2. An Overview of the System
The instrumentation of the 140 Foot Telescope can be
arbitrarily divided into: the antenna, the front-end and Universal
Local Oscillator (ULO), mount and drive, Model IV Autocorrelator
(A/C), Analog-to-Digital converters (A/D), Digital Continuum Receiver
(DCR), Control and Analysis Computers. The observer should also be
aware of the roles played by telescope operators and other members of
the Green Bank staff in the running of the telescope.
a. Antenna
The optics of the telescope consist of a 140 foot (43 m)
diameter main reflector with a focal ratio of 0.43 for prime focus
front-ends. For Cassegrain front-ends, an off-axis hyperbolic
subreflector, mounted near the prime focus, increases the focal ratio
to 4.0. The half power beamwidth of the telescope in arc minutes
approximately equals the wavelength of the observations when expressed
in cm; that is, HPBW (arcmin) ~ wavelength (cm).
The average rms surface accuracy of the main reflector, about
0.9 mm when pointing at an elevation of 45 degrees, gives an aperture
efficiency of 56% across the sky at frequencies less than 5 GHz. At
higher frequencies, the aperture efficiency is less and, because of
the deformation of the backup structure and surface of the main
reflector, depends upon hour angle and declination. The highest
useable frequency is presently 25 GHz.
Since the Cassegrain system is used at the higher frequencies
(> 5 GHz), some of the fall-off in efficiency caused by astigmatism
and coma is eliminated by deforming and tilting the subreflector. If
desired, an observer can turn off either capibility. When both are
used, typical maximum aperture and beam efficiencies are given in
Table 1.
Table 1
Typical Efficiencies
Frequency Antenna Beam
(GHz) Efficiency Efficiency
========================================
5 54% 75%
8 52% 72%
11 48% 66%
15 42% 58%
22 30% 45%
========================================
With the Cassegrain system, nutating the subreflector at a
rate of 5.0 Hz or less with a beam throw of 8 arc minutes or less
reduces some of the effects of the atmosphere. The observer can vary
the nutation rate and throw but not the direction of the throw (see
CAO, App. G).
For those who look at objects within the solar system, the
astronomical longitude, latitude, and elevation of the telescope are
-79 50' 06."365, +38 26' 12."448, and 0.88087 km, respectively.
b. Front-End Receivers and Universal Local Oscillator
Cassegrain receivers are available at frequencies above 5 GHz.
You can quickly switch between recievers by rotating the subreflector.
Currently four Green Bank Telescope (GBT) receivers are installed
which consist of cooled HFET amplifiers. Prime focus receivers,
available at 5 GHz and lower frequencies, require a box change which
is typically performed at the begining of the work day. See the
FrontEnd Status Sheet for more details.
The frequency generated by the ULO dictates what the center
frequency of the front-end receiver will be. For continuum observing,
the ULO is usually left in manual control. For spectral line
observing, the observer provides the control computer with the rest
frequency of the spectral line; the intermediate frequency (IF) of the
front-end receiver; and a source velocity and its frame of reference.
The control computer instructs what the proper ULO frequency should be
for each source observed. The range of useable ULO and IF frequencies
can be obtained from the CAO (App. F) or the receiver engineer.
c. Mount and Drive
Because of the equatorial mounting of the telescope, the
telescope can observe an object when it is within the hour angle and
declination limits shown in Figure 1 (see CAO, p.8 for more details).
If D is the number of degrees seperating two positions in the sky,
then the time needed to move the telescope is:
(D - 2) * 3 + 20 seconds (D > 6.75 degrees)
D * 6 + 8 seconds (D < 6.75 degrees).
