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,
ΔT_{R}* , (T_{R}* = T_{A}* /η_{fss}, where η_{fss} 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

ΔT_{A} * = (2 T_{sys} ) / (t Δν)^{0.5}

Here Δ T_{A} * is the antenna temperature corrected for ohmic losses and rear spillover.
T_{sys} 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 t_{ON} = t_{OFF},
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

ΔT_{A} * = T_{sys} (1 + (1/N_{on})^{0.5})^{0.5} / (t Δν)^{0.5}

where t the ON-source integration time per map point only and N_{on} 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 T_{sys}, 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 T_{R}*) based on the input
integration time.

** Brightness Temperature, T _{R}* (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,
T

** 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
N_{on} 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 N_{on} 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 N_{on} 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 N_{on} field. You may ignore the Map Size and Map
Type fields.

For questions or issues with the webpage please contact the helpdesk

Software version 1.0.9 06 Apr 2018