The Proposer’s Guide for the Green Bank
Telescope
GBT Support Staff
December 19, 2012
This guide provides essential information for the preparation of observing
proposals on the Green Bank Telescope (GBT). The information covers
the facilities that will be offered in Semester 13B.
i
ii
Important News for Proposers
Deadline Proposals must be received by 5:00 P.M. EST (22:00 UTC) on Friday, 1 February 2012.
Technical Justification is Required All GBT proposals must include a Technical Justifica-
tion section (see Section 8.2)). Any proposal that does not include a technical justification may
be rejected without consideration.
VErsitile GBT Astronomical Spectrometer (VEGAS) We will accept shared-risk ob-
servations using the new VErsitile GBT Astronomical Spectrometer (VEGAS) which is an FPGA
based backend (see Section 3.3.2)).
PF1/450 Feed RFI Digital TV signals at frequencies above 470 MHz will make observing very
difficult with this receiver. Available RFI plots do not show the strength of these signals very well
as they overpower the system. Observers should consult the support scientists before submitting
a proposal for this feed.
PF1/600 Feed RFI Digital TV signals at frequencies covering most of this feed will make observ-
ing very difficult with this receiver. Available RFI plots do not show the strength of these signals
very well as they overpower the system. Observers should consult the support scientists before
submitting a proposal for this feed.
C-band Receiver The C-band receiver will be upgraded to include the 6-8 GHz frequency range.
We will consider shared-risk proposals for the 1 February 2013 deadline for observations in the 6-8
GHz range.
Ku-wideband Receiver The Ku-wideband receiver has nominal frequency range to cover 12.0
- 18.0 GHz. We will consider shared-risk proposals for this new feed (Ku-wideband) at the 1
February 2013 proposal deadline. When proposing, please use the nominal system temperature for

3.1.3 Efficiency and Gain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.2 Receivers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.2.1 Prime Focus Receivers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.2.2 Gregorian Receivers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.2.3 Receiver Resonances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.3 Backends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
3.3.1 GBT Spectrometer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
3.3.2 VErsitile GBT Astronomical Spectrometer . . . . . . . . . . . . . . . . . . . . . . 17
3.3.3 Spectral Processor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
3.3.4 DCR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
3.3.5 Guppi . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
3.3.6 CCB . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
3.3.7 Mark5 VLBA Disk Recorder . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
3.3.8 User Provided Backends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
4 GBT Observing Modes 21
4.1 Utility modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
4.2 Standard Observing Modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
4.3 Switching Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
4.4 Spectral Line Modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
4.4.1 Sensitivity and Integration Times . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
4.5 Continuum Modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
4.6 Polarization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
4.7 VLBI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
iv
5 Defining Sessions 26
6 Estimating Overhead Time 27
7 RFI 27
8 Tips for Writing Your Proposal 28
8.1 Items To Consider . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
8.2 Advice For Writing Your Technical Justification . . . . . . . . . . . . . . . . . . . . . . . . 28

14 GBT Contacts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
15 Useful Web Sites for Proposal Writers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
1
1 Introduction to the GBT
Location Green Bank, West Virginia, USA
Coordinates Longitude: 79
◦
50

23.406

West (NAD83)
Latitude: 38
◦
25

59.236

North (NAD83)
Track Elevation: 807.43 m (NAVD88)
Optics 110 m x 100 m unblocked section of a 208 m parent paraboloid
Offaxis feed arm
Telescope Diameter 100 m (effective)
Available Foci Prime and Gregorian
f/D (prime) = 0.29 (referred to 208 m parent parabola)
f/D (prime) = 0.6 (referred to 100 m effective parabola)
f/D (Gregorian) = 1.9 (referred to 100 m effective aperture)
Receiver mounts Prime: Retractable boom with
Focus-Rotation Mount
Gregorian: Rotating turret with

