Some details working with NCDRT

The source files are all human readable files. All the programmes are .pro files. The .CB files are the Common Blocks. The ncdrt_v1.0.pro file contains the first version of NCDRT, which is quite primitive. NCDRT uses the READFITS.PRO routine from the IDL astro routines. This routine was found at the time of development of this application at URL idlastro.gsfc.nasa.gov/homepage.html, which does not exist anymore. A search gives this IDL Astro GitHub repository.

The (old) routine is included in this distribution of NCDRT.

SHOW_LIN_CUT and APPLY_LIN_CUT

A NRT drift scan is a measure of the flux as a function of time. The first three seconds of the scan are used to observe the calibration noise tube. NCDRT ignores these first three items. The rest of the scan sees the system noise and the source, as it drifts through the field of view of the system. In order to measure the source’s flux, the system noise needs to be subtracted. It is assumed that this noise, the offset, is a line as a function of time.

The offset sliders determine which part of the scan is used for the offset line determination. This is from t=1 to t=offset_low plus t=offset_high to t=end_scan. The offset limits are defined and the same for all the channels. The line is then subtracted from the whole scan. Now the peak value of the object can be measured.

The offset sliders determine the part of the data to be used for the determination of the offset. There is a build-in protection that prevents these sliders from having inappropriate values: the peak sliders must be at least 4 time units apart for the curve_fit procedure in the apply_lin_cut.pro routine to work, and the low offset slider cannot be larger than the high offset slider. low sliders cannot be set to more than half the range, Also, the offset sliders cannot be set to 0 or to the maximum of points. This is checked in the show_lin_cut.pro routine. The show_lin_cut.pro routine then plots the first channel (by default, this can be changed by the user), corrected by the offset, in a small plot window (scissors) and shows the offset lines.

The peak sliders determine which part of the scan is to be used to fit the peak of the object. This is also plotted in the scissors-plot by show_lin_cut.pro.

The routine apply_lin_cut.pro fits a gaussian function to the selected peak. It uses the IDL routine GAUSSFIT with initial estimates of the four parameters \(a(0), a(1), a(2), a(3)\):

\[f(x) = a (0) * \exp { \frac {-z^2}{2} } + a (3)\]

with

\[z = \frac{ x- a (1) }{ a (2) }\]

The results are shown in the result plot window and in the result text box in the right hand colomn of the NCDRT widget. The flux at the peak is

\[a (0) + a (3)\]

Three estimates of the noise level are calculated and reported:

  1. standard deviation of the (peak - gaussian fit) for the peak selection of the scan only;

  2. standard deviation of the (scan - offset_line) for the offset selection of the scan only;

  3. standard deviation of the combination of (1) and (2).