You can observe with the telescope tracking a position at the
sidereal rate, slewing in any direction at any rate less than 480' /
min in declination and 480' cos(dec) / min in right ascension, or with
the telescope stationary. The observer can choose between apparent
RA, Dec; the RA, Dec for any reasonable epoch; galactic l, b; Azimuth
and Elevation; Hour Angle, Dec; and a user defined coordinate system
(CAO, App. A). Pointing accuracy is discussed below. [The epoch
coordinates must be 1950].
d. Mark IV Autocorrelator
The 1024 channel, 3 level Mark IV autocorrelator, used for
spectral line observations, can be divided into one through four
separate receivers, each of which can be centered at different
frequencies. The bandwidths of the A/C receivers are independent of
each other and can range from 78 kHz to 80 MHz. The versatility of
the A/C allows the observer to pick the best A/C setup (i.e., velocity
resolution and coverage, number of lines observed simultaneously) from
an almost unlimited number of possible setups.
Depending upon the front-end, you may be able to observe four
different spectral lines that lie hundreds of MHz apart, or two lines
each observed with two different velocity resolutions, or a single
line with very high velocity resolution, or many lines within a single
wide A/C receiver, or two different spectral lines observed in
different polarization.
The A/C can oversample reducing the number of available
channels by a factor of two with an approximate gain of 9% in signal
to noise. If you observe with two A/C receivers centered at the same
frequency and polarization, averaging the results from the two
receivers will increase the signal to noise by 9%. If you feed
different polarizations into different A/C receivers, averaging the
results from the two A/C receivers will increase the signal to noise
by the square root of 2 for unpolarized lines. (CAO, p 26, and
"Autocorrelation Receiver Model IV: Operational Description -
Electronics Division Internal Report No. 234" provide more information
on the A/C.) The different A/C configureations and Bandwidth codes
can also be found here.
e. Analog-to-Digital Converters
For continuum observations, observers use either the DCR,
described below, for the greatest sensitivity or the A/D converters.
Observers check the pointing of the telescope with the A/D's, which
are simpler to use than the DCR. Most receivers supply two outputs,
typically two different polarizations or total and switched power, so
two A/D's are usually needed. The computer can handle up to 8 A/D's.
f. Digital Continuum Receiver
The DCR, for sensitive continuum observations, can handle up
to four different inputs. If you plan to use the DCR, contact the
engineer in charge of the receiver or the "Friend of the Telescope" so
that they can set up the DCR. A separate manual, "Digital Continuum
Receiver User's Manual -- Electronics Division Internal Report
No. 243" describes the DCR in depth.
g. Control and Analysis Computers
A Honeywell 316 and a Modcomp II computer move the telescope,
acquire the data, and monitor the system. The observers enter their
"Observing" decks into the Modcomp before observations begin. The
Modcomp checks the deck for errors and, when commanded to by the
telescope operator, begins the observations. The raw data, stored on
disk drive and occasionally dumped to tape, is passed to the analysis
computer and stored on a second disk drive.
The analysis computer is a SUN SPARCstation 4 workstation.
The present analysis software is Unipops (V3.3). Observers can take
with them tapes containing their data in binary or FITS representation
for further processing with their home institute's computer.
h. Telescope Operators and Green Bank Staff
After your observations have been scheduled by the site
director, you will receive a form that concerns the technical setup of
your experiment; fill out and return the form as soon as possible so
that the NRAO staff can prepare for the observations. If your
observations require special hardware or software, please contact the
"Friend of the Telescope" or the person in charge of that part of the
telescope, if they do not contact you first.
The observer works mostly with the telescope operators who are
responsible for the safety of the telescope and personnel and are the
only ones who can command the telescope. The observer instructs the
operator what observation to perform next and the operator sees to its
proper execution. Since the operator oversees the observations, you
can set up a series of things for the telescope and operator to do,
and go off to dinner or lunch confident that all will go well in your
absence.
You should contact the appropriate person from the following
list, correct as of November 1996, if you have any questions before
arriving or during your stay at Green Bank.
Frank Ghigo -- Scheduling : [fghigo@nrao.edu] (304) 456-2208
Becky Warner -- Transportation/Housing: [bwarner@nrao.edu] (304) 456-2227
Dana Balser -- Friend of the 140 Foot: [dbalser@nrao.edu] (304) 456-2204
140 Foot Telescope Main Console (304) 456-2345