This form can be used to estimate the integration time needed to reach a requested signal-to-noise for a given brightness temperature.
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. Subsequent modifications and revisions have been made by Bill Vacca.
The time estimator calculates the time required to reach an rms brightness temperature, ΔTR* , (TR* = TA* /ηfss, where ηfss is the forward scattering efficiency and equal to 0.97 for GREAT at all bands) for a line at a frequency ν by solving the standard radiometric formula for a single point
ΔTA * = (2 Tsys ) / (t Δν)0.5
Here Δ TA * is the antenna temperature corrected for ohmic losses and rear spillover. Tsys is the single sideband system temperature outside the earth's atmosphere, t is the integration time and Δν is the desired frequency resolution. For single point observations, the integration time is the total ON+OFF time.
For Total Power OTF mapping observations the corresponding equation is
ΔTA * = Tsys (1 + (1/Non)0.5)0.5 / (t Δν)0.5
where t the ON-source integration time per map point only and Non is the number of on-source positions for each off-source observation. For further details, see Guide to GREAT . (Note: The OTF mapping sensitivity is for GREAT's Total Power mode only. If you plan to map in 'Beam-Switched' mode, the on-source and off-source times are the same, and you can use the single point sensitivity calculator.)
If Non is not input by the user (or set to 0), it is calculated using the input map size and the observing frequency. The calculator evaluates scanning in both x- and y-directions, and selects the direction that has fewer scan lines. It then estimates N_on using the length of the scans and a frequency-based receiver stability time. The step sizes assumed for each frequency are: HFA: 3 arcsec, M: 5 arcsec, LFA: 6 arcsec, L1: 8 arcsec. When mapping in the 4.7 THz or 1.8-2.1 THz range, it is assumed that the HFA (size 29.1 arcsec) or LFA (size 72.6 arcsec) configuration is used. Note that there are many ways to configure a mapping observation, and the calculated value of Non is only one of many possible values.
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 Line Width field sets the frequency/velocity window that is used to calculate the atmospheric transmission. Modifying this parameter may be important if the line you wish to observe falls close to a narrow atmospheric feature.
The time estimator can also compute the required integration time for a line with a given peak brightness temperature, desired signal to noise ratio (SNR) and frequency or velocity resolution. For single pointing observations, the integration time is the total ON+OFF time; for mapping observations this is the ON-source time per map point only. In either case, this is the time to input into USPOT. 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.
GREAT is a dual sideband receiver, meaning it receives signal in two frequency bands, the upper sideband (USB) and the lower sideband (LSB). The transmission plot shows the location of both sidebands (separated by +/- 3.0 GHz for L1, L2/LFA, and H, and +/- 6.5 GHz for Ma). It is possible to put the line to be observed in either the USB or the LSB (the two possible tunings). Integration times are calculated for both tunings. 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 emission from both the signal and the image band contributes equally to the system temperature. (Noise arises from both the USB and the LSB.)
For questions or issues with the webpage please contact the helpdesk