• Fully-steerable antenna 5–90 degrees elevation range and 85% coverage of the celestial sphere
1
• Unblocked aperture reduces sidelobes, Radio Frequency Interference (RFI), and spectral standing
waves
• Active surface allows for compensation for gravity and thermal distortions, and includes near real-
time adjustments to optics and pointing.
• Frequency coverage of 290 MHz to 100 GHz provides nearly 3 decades of frequency coverage
for maximum scientific flexibility
1
Because the GBT is an alt-az mounted telescope it cannot track sources that are near the zenith.
2
• Location in the National Radio Quiet Zone ensures a comparatively low RFI environment
The GBT is operated by the National Radio Astronomy Observatory, a facility of the National
Science Foundation operated under cooperative agreement by Associated Universities Incorporated. The
GBT is intended to address a very broad range of astronomical problems at radio wavelengths, and is
available to qualified observers on a peer-reviewed proposal basis. It is run primarily as a facility
for visiting observers, and the NRAO provides extensive support services including round-the-clock
operators.
Technical specifications for the telescope are given in Table 1.
Source rising and setting times can be estimated using Figure 1.
Figure 1: Plot of elevation vs azimuth, with lines of constant Hour Angle (HA; cyan lines) and Declination
(DEC; brown lines) for the GBT. The horizon (magenta line) is shown at 5 degrees elevation, except for
the mountains in the west and the 140–foot (43-m) telescope at azimuth = 48
◦
. The lines of constant
DEC are shown in increments of ± 10
◦
, while the lines of constant HA are in increments of ± 1 hour.
2 Submitting a proposal
General proposal information is available at The NRAO proposal

The resolution of the GBT is given by
FWHM = (1.02 + 0.0135 ∗Te(Db))
λ
100 m
rad (1)
where FWHM is the Full-Width at Half-Maximum of the symmetric, two-dimensional Gaussian shaped
beam and Te(Db) is the edge taper of the feed’s illumination of the dish in decibels. The edge taper
varies with frequency and polarization for all of the GBT feeds. For the Gregorian feed the edge taper
is typically 14 ± 2 Db which results in
F W HM
>1GHz
=
12.46 → 12.73

f
GHz
=
747.6 → 763.8

f
GHz
(2)
For the prime focus receivers the edge taper is typically 18 ± 2 Db which results in
F W HM
<1GHz
=
12.73 → 13.29

f
GHz

The prime focus receiver is mounted in a focus-rotation mount (FRM) on a retractable boom. The boom
is moved to the prime focus position when prime focus receiver is in use, and retracted when Gregorian
receivers are required. The FRM has three degrees of freedom: Z-axis radial focus, Y-axis translation
(in the direction of the dish plane of symmetry), and rotation. It can be extended or retracted at any
elevation. This usually takes about 10 minutes.
As the FRM holds one receiver box at a time, a change from PF1 to PF2 receivers requires a
box exchange. Additionally, changing frequency bands within PF1 requires a change in the PF1 feed.
Changes of or in prime focus receivers are usually made during routine maintenance time preceding a
dedicated campaign using that receiver.
Prime Focus 1 (PF1)
The PF1 receiver is divided into 4 frequency bands within the same receiver box. The frequency
ranges are (see Table 3) 290 - 395 MHz, 385 - 520 MHz, 510 - 690 MHz and 680 - 920 MHz. Each
frequency band requires its specific feed to be attached to the receiver before that band can be used.
The receivers are cooled FET amplifiers. The feeds for the first three bands are short-backfire dipoles.
The feed for the fourth is a corrugated feed horn with an Orthomode transducer (OMT) polarization
splitter.
A feed change is required to move between bands. This takes 2-4 hours, and is done during routine
maintenance days (see above).
The user can select one of four IF filters in the PF1 receiver. These have bandwidths of 20, 40, 80
and 240 MHz.
6
Prime Focus 2 (PF2) (0.910 - 1.23 GHz)
PF2 uses a cooled FET and a corrugated feed horn with an OMT. The user can select one of four
IF filters in the PF2 receiver. These have bandwidths of 20, 40, 80 and 240 MHz.
3.2.2 Gregorian Receivers
The receiver room located at the Gregorian Focus contains a rotating turret in which the Gregorian
receivers are mounted. There are 8 portals for receiver boxes in the turret. All 8 receivers can be kept
cold and active at all times.
More information on individual Gregorian receivers follows, which includes design types and internal
switching modes, i.e., those switching modes activated inside the receiver (e.g., frequency, beam, or

