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VLTI Visibility Calculator |
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The VLTI Baseline Preparation Tool is a tool intended to assist in the preparation of VLTI observation baselines. It uses a model of the VLTI and of various instruments to assess the fringe visibility, given the observed bandpass, the geometry, spectrum and coordinates of the target and the time at which the observations are to be made. It also computes quantities such as shadowing by telescope structures and stroke of the delay lines.
This tool provides an HTML/Java based interface and consists of two pages. The observation parameters page presents the entry fields and widgets for the target spectrum and geometry, time of observation, instrument configuration and results selection. An "Apply" button submits the parameters to the model executed on the ESO Web server. The results page presents the computed results. The optional graphs are displayed within Java applets allowing interactive manipulation. The results are also provided in ASCII and GIF formats for further analysis and printing. Finally a summary of the input parameters is appended to the result page.
In this section, the spectrum and geometry of the target to be observed must be defined.
A coord between 0 and 24. The sky is represented as 24 sections.
Based on latitudes. A value between -90 and +90.
The target name is passed to a software package called Simbad. This softare determines the coords of the target automatically.
There exists a web based application for finding the coordinates of a target using a target name - Simbad Web App
The coordinate Epoch used in the calculation.
In this field you must specify the spectral profile of the target source.
If this option is selected, the relative flux is assumed to be the same at all wavelengths; i.e. a flat-spectrum source.
If this option is selected, the flux destribution is assumed to be that of a black body with the specified temperature.
By selecting this option, the flux destribution is assumed to match a star of the selected spectral type.
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.
The target is represented by a single disc with uniform intensity. See also the single disc visibility formula
Measured in milli arc seconds.
The target intensity is represented by a gaussian model. See also the gaussian disc visibility formula
Full Width Half Maximum. Measured in milli arc seconds.
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
The diameter, in milli arc seconds.
The separation, in milli arc seconds.
The position angle between the two objects in the binary system from North , in degrees.
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
Allows the user to select a fits image that represents the geometry of the target.
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.
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.
UT Date on which the observation shall take place
Time at which the observation shall take place. Measured in Universal Time (UT).
The length of time the observation shall take.
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.
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.
Wavelength which has perfect transmission of the targets intensity.
Full Width Half Maximum. Measured in microns.
The user can enter wavelengths to exactly define a "hat" shaped filter.
The user can either select the location of Paranal or enter an abritrary location.
This is the VLTI site.
Possible Unit Telescope configurations.
Possible auxiliary telescope configurations.
The coords of any observatory can be set by the user.
Coordinates of an arbitrary location in degrees. (North and East are positive)
User specific telescope configuration baseline lengths can be entered. This is the length between two telescopes.
User specific telescope configuration baseline angles can be entered. This is the angle from north of the line connecting the two telescopes.
UV tracks are not plotted if the target airmass falls above the maximum threshold allowed.
UV tracks are not plotted if the sun altitude rises above the maximum threshold allowed. The horizon is zero degrees.
The separation in time of the points on the UV plane. Reducing the integration time increases the computation time.
Difference in delay line at which the uv tracks will not be plotted or calculated.
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.
Calculates all results.
The image of the fits file used. East points to the left. East is 90 degrees.
Sometimes the target may be obscured or hidden behind a telescope building.
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.
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.
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.
This will show the position of the UV tracks in details.
The graph will show the UV plane overlaid with the UV tracks from all telescope configurations used.
The interferometric visibility is normalised between 0 and 1 and this visibility can be seen against time as the observation progresses.
The interferometric visibility simply squared. This value lies between 0 and 1.
The correlated magnitude is calculated as -2.5log10( visibility ).
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.
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.
The illumination is a measure of the flux of the target multiplied by the transmission of the filter used.
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.
* VisCalc calculates a weighted average curve to calculate the final visibility value and depends on the following
aU coord [metres] aV coord [metres] lambda [metres] diameters [radians] angles [radians] gaussFwhm [radians]
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;
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)) );
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
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;
}
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