[evlatests] how often do we observe in narrow bandwidths?
Bryan Butler
bbutler at nrao.edu
Thu Dec 20 15:56:24 EST 2007
all,
a few responses to michael's comments.
. the july cutoff was only because i had heard that "since the summer"
the narrow line observing rate had gone up significantly. i can
change the date to any one you like and re-run the numbers - the
program is in place and it's trivial to modify it to do so. it
takes a bit of time to run, but it's not too bad.
. i think our estimates of how long potential fixes will take are
further along than you say they are. they are certainly beyond
the "boogeyman" stage. i based my "many-months-long" delay on a
discussion i had with mark yesterday morning about the possible
fixes to the problem, both hardware and software. mark knows how
long the hardware fixes will take, and i think i have a pretty good
handle on how long the software fixes will take. to assume that
any such fix is simple and might only take a week is naive at best.
. i totally agree with your final point regarding the large proposals.
we made a mistake. a doozy of one. but we now have to weigh what
it will take to fix that mistake (to keep the large proposal folks
happy) against the cost to the entire community because of a delay
in the overall EVLA.
-bryan
Michael Rupen wrote:
> Hello all,
>
> a couple comments on Bryan's numbers, and his conclusions.
>
> * The July 1 cutoff for discussing recent observations is a rather odd
> one.
> - This corresponds to sometime in A configuration. The distribution of
> projects (esp. line vs. continuum) varies enormously with the array
> configuration, since surface brightness sensitivity prohibits
> most emission line work in the larger configurations, and emission line
> projects are much more time consuming in general than absorption work.
> Bryan's July 1 cutoff will likely show many more continuum observations
> than an average over all configurations. In this context I'm surprised
> that we don't see a substantially *smaller* fraction of observing hours
> in the narrow-band modes.
>
> - The two large proposals affected by aliasing are those of Ott and
> Hunter. Both projects center on HI in dwarf galaxies, and use the 0.78
> and 1.56 MHz bandwidths exclusively. Ott's observations began in late
> November (BnA); those of Hunter began on Dec. 19th. The fraction of
> observing hours devoted to those projects will become even higher as
> B configuration continues, with the bulk of Hunter's observations at
> least scheduled for January.
>
> * Comparing fractions of *total* observing hours mostly reveals the
> distinction between continuum and line observations, and shows that
> most VLA observations -- esp. in the A and B configs. -- are continuum
> ones.
>
> - BWcodes 0 and 1 (50 and 25 MHz) are primarily continuum experiments.
>
> 2003-now:
> > BWcode all observing science observing
> > 0 787.5 (62.4%) 433.3 (50.9%)
> > 1 138.4 (11.0%) 122.7 (14.4%)
>
> July 1-now:
> > BWcode all observing science observing
> > 0 90.6 (58.2%) 41.5 (42.3%)
> > 1 30.9 (19.9%) 27.4 (27.9%)
>
> The increase in BWcode 1 corresponds to the use of pseudo-continuum
> line
> mode to take care of BW smearing in the A & B configurations.
>
> These continuum observations account for about 65% of all observations,
> and about 70% of the recent ones.
