Reducing back ground data
I recommend reducing the back ground data first, simply because they are
needed when averaging point data for the visibility normalization. (You get
there by clicking on Reduce|BG Data|PLOT.) Since
back ground data are technically scan data, all scans are loaded and are
available for editing. I plot 4 channels at a time, by setting the selection
directive initally to Selected and entering the selection 1-4 into the field
just below the directive button. After plotting the data, I set the directive
to Next, which will then increment the selection by 4 before each of the
following plots. This is a convenient way to get through all 32 channels
of a NPOI spectrometer (or output beam, as they are called in OYSTER).
Do not forget to look at all three spectrometers!
Note in this example the peculiar behaviour of channel 1 of the first
spectrometer, which seems to be bimodal. We have not fully investigated
the effect this has on the visibility amplitudes, but my impression is that
the channel is OK. In other matters, I write down the channels which have
back ground rates (per 2 ms) in excess of 1.0. For this I use formsheets
which can be printed using print_specform, or Utilities|Forms|SpecForm.
An example is shown at the bottom of this page.
Here I have selected channel 11 for editing. This channel has stray light
from the metrology lasers in it, which has corrupted the stellar data. The
way the editing is done is by selecting Util|Edit, and placing a box over
the data which needs to be flagged. At this point we need to select 'Delete
inside boxes' to complete this flagging session. The flagged data will be
colored in red, and if you try to plot this again, nothing will show up
in the plot window, unless you select Options|Flagged. Internally, flagged
data is recognized by their error bars set to negative values, so that they
can be easily restored, if needed. After having flagged all bad data, save
the flag table using Reduce|BG Data|Flagtable|Save. The flag table (which
comes in three flavors for back ground, point, scan, and metrology data)
contains the indices of the flagged data. When saved to a disk file (e.g.
1998-10-07.flg), it could be loaded and applied to the data if for some
reason the latter had to be re-loaded.
This is how one of the spectrometer formsheets looks like. Its purpose is
only to help you with preparing an overview of the APD performances, and
it is entirely up to you if you want to use it. Write an X into the BG column
if you delete this channel based on the back ground data (e.g. channel 11),
or write the rate into it when larger than 1.0. The third column is for flags
based on inspection of the point data. The following three columns (SM =
system magnitude, Sl = slope, and RMS) can be used for channel calibration
results. (I must admit that I hardly ever do this, but feel free to improve
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