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 and Ed Chambers.
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. This formula applies when tON = tOFF, as is the case for single point total power observations, and all beam-switched observations.
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 the GREAT chapter of the observer's handbook.
The calculator uses the most recent measured receiver temperatures 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.
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 +/- 1.5 GHz for the LFA, HFA, and 4GREAT bands 3 and 4, and +/- 5.5 GHz 4GREAT bands 1 and 2). 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).
Type of Observation: Select "Single Point or Beam Switch OTF/Raster Map" for single point observations in both total power and beam-switched mode, and OTF/raster map observations in beam-switched mode. Select "TP OTF/Raster Map" for OTF/raster maps in total power mode.
Observatory Altitude: Enter the flight altitude for the observations (in feet or meters). If unknown, leave at the default value of 41000 feet.
Water Vapor Overburden: Enter the precipitable waver vapor overburden (in microns) to use in the calculation of the atmospheric transmission. If unknown, leave at the default value of 0 (ATRAN will use 7.3 microns for an altitude of 41000 feet).
Telescope Elevation: Enter the telescope elevation for the observations (in degrees). If unknown, leave at the default value of 45 degrees.
Signal to Noise Ratio / Integration Time: If the SNR radio button is selected, enter the desired signal to noise ratio in this field and the esimated line strength in the Brightness Temperature field. The time estimator will calculate the integration time required to reach this SNR. If the "Integration Time" radio button is selected, enter the integration time (in seconds) for your observation. If your observation is a TP OTF map or a TP raster map, enter the ON-source time per map point. Otherwise, enter the ON+OFF integration time. The time estimator will calculate the 1-sigma rms sensitivity (in units of TR*) based on the input integration time.
Brightness Temperature, TR* (K): Enter the estimate of the peak brightness temperature of your line. This field only appears if the SNR radio button is selected (see above). 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 0.70 for the LFA, 0.63 for the HFA. For the latest 4GREAT main beam efficiencies, please contact the helpdesk
Rest Frequency: Enter the rest frequency (in THz, using 7 decimal places) of the line you wish to observe. The current tuning ranges for the GREAT receivers are listed in the table below.
|Reciever||Frequency Range (THz)|
|LFA||1.830 - 2.006|
|HFA||4.744 - 4.745|
|4GREAT1||0.490 - 0.635|
|4GREAT2||0.890 - 1.100|
|4GREAT3||1.200 - 1.529|
|4GREAT4||2.480 - 2.700|
Source Velocity: Enter the source velocity (in km/s) in the LSR reference frame.
Observer Velocity: Enter the velocity of the observatory with respect to the LSR on the date of the observation. If this is unknown, you may either leave the default (0 km/s) or enter the date, time, coordinates, and location for your observation and the time estimator will calculate the observer velcity for you. Note that 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.
Frequency or Velocity Resolution: Enter the frequency (in MHz; select the "MHz" radio button) or velocity (in km/s; select the "km/s" radio button) resolution that you want in your final spectrum.
Line Width: Enter the frequency(in MHz; select the "MHz" radio button) or velocity (in km/s; select the "km/s" radio button) window that will be 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.
TP Map Parameters:
For OTF maps: Enter the number of on positions (dumps) in each OTF scan row in the Non field or have the time estimator calculate this value for you. If you choose to have the estimator calculate it, you should enter the dimensions of the map (in arcsec) and select a "Map Type" option (Classical OTF or Array OTF). For a Classical OTF map, the "Map Size" refers to the area mapped by the central pixel only. For an Array OTF map, the "Map Size" refers to the area that will be fully-sampled (i.e., the array width is added to the length of each scan). The Array OTF map should only be selected if the frequency falls within the tuning range of the LFA or HFA (see table above). With the inputs, the calculator evaluates scanning in both x- and y-directions, and selects the direction that has fewer scan lines. It then estimates Non using the length of the scans and a frequency-based receiver stability time. The step sizes assumed for each frequency band are: HFA: 3 arcsec, LFA: 6 arcsec, 4GREAT1: 25 arcsec, 4GREAT2: 12 arcsec, 4GREAT3: 8 arcsec, 4GREAT4: 5 arcsec. Note that there are many ways to configure a mapping observation, and the calculated value of Non is only one of many possible values.
For Raster maps: Enter the number of on positions that will be used for each reference position in the Non field. You may ignore the Map Size and Map Type fields.
For questions or issues with the webpage please contact the helpdesk