This form can be used to estimate the integration time needed to reach a requested signal-to-noise for a given brightness temperature.
GREAT receives signal in two frequency bands, USB and LSB. The transmission plot shows the two possible tunings, putting the line in the USB or in the LSB. Noise comes from both USB and LSB. Integration times are calculated for both tunings.
The original versions of this form and the program to estimate the desired quantities were written by Riccardo Melchiorri based on a previous PHP code version.
The time estimator calculates the time required to reach an rms brightness temperature , ΔTR* , (TR* = TA* /ηfss, where ηfss is the forward scattering efficiency, = 0.97 for GREAT at all bands) for a line at a frequency ν by solving the standard radiometric formula
ΔTA * = (2 Tsys ) / sqrt(t Δν)
Here Δ TA * is the antenna temperature corrected for ohmic losses and rear spillover. Tsys is the single sideband system temperature outside the earth atmosphere, t is the integration time (ON+OFF) and Δν is the desired frequency resolution. For further details, see Guide to GREAT .
The calculator uses the most recent measured receiver temperatures (December 2015) and calls the atmospheric transmission program ATRAN to estimate the atmospheric transmission for a given frequency, altitude, telescope elevation and water vapor overburden. The transmission is used to calculate Tsys, assuming an ambient temperature of the atmosphere of 220 K and a telescope temperature of 230 K.
The time estimator can also compute the required integration time (ON + OFF) for a line with a given peak brightness temperature, desired signal to noise ratio (SNR) and frequency or velocity resolution. To estimate the total time needed for your observations, enter the integration time in the SOFIA Proposal Tool. For all observing modes the overheads are currently assumed to be 100% (2 x integration time). Add 2 minutes for tuning and calibration.
If your line estimates are in main beam brightness temperature, Tmb, convert to radiation temperature using TR* = ηmbTmb, where ηmb is the main beam efficiency. The main beam efficiency has been measured from planetary observations and determined to be 0.69 for L1 (December 2015), 0.68 for L2 (December 2015), 0.70 for Ma, and 0.69 for H (December 2015). For the upGREAT Low Frequency Array (LFA), the L2 parameters should be assumed.
If your desired line rest frequency falls close to or in an atmospheric absorption feature, you may still be able to observe the line if you choose the right time of the year and your source is blue or redshifted to move you out of the atmospheric feature. The time estimator therefore also allows you to put in a velocity correction. The first term in this velocity correction calculates the radial velocity of the observer with respect to your source for a given date and location and then you still need to add the VLSR of your source.
The time estimator also plots the position of both sidebands (separated by +/- 3.0 GHz for L1, L2/LFA, and H, and +/- 6.5 GHz for Ma). If the transmission is poor at the lower frequency but very good at the higher frequency, you would tune your line to the lower sideband. If the opposite is true you would tune your line to the upper sideband (USB). If both sidebands have poorer transmission than your signal band, your system temperature will be underestimated and your time estimate will be too optimistic, since GREAT is a dual sideband receiver and emission from both the signal and the image band contributes equally to the system temperature.
For questions or issues with the webpage please contact the helpdesk