Table of Contents
I. Introduction ........................................................... 1
II. The 140 Foot Telescope -- An Overview of the System .................... 2
b. Front-End Receivers and Universal Local Oscillator
c. Mount and Drive
d. Mark IV Autocorrelator
e. Analog-to-Digital Converters
f. Digital Continuum Receiver
g. Control and Analysis Computers
h. Telescope Operators and Green Bank Staff
III. Observational Techniques .............................................. 5
a. Continuum Observing, Pointing, and Calibration
b. Spectral Line Observing
IV. Card Decks -- The Way to Tell the Computer What to Observe and How ..... 9
a. Setup Cards and Decks
b. Source Cards and Decks
V. Observing Procedures ................................................... 10
Appendix A -- Rules and Regulations for Creating Setup and Source Decks .... 11
Appendix B -- Examples of Card Decks ....................................... 24
Appendix C -- Commonly Used Observing Procedures........................ 27
You've submitted an observing proposal to use the 140 Foot Telescope and it has been accepted! But, you never used the telescope before or it's been years since you were in Green Bank and you forgot how to use thetelescope. And, to top it off, you begin to wonder from the size of the telescope manuals whether Tolstoy has been reincarnated as an NRAO radio astronomer.
This handbook, a condensed version of the larger manual "Computer
Assisted Observing: The 140 Foot Manual -- 3rd Edition, November 1987"
(hereafter, CAO), is designed to assist observers who are not very familiar with
the 140 Foot Telescope. Keep the CAO by your side while reading the handbook
since the CAO contains more complete details on different subjects. Some
of the material in the handbook does not originate from the CAO but from other
manuals, internal memos, technical reports, and the undocumented, almost
folkloric practices of long-time users of the telescope. If you need any
telescope documentation, contact Telescope Services at Green Bank.
First-time users of the telescope should arrive in Green Bank one or two WORKING days before the start of the observing run so that they can meet with the staff to discuss the proposed observations. Ph.D. advisors should observe with their students on their student's first experiment. The "Friend of the Telescope", who advises observers in planning the observations and assists if problems arise, should be able to either answer most questions or direct you to the correct person. However, staff members expect that you have at least tried to learn something about the telescope and its software before seeking help. Don't look for help if you have not looked through the manuals. We do not support remote observing.
Newcomers to the telescope should read the complete handbook; experienced users may need to skim through a few sections.
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.
The optics of the telescope consist of a 140 foot (43 m) diameter
mainreflector 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.
|Frequency (GHz)||Antenna Efficiency||Beam Efficiency|
With the Cassegrain system, nutating the subreflector at a rate of 5.0 Hz or
less with a beam throw of 18 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.
Most observations at frequencies higher than 5 GHz are made with either of
the two maser receivers located at the Cassegrian focus. You can quickly switch
between maser receivers by rotating the subreflector. With the Beam Splitter,
useable at frequencies higher than 7.5 GHz, you can simultaneously observe two
orthogonal linear polarizations or two different frequencies.
Prime focus receivers, available at 5 GHz and lower frequencies, can be rotated in their mount so that you can choose different polarizations. Some receivers allow for the simultaneous observing of orthogonal polarizations. Technical data about the available receivers can be found in the "NRAO Front- End Box Status" memo and from the Electronics division at Green Bank.
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.
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
(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 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.)
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.
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.
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 the Modcomp's 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 second Modcomp II. You can further reduce data on
the Modcomp or the Masscomp in Jansky Lab, or on the VAX in Charlottesville. The
analysis software, called POPS for spectral line data and CONDAR for continuum,
is described in "Spectral Analysis for the NRAO Single Dish
Telescopes" and "On-Site Data Reduction for the NRAO Single Dish
Telescopes". Observers can take with them tapes containing their data in
binary or FITS representation for further processing with their home institute's
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 1987, if you have any questions before arriving or during your stay at Green Bank. If you don't know who to call, try the appropriate "Friend of the Telescope".