calibration.
If funds and resources become available the frequency range of this receiver may be extended up to
8 GHz.
X-Band (8.0 – 10.0 GHz)
1
1
The frequency range of the X-band receiver has been extended up to 11.6 GHz. However, users are cautioned that
above 10 GHz, the polarization purity degrades, and the low level noise-diode strength drops off.
7
This receiver has one beam, with dual circular polarizations. The feed has a cooled polarizer pro-
ducing circular polarizations. The internal switching modes are frequency switching and polarization
switching. The user can select IF Bandwidths of 500 or 2400 MHz. There is a single noise diode (∼ 10%
of the system temperature) for flux calibration.
Ku-Band (12.0 – 15.4 GHz)
This receiver has two beams on the sky with fixed separation, each with dual circular polarization.
The feeds have cooled polarizers producing circular polarizations. Internal switching modes are frequency
and/or IF switching (the switch is after the first amplifiers). The user can select IF Bandwidths of 500
or 3500 MHz. The two Ku-band feeds are separated by 330

in the cross-elevation direction. There is a
noise diode for each beam (∼ 10% of the system temperature) for flux calibration.
Ku-Wideband (11.0 – 18.0 GHz)
This receiver has one beam on the sky with dual linear polarization. The receiver will cover 11 to
18 GHz simultaneously in both polarizations for pulsar observations. Spectral line observations will also
be possible with this receiver but the observer should be aware that spectral baselines are not expected
to be very good.
K-Band Focal Plane Array (18.0 – 27.5 GHz)
The K-band Focal Plane Array has seven beams total, each with dual circular polarization. Each
beam covers the 18-27.5 GHz frequency range with fixed separations on the sky. The feeds have cooled
polarizers producing circular polarization. The only internal switching modes is frequency switching.

in the cross-elevation direction.
W-band 4mm (67–93.3 GHz)
A W-band 4mm two pixel receiver is currently under development. The receiver will cover the fre-
quency range 67–93.3 GHz. Please see 12b.shtml for more details.
Note that the receiver’s instantaneous bandwidth is limited to 1280 MHz. An ongoing update to
the receiver may change this limitation. Please see 12b.shtml for
the latest bandwidth limitation information.
8
The IF system for the 4mm system is broken into four separate bands: 67-74 GHz, 73-80 GHz, 79-86
GHz, and 85-93.3 GHz, and you can only use one of these bands at a time.
Mustang (80–100 GHz Bolometer Array)
Mustang, built by a collaboration that includes the University of Pennsylvania, NRAO, GSFC,
NIST, and Cardiff University, is a 64 pixel bolometer array which operates with a 20 GHz bandwidth
centered at 90 GHz. As of March 2009, the demonstrated sensitivity of MUSTANG on the GBT yields
a 0.4 mJy RMS in one hour of integration time mapping a 3

x3

region. The noise scales as the square
root of the integration time, and with the square root of the area covered. Mapping smaller areas is
not efficient in terms of noise performance. For significantly larger areas, faster scanning will reduce the
noise by up to ∼ 35%. For photometry of compact (D = 1

or less) objects, center-weighted “daisy”
mapping scans may be used which further reduce the RMS by a factor of two in the central region.
Finally, smoothing will reduce the map RMS by a factor of ∼ (FWHM/4

), where F W HM is the
full-width at half max of the smoothing kernel (the default gridding and pixel size parameters provide
an effective 4

(OMTs) which separate the two polarizations of the incoming signal. Although valid data can be ob-
tained within the receiver resonances, the observer should be aware that this might not always be the
case. As a general rule, polarization observations will be affected much more strongly than total intensity
observations in the regions of the resonances.
The receiver resonances have been measured in the lab and are listed in Table 2. However, these
data should not be taken as complete as there may be resonances that could not be detected in the lab
due to sensitivity limits. The center frequencies of the resonances are determined with an accuracy of
only a few MHz at best. The widths of the resonances are typically less than 5 MHz.
9
Receiver Frequency FWHM
MHz MHz
PF1 796.6 2.09
PF1 817.4 3.29
PF2 925.9 0.17
PF2 1056.0 –
PF2 1169.9 3.28
L 1263.0 0.60
L 1447.0 0.68
L 1607.0 0.90
L 1720.0 –
S 1844.0 2.00
S 2118.0 0.96
S 2315.0 –
S 2561.0 0.91
C 4163.0 4.7
C 4747.0 28.6
C 5150.0 –
C 5248.0 4.3
C 5680.0 5.0
X 9742.0 6.7


