GBT 4mm Receiver 2011B Shared-Risk Call for Proposals

The National Radio Astronomy Observatory announces the opportunity for shared-risk proposals for the 2011 February 01 deadline using the new GBT 4mm Receiver. The 4mm system is a traditional dual-beam feed horn receiver designed to cover the low-frequency end of the 3mm atmospheric window from 68-92 GHz (GBT 4mm Receiver). Active design work is ongoing, and the goal is to have the system ready for early commissioning on the GBT in 2011 December. Under the current schedule, shared-risk observations for the general community are planned for early 2012.

System Performance

The current hardware was designed for the 68-92 GHz band.
We expect initial system temperatures of 100-150 K for the receiver in good weather (zenith tau<~0.1).
We initially expect night-time only observing.
The aperture efficiency for night-time observations is expected to be about 32% at 90 GHz. The performance should be better at lower frequencies.
You should allow for additional overheads of 20 minutes for the AutoOOF procedure, every 1.5-2 hrs.
The FWHM beam size is expected to be about 9 arcsec at 90 GHz.
Upgrades to the servo system are planned for the summer of 2011. Until improvements are made to the pointing and control system (planned in the fall of 2011), you may expect to have pointing/tracking errors of 1.5-2.5 arcsec, which is a significant fraction of the beam size. The upgraded servo system is expected to have tracking errors of 1 arcsec or less.
Weak, broad lines may be difficult to detect due to baseline issues. The baseline performance has yet to be determined for the 4mm receiver, and fast optical nutating/chopping is not available for the GBT to help mitigate potential baseline issues.
Not including potential pointing errors, we expect relative calibration errors of 2-5%. You should allow additional overheads of 1 minute every 10-30 minutes for the calibration sequence, consisting of observations of the cold, ambient, and sky loads.

Available Observing Modes

For the current call, only spectral-line observations using the current GBT spectrometer will be accepted.
Proposals using the following modes will be accepted:
- Point-source spectral line observations using position nodding or frequency switching.
- On-The-Fly spectral line mapping of extended regions.
Linear dual polarization is available for this call.
Continuum observations will not be accepted and should be done with Mustang.
No GBT 4mm VLBI proposals will be accepted for the 2011B semester.

Sensitivity Estimate

The noise on the T_A temperature scale for ON-OFF observations is
sigma(T_A) = 2*Tsys(np*dnu*t)^(-0.5),
where Tsys is the system temperature in K, np is the number of polarizations, dnu is the bandwidth in Hz, and t is the time in seconds for the ON+OFF observations. For 1hr of ON+OFF observations, dnu=1MHz, np=2, and Tsys=125K, sigma(T_A) =~ 3mK.

Correcting for the atmosphere, the noise on the main beam temperature scale T_mb for sources larger than the beam is
sigma(T_mb) = sigma(T_A)*exp(tau_o*Airmass)/eta_mb,
where tau_o is the zenith opacity and eta_mb is the main beam efficiency which is about 1.32 times that of the aperture efficiency for the GBT (but eta_mb still needs to be measured and verified at 4mm). For the same sigma(T_A) =3mK (1hr of ON+OFF, dnu=1MHz, np=2, and Tsys=125K), tau_o=0.1, Airmass=1.4, and eta_mb = 0.42 for an aperture efficiency of 32% at 90 GHz, sigma(T_mb) =~ 8 mK.

For point sources, the noise in flux density units is
sigma(S_nu) = 2k*sigma(T_A)*exp(tau_o*Airmass)/(eta_a*A_GBT),
where k is the Boltzmann constant, eta_a is the aperture efficiency, and A_GBT = the projected area of the telescope (7854.0 m^2). For the same sigma(T_A) =3mK (1hr of ON+OFF, dnu=1MHz, np=2, and Tsys=125K), tau_o=0.1, Airmass=1.4, and eta_a = 0.32 (at 90GHz), sigma(S_nu) =~ 4 mJy.

For estimating the total amount of time required, include 15 minutes for session setup, add 10 minutes per hour for the standard point+focus, 15 minutes per session for absolute flux calibration, 1 minute per 10-30 minutes of observation time for the calibration sequence, and include an extra 20 minutes per 1-2 hours for the AutoOOF procedure.

The ongoing status of the instrument will be reported on the (GBT 4mm Receiver Web Page). Feel free to contact David Frayer if you have any questions: ().