[alma-config] parameters for IMAGR, and integration times

[Steven]

Thanks for clarifying the calculation, which indeed seems highly sensible. I
was was inquiring about the integration times more in hope than expectation-
however I am more hopeful of the practicalities of the situation now, having
recalled that the A arrays simulated have maximum baselines of approx 3km
rather than 10km, C approx 700m, and E approx 150 m, suggesting maximum
integration times of 28 seconds, 126 seconds and >500 seconds respectively.
With this in mind, would it be acceptable to leave the E arrays' data as it
is, as it is oversampled by a factor of three even at 180 seconds, redo the
C arrays at an integration time of 60 seconds to get two times oversampling,
and redo the A arrays at an integration time of 14 seconds to get two times
oversampling there also? The A arrays will still necessitate a plea to my
employers for an additional large harddisk, but that can probably be
obtained and fitted in about a week. In the meantime I can work at producing
prototype CLEAN and difference images with the redone C arrays, prior to
automating the process when deemed suitable, and indeed could possibly
retain the A snapshots integrated for 60 seconds as a temporary measure.

By the time you read this my hard disk will have been thrashing away at the
C arrays for a few hours, so I hope that this seems reasonable!

Cheers,
    Steven


> > >> Steven wrote:
> > >> The A array simulations have been redone with an integration time of
60
> > >> seconds rather than the 180 seconds used to date, and for the other
> > arrays,
> > >> as this was felt to be too long for the longer baselines. The redone
> > results
> > >> have been posted to the site, accessible from
> > >> http://www.stevenheddle.co.uk/ALMA/ALMA_IND.HTM
> > >>
> > >> Cheers,
> > >>     Steven
> > >>
> > >>
> > >
> > >Hi Steven,
> > >
> > >I do not believe this is correct. The Nyquist sampling for
> > >a 10 km baseline is 8 seconds... It is 27 seconds for a 3 km baseline.
> > >So your UV data are fairly heavily undersampled at their longest
baselines.
> > >This will add significant sidelobes, and perhaps explain why you get
> > >large negative sidelobes than theory says.
> > >
> > >Stephane
>
> it depends a bit on where the source is (how fast it moves through the
> u-v cells), how far you are from the phase center, and how much error you
> are willing to accept, at least as i understand it.  stephane has probably
> made some sensible assumptions here and just calculated a rule of thumb.
>
> here is probably something similar to what he did...
>
> take for illustration purposes an east-west baseline, and a source passing
> overhead at zenith.  then the rate of change of u is:
>
>   du    B
>   -- ~ --- w
>   dt    l
>
> where B is the baseline length, l is the wavelength, and w is the earth
> rotation rate.  now, convert the u-coordinate back to physical distance
> (instead of wavelengths), and you have:
>
>   du ~ B w dt
>
> so, for a 10 km baseline, a 1 second integration time gives du ~ 0.7 m.
> now, we'll want u-v cells of order an antenna size divided by 2 (at the
> *very* least), or about 6 m, so the max integration time is of order 8
sec...
>
>
> >
> > My problem is mainly the practical one of dealing with the UV data files
> > this implies- at 60 seconds integration time the files are already 48MB,
so
> > at 8 seconds they will balloon to approximately 360 MB per file. As it
> > stands (60 seconds) I reckoned I could shoehorn the A array simulations
> > (including CLEANing)  into 6GB of AIPS disk, but am now faced with the
> > prospect of requiring 45GB for only the A arrays!
>
> yes, this is a serious problem...  why are we concentrating on the A array
> first, just out of my own curiosity?
>
> > Also I was not aware that
> > there was a problem with large negative sidleobes, but if there is (or
> > indeed any other problems) please tell me as soon as possible.
>
> well, if you don't sample your visibilities rapidly enough, then you can
> certainly get artifacts.  it's not clear to me that larger sidelobes would
> be a result, but i imagine it could be...
>
> > Are these the integration times I should be using? Are there other
issues
> > such as noise which work against such short integration times?
>
> the noise of each visibility goes up, but the overall noise in the final
> image should be the same (barring no systematics) because you're
> effectively vector adding the visibilities together to make the image.
>
> > What
> > integration times are commonly used on the VLA with its >20 km
baselines?
>
> at short wavelengths, 3.33 sec is what i would consider "standard".  but
> this is as much to combat atmosphere as to combat time-average smearing.
> even at the longer wavelengths (6 cm, e.g.), i don't think anybody would
go to
> integration times longer than 10 sec, in practice.
>
>
> -bryan
>
>