SAAO CCD Camera (STE3/STE4)

Introduction

Two SAAO CCD cameras are available for direct imaging on the 1.0-m telescope, but currently unavailable on the 1.9-m telescope due to failure of the DOS control system. The two cameras are distinguished by their detectors: STE4, a 1024×1024 pixel back-illuminated CCD, and STE3, a 512×512 pixel version of the same chip, with one quarter of the field of view of STE4.  The control software is Linux-based (manual available here).  Software is available at the telescope for determining guide star positions, as described in the TCS manual for each telescope.

Filters

Bessell U B V R I filters are available for use with the SAAO CCDs and are mounted at all times (click on filter name to view transmission curve); 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:

Please be sure to list every filter you require for your program in Section 4 of the SAAO Telescope Time Application form.  This will ensure that your desired filters are scheduled for your run.

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.  Where the attributes of STE3 differ, they are given in square brackets.

Device Name STE4 [STE3]
Manufacturer SITe
Chip Type Back-illuminated
Number of Pixels 1024×1024 [512×512]
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″x317″ [158″x158″]
Field of view (1.9-m) 146″x146″ [73″x73″]
Read Noise 6.5 e [5 e]
Scale Factor 2.8 e/ADU [1.9 e/ADU]
Linear Count Limit 65535 ADU
Readout Time (unbinned) 43 sec [17 sec]
Prebin options 1×1, 2×2
U zero point (1ADU/sec)(STE4 on 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 the local STAR package 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.