1.9 70% 970
C-Band —— 3.95-6.1 Greg. Lin/Circ 1 2 —— 2.5

1.85 70% 2000
X-Band —— 8.00-10.0 Greg. Circ 1 2 —— 1.4

1.8 70% 2400
Ku-Band —— 12.0-15.4 Greg. Circ 2 2 330

54

1.7 70% 3500
Ku-wideand —— 11.0-18.0 Greg. Linear 1 2 54

1.7 70% 7000
KFPA —— 18.0-27.5 Greg. Circ 7 2 96

32

1.5 67% 1800
Ka-Band MM-F1 26.0-31.0 Greg. Circ 2 1 78

26.8

1.5 56-64% 4000
MM-F2 30.5-37.0 22.6

MM-F3 36.0-39.5 19.5


intended to optimize the scientific return on experiments. The Spectrometer is a modular system, with
four quadrants. Quadrants may be used independently or grouped together into banks of 1, 2 or
4 quadrants. This provides the observer with 1 to 3 different levels of spectral resolutions for each
observing mode, as described below. When the 4 quadrants are independently operated, they can be
configured to acquire data at up to 8 different frequencies.
The spectrometer performs auto correlations of the input signals. The input signals may be a) both
polarizations in a spectral window (i.e the selected bandwidth centered on a specified spectral line), b)
both polarization inputs from different feeds of multi-feed receivers, or c) combinations of the preceding
in different spectral windows.
The spectrometer modes are divided into two major types, wide bandwidth, low resolution and
narrow bandwidth, high resolution.
The spectrometer has a dynamic range of about ±2.5 dB from its optimal balance point
2
Off/On
observations of bright continuum sources may be affected by this.
The GBT IF system limits the number of spectral windows available to a maximum of eight. Our
observing software assumes that polarization pairs (i.e. both polarizations) will be routed to the spec-
trometer. Consequently, it is best to write your proposal assuming that you will use both polarizations.
Using only one polarization for an observation requires the observer to be a GBT expert, and requires
very long setup times.
Wide Bandwidth, Low Resolution
The spectrometer can be configured to produce 1, 2, or 4 spectra simultaneously, each with up to
800 MHz bandwidth. Hence the maximum total spectral coverage is 3200 MHz (4 spectral windows each
with 800 MHz bandwidth and no overlap) using both polarizations. The maximum spectral resolution
is dependent on the number of spectral windows and the number of quadrants used. For a given number
of spectral windows up to three different spectral resolutions are possible.
A sub-mode of the wideband operation is the 200 MHz bandwidth option, which provides increased
spectral resolution.
Table 4 shows the possible spectral resolutions with both the 800 MHz and 200 MHz bandwidths.
Narrow Bandwidth, High Resolution

