Visibility


Raw Visibilities

Following the steps described in the previous chapters we will calculate one raw visibility curve for the calibrator HD 10380 and one for the AGN of NGC 1068. Although the thresholds have been determined interactively I set it here with set_thresholds() to get reproducable results. Here it is more important to show the basic steps than getting optimized results. Thus improved visibilities are probably easy to find!

HD 10380

  • calfile=midigui(dir='<path>')
  • select files with index 1, 2, and 3
  • cal = xmdv(calfile)
  • confirm masks
  • cal -> gui,/true
  • cal -> set_thresholds,15523.9,0.00000,3.89843e-05


NGC 1068

  • scifile=midigui(dir='<path>')
  • select files with index 5, 6, and 7
  • sci = xmdv(scifile)
  • confirm masks
  • sci -> gui,/true
  • sci -> set_thresholds,3172.32,0.00000,4.00620e-05


Instrumental Visibility

In a next step we want to determine the instrumental visibility, i.e. the visibility of an unresolved point source, from a calibrator object of known diameter with the routine

  insvis = x->instruvisi(diameter,NAME=name)

where diameter is the diameter of the calibrator in mas. If the object is listed in the calibrator database, i.e. in 'heidelberg/calibrators.pro', the diameter can be omitted. The name that is used to find the object in the database is either read from the MIDI data files (can be checked with object = cal -> get_objectname()) or can be given with the parameter NAME. The instrumental visibility is calculated by dividing the raw visibility with the expected visibility of the calibrator. When /VISPLOT is specified one gets a plot of the instrumental visibility, i.e. insvis[2,*] vs. insvis[0,*]. The parameter /PLOT launches the display for the wavelength-binned spectral power of the fringes.

HD 10380 is in the database. Thus the output of the command

  insvis = cal -> instruvisi()

is

  Looking up hd10380 in calibrator database
  Found:   2.85000 mas
  object:   hd10380
  Baseline:   U1 - U3
  ESO's baseline len.:   72.7130
  Date of observation:   2003-06-15T09:37:19.000
  Local Sid Time:   80914.915
  Right Ascension:   25.357838
  Declination:   5.4878200
  Our baseline length:   72.7233, p.a.:   7.63431
  Diameter 2.85000mas = 1.38172e-08rad
 lambdax_besselvisibility
12.92990.2441450.992568
12.76540.2472920.992375
12.59590.2506200.992169
12.42140.2541390.991948
12.24210.2578620.991711
12.05780.2618030.991457
11.86860.2659760.991183
11.67450.2703980.990888
11.47550.2750880.990571
11.27150.2800660.990227
11.06260.2853550.989856
10.84880.2909790.989454
10.63010.2969660.989017
10.40640.3033490.988541
10.17780.3101620.988023
9.944290.3174460.987456
9.705850.3252440.986835
9.462490.3336090.986152
9.214200.3425990.985400
8.960990.3522800.984567
8.702840.3627290.983643
8.439780.3740350.982614
8.171790.3863010.981462

The last column is the expected visibility of the calibrator and thus the instrumental visibility is close to the raw visibility of HD 10380.


Calibrated Visibility

To calculate the calibrated visibility of the science object, i.e. to divide the raw visibiliy of the science object by the instrumental visibility, please use the routine

  calvis = sci->calibratedvisi(cal,diameter,NAME=name,/PLOT,/VISPLOT)

where diameter is the diameter of the calibrator in mas. Alternatively, the diameter can be read from the database ('heidelberg/calibrators.pro') by using the name of the calibrator that is either set automatically from the fits header (can be checked with object = cal -> get_objectname()) or manually by using the paramter NAME. The advantage of the routine calibratedvisi() over calling instruvisi() manually and dividing the raw visibility of the science target by the result is the adjustment of the lambda-binning for the visibility of the calibrator, i.e. calibratedvisi() ensures that the same lambda-binning used for the science target is applied to the calibrator. The option /VISPLOT leads to a plot of the calibrated visibility. /PLOT launches the display for the wavelength-binned spectral power of the fringes for both the raw visibility of the science target and the now lambda-bins-adjusted instrumental visibility.

  calvis = sci->calibratedvisi(cal,/VISPLOT)

leads to

The wavelength and the calibrated visibility is stored in calvis[0,*] and calvis[2,*].