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. a. Continuum Observing, Pointing, and Calibration 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. Table 2 Suggested Pointing Strategy ================================================ Frequency How Often How Far ------------------------------------------------ 22 GHz 1h best, 2h at most 10-15 degrees 11 GHz 2-4h 30 degrees 5 GHz 4-6h 30 degrees 1.4 to 5 GHz 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. b. Spectral Line Observing 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 good scans. 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. i. Frequency Switching 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 bandpass. 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. ii. Total Power or Position Switching 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. iii. Beam Switching and Double Beam Switching 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.