ETCs: Frequently Asked Questions

Frequently Asked Questions

Q: Are the magnitudes in the ETCs AB or Vega magnitudes?

A: All magnitues in the ESO ETCs refer to the Vega magnitude system unless something else is explicitly indicated.

Q: How is the signal to noise defined?

A: In optical and infrared imaging, the applications display a signal to noise over the PSF area, which is computed in a circle of radius seeing/plate_scale. The number of pixels for this S/N computation is displayed as "Number of pixels in the PSF area". In optical imaging only, a second signal to noise is computed which is the signal to noise at the central pixel. In optical and infrared spectroscopy, the signal to noise corresponds to the signal per pixel along the dispersion relation and integrated over seeing/plate_scale pixels along the slit.

Q: My browser does not display the output graphs! How can I visualize the outputs?

A: You need a java-capable browser! The latest versions of Netscape and Explorer should do a fine job displaying these pages. Also: Output graphs are produced as GIF images, so that even without Java it should be possible to visualize the outputs. Of course, Java displays are interactive but not the GIFs.

Q: The redraw function of the Java graphics applet (key R) does not always redraw...

A: The applet remains active and can be redrawn as long as the page remains displayed. Navigating back and forth to other pages (e.g. using the Back and Forward buttons) will make the graphics inactive.

Q: What are the zero points and how are the spectra scaled to the specified magnitude ?

A: The effective central wavelengths are respectively B=440nm, V=550nm, R=640nm, I=790nm, J=1250nm, H=1650nm, K=2200nm. The spectrum is scaled after integration over the corresponding photometric filter. The photometric zero points and effective wavelengths are taken from the two following references:

 
        (B,V,R,I) filters: Bessel, 1979, PASP, 91, 589
        (J,H,K) filters:   Wilson, 1972, ApJ, 177, 533

Q: What does the message "ETC Warning: Wrong input range" mean?

A: The template spectra stored in the database are measured or simulated spectra covering different wavelength range. Although computations can always be performed with the continuum and blackbody input flux distributions, the ETC will not perform any computation if the bandpass of the instrument is larger than the template spectra wavelength range, the reason being that it would be misleading to assume the flux of the object is zero or constant in these regions. The template spectra with the largest wavelength range are the Kurucz models. The message may occur in particular in imaging with U or Z filters, and in spectroscopy with low resolution configurations.

Q: How is the sky brightness determined?

A: The sky magnitude is determined by the moon-phase and by the instrument bandpass, according to the table mentioned in the documentation. It should be noted that values for the Z band correspond to an extrapolation and assume a distribution of intensity constant with the wavelength. The sky spectrum in the 850-1100 nm and beyond present a number of atmospheric lines which are not modeled in the optical ETC.