• Target = Sirius
• Date = 01 Jan 2003
• Config = U1-U3
• Bandpass = 2200nm

In this section, the spectrum and geometry of the target to be observed must be defined.

• Target Coordinates

  • Right Ascension

    A coord between 0 and 24. The sky is represented as 24 sections.

  • Declination

    Based on latitudes. A value between -90 and +90.

  • Target Name

    The target name is passed to a software package called Simbad. This softare determines the coords of the target automatically.

  • Simbad

    There exists a web based application for finding the coordinates of a target using a target name - Simbad Web App

  • Epoch

    The coordinate Epoch used in the calculation.

• Target Spectrum

  In this field you must specify the spectral profile of the target source.

  • Uniform

    If this option is selected, the relative flux is assumed to be the same at all wavelengths; i.e. a flat-spectrum source.

  • Blackbody

    If this option is selected, the flux destribution is assumed to be that of a black body with the specified temperature.

  • Template Spectra

    By selecting this option, the flux destribution is assumed to match a star of the selected spectral type.

• Target Geometry

  In this field, the geometry of the target must be defined. This is either done by a user-supplied FITS image, or by selecting one of the geometry templates.

  • Single Disc

    The target is represented by a single disc with uniform intensity. See also the single disc visibility formula

    • Diameter

      Measured in milli arc seconds.

  • Gaussian

    The target intensity is represented by a gaussian model. See also the gaussian disc visibility formula

    • FWHM

      Full Width Half Maximum. Measured in milli arc seconds.

    • Elliptical

  • Binary Disc

    If this option is selected, the target is assumed to be a binary. Each star of the binary is represented by a single disc. Diameters for both disc in the binary system must be entered. See also the binary disc visibility formula

    • Diameter

      The diameter, in milli arc seconds.

    • Separation

      The separation, in milli arc seconds.

    • Angle

      The position angle between the two objects in the binary system from North , in degrees.

    • Brightness Ratio

      The ratio between the integrated intensities of the primary and the secondary must be entered. 1 means that the primary is as bright (per surface unit) as the secondary, whereas, for example, 0.5 means that the primary is only half as bright (per surface unit) as the secondary. If a brightness ratio greater than 1 is entered then it will automatically be inverted so the following condition applies 0 <= brightnessRatio <= 1. See also the binary disc visibility formula

• FITS Image

  Allows the user to select a fits image that represents the geometry of the target.

• Pixel scale

  The pixel scale measured in mas. Optimal pixel scales are close to the resolution limit of the system determined by the baseline and the observation wavelength (typical values for MIDI at 10 microns with the UT1-UT3 or UT2-UT3 baselines range about 20 mas). Oversampled FITS files with pixel scale much below the system resolution yield to inaccurate visibility estimates in the (u,v) plane.

• FITS Upload

  A fits file can be uploaded using an upload script. To upload a fits file, select the "Upload Fits File" button and then make a selection of the fits file to be uploaded. It should be noted that after uploading the user must change the name of the fits file displayed in the edit box. The user can also use any of the fits files have have previously been uploaded. To see a list of previously uploaded fits files, select the "Available Fits Files" button. Again, the user must edit the fits file name displayed to use a previously uploaded file. The file "demo.fits" is stored internally to the application.The maximum size is 1024x1024 and the data type must be FLOAT.

• Time

  • Observation Date

    UT Date on which the observation shall take place

  • Observation Time

    Time at which the observation shall take place. Measured in Universal Time (UT).

  • Observation Timespan

    The length of time the observation shall take.

• Wavelength Domain

  • Filter

    A number of predefined filters are available. The filter reduces the flux from the target by varying amounts at different wavelength. The selection made will alter the calculated visibilities.

  • Bandpass

    Instead of a defined filter a bandpass may be selected. This assumes a perfect transmission at the central wavelength leading to zero transmission away from the central wavelength. The reduction in transmission is defined by a gaussian decay.

    • Central Wavelength

      Wavelength which has perfect transmission of the targets intensity.

    • FWHM

      Full Width Half Maximum. Measured in microns.

  • Multiwavelength (Disperse Mode)

    The user can enter wavelengths to exactly define a "hat" shaped filter.

• Observatory Location

  The user can either select the location of Paranal or enter an abritrary location.

  • Paranal

    This is the VLTI site.

  • Telescope Configuration

    Possible Unit Telescope configurations.

  • Auxiliary Telescope

    Possible auxiliary telescope configurations.

  • Arbitrary Location

    The coords of any observatory can be set by the user.

  • Latitude,Longitude

    Coordinates of an arbitrary location in degrees. (North and East are positive)

  • Baseline Length

    User specific telescope configuration baseline lengths can be entered. This is the length between two telescopes.

  • Baseline Angle

    User specific telescope configuration baseline angles can be entered. This is the angle from north of the line connecting the two telescopes.

• Altitude

  • Minimum Allowed Target Altitude

    UV tracks are not plotted if the target altitude falls below the minimum threshold allowed. The horizon is zero degrees.