Telephone Operator (304-456-2201; FTS: 924-6201)
George Seielstad -- Site director; telescope scheduling (ext. 301)
Fred Crews -- Telescope Services; manuals (ext. 215)
Roger Norrod -- Head of Electronics; receiver design (ext. 122)
Chuck Brockway -- Cassegrain receiver and subreflector (ext. 132)
Bob Vance -- Computer system and software (ext. 222)
Becky Warner -- Transportation and housing (ext. 227)
Harry Payne -- "Friend of the Telescope", obs. below 1 GHz (ext. 209)
Ron Maddalena -- "Friend of the Telescope", obs. above 1 GHz (ext. 207)
140 Foot Telescope Main Console (ext. 345)
More information about the site and personnel may be found in the
"Visitors Information" pamphlet available from Telescope Services.
The observing techniques used at the 140 foot depend not only on whether the mode is continuum or spectral line but also depend on the type of object observed, the frequency, and the available hardware. The following sections offer suggestions on how best to conduct certain standard experiments. If you plan observations that are unusual, or that press the hardware to its limits, then contact the "Friend of the Telescope" for suggestions.
All observers who use the telescope at frequencies higher than 5 GHz or whose observations require critical calibration or pointing of the telescope should familiarize themselves with the continuum capabilities of the 140-ft since the necessary data are obtained with continuum measurements.
The unit of observation for continuum and spectral line work is called a
scan, each of which is given a unique designation called a scan number. Maps and
surveys are built out of individual scans.
Depending upon the receiver, continuum observations can be in one of the following modes: total power, noise adding, load switching, and beam or position switching; check with the receiver engineer as to which are possible. Observations with the Cassegrain reciever can take advantage of the quick beam switching provided by nutating the subreflector; position switching with prime focus receivers requires physically moving the telescope. The excellent description of continuum techniques found in Chapter IV of the "Digital Continuum Receivers User's Manual -- Electronics Division Internal Report No. 243" should be consulted by those who wish to make, but are unfamiliar with, continuum measurements.
A scan is built out of one or more samples of the incoming signal; the sample times must be multiples of 0.1 seconds. While taking data, the telescope can be either stationary, tracking at the sidereal rate, slewing in any direction, or moving back and forth between positions in the sky (ON-OFF observations).
If you only need continuum data in order to check the pointing, then total power with prime focus receivers and beam switching with the Cassegrain system are adequate. At high frequencies (> 5 GHz) we highly recommnend that you check the pointing occasionally. Table 2 suggests how often pointing should be done and how close a continuum source must be to your region of interest in order to ensure adequate pointing accuracy (better than 1/5 of the beamwidth). You should also check the pointing every time a move is made to a source at a new and widely different position in the sky.
|Frequency (GHZ)||How Often||How Far|
|22||1h best, 2h at most||10-15 degrees|
|1.4 to 5||6-12h||45 degrees|
Note that bad weather while observing at high frequencies can make pointing
difficult on even the strongest sources. You can schedule pointing in an
observing deck with the PEAK observing procedure or, to eliminate the
possibility of erroneous pointing corrections, you can tell the operator when
and on what object to point. The operator will set up the control panel, execute
the observation, and only update the pointing corrections if the observations
were good (CAO, pp 13-16). The operators have an extensive list of sources
suitable for pointing.
The observers are responsible for correcting their data for telescope efficiency, atmospheric opacity, and errors in the value of the noise tube temperature. You can obtain most of these corrections from continuum measurements of sources with known fluxes. Some types of observing require as much time checking the calibration as observing. Some types require only a quick check. In order to correct your data for the shape of the beam of the telescope, you can make continuum maps toward strong point sources. Upon request, the "Friend of the Telescope" will recommend ways to calibrate data and make beam maps.
