SALTICAM: the SALT imaging and acquisition camera
SALTICAM was designed as a multi-purpose device, acting as the telescope acquisition camera as well as a science imager/photometer. It is situated at the prime focus, but fed by a 45° fold mirror. SALTICAM was built at the SAAO with D. O'Donoghue as PI.
It is a simple instrument with the following functional flow:
If you already know the instrument, SALTICAM detailed technical information can be found here.
The lens system reduces the SALT f/4.2 prime focal ratio to f/2, thereby enabling the ful 4-arcmin science field of view, as well as almost all of the guide star field of view, to be captured on the 2x1 CCD mosaic. The lenses are made from UV-transmitting crystals, and the CCDs have excellent UV performance, so the instrument is expected to be very efficient at short wavelengths. The optical design is illustrated below.
Fields Of View
The next illustration shows the fields of view superimposed on the detector and includes:
The CCD Detectors
The detector is a 2x2 mosaic of 2kx4kx15 micron pixel CCD 44-82 chips from E2V Technologies. These devices are thinned, back-illuminated and coated with the E2V Astro Broad Band coating. They are also deep-depletion devices for better near-infrared sensitivity and lower fringing. A schematic of one of the chips is shown below.
An example raw SALTICAM frame is shown below. The two CCDs with the 1.5 mm gap between them are obvious. Also obvious are the two amplifiers in each chip (with different bias levels) and the vignetting of the field in the corners. Image quality at the time of this image was not optimal, which accounts for the poor star images at the edges.
High Time Resolution
The illustrations below show how high time resolution
imaging can be obtained. For moderate time resolution (a few sec), frame
transfer operation is used. This is explained by the left hand diagram: a
mask (shown in grey) covers the lower half of each chip. At the end of
each exposure, the image in the top half of the chip is rapidly (200
millisec) shifted to the lower half where it is readout while the next
image in the top half accumulates during the next exposure.
Even faster sampling can be obtained with so-called slot mode: in this mode a mask is advanced over the entire chips except for a slot just above the frame transfer boundary. Instead of half frame transfers at the end of each exposure, 144 rows are moved and this allows exposure times as short as 100 millisec. The slot position is illustrated in the right hand diagram above and the readout scheme is shown in the illustration below.
Of course both techniques require field of view to be sacrificed for time resolution: in frame transfer mode, half the field of view is lost; in slot mode, only the slot is available for imaging. The intended use of slot mode is to position a rapidly varying target star and a brighter nearby companion star in the slot to perform differential photometry of the variable with respect to the comparison star. See further discussion on photometry with SALTICAM below. The telescope rho stage can be rotated to locate comparison stars at arbitary position angles within the slot.
Implications Of Doing Photometry With SALTICAM
The moving pupil inherent to the basic operation of SALT presents special problems for doing photometry with SALTICAM. While it is true that if the tracker position is known at all times, the fraction of the primary mirror within the pupil can be calculated (including gaps between mirrors) and the photometric "response" function of the telescope can be worked out. However, this assumes equal reflectivity for all mirrors; clearly this will not be true and furthermore it will be variable as the cycle of mirror recoating runs. (Typically, at least one and possibly two segments per week will be recoated).
Measuring reflectivities of mirrors is an uncertain process so it seems very difficult to provide calibrations sufficient to estimate the response function to at least 1 per cent (preferably better) for all tracker positions.
So those carrying out photometry with SALTICAM should bear in mind:
More Information about SALTICAM