  • Maximum Allowed Sun Altitude

    UV tracks are not plotted if the sun altitude rises above the maximum threshold allowed. The horizon is zero degrees.

• Special

  • Integration Time

    The separation in time of the points on the UV plane. Reducing the integration time increases the computation time.

  • Max delay line difference

    Difference in delay line at which the uv tracks will not be plotted or calculated.

• Shadowing Threshold

  When a target passes behind a telescope building then shadowing occurs. While the target is half behind a building then the shadowing is 0.5 or 50%. VisCalc sets the default shadowing threshold to 0.1 or 10%. If the shadowing is greater than this then the results are not recorded and the uv tracks not shown. This option allows the threshold to be manually set. Zero is the minimum and 1 is the maximum.

• All Results

  Calculates all results.

• Fits File

  The image of the fits file used. East points to the left. East is 90 degrees.

• Shadowing

  Sometimes the target may be obscured or hidden behind a telescope building.

• Delay Lines

  Delay lines have a finite length and this is a limiting factor for the execution of the full observation. Any delay line value greater than +100 or less than -100 prevents the UV plot from being displayed.

• Target Observability Period

  The results from this graph will show if the target is above the horizon and therefore visible. It will also show the position of the sun.

• Time Window Table

  The target is not always visible due to shadowing restrictions and delay line limitations. The Paranal specific table shows shadowing and delay lines limitations for each station configuration.

• UV Tracks

  This will show the position of the UV tracks in details.

• Fourier transform image with uv tracks overlaid

  The graph will show the UV plane overlaid with the UV tracks from all telescope configurations used.

• Visibility vs Time

  The interferometric visibility is normalised between 0 and 1 and this visibility can be seen against time as the observation progresses.

• Visibility Squared vs Time

  The interferometric visibility simply squared. This value lies between 0 and 1.

• Loss of Correlated Magnitude vs Time

  The correlated magnitude is calculated as -2.5log₁₀( visibility ).

• Phase

  The phase is either PI or zero. Phase Closure is the sum of the 3 phases in a triplet. A phase change occurs where the fringe visibility goes through a zero point.

• Finito

  This option shows the UV Visibility versus Time at the Finito Wavelength 1640 nm. There is an option to select a single disc diameter that is different from the Target. This Finito option is only available for the Single Disc Target setup.

• Illumination

  The illumination is a measure of the flux of the target multiplied by the transmission of the filter used.

• Process Execution Time

  This is the time it takes for the C++ code to complete on the server. This is not the time for the web page to load.

• Calculating normalised visibilities

  * VisCalc calculates a weighted average curve to calculate the final visibility value and depends on the following

• Filter transmission
• Uniform or blackbody or spectral target fluxes

  • General (parameter units)

    	aU coord [metres]
    	aV coord [metres]
    	lambda [metres]
    	diameters [radians]
    	angles [radians]
    	gaussFwhm [radians]
    	

  • Single Disc

    	
    	double uCoord = aU/lambda;
    	double vCoord = aV/lambda;
    	double rho = sqrt(uCoord*uCoord + vCoord*vCoord);// uv plane radius
    	double uda = pi*singleDiscDiam*fabs(rho);
    	double visSigned = (2*j1(uda)/uda);
    	udv = fabs(visSigned);// modulus |vis|
    	vis = udv;	
    	

  • Gaussian

    	
    	double uCoord = aU/lambda;
    	double vCoord = aV/lambda;
    	double rho = sqrt(uCoord*uCoord + vCoord*vCoord);// uv plane radius
    	
    	double vis = exp( (-2 * _pi * _pi * gaussFwhm * gaussFwhm * rho * rho)/(8 * log(2)) );
    	

  • Binary Disc

    	
    	double uCoord = aU/lambda;
    	double vCoord = aV/lambda;
    	double rho = sqrt(uCoord*uCoord + vCoord*vCoord);// uv plane radius
    	
    	double visSquared = 0.0;
    	double bessel1 = 0.0;
    	double bessel2 = 0.0;
    	double denominator = 0.0;
    	double d1 = _pHtmlInputPage->_binDiscDiam1/_mas2rad;// convert from mas to radians
    	double d2 = _pHtmlInputPage->_binDiscDiam2/_mas2rad;// convert from mas to radians
    	double binarySeperation = _pHtmlInputPage->_binDiscSep/_mas2rad;// convert from mas to radians
    	double binaryAngleTheta_Radians = ( _pHtmlInputPage->_binDiscAngle - 90.0 )/_deg2rad;
    	double binaryBrightnessRatio = _pHtmlInputPage->_binDiscBrightRatio;
    