800 No 2 1 2048 – 390.6250 kHz 4096 – 195.3125 kHz 4096 – 195.3125 kHz
800 No 1 2 2048 – 390.6250 kHz 4096 – 195.3125 kHz 4096 – 195.3125 kHz
800 No 4 1 2048 – 390.6250 kHz 2048 – 390.6250 kHz 2048 – 390.6250 kHz
800 No 2 2 2048 – 390.6250 kHz 2048 – 390.6250 kHz 2048 – 390.6250 kHz
800 Yes 1 1 1024 – 781.2500 kHz 2048 – 390.6250 kHz 4096 – 195.3125 kHz
800 Yes 2 1 1024 – 781.2500 kHz 2048 – 390.6250 kHz 2048 – 390.6250 kHz uncommissioned
800 Yes 1 2 1024 – 781.2500 kHz 2048 – 390.6250 kHz 2048 – 390.6250 kHz uncommissioned
800 Yes 4 1 1024 – 781.2500 kHz 1024 – 781.2500 kHz 1024 – 781.2500 kHz uncommissioned
800 Yes 2 2 1024 – 781.2500 kHz 1024 – 781.2500 kHz 1024 – 781.2500 kHz uncommissioned
200 No 1 1 8192 – 24.4141 kHz 16384 – 12.2070 kHz 32768 – 6.1035 kHz
200 No 2 1 4096 – 48.8281 kHz 8192 – 24.4141 kHz 16384 – 12.2070 kHz
200 No 1 2 4096 – 48.8281 kHz 8192 – 24.4141 kHz 16384 – 12.2070 kHz
200 No 4 1 8192 – 24.4141 kHz 8192 – 24.4141 kHz 8192 – 24.4141 kHz
200 No 2 2 8192 – 24.4141 kHz 8192 – 24.4141 kHz 8192 – 24.4141 kHz
200 Yes 1 1 4096 – 48.8281 kHz 8192 – 24.4141 kHz 16384 – 12.2070 kHz
200 Yes 2 1 2048 – 97.6563 kHz 4096 – 48.8281 kHz 8192 – 24.4141 kHz uncommissioned
200 Yes 1 2 2048 – 97.6563 kHz 4096 – 48.8281 kHz 8192 – 24.4141 kHz uncommissioned
200 Yes 4 1 4096 – 48.8281 kHz 4096 – 48.8281 kHz 4096 – 48.8281 kHz uncommissioned
200 Yes 2 2 4096 – 48.8281 kHz 4096 – 48.8281 kHz 4096 – 48.8281 kHz uncommissioned
Table 4: Commonly used configurations of the GBT Spectrometer in its Wide Bandwidth, Low Resolution Modes. All modes use 3 level sampling.
15
Bandwidth Polarization Level Number of Number of Lags - Approximate Resolution
(MHz) Cross-Products Sampling Spectral Beams Low Medium High
Windows
50 No 3 1 1 32768 - 1.5259 kHz 65536 - 0.7629 kHz 131072 - 0.3815 kHz
50 No 3 2 1 16384 - 3.0518 kHz 32768 - 1.5259 kHz 65536 - 0.7629 kHz
50 No 3 1 2 65536 - 0.7629 kHz 65536 - 0.7629 kHz 65536 - 0.7629 kHz
50 No 3 4 1 16384 - 3.0518 kHz 32768 - 1.5259 kHz 32768 - 1.5259 kHz
50 No 3 2 2 32768 - 1.5259 kHz 32768 - 1.5259 kHz 32768 - 1.5259 kHz
50 No 3 8 1 16384 - 3.0518 kHz 16384 - 3.0518 kHz 16384 - 3.0518 kHz Single Beam Receivers Only

