<#5085#>NOTE: This is an old Lick Observatory data-taking system command
that is no longer valid (unless you have to reduce data taken before
1990). It is <#5074#>not<#5074#> the same as overscan subtraction.<#5085#>
BL removes the noise introduced through the digital baseline-restoration
procedure used by the data collection program. The last column of the CCD,
called the 'baseline column' is not illuminated during an exposure. Each
pixel in this column determines the zero-level for the corresponding row in
the rest of the chip, which is subtracted from each pixel in that row as
the image is read off the chip into the data-taking system. NOTE THAT THIS
IS THE PROCEDURE IMPLEMENTED AT LICK OBSERVATORY, BUT NOT MOST OTHER
PLACES! Lick subtracts a row by row baseline value before storing the
image at the telescope. If this is not done at your site, you should not
use the BL command! Write a procedure to do the appropriate bias and/or
baseline subtraction using the information supplied by your data taking
system.
The baseline measurement is rather noisy, so when the zero-levels are
subtracted, significant variations in level from one row to another are
introduced. The BL program corrects for this by fitting a straight line by
least-squares to the values in the baseline column. The original baseline
measurements are then added back into the image data and the mean baseline
values evaluated from the linear least-squares fit are subtracted. The
best fit baseline is stored in the baseline column, unless the keyword KEEP
is specified, in which case the original baseline is left in the baseline
column.
The baseline column can sometimes exhibit several jumps in level along its
length. BL detects these jumps, fitting a series of linear functions to
the baseline column between these jumps. To detect the jumps, thus fitting
the entire column with more than one linear function, use the JUMP keyword.
The BL procedure should be applied to every raw image before analysis or
processing.