## Hadravas Program FOTEL

FOTEL (Hadrava 1997, 2004) is a code developed at the Ondrejov observatory by Hadrava for the simultaneous solution of light curves and radial-velocity curves of EBs. It uses (in its version from 1995) a very simplified model of the physics6 and geometry for the flux calculations, but it has some options rarely available in more physically sophisticated codes. It is designed to handle a large number of original observational data (typically up to several thousand points) from up to 30 data sets of radial velocities and different passband magnitudes and is designed to run efficiently on a PC. The zero point for each data set is determined by fitting, i.e., the program corrects for systematic instrumental shifts or for different comparison stars. The mean quadratic error for each data set is calculated and the weight of the set can be chosen according to its estimated reliability. The code can take into account a third body or component including observations of its radial velocities and the corresponding light-time effect. Secular changes of several parameters (e.g. , the periastron advance, change of period, or amplitude of radial velocities) can be fitted. An option is also included to look for a change of radius (and volume) due to the variations of tidal force in an eccentric orbit. The code uses a Simplex method (for up to 10 parameters in one run) combined with a direct least-squares solution of multiplicative parameters (i.e., K-velocity and zero points of data sets). Errors and cross-correlations of all these fitted parameters are also calculated.

6 The most recent version, fotel 3, allows the user to choose between the approximation of stellar shapes by triaxial ellipsoids (which is faster and usually satisfactory) and by Roche equipotentials (which is more accurate).

## Telescopes Mastery

Through this ebook, you are going to learn what you will need to know all about the telescopes that can provide a fun and rewarding hobby for you and your family!

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