12.5 No 3 4 1 16384 - 0.7629 kHz 32768 - 0.3815 kHz 32768 - 0.3815 kHz
12.5 No 3 2 2 32768 - 0.3815 kHz 32768 - 0.3815 kHz 32768 - 0.3815 kHz
12.5 No 3 8 1 16384 - 0.7629 kHz 16384 - 0.7629 kHz 16384 - 0.7629 kHz Single Beam Receivers Only
12.5 No 3 4 2 16384 - 0.7629 kHz 16384 - 0.7629 kHz 16384 - 0.7629 kHz
12.5 Yes 3 1 1 16384 - 0.7629 kHz 32768 - 0.3815 kHz 65536 - 0.1907 kHz uncommissioned
12.5 Yes 3 2 1 8192 - 1.5259 kHz 16384 - 0.7629 kHz 32768 - 0.3815 kHz
12.5 Yes 3 1 2 32768 - 0.3815 kHz 32768 - 0.3815 kHz 32768 - 0.3815 kHz uncommissioned
12.5 Yes 3 4 1 8192 - 1.5259 kHz 16384 - 0.7629 kHz 16384 - 0.7629 kHz
12.5 Yes 3 2 2 16384 - 0.7629 kHz 16384 - 0.7629 kHz 16384 - 0.7629 kHz uncommissioned
12.5 Yes 3 8 1 8192 - 1.5259 kHz 8192 - 1.5259 kHz 8192 - 1.5259 kHz uncommissioned, Single Beam Receivers Only
12.5 Yes 3 4 2 8192 - 1.5259 kHz 8192 - 1.5259 kHz 8192 - 1.5259 kHz uncommissioned
12.5 No 9 1 1 8192 - 1.5259 kHz 16384 - 0.7629 kHz 32768 - 0.3815 kHz
12.5 No 9 2 1 4096 - 3.0518 kHz 8192 - 1.5259 kHz 16384 - 0.7629 kHz
12.5 No 9 1 2 16384 - 0.7629 kHz 16384 - 0.7629 kHz 16384 - 0.7629 kHz
12.5 No 9 4 1 4096 - 3.0518 kHz 8192 - 1.5259 kHz 8192 - 1.5259 kHz
12.5 No 9 2 2 8192 - 1.5259 kHz 8192 - 1.5259 kHz 8192 - 1.5259 kHz
12.5 No 9 8 1 4096 - 3.0518 kHz 4096 - 3.0518 kHz 4096 - 3.0518 kHz Single Beam Receivers Only
12.5 No 9 4 2 4096 - 3.0518 kHz 4096 - 3.0518 kHz 4096 - 3.0518 kHz
12.5 Yes 9 1 1 4096 - 3.0518 kHz 8192 - 1.5259 kHz 16384 - 0.7629 kHz uncommissioned
12.5 Yes 9 2 1 2048 - 6.1035 kHz 4096 - 3.0518 kHz 8192 - 1.5259 kHz uncommissioned
12.5 Yes 9 1 2 8192 - 1.5259 kHz 8192 - 1.5259 kHz 8192 - 1.5259 kHz uncommissioned
12.5 Yes 9 4 1 2048 - 6.1035 kHz 4096 - 3.0518 kHz 4096 - 3.0518 kHz
12.5 Yes 9 2 2 4096 - 3.0518 kHz 4096 - 3.0518 kHz 4096 - 3.0518 kHz uncommissioned
12.5 Yes 9 8 1 2048 - 6.1035 kHz 2048 - 6.1035 kHz 2048 - 6.1035 kHz uncommissioned, Single Beam Receivers Only
12.5 Yes 9 4 2 2048 - 6.1035 kHz 2048 - 6.1035 kHz 2048 - 6.1035 kHz uncommissioned
Table 6: Commonly configured GBT Spectrometer 12.5 MHz Bandwidth, High Resolution Modes.
17
Bandwidth Number of Number of Channels - Approximate Resolution Mininum Integration Notes
(MHz) Spectral Windows Beams (kHz) Time (sec)
1500 1 or 2 1 1024 – 1464.844 0.5 1st priority mode

YY) without a loss in spectral resolution.
Minimum integration times are listed in Tables 7, 8 and 9.
The modes that will be available for this proposal call are listed in Tables 7, 8 and 9.
3.3.3 Spectral Processor
The Spectral Processor is an FFT spectrometer primarily designed for high time resolution observations.
Because of its wide dynamic range it is also useful for spectral line observations at low frequencies where
strong interference is a problem. It contains two FFT engines, each with 1024 channels over a maximum
bandwidth of 40 MHz which may be divided into 1, 2, or 4 separate pass bands. The two FFT engines
are synchronous and their outputs may be cross-multiplied to measure polarization. The most commonly
used types of observing with the spectral processor either total power or frequency-switched spectral
line observations.
Table 10 gives the general specifications for the spectral processor, and Table 11 shows the possible
combination of IF’s, frequency channels per IF, and bandwidth per IF.
18
Bandwidth Number of Number of Channels - Approximate Resolution Mininum Integration Notes
(MHz) Spectral Windows Beams (kHz) Time (sec)
250 1 or 2 1 32768 – 7.629 10
250 1 2 32768 – 7.629 10
100 1 or 2 1 32768 – 3.052 10
100 1 2 32768 – 3.052 10
50 1 or 2 1 32768 – 1.526 10
50 1 2 32768 – 1.526 10
25 1 or 2 1 32768 – 0.763 10
25 1 2 32768 – 0.763 10
10 1 or 2 1 32768 – 0.305 10 3rd priority mode
10 1 2 32768 – 0.305 10 3rd priority mode
5 1 or 2 1 32768 – 0.153 10
5 1 2 32768 – 0.153 10
1 1 or 2 1 32768 – 0.031 10 4th priority mode
1 1 2 32768 – 0.031 10 4th priority mode

