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 this situation!)

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