Photometry with Chop_Nod_Phot

As shown above the flux from the calibrator is still hidden in the overwhelming background that dominates observations in the mid-infrared. To overcome this, the background has to be determined immediately after a certain number of frames with the source have been taken. This is done by observing with the telescope a nearby part of the sky ("chopping").Whether a exposure is taken on the sky or on the source is given in the fitsheader belonging to that frame.

A routine called chop_nod_phot makes use of this keyword. The routine is started just by typing

  chop_nod_phot,calfile[0]

  Options

  • THRESHOLD=x.xx (by default 0.1)
  • /SILENT, tells routine to be non interactive
  • /REVERSE, flags for object and sky are reversed
  • /FRAMES, determines the position, flux, and FWHM of the source in the individual frames and plots the results
  • /CHOPDIR, determines the chopping direction with the help of the "U"-typ ("uncertain") frames

This routine subtracts in both beams the sky frames from the acquisition frames containing the source, combines them ...

 

... and calculates the relative positions of the source by fitting two-dimensional gaussians:

----------------------------------------
Star gauss. B: 29.8812 - 32.7990
FWHM x, y: 1.46956 - 1.53640
Max image: 521.918, Pos X = 30, Pos y = 33
----------------------------------------
Star gauss. A: 28.7899 - 32.6536
FWHM x, y: 1.37927 - 1.34973
Max image: 374.207, Pos X = 29 Pos, y = 33
----------------------------------------

 

After asking the user whether the dataset looks satisfactory, the routine continues with the aperture photometry for radii from 1 to 10 pixels (i = 1..10). The sky annuli are defined with an inner radius i+6 and an outer radius i+8. The calculated flux is given for both beams:

----------------------------------------
> Is the dataset satisfactory from display inspection? (y/n) ? y
aperture radius 1 - 1033.97 - 1358.51
aperture radius 2 - 2840.84 - 3915.91
aperture radius 3 - 3965.85 - 5647.27
aperture radius 4 - 4514.09 - 6806.48
aperture radius 5 - 4849.03 - 7825.82
aperture radius 6 - 4938.91 - 8515.50
aperture radius 7 - 4849.72 - 8890.86
aperture radius 8 - 4702.60 - 9034.31
aperture radius 9 - 4610.37 - 9080.96
aperture radius 10 - 4736.34 - 9101.36
----------------------------------------

 

When the convergence defined as (Flux[i]-Flux[i-1])/Flux[i] reaches a certain threshold (by default 0.1) in one of the two beams, the photon flux is given for this radius j (tick in the plot) and the following one (j+1).

----------------------------------------
Aperture Photometry of
MIDI.2003-06-15T09:26:17.000.fits
----------------------------------------
rA= 5 : rB= 5
----------------------------------------
Flux (photons) - error - Sky - error
MIDI Beam B: 2.8368597e+08 - 747517.64 - 68528.595 - 63923.805
----------------------------------------
MIDI Beam A: 1.7577727e+08 - 865017.33 - 80041.702 - 76718.819
----------------------------------------
----------------------------------------
rA= 6 : rB= 6
----------------------------------------
Flux (photons) - error - Sky - error
MIDI Beam B: 3.0853491e+08 - 940967.04 - 68528.595 - 63923.805
----------------------------------------
MIDI Beam A: 1.8071686e+08 - 1101247.1 - 80041.702 - 76718.819
----------------------------------------

All output is written to an ascii file within the newly created subfolder

  '<path>/PHOTOMETRY'


Spectroscopy with Chop_Nod_Disp

A similar routine exists for dispersed data. It is called chop_nod_disp. To start it, e.g. for the photometry of beam A, just type

  chop_nod_disp,calfile[2]

  Options

  • WIDTH=w, width of 1D-gaussian relative to width of fit (by default 1.0)
  • TRACE_ORDER=o, order of polynopmial used to fit position (by default 2)
  • FWHM_ORDER=o, order of polynopmial used to fit FWHM (by default 1)
  • BEFORE=b, number of frames to skip before chop (by default 1)
  • AFTER=a, number of frames to skip after chop (by default 1)
  • /SILENT , tells routine to be non interactive
  • /REVERSE , flags for object and sky are reversed

 

The image above is composed by combining the content of two IDL panels partially. It shows the graphical output of chop_nod_disp for the interferometric beam I1 when it has been fed only by beam A during the photometric measurement. The uppermost spectra is the "raw" spectrum one gets when the chopped frames are subtracted. A few additional steps are needed to determine the "real" spectrum:

  1. columnwise median smoothing to remove badpixels
  2. columnwise one-dimensional gaussian fit (centers = photocenters = crosses in first graph)
  3. fit to the photocenters (solid line): c(x) = 22.1029+0.0130640x-9.98348e-5x^2.
  4. fit to the FWHMs of the gaussians (dotted lines): s(x) = +/- 2.20357+0.000111095x
  5. creation of a gaussian mask by using for each column x the fitted center c(x), the fitted FWHM s(x), and the user-specified WIDTH
  6. linear fit of the background for each column (integrated background plotted as dotted line in the last graph)
  7. subtraction of the background
  8. weighting of the remaining spectrum with the mask (see image above)
  9. columnwise integration of the weighted flux (solid line in the last graph)

The spectrum is written to an ascii file within the newly created subfolder

  '<path>/PHOTOMETRY'