Spectral line observations with the Mark IV A/C can be either frequency
switched, position switched, or, with a Cassegrain receiver, beam switched (see
description above and CAO, p. 26 for details about the A/C). The
integration time for a scan must be multiples of the A/C dump time which itself
must be multiples of 20 seconds. We advise that the duration time for a scan
should not exceed 1 hour; you can always average scans together during the
analysis. In the presence of spurious interference, as found at low frequencies,
we suggest that you start off with short (20 or 60 sec.) scans. By analyzing
individual scans, you can throw away scans plagued by interference and keep the
During the integration time, the telescope can be stationary, tracking an object at the sideral rate, or slewing in any direction. The appropriate way for your observations to set the internal attenuators of the A/C, called balancing the A/C, can be discussed with the "Friend of the Telescope".
Continuum emission from the sky, ground, and astronomical sources generates standing waves between telescope components above the dish of the telescope. The standing wave, which will appear as a 8 MHz ripple in the data, can be partly removed by including MODFOCUS on an unlabelled card in a "Setup" or Source" deck. (MODFOCUS alternates the position of the subreflector or receiver by +1/8 and -1/8 of a wavelength during an observation - see CAO, p. 34.) The problems is most severe at high frequencies, because of atmospheric variablity, and with wide A/C bandwidths (5-80 MHz).
In the discussion below, I am considering the case of an emission line; the extension to absorption lines is straightforward.
During frequency switched or SPOWER observations, the observed frequency
alternates between frequencies chosen by the observer. The observer supplies on
the "L" card one on-frequency offset and two off-frequency offsets.
The A/C cycles the ULO through the sequence: on - off 1 - on - off 2 - on - off
1 ... with each step in the cycle lasting 1 second. The A/C subtracts the off
observations from the on observations in order to remove the instrumental
Most observers use identical off-frequency offsets that are < 1/4 the A/C bandwith and an on offset of minus the off offsets. For example, an observation at 15 GHz with a 20 MHz bandwidth for the A/C could switch between 15.005 GHZ and 14.095 GHz by specifying frequncy offsets of 5, -5, and -5 MHz on the "L" card; the spectral line will appear twice in the bandpass of the A/C. One of the lines will be the normal emission line at the correct velocity while the second will appear 10 MHz away but as an absorption feature. The observer can average the two lines with the analysis software thereby doubling the effective integration time (see CAO, Appendix E).
If the amount of frequency change is small (< few MHz), SPOWER provides one of the best ways of removing the shape of the instrumental bandpass and provides the best signal-to-noise. If the expected spectral line is wide or could fall over a large range of velocities, or if more than one line is expected along the line-of-sight, than SPOWER may not be the best method. In some applications, SPOWER, because each channel in the A/C is sensitive to two frequencies, reduces the number of useable channels by the amount of the frequency switch.
In position switched or TPOWER observations, the telescpe collects data not only at the position in the sky the observer is interested in (the ON position), but also a nearby OFF position that is assumed or known to be free of emission. The instrumental bandpass of the telescope, measured at the OFF position, is subtracted from the ON positions using the analysis software. Unlike frequency switching, both the on and off measurements are kept as individual scans. TPOWER provides more useable channels than SPOWER but can produce exceedingly bad baselines in some cases.
Problems with TPOWER are: the off position may not be free of emission; the atmospheric contribution to the system can differ between the ON and OFF position; the receiver characteristics can change between the ON and OFF measurements; and standing waves can be more troublesome.
An alternate way of position switching with Cassegrain receivers, called beam
switching, uses the nutating subreflector. The observer specifies ZEROS for the
frequency offsets on the "L" card and takes SPOWER observations. The
A/C, controlling the nutation of the subreflector, measures the bandpass of the
system every 1 sec when the subreflector is in its reference position.
Next to frequency switching, beam switching provides the best way of removing the changes in the atmosphere and response of the instrument; it does not provide twice the signal as frequency switching sometimes does. Since the beam throw of the nutator cannot exceed 18' (CAO, App. G), the reference position could lie on the source if it is extended; you then can't beam switch.
In double beam switching, an improvement to standard beam switching, the telescope takes pairs of scans with the source first in the signal beam of the nutator and, after the telescope moves by a specified amount, with the source in the reference beam. The baselines usually are good, but you cannot use double beam switching with extended sources. A memo attached to the CAO explains the mechanics of double beam switching.
