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Hi Chris,

Thanks for the comments. I got the green light from Deputy Director
Dave Hogg in Charlottesville today, so we are cleared to move on this.
Below you will find a version of the NRAO newsletter article as submitted.
I'll also be sending that article into the next AAS newsletter, but
the deadline for that is not until April 16.

I had no problem incorporating all of your suggestions into the
newsletter article, but a few issues warrant further discussion:

On Fri, 5 Mar 2004 csalter@naic.edu wrote:

> % "The first part is a bit unfair on Arecibo and the GBT methinks!
> % Here, we currently have a borrowed JPL H-maser, with our own H-maser
> % coming on-line very soon, plus a VLBA4 recorder capable of
> % recording data at 500 Mbits/s! We also having a tone cal unit kindly
> % provided some years back by Alan Rogers at Haystack. Hence, the HSA
> % being restricted to 256 Mbits/s would not be fully using our already
> % available capacity! In addition, we are about to order a Mk-5A unit,
> % and when this is upgraded to Mk-5B within the next year or so, full 1
> % Gbit/s recording will be possible."

Point taken. No slur on Arecibo or the GBT was intended. I think
the committee was reacting to the relatively limited participation
in VLBI by the GBT and Arecibo, where these two antennas can have
a big impact. If the technical groundwork is already in place
then that is terrific and we should proceed.

> % "The sensitivity and UV coverage with the large antennas would be
> % further improved if you could add WSRT-phased, Jodrell-Bank, SRT
> % (eventually), the 2 x DSN dishes, and the 4 x European 32-m antennas.
> % However, I appreciate that this would make the HSA just the Global Array,
> % which would only be schedulable during EVN sessions three times per
> % year, with a limited frequency availability in any single session. It
> % is interesting that if one just considers the "Big Antennas", i.e.
> % phased-VLA, Eb, GBT, JB, phased-WSRT and Ar, as an array, then
> % Arecibo provides most of the North-South UV-coverage."

The HSA is not meant to supplant the globals. In fact some people in the
EVN have already expressed mixed feelings about this initiative. And since
one selling point for the EVN is improved sensitivity over the VLBA at low
frequencies, they may even resent this competition. This may explain why
we haven't heard anything from EB. It may well be cleaner not to
incorporate them anyway, so that we can clearly identify this as a US
initiative. In that regard it is tempting to look into availability of
the DSN antennas. I may do that, but we may not want to go beyond that if
this looks like it will conflict with our other goal that the data should
be easy for the users to deal with (high quality calibration tables
included).

> % "Tapasi and I would be very happy to accept standard VLBA
> % refereeing. HOWEVER, elaborating on Bob's remarks of yesterday, we %
> % also think it would be a good idea to have a TAC with input from
> % representatives from the stations participating in the HSA. We
> % presently send a technical audit to the EVN PC for all EVN proposals
> % that request Arecibo participation. Because of the somewhat unusual
> % features of the Arecibo telescope, this sometimes reveals inconsistencies
> % between a proposal as submitted and Arecibo's capabilities. This
> % often results in improvements in the way the way the experiment is
> % carried out that would not have happened without this audit. I may add
> % that because of this we see all EVN proposals as early as possible,
> % while, as Bob notes, VLBA projects including Arecibo are often already
> % scheduled without Arecibo having seen the proposal!"

I brought this point up with both Dave Hogg and Jim Ulvestad. Both of
them agree having input from Arecibo to the scheduling committee
is a good idea. Details will need to be sorted out before the June
proposals are reviewed.

> % "Jonathan Friedman & Tapasi are presently putting together
> % our next NAIC Newsletter, and it would indeed be nice to put your
> % piece in this too. The web page should also be linked to the relevant
> % places in the web sites of participating stations."

The revised article is attatched below. The web site mentioned at the end
is under construction. When I have something worth looking at I'll ask
for your help looking it over.

> % "Agreed. It would be a good thing to also send notification via the
> % vlbi exploder. We could also circulate it via the "aousers" alias
> % that reaches all who have proposed to Arecibo (in all fields) since our
> % 1990's upgrade."
>
I suggest we send our announcements out to e-mail lists in early May
so as to get people thinking about it in those critical weeks before
the proposal deadline. Ciao,
- Greg



Newsletter Article:

Call for Proposals for the High Sensitivity Array (HSA)

Proposals for a high sensitivity VLBI array consisting of the VLBA,
Green Bank Telescope (GBT), phased VLA (Y27), and Arecibo (AR) are now
being accepted with priority. A total of up to 100 hours in each
trimester (~25 hours/month) may be scheduled for highly rated projects
that can justify the increase in sensitivity. By adding the GBT, Y27,
and AR to a VLBA experiment the sensitivity can be increased by
more than an order of magnitude. This capability opens up promising
new avenues for scientific discovery. In a 1 hour integration at 1.4
GHz the predicted image sensitivity is 6.5 microJy/beam, compared to
94 microJy/beam for the VLBA alone. At 22 GHz, the above array
(without Arecibo which is limited to 10 GHz and below) the thermal
image noise in 1 hour is 43 microJy/beam compared to 275 microJy/beam
for the stand-alone VLBA. For further estimates, observers can make
use of the EVN Sensitivity Calculator at
http://www.evlbi.org/cgi-bin/EVNcalc. Trial runs with SCHED may also
be desirable in order to determine the mutual visibility of the array
on the target source. In general a source that transits over Arecibo
(with transit time 1-2.75 hours for 0 < declination < 37 degrees) is
visible during an Arecibo transit by all other HSA antennas.

The High Sensitivity Array (HSA) is available at frequencies of
0.33, 0.61, 1.4, 5, 8.4, 15, 22, and 43 GHz starting with the June 1,
2004 proposal deadline. The proposal should be sent to NRAO only. To
request HSA time, the observer should indicate in item 11 of the
proposal coversheet on the line starting "VLBA" that the antennas
requested are "HSA = VLBA+GBT+Y27+AR". If the celestial position,
or required frequencies, prohibit participation by Arecibo then the
desired subset should be described. Proposals requesting less than 3
of the 4 instruments will not be considered for the HSA time. It is
also assumed that the VLBA will participate. Due to the coordinated
nature of the observations, dynamic scheduling is not possible.
Proposals to the HSA are limited in recording bandwidth to 256 Mbps,
but all continuum observations are encouraged to request the use of
this maximum bandwidth.

Observers are reminded that with the extremely high gains of these
antennas, fields with an integrated flux density of more than ~1 Jy
will contribute substantially to the thermal noise. The good news
though is that sources with even a few mJy of emission on
milliarcsecond scales may be suitable for self-calibration, and
phase-calibrators can be similarly weak. Phase referencing by fast
switching to calibrators within 2 degrees of a faint target source
with cycle times of ~3 minutes are feasible. All HSA observations
will be correlated in Socorro. For further details consult the web
page at (http://www.nrao.edu/HSA) or direct your questions to the
undersigned.

- Greg Taylor