Currently the TIMMI2 Exposure Calculator is under development and testing. At this stage calculations are for imaging mode in N1, N10.4, N11.9, N12.9, M and Q bands. The output signal-to-noise values are highly dependable on sky conditions and should be used as a guide only.
The TIMMI2 exposure calculator provides a plot of the signal-to-noise versus time, for a given source brightness.
The calculations are based on the assumption that the sky background is the dominant noise source. The following formula is used
sn = [source x (sqrt(time)] / [convf x Npixel x sky]
|source||=||source brightness (Jy)|
|time||=||exposure time (sec)|
|convf||=||conversion factor (Jy sec/ADU)|
|Npixel||=||number of pixels|
|sky||=||sky brightness (ADU/sqrt(sec)/pixel)|
The conversion factor and sky brightness have been derived from the TIMMI2 monitoring program.
The number of pixels can be determined as follows:
For point source: Npixel = [size/(2 x scale)]^2 x pi
For extended source: Npixel = (1/scale)^2
|scale||=||pixel scale (0.2" or 0.3")|
|size||=||diffraction limit (1.4" in Q band and 0.7" for N and M bands)|