Observers must be aware of problems with double beam switching at frequencies
higher than 10 GHz. Because the reference beam is off the axis of the telescope,
the shape of the beam is distorted; the distortion depends upon the telescope's
position and is unpredictable. The efficiency of the reference beam is variable
and can be 1/2 the efficiency of the signal beam. The software must know the
location of the reference beam, but the center of the distorted beam changes
position so the observer must occasionally measure the location of the beam.
Once you have decided on the methods you wish to use to observe, you must
prepare either a "Setup" or a "Source" deck. These decks can
be either a stack of computer cards or a file of card images written on tape or
PC floppy Long decks of cards (> 50 cards) should be stored as a file on tape
or floppy; short decks on tapes are inconvenient.
Our hardware accepts 1600 BPI, 9 track tapes consisting of formatted, 80 bytes blocked records with no lower case or special characters like tabs, escape sequences, line feeds, or carriage returns. Since tape files cannot be edited at the telescope, we recommend that all tape files should be written to a floppy using the Masscomp computer in Jansky Lab.
Some observers may want to transfer their decks to the Masscomp in Jansky Lab from their home computer via a modem or create their decks on that computer. The observer can then modify the file and write it to a floppy disk.
Floppy files must contain lines that are 80-82 characters long; carriage
returns, line feeds or other characters in columns 81-82 are ignored. Columns 1
- 80 must not contain lower case or special characters like tabs, escape
sequences, carriage returns, or line feeds. We support double-sided,
double-density (2S/2D), 5 1/4 inch floppies. File-ed, a menu driven editor
available on PC's at Green Bank, can help first-time users of the telescope in
creating observing files (documentation attached to the CAO). You can use
Checker, a program available on PC's and the Masscomp in Jansky Lab, to examine
decks for errors.
Regardless of the physical medium of the deck, the term card in the following discussion will mean either a computer card, a record in a tape file, or a line in a floppy file. The rules and regulations on what these cards must contain and examples of decks are given in Appendix A and B, respectively.
The telescope operator or the "Friend of the Telescope" will instruct you on how to read a deck of cards, a file on a tape, or a floppy into the control computer.
All observers must prepare "Setup" cards that tell the computer how to set up the hardware or how the hardware has already been set up. The deck consist of an "O" card, up to four "R" cards, "A", "L", P", and optional "C", "D", and "Unlabelled" cards that are formatted exactly as shown in Appendix A.
A deck consisting of only "Setup" cards is a "Setup" deck
and is for interactive types of observing programs using the operator's console.
Interactive observing can be very powerful for very short list of objects that
are observed for long stretches of time. However, we only recommend observing
from the operator's console to seasoned veterans of radio astronomy. Most
first-time users of the telescope should prepare "Source" decks and
run their observing program under computer control. The CAO and "Friend of
the Telescope" will provide help to those who want to observe from the
A "Source" deck consists of "Setup" cards followed by
source ("S") cards containing information like the name, position,
velocity for the objects the observer wishes to look at, and the name of the
procedure to use for the observations. You can change a "Setup"
parameter in the middle of a "Source" deck by inserting the new
"Setup" card at the location in the deck where you wish to make the
change (examples in App. B).
Since the operator cannot change observing parameters when a "Source" deck is used, the deck must contain all the necessary information. Observations can start from any card in the deck, and will proceed down the deck until the operator manually terminates the observations or the end of the deck is reached. You can jump back and forth within a deck.
Observing procedures tell the computer how to perform the observations and consist of commands on where to move the telescope and what action to perform (e.g., balance the A/C, take an off or on TPOWER scan, a continuum scan with the A/D's or DCR, or an SPOWER scan). The language of procedures includes assignment statements, arrays, loops, and if and while statements. Procedures can write messages to the operator's CRT and assign values to variables through read statements from the CRT. The language is identical to that used for procedures in the analysis systems POPS, CONDAR, or AIPS; only the list of adverbs or verbs differ between the systems. Observers familiar with procedures in these systems will have no dificulty in understanding observing procedures although, since the verbs differ, they may have difficulty writing one. The limitations as to what a procedure can do depend upon the storage space of the computer and the ingenuity and experience of the person writing the procedure.