>
> - Taking BWC 3 and up (6.25 MHz and narrower) as line experiments, I've
> added columns corresponding to the fraction of line data taken in the
> narrower BWCs. The tables are then as follows:
>
> 2003-now:
> > BWcode all observing science observing % of line expt
> > 3 68.4 (5.4%) 59.5 (7.0%) 21.4%
> > 4 101.7 (8.1%) 91.5 (10.7%) 32.9%
> > 5 99.0 (7.8%) 95.2 (11.2%) 34.2%
> > 6 29.3 (2.3%) 28.0 (3.3%) 10.1%
> > 7 0.1 (0.0%) 0.1 (0.0%) 0.0%
> > 8 0.8 (0.1%) 0.5 (0.1%) 0.2%
> > 9 4.0 (0.3%) 3.2 (0.4%) 1.2%
>
> July 1-now:
> > BWcode all observing science observing % of line expt
> > 3 7.9 (5.1%) 6.6 (6.7%) 23.6%
> > 4 2.7 (1.7%) 1.7 (1.7%) 6.1%
> > 5 11.6 (7.5%) 11.1 (11.3%) 39.6%
> > 6 8.8 (5.6%) 8.4 (8.6%) 30.0%
> > 7 0.0 (0.0%) 0.0 (0.0%) 0.0%
> > 8 0.0 (0.0%) 0.0 (0.0%) 0.0%
> > 9 0.3 (0.2%) 0.2 (0.2%) 0.7%
>
> I read this as a major increase in the fraction of time spent at BWC 6
> (0.78 MHz), at the expense of BWC 4 (3.125 MHz). BWC 5 (1.56 MHz)
> also increases somewhat.
>
>
> * Conclusions
> Bryan's conclusion was as follows:
> > so, the question is, do we want to go to much effort, including a
> > potential many-months-long delay in EVLA, to make a change to fix
> > something that might affect something like 10%-20% of observing in
> the
> > transition system (i'm counting modes 5 and above here)? i leave
> it to
> > those more erudite than i to decide.
>
> I have several issues with this.
>
> 1- The "potential many-months-delay" seems more a boogeyman than a
> reality.
> * I have not seen the cost analysis of this, and my impression
> from speaking with Bob and Mark yesterday is that such an analysis
> has
> not been done yet. There are several possible avenues being
> explored,
> from hardware to software; some of these would likely be rather easy
> to implement (cf. Eric's ridiculously rapid modification of UVLSF).
>
> I presume this very long delay estimate comes from Mike Revnell's
> early off-the-cuff guess as to the time required for a hardware fix.
> He had revised that downwards by an order of magnitude when last we
> spoke.
>
> * Clearly we cannot tolerate a huge delay in the EVLA, and that is
> being factored into the discussions of possible approaches.
> Significant delays in EVLA developments are NOT on the table.
>
> * The nature and severity of the problem are still being analyzed.
> It seems
> early to panic about a fix, when we don't have a clear understanding
> of the impact of the problem on scientific observations. Taking the
> worst case, which has NOT been demonstrated, we might decide that the
> problem isn't fixable in post-processing. [Note that the primary
> responsibility for gathering the data leading to that decision has
> been given to the *observers*, not NRAO staff.] One -- extreme! --
> approach would simply be to allocate more observing time to make up
> for the increase in the noise. This would have a major impact on the
> array configuration schedule, for instance, but not on the EVLA
> project. I do not advocate this, but even this worst case need not
> lead to an EVLA meltdown.
>
> This is a serious problem, and we should take it seriously. We cannot
> allow a transition issue to dominate our thinking; but neither
> should we
> stop thinking, on the off chance that all possible approaches might
> lead
> to significant delays in other projects.
>
> 2- It's not obvious that the major impact of even the most pessimistic
> version of a fix would be on the EVLA. Mike for instance is primarily
> involved in VLBA developments at the moment. Those developments are
> of course also very important -- the point is that we should not focus
> exclusively on the EVLA when considering what to do about the aliasing
> problem.
>
> 3- Counting the impact purely in overall observing hours is rather naive.
> * The extreme of this approach would be to say the transition system
> is purely a continuum instrument: spectral line observations are not
> allowed. That might be a fine policy, but it is not the current
> one.
>
> * We have made a special commitment to the Large Proposals, to the
> extent of (for instance) changing the array configuration schedule.
> Those proposals have been subjected to much more extensive scientific
> and technical reviews. Further, the PIs have committed to spending
> much of their time over the next several years on these projects,
> have gotten NSF and other grants to support them, and have hired
> graduate students and postdocs with those moneys. We have a duty to
> at least try to mitigate major, unanticipated, unadvertised
> instrumental problems which have a major impact on these Large
> Proposals.
>
> Cheers,
>
> Michael
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