EVLA Advisory Committee Meeting

1
Scientific Impact of Descopes

• Rick Perley
• Version 3may06, 12PM

2
The Problem

• Contingency is getting short.
• It may be necessary to consider descoping aspects
  of the EVLA
• We can consider descopes in one or both
• Hardware
• Software
• Alternatively, there are some delay options.
• I review here the impact of these options on the
  scientific productivity of the EVLA.

3
Hardware Descopes

• We have considered
• Removal of one or more bands
• X, Ku, S, Ka are the possibilities
• Reduction of bandwidth from 16 to 8 GHz.
• Reduce number of outfitted antennas.
• Removal of special solar observing hardware.
• Items 1 and 2 were discussed last year.

4
Bandwidth Reduction

• Significant savings possible but mostly in
  correlator. Not our money to save, and
  politically very tricky.
• Our savings come from DTS system. But, much of
  the needed components for all antennas already
  purchased.
• Scientific impact 40 loss of continuum
  sensitivity per unit time at K, Ka and Q bands,
  less on Ku. None other bands.
• It is too late to obtain significant savings
  through this descope option.

5
Band Descopes

• Bands we could consider
• Total Parts Only
• S-Band 1.4M 400K
• X-Band 1.0M 200K
• Ku-Band 1.3M 200K
• Ka-Band 1.2M 200K
• I consider the scientific implications of each
  band listed.

6
X-Band

• The VLA has a good X-band system operating from
  7.8 8.8 GHz.
• It has been argued that simply leaving this
  legacy system in place would be sufficient.
• This is a poor argument
• Continuum sensitivity will be less than half that
  of C-band.
• Only spectral observations of transitions lying
  between 8 and 8.8 GHz would be useful.
• X-band would become both legacy and vestigial.
• Nevertheless this is the easiest band descope.

7
S-Band vs. U-Band?

• Removal of X-band might be considered an easy
  decision but the decision of which band is next
  to go will not be easy.
• The debate of which is more scientifically
  productive will surely pit non-thermal vs.
  thermal science interests.
• Who is to say what is more valuable?
• Consider the following

8
S-Band

• This band will become the band of choice for
  non-thermal imaging
• Likely better than twice the sensitivity of
  L-band.
• Twice the total bandwidth,
• At least three times the available (RFI-free)
  bandwidth
• Nearly 1.5 times the efficiency.
• We anticipate superb performance from this band.

9
Examples of Key Lost Science

• TBD? (Do we need to)?

10
Ku-Band

• This is a key thermal science band.
• Transition zone (from optically thick to
  optically thin) for the highest EM HII regions.
• Dozens of molecular and atomic spectral
  transitions.
• Lowest frequency where precise dust emission
  observations can be made (lowest optical depth to
  dust extinction).
• Lowest frequency ( highest redshift) for key
  high frequency molecular studies (e.g. CO 1-0 at
  z 5.8).
• Could be argued this is the highest really good
  band for non-thermal science.

11
Examples of Key Lost Science
12
Ka-Band?

• Early in the project, this badn was identified
  for accelerated development rich science mine!
  
• Relatively unexplored spectral region, esp. good
  for thermal emission processes.
• Hundreds of molecular spectral transitions.
• Low sky emission between H2O and O2 emission.
  Decent efficiency, excellent Tsys.
• Testing of the designed horn/polarizer is about
  to begin.
• We could review this decision, if necessary.

13
Ka-Band Science

• Thermal Science!
• HII regions, dusty obscured disks, star forming
  regions.
• Hundreds of spectral transitions.
• Hi-z lines.
• Excellent continuum sensitivity far better than
  Q-band, even for optically thick thermal
  emission.

14
My Opinion

• Full Frequency Coverage a Primary Goal of this
  project.
• Any retreat is an admission of failure by NRAO
  (and by AUI!)
• The science impact of band descopes is enormous.
  At least 10 years would be needed to recover a
  descoped band.
• If we must cut back, then an acceptable
  compromise is to simply purchase the parts, and
  build the new systems in post-construction years.
• This, in essence, shifts the implementation costs
  to operations.

15
Solar Mode Descopes

• Not a lot of money (200K)
• Scientific impact limited to the solar
  community and solar science.
• Solar community use of VLA is now very light
  (nearly non-existent).
• Only two bands (L-band and one other, currently
  not identified) budgeted for special solar mode