Since the size of the handbook is limited, a full description of the language
of observing procedures will not be given here; descriptions can be found in the
manuals describing the analysis system. Usually, the "Friend of the
Telescope" or Bob Vance will confer with the observer and write the desired
procedure or alter an existing procedure. Appendix C lists some existing
The following section describes the cards needed to produce either a "Setup" or "Source" deck. If the deck has been specified as a "CONT" deck on the "O" card, then some of the parameters dealing with spectral line observations will not be used and must be left blank. If the deck is a "LINE" deck, then the continuum parameters must be left blank. We recommend that neither "LINE" or "CONT" be specified on the "O" card and that you supply all the information on all the cards. "Setup" and "Source" decks differ in whether or not the deck contains one or more 'S' cards. "Setup" decks are used for interactive programs run from the operator's console while source decks are used for less interactive programs.
In the following, integer fields, specified by I's, must be right justified. Floating point fields, designated by XXX.XX can be placed anywhere in their fields; the decimal point MUST be supplied but it need not fall in the depicted column. Character fields must be left justified and are designated by A's. 'H', 'M', 'S', and 'D' refer to angular coordinates. The type of card is designated by the character you place in the first column (i.e., 'O', 'R', 'A', 'L', 'P', 'D', 'C', 'S', and ' ' [unlabelled cards). Column 2 must be left blank. (The unlabelled and descriptive origin cards are described fully in the CAO, pp. 41-42 and App. A)
You cannot exceed the column limits for any field on any card. Each deck must start with an "O" card and end with an end-of-file ($$ in columns 1-2 of a physical card; do not insert a $$ in a floppy or tape file).
CAL -- Measures the level of the noise source for A/D observations at the current position of the telescope.
OFFON - Moves the telescope between two specified positions and collects A/D data at those locations; used for measuring the antena temperature of a source.
POINT - Pointing procedure that will display the pointing corrections on the operator's CRT. The corrections will not be used unless the operator is satisfied with the observation and dials them into the console.
PEAK -- Same as POINT except used when pointing within a deck; pointing corrections, regardless of how good, will be used for all subsequent observations until another PEAK or manually reset.
CROSS - Slews the telescope both vertically and horizontally through a source.
CTRK -- Tracks a position in the sky for a specified duration.
HMAP -- Performs a series of slew scans in the horizontal direction; the vertical coordinte is incremented for each scan, producing a map of specified size.
VMAP -- Same as HMAP except vertical slew scans are performed and the horizontal coordinates is incremented.
TIP --- Measures atmospheric opacities by tipping the antenna.
NOTE: The above procedures use the A/D's; except for CAL, OFFON, and TIP,
they can be modified to use the DCR.
SPWR -- SPOWER scan, either beam switched with the nutator or frequencyswitched.
SMANY - Repeats SPWR scans for as many times as desired
SGRID - Moves the telescope through a grid of points taking SPOWER scans ateach position. The observer specifies the size of the grid.
SFOC -- Changes the focus of the telescope between SPOWER scans by a specifiedamount (usually +_ 1/8 of a wavelength); the observer specifies thenumber of times to repeat the series of observations.
TON --- Takes a single on TPOWER scan at a specified location.
TOFF -- Takes a single off TPOWER scan at a specified location.
TMANY - Alternates between on and off TPOWER scans at two specified locations. The observer chooses how many times to repeat the cycle.
TGRID - Moves the telescope through a grid of points taking on TPOWER scans at each position. Off TPOWER scan are performed at a specified location before the start of the map, before the start of strips in the map, or before every on TPOWER scan. The observer specifies the grid size.
DBLBO - Repeats a double beam switched observations for as many times as
NOTE: In the above procedures, the observer can choose between various
options as to how to balance the A/C.