Time Resolution

The minimum integration time at which all data can be written is a function of the total number of channels of data produced by the correlator. This minimum time varies from $1\frac{2}{3}$ seconds for the continuum mode to 5 seconds for 512 channel spectral line modes. Note that in order to ensure the complete removal of correlator offsets, averaging times should be an integer multiple of $3\frac{1}{3}$ seconds (which is the period of the Walsh functions that are used to negate cross-coupling between antenna signal lines).

Averaging times shorter than those listed above can be selected, but only at the cost of removing antennas from the array. For the spectral line modes, Table 13 gives the minimum allowed integration time as a function of the numbers of IFs and channels. In continuum mode, integration times as short as 0.4 seconds are available, but are appropriate only for EVLA testing and for fast flaring activity such as solar flares. Contact Ken Sowinski for details on their use.

Table 13: Minimum Integration Times for One Subarray of 27 Antennas

N$_{\rm ch}$	1 IF	2 IFs	4 IFs
16	1-1/3 sec	1-2/3 sec	3-1/3 sec
32	1-1/3 sec	1-2/3 sec	3-1/3 sec
64	3-1/3	3-1/3	3-1/3
128	3-1/3	3-1/3	3-1/3
256	3-1/3	3-1/3	3-1/3
512	5	5	5

Note: The number of channels, N$_{\rm ch}$, is the number of channels per IF multiplied by the number of IFs in Normal mode (Table 18). For Hanning smoothing multiply by another factor of 2.

The maximum recommended integration time for any VLA observing is 60 seconds, but given current computing capabilities there is no reason to use integration times longer than 10 seconds. For high frequency observers with short scans (e.g., fast switching, as described in Section 3.13.2), a $3\frac{1}{3}$ second integration time may be preferable.

Users should keep in mind the data rate of the VLA when planning their observing. The array's maximum data rate of more than 3 GByte per day presently is only a minimal problem for the modern computers that most astronomers use for their data reduction. If necessary, this rate may be reduced by increasing the averaging time and/or decreasing the number of spectral channels. Consult one of the spectral line experts listed in Table 20 for advice. `