saltslot – Clean raw SLOT mode data


saltslot images outimages outpref gaindb xtalkfile logfile (verbose)


String. List of images to reduce, including, if necessary, absolute or relative paths to the data.. Data can be provided as a comma-separated list, or a string with a wildcard (e.g. ‘images=S20061210*.fits’), or a foreign file containing an ascii list of image filenames. For ascii list option, the filename containing the list must be provided preceded by a ‘@’ character, e.g. 'images=@listoffiles.lis‘. Note that SLOT mode fits files often contain more than one exposed frame. In such cases, all frames will be reduced by default from any file specified in the list of images. Because the gaindb and xtalkfile arguments can only point to a single reference file, the images list must not contain data from more than one instrument. saltslot works specifically only on raw image data. Do not provide any files that have already undergone processing with SALT reduction software.
String. A list of images. Data can be provided as a comma-separated list, or a string with a wildcard (e.g. ‘outimages=rS20061210*.fits’), or a foreign file containing an ascii list of image filenames. For ascii list option, the filename containing the list must be provided preceded by a ‘@’ character, e.g. 'outimages=@listoffiles.lis‘. This list must be of the same size as the images argument list. If the output is intended for a different directory the absolute or relative path must be supplied with the file name.
String. If the outpref string is non-zero in length and contains characters other than a blank space, it will override any value of the outimages argument. Output file names will use the name list provided in the images argument, but adding a prefix to each output file defined by outpref. An absolute or relative directory path can be included in the prefix, e.g. ‘outpref=/Volumes/data/x’.

String. This is the name of an ascii table that contains the amplifier gains specific to a SALT instrument. The table is used to gain-correct amplifier raw count images. Gain is assumed to be uniform across an individual amplifier but assumed to vary from amplifier to amplifier. An example of the table format follows:

# Database of SALTICAM CCD amplifier properties
# 10 Aug 2006 - Telescope Data
SLOW FAINT  1.06  3.60  300 amp1
SLOW FAINT  0.99  3.43  300 amp2
SLOW FAINT  1.06  3.71  300 amp3
SLOW FAINT  1.07  3.69  300 amp4
SLOW BRIGHT 2.32  3.98  300 amp1
SLOW BRIGHT 2.17  3.78  300 amp2
SLOW BRIGHT 2.32  4.07  300 amp3
SLOW BRIGHT 2.33  3.96  300 amp4
FAST FAINT  1.55  5.22  300 amp1
FAST FAINT  1.45  5.11  300 amp2
FAST FAINT  1.53  5.51  300 amp3
FAST FAINT  1.58  5.61  300 amp4
FAST BRIGHT 4.26  6.39  300 amp1
FAST BRIGHT 3.96  5.88  300 amp2
FAST BRIGHT 4.21  6.35  300 amp3
FAST BRIGHT 4.32  7.02  300 amp4

RDNOISE refers to readout noise and BIAS refers to typical CCD bias levels. These data are calibrated regularly at the telescope and provided by the SALT project. Recent versions of the table are provided in the SALT IRAF distribution at salt/salticam/data/SALTICAMamps.dat and salt/pfis/data/PFISamps.dat, and updates will be publicized on the SALT web site at


String. This is the name of an ascii table that contains the CCD amplifier crosstalk coeffcients. The table is used to subtract cross-talk contamination from the CCD amplifier images. Crosstalk is assumed to occur at a constant level across an individual amplifier, but the coefficients are assumed to vary across amplifer pairs. An example of the table format follows:

# PFIS CCD amplifier crosstalk data
# from 20041201 gain = bright distortion image, outer amps duplicated
# Date    VCTM     2       1       4       3        6        5
#         SRC      1       2       3       4        5        6
2004-01-01     .001474 .001474 .001166  .001111  .001377  .001377

The crosstalk-corrected amplifier image is given by SRC - VCTM * coeff.

