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Data

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Brian Kloppenborg edited this page Aug 7, 2015 · 10 revisions

SIMTOI has native support for two types of data:

Optical Interferometry Exchange Format (OIFITS)

SIMTOI has built-in support for the OIFITS (version 1.0) format squared visibility (V2), triple amplitude (T3Amp), and triple amplitude phase (T3Phi) data. At the present time SIMTOI does not support high spectral dispersion observations.

How OIFITS data is treated

SIMTOI uses liboi for reading and simulating OIFITS data. SIMTOI regards an OIFITS file as a complete observation which should be simulated. Thus the average Julian date and average wavelength are computed for each file and used during the simulation as described below.

How SIMTOI simulates OIFITS data

For every OIFITS file SIMTOI completes the following steps to simulate OIFITS data:

  1. Set the average Julian Date (computed during OIFITS file import)
  2. Set the average wavelength (computed during OIFITS file import)
  3. Sets model parameters as given in the GUI or specified by the minimization engine
  4. Converts model temperature to model fluxes for every model
  5. Normalizes the fluxes using the maximum flux computed in step 4. This step is necessary because of limitations imposed by OpenGL.
  6. Renders each model to a 4x multi-sample anti-aliasing (4x MAA) off-screen render buffer
  7. Hands the OpenGL image to liboi which completes the necessary image to OIFITS / chi / chi-squared / log Z computations.

Broadband photometric data

Starting in SIMTOI 1.1.1 the program has built-in support for broadband photometry in UBVRIJHKLMN and custom-defined filters. Broadband photometry is calculated using the mean wavelength of the specified filter, thus emission/absorption due to atomic/molecular lines are not considered.

Data format

SIMTOI can read photometric data files that conform to the AAVSO data format. In short, this format consists of a series of comma-separated fields in which a single line denotes a single observation. Although SIMTOI parses each line, only the first five fields in the format are used. These files are as follows:

JD, Magnitude, Uncertainty, HQUncertainty, Band, ...

Where JD is the Julian Date, HQUncertainty is the uncertainty assigned by the AAVSO (often blank), and Band is one of the standard photometric filters discussed below. Band may also be a floating point value interpreted to have meters as units.

Because OpenGL permits only normalized fluxes, SIMTOI will use the first observation in the file to set a zero point. Thus it is suggested that the user provide only one photometric filter per data file.

Supported filters

Band Wavelength Band Wavelength
U 365 nm J 1.250 um
B 445 nm H 1.635 um
V 551 nm K 2.200 um
R 658 nm L 3.450 um
I 806 nm M 4.750 um
N 10.5 um
Q 21.0 um

The UBVRI wavelengths are based upon the mean of the Johnson-Cousin filter system. The JHK filters are based upon the Mauna Kea Observatory NIR filter set. The filter values for LMN and Q were taken from the Wikipedia article on photometric systems, but should be close enough to be useful.

How a photometric point is simulated

  1. Set the Julian Date
  2. Set the wavelength (via. lookup value computed during import)
  3. Sets model parameters as given in the GUI or specified by the minimization engine
  4. Converts model temperature to model fluxes for every model
  5. Normalizes the fluxes using the maximum flux computed in step 4. This step is necessary because of limitations imposed by OpenGL.
  6. Renders each model to a 4x multi-sample anti-aliasing (4x MAA) off-screen render buffer
  7. Hands the OpenGL image to liboi which computes the sum of the image
  8. Computes the photometric value by value = -2.5 log10(flux)
  9. Normalizes the simulated value to the first photometric point in the file

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