITe
Chip Type Back-illuminated
Number of Pixels 1024 x 1024
Pixel size 24 microns square
Scale (1.0-m) 0.31 arcsec/pix
Scale (1.9-m) 0.14 arcsec/pix
Field of view (1.0-m) 317 x 317 arcsecs squared
Field of view (1.9-m) 146 x 146 arcsec squared
Read Noise 6.5 electrons
Scale Factor 2.8 electrons/ADU
Linear Count Limit 65535 ADU
Readout Time (unbinned) [STE3] 43 sec [17 sec]
Prebin options 1x1, 2x2
U zero point (1ADU/sec)(1.0-m) 19.20 mag
B 22.00 mag
V 22.35 mag
I 21.80 mag

Data reduction

There are Linux PCs located in the domes for image processing using IRAF and a pipeline based on DoPHOT.  The latter can be used for real-time reductions, to obtain reduced CCD photometry of stellar fields while observing. As each image is written to disc, the program picks out the correct flatfield, cleans and flatfields the frame and writes it out as a FITS file. It then runs DoPHOT on the image to produce aperture and profile-fitted magnitudes. If you are observing the same field repeatedly, DoPHOT will display differential profile-fitted magnitudes as a function of time. The results from DoPHOT can be further processed using routines in STAR to produce the final results. 

Data backup & archiving

Observers can backup their data to their laptop, or transfer it to their home institute via ftp.  Alternatively, observers may arrange with IT staff to write their data to DVD for them.  The computers in the domes do not have DVD-writing facilities, nor functioning USB ports.