    	
    	// Diameter to radius conversion
    	double r1=d1/2;
    	double r2=d2/2;
    
    	bessel1 = CInterferCalcEngine::besselJ( 2*pi*rho*r1 );
    	bessel2 = CInterferCalcEngine::besselJ( 2*pi*rho*r2 );
    
    	denominator = ( pi*pi*
    				(binaryBrightnessRatio*binaryBrightnessRatio*r1*r1*r1*r1*uCoord*uCoord
    				+ binaryBrightnessRatio*binaryBrightnessRatio*r1*r1*r1*r1*vCoord*vCoord
    				+ 2*binaryBrightnessRatio*r1*r1*r2*r2*uCoord*uCoord
    				+ 2*binaryBrightnessRatio*r1*r1*r2*r2*vCoord*vCoord
    				+ r2*r2*r2*r2*uCoord*uCoord
    				+ r2*r2*r2*r2*vCoord*vCoord ));
    
    	visSquared = ( 2*r2*bessel2*binaryBrightnessRatio*r1*bessel1*cos(2*pi*binarySeperation*(cos(binaryAngleTheta_Radians)*uCoord + sin(binaryAngleTheta_Radians)*vCoord))
    				+ binaryBrightnessRatio*binaryBrightnessRatio*r1*r1*bessel1*bessel1
    				+ r2*r2*bessel2*bessel2)
    				/ denominator;
    	vis = sqrt(visSquared);
    	
    	if (uCoord==0 && vCoord==0)
    		vis=1;// special case
    
    	
    	

  • Bessel

    double besselJ(const double& aInputParam)
    {
    	double x = aInputParam;
    	double ax,z;
    	double xx,y,ans,ans1,ans2;
    
    	if ((ax=fabs(x)) < 8.0) {
    		y=x*x;
    		ans1=x*(72362614232.0+y*(-7895059235.0+y*(242396853.1
    			+y*(-2972611.439+y*(15704.48260+y*(-30.16036606))))));
    		ans2=144725228442.0+y*(2300535178.0+y*(18583304.74
    			+y*(99447.43394+y*(376.9991397+y*1.0))));
    		ans=ans1/ans2;
    	} else {
    		z=8.0/ax;
    		y=z*z;
    		xx=ax-2.356194491;
    		ans1=1.0+y*(0.183105e-2+y*(-0.3516396496e-4
    			+y*(0.2457520174e-5+y*(0.240337019e-6))));
    		ans2=0.04687499995+y*(-0.2002690873e-3
    			+y*(0.8449199096e-5+y*(-0.88228987e-6
    			+y*0.105787412e-6)));
    		ans=sqrt(0.636619772/ax)*cos(cos(xx)*ans1-z*sin(xx)*ans2);
    		if (x < 0.0) ans = -ans;
    	}
    	return ans;
    }
    
    

• Version 4.1.3 (29 February 2012)

  New baselines for Period 90 have been added.

• Version 4.1.2 (1 September 2009)

  Support for the quadruplets.

• Version 4.1.1 (20 May 2008)

  Fixed visibility plots for user-supplied FITS models.

• Version 4.1.0 (25 April 2008)

  Removed some superfluous debug text

• Version 4.1.0 (7 March 2008)

  Elliptical geometry added.

  Shadowing threshold option added.

  Corrected the formula computing the loss of correlated magnitude.

• Version 4.0.0 (30 November 2007)

  New Bessel Function used for single disc & binary fringe visibility. Using j1() from math.h

  User defined template spectra function added.

  Process execution time added.

• Version 3.0.1 (24 August 2007)

  New Observation Time Window Html Page.

  Warnings added for split observation time periods.

  Triplet telescope configurations implemented.

  Target Azimuth added.

  Normalized wavelength corrected for bug in QucFilter (50bins).

  Graphs not displayed when baseline not visible. Bug fixed. eg. HourAngleMode G1-H0 Declination>=30

  Shadowing azimuth convention bug fix.

• Version 2.1.0 (22 December 2006)

  Finito Checkbox added with separate diameter for Single Disc.

• Version 2.0.0 (01 December 2006)

  UV plane orientation now changed so that East points to left. UV plane icons added where relevant.

  Finito Visibilities added.

  North East convention used for all user input and output on html pages. Internal interferometry code engine still uses XY trigonometry convention.

  BaselineLength and Azimuth graphs added.

  Shadowing added - this will appear when shadowing is present.

  Target Altitude and Airmass added.

  Simbad web link added.

  Expert VisCalc link added.

• Version 1.2.3 (21 December 2005)

  Min.elevation raised to 30 deg., Max. delay changed to 105 meter, Activated internal Optical Path Delay.

• Version 1.0.4 (1 March 2005)

  Added support for AMBER.

• Version 1.0.3 (23 Apr 2003)

  Added a third configuration for the auxiliary positions. The graphs have been given a second axis showing the HourAngle in addition to the Universal Time

• Version 1.0.2 (12 Mar 2003)

  Updated the demo.fits file so that the file is stored internally to the application.

• Version 1.0.1 (3 Mar 2003)

  Updated calculation of visibilities on uploaded FITS files and frequency scale calculation

• Version 1.0.0 (17 Feb 2003)

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