De-dispersed time series
Table 10: Spectral Processor Specifications.
19
Bandwidth (MHz) Number of Spectral Windows Multiplied by Number of Channels
1 × 1024 2 × 512 2 × 256 4 × 256
40 Auto, Cross
20 Auto, Cross Auto, Cross, Both Auto, Cross, Both
10 Auto, Cross, Both Auto, Cross, Both Auto, Cross, Both Auto, Cross, Both
5 Auto, Cross, Both Auto, Cross, Both Auto, Cross, Both Auto, Cross, Both
2.5 Auto, Cross, Both Auto, Cross, Both Auto, Cross, Both Auto, Cross, Both
1.25 Auto, Cross, Both Auto, Cross, Both Auto, Cross, Both Auto, Cross, Both
0.625 Auto, Cross, Both Auto, Cross, Both Auto, Cross, Both Auto, Cross, Both
0.3125 Auto, Cross, Both Auto, Cross, Both Auto, Cross, Both Auto, Cross, Both
0.15625 Auto, Cross, Both Auto, Cross, Both Auto, Cross, Both Auto, Cross, Both
0.078125 Auto, Cross, Both Auto, Cross, Both Auto, Cross, Both Auto, Cross, Both
Table 11: GBT Spectral Processor Modes. Columns are Number of Spectral Windows by Number
Channels while rows are for each available bandwidth. Entries indicate availability of auto-correlations
(Square mode), cross-correlations (Cross mode), or both (Sqrcross mode). When configuring the GBT,
note that: a) High resolution modes are the 1 × 1024 and 2 × 512 modes; and b) Low resolution modes
are the 2 × 256 and 4 × 256 modes.
3.3.4 Digital Continuum Receiver (DCR)
The digital continuum receiver is the GBT’s general purpose continuum backend. It is used both for
utility observations such as pointing, focus, and beam-map calibrations, as well such as for contin-
uum astronomical observations including point-source on/offs and extended source mapping. It has the
following specifications and characteristics:
• Number of input channels: 32 in two banks of 16, only one bank usable at a time.
• Switching modes: A user defined mode and Four pre-defined modes: total power with and without
continuous calibration and switched power, with and without continuous calibration.
• Maximum number of switching phases: 10 (determined by software).
• Minimum phase time: 1 millisecond. Switching frequency is the reciprocal of the sum of the phase

with the GBT Ka-band receiver over the frequency range of 26–40 GHz. It provides carefully optimized
RF (not IF) detector circuits and the capability to beam-switch the receiver rapidly to suppress instru-
mental gain fluctuations. There are 16 input ports (only 8 can be used at present with the Ka-band
receiver), hard-wired to the receiver’s 2 feeds x 2 polarizations x 4 frequency sub-bands (26-29.5 , 29.5-
33.0; 33.0-36.5; and 36.5 - 40 GHz). The CCB allows the left and right noise-diodes to be controlled
individually to allow for differential or total power calibration. Unlike other GBT backends, the noise-
diodes are either on or off for an entire integration (there is no concept of “phase within an integration”).
The minimum practical integration period is 5 milliseconds; integration periods longer than 0.1 seconds
are not recommended. The maximum practical beam-switching period is about 4 kHz, limited by the
needed 250 micro-second beam-switch blanking time (work is underway to reduce the needed blanking
time). Switching slower than 1 kHz is not recommended.
Under the best observing conditions (clear, stable, and few or no clouds) the combination of the
Ka-band receiver and the CCB deliver a photometric sensitivity of roughly 0.2 mJy for a single one-
minute, targeted nod observation. The median sensitivity is 0.4 mJy RMS. These numbers apply to the
most sensitive (33-36 GHz) of the four frequency channels; averaging all channels together will slightly,
but not significantly, improve performance because of noise correlations and variations in the receiver
sensitivity between channels. The analogous noise performance figures for sensitive mapping projects
are still to be determined.