(This chapter is slightly outdated, the new chop_nod_phot is much better than shown here).
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 by typing
chop_nod_phot,calfile[0]
Options
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'