Minimum and Maximum Acceptable Integration Times for VLA Observing
Ken Sowinski (original November 26, 1996)
Gustaaf van Moorsel (update March 13, 2007)
Gustaaf van Moorsel (update November 21, 2007)
A word of caution
As we are finding out details of the constraints posed by the new
Modcomp independent system, the numbers given below are subject to
change. Though we will attempt to keep this table up-to-date, this
should be taken as a warning that in the future it may not always be
accurate. When in doubt, please consult Ken Sowinski or Gustaaf van Moorsel.
Minimum Integration Time - Continuum
For continuum observing we routinely support an integration interval
of 1-2/3 seconds. It is possible to specify integration intervals as
short as 5/12 seconds if needed, but this places a noticable strain on
the system and is discouraged.
Minimum Integration Time - Spectral Line
In the case of spectral line observations, the greater amount of data
and processing make a short integration interval impossible, and the
limits cannot be expressed as succinctly. The table below shows the
minimum achievable integration time (in seconds) as a function of the
total number of channels per baseline, and the number of IFs, for a
single 27 antenna subarray. Note that the total number of antennas
per baseline is the total number of channels multiplied by the
total number of IFs.
A few notes:
- The total number of channels per baseline is the total number
of channels per IF multiplied by the number of IFs. So, for BW
code 2 in Two IF mode (normal mode) the number of channels to use
is 32 times 2 = 64, leading to a minimum integration time of 3-1/3
seconds
- The number of channels is that without Hanning
smoothing: N channels with Hanning smoothing equals 2N channels in
this table. For BW code 4 in Four IF mode (online Hanning
smoothing), the number of channels to use is 32 times 4 times 2 is
256, leading to a minimum integration time of 3-1/3 seconds
- Full polarization modes (PA, PB) count as 4 IFs in this table.
So for BW code 3, mode PB, and Hanning smoothing the number of
channels to use is 16 times 4 times 2 = 128, and the minimum
integration time is 3-1/3 seconds.
Minimum Integration Times: One Subarray of 27 Antennas
| No of Chs* |
1 IF |
2 IFs |
4 IFs |
| 16 |
1-2/3 sec |
1-2/3 sec |
3-1/3 sec |
| 32 |
1-2/3 |
1-2/3 |
3-1/3 |
| 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 |
|
* See notes above!!
|
In all cases baselines (antennas) may be given up to allow shorter
integration intervals. The processing time is, more or less, linear
to first order. In case not all IFs will produce the same number of
channels, straightforward averaging should produce a sensible limit.
Maximum Integration Time
Given current computer capabilities there does not appear to be a
compelling reason to use integration times exceeding 5 seconds,
except for scans to determine reference pointing. Longer
integration times may lead to unnecessary data loss, especially when
using short scans.