String. Name of an ascii file for storing log and error messages from the tool. The file may be new, or messages can also be appended to a pre-existing file.
Boolean. If verbose=n, log messages will be suppressed.


saltslot provides basic image reduction processing for SALTICAM and RSS SLOT mode data. SLOT mode is the continuous readout of the detector CCDs with no deadtime overheads. It is achieved by clocking only a small number of CCD rows towards the readout amplifiers between each exposure. The CCD is covered by a mask with only a thin slot cut from the center through which the detector can expose on the sky. The width of the slot is 144 raw pixels, which corresponds to the number of rows clocked after each exposure. saltslot will also work on other SALT modes and probably has value reducing Frame Transfer data.

With a maxmimum frame frequency of 10 Hz, SLOT mode is capable of producing a large number of image frames during a track. Traditional IRAF tools, although capable of reducing SLOT mode data, are not well suited to large amounts of frames. IRAF is flexible and generally robust but at the expense of economy. The IRAF approach is to perform operations in parallel, i.e. open all files, perform one operation, close all files, and repeat for all operations. The amount of time performing I/O operations rapidly becomes prohibitive with large number of images. saltslot is a python-based script which sacrifices sophistication and flexibility for speed. It has the ability to reduce SLOT data faster than realtime. The algorithms used in saltslot are generally identical to those used in other IRAF and PyRAF tools, e.g. salt.salticam.sgain, salt.salticam.sbias, mscred.xtalkcor. Much of the tools speed is in minimizing the amount of file I/O. Each image is opened and closed once, and only once. All data operations are performed in series.

The one departure from the standard IRAF algorithms is the calculation and subtraction of the bias level. Typically, it is assumed that the bias level varies across the detector; this assumption sacrificed for the sake of speed. saltslot assumes that the bias level is constant and calculates the median value in the overscan region, subtracting this value from all image pixels. This is not usually a good approximation, but in the case of SLOT mode, the active area of the chip is so small that bias derivatives across the CCD rows are almost negligible. Typically there is a one count difference between the top row and bottom row. Investigators concerned about this difference can remove it either by constructing and subtracting a master bias frame or using the slower, but more robust tools salt.saltprepare, salt.saltgain, salt.saltxtalk and salt.saltbias.

The saltslot procedure is as follows.

1. Ensure keyword and images structures are consistent with SALT standards.

2. Gain correct images. This is simply a multiplication of all pixels by a scalar quantity specific to amplifier, gain setting and readout speed. The gain factor is provided by an ascii table pointed to by the gaindb argument.

3. Subtract the bias level from images. The overscan region defined by the BIASSEC FITS keyword is used to estimate the bias level. The median pixel value in the overscan is adopted, and this value is subtracted from all pixels in the image.

4. Correct for amplifier crosstalk. Each SALTICAM and RSS CCD has two readout ammplifiers. There is crosstalk between them at the level of ~ 0.1% which, provided images are not saturated or non-linear, can be removed adequately by simple subtraction of a scaled image of one amplifier from it’s neighbour. The scaling factors are supplied as an ascii table through the xtalkfile argument.

5. The overscan region, defined by the BIASSEC keyword, is trimmed from each image.


  1. To extract counts from two field stars over a series of exposures:

    --> saltslot images='S*.fits' outimages='' outpref='r'
    logfile='salt.log' verbose='yes'

Time and disk requirements

Unbinned raw slot mode images are 400KB. While saltslot processing does result in a slightly smaller set of images after overscan stripping, it also converts the raw images from signed 32-bit to unsigned 32-bit pixels. This increases the size of unbinned slot mode images to 650KB, but the advantage is a significant decrease in run time. It is recommended to use workstations with a minimum of 512MB RAM. On a fast linux machine with 2.8 Ghz processor and 2 Gb of RAM, one FITS file containing two 4x4 binned slotmode exposures with a total of 1124x36 pixels can be processed in 0.09 sec.

Bugs and limitations

The current version of SALTSLOT has been tested only on SALTICAM slot mode data. RSS SLOT mode has not yet been commissioned. Currently, no flat fielding is performed. It is not yet clear whether SLOT mode flat fields are useful.

Send feedback and bug reports to

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