The telescope supports various different types of observing mode. The basic unit is a scan, which is the observation of one position on the sky. A grid consists of one or more scans in a regular pattern. A smaller unit is the subscan, which can contain a signal and a reference observation (e.g. for a frequency-switching mode). There are computer limits to the length of a subscan. Within a subscan there must be at least one correlator cycle, and there are hardware limitations to the length of this cycle.
The correlator has a maximum of 1024channels (at 20MHz bandwidth or less) and this reduces to 512channels for 40 MHz bandwidths. For bandwidths up to 10MHz there is an option to use the correlator in split mode also.
The correlator hardware allows the following combinations of numbers of inputs, bandwidth per input and numbers of channels per input:
code #inputs Bandwidth/MHz #channels/input 0 1 40 512 1 2 40 256 2 1 20 or less 1024 3 2 20 or less 512
It is setup with 5 parameters, one of which allows manual control. These are described in the section about section Schedule format.
The correlator can operate in 1bit or 2bit mode but these have different normalization and van Vleck corrections.
Data is read out in increasing lag order as 1024 32-bit integers. Only 24 bits are significant.
There is a formula to correct the observed (digital) correlation to the true (analogue) correlation. This formula is the `van Vleck correction', and is different for observing modes, but for low levels of correlation it can be approximated to by a simple correction factor. After this has been done the correlation function must be scaled by the measured total power to give physical units and fourier-transformed to produce a spectrum, but these corrections have to be done in the offline software.
The single-bit (=two-level) mode gives a loss in the signal to noise ratio (relative to the ideal analogue case) of a factor 1.57. Because there is an exact analytical expression for this case, the single-bit mode is more suited to observations where high correlation values are expected (strong sources or interference).
The two-bit (=four-level) mode gives a loss in the signal to noise ratio (relative to the ideal analogue case) of a factor 1.14. There is no exact analytic expression for the van Vleck correction so a power series approximation is used.
For bandwidths up to 10MHz there are 2 IFs which can be used. The Split mode allows the user to observe two separate lines at a fixed frequency separation (about 2MHz, given by the difference in the synthesizer frequencies at 33.043 and 35MHz) and these observations are done in 2 halves of the correlator. This mode has only been used to observe the 2 OH spectral lines at 1665 and 1667MHz.
To measure a spectral line if this line is widely distributed across the sky (as is the case with galactic neutral hydrogen, HI) it is possible to use an observation with the frequency shifted as a reference. This assumes that the bandpass does not vary with frequency.
It is possible by chosing the correct shift with respect to the bandwidth to have the line in both the 'source' and 'reference' positions and so lose very little observing time, but only half the bandwidth can be used. This is known as `Dual-Dicke' switching: the reduction is slightly complicated.
If the spectral line is only present in small areas of the sky, so that a few beamwidths away there is no spectral line signal (as is the case for OH maser observations, or extragalactic HI) it is simplest to use a blank patch of sky nearby as a reference field. This will involve a separate measurement.
For survey observations the telescope measures spectra at a series of points on a grid. The coordinate system of the grid can be chosen from the following, on the basis of what is the most convenient for the project. For all grids the user must specify the following items:
The online system only supports coordinates in decimal degrees.
For objects that move in Right Ascension and Declination (planets, comets, etc) the most convenient reference frame is that of apparent RA/dec for the observing date.
Galactic coordinates (latitude and longitude) are the most convenient for galactic observations. The standard astronomical notation is used.
For extragalactic objects fixed in Right Ascension and Declination (e.g. some calibrators) the most convenient reference frame is that of RA/dec at a standard epoch. Only B1950 is supported.
This mode is little used, but can be derived from apparent Right Ascension and Declination and a sidereal time. Time information is not needed to point to a given Hour Angle and declination.
This mode is not used for astronomy, and does not need any clock information. It can be used for tests, interference searching and to move the telescope toward the correct position for maintainance.
This mode can be used to scan a region and take data continously while doing this. The same coordinates are supported as in the grid mode.
This mode can be used to point at the same position continuously during an observation. The same coordinates are supported as in the grid mode. Clearly the items on step lengths and numbers of points are irrelevant.
This mode is only used to determine, and check, the pointing corrections used by the telescope control software. It is not used by astronomers.
The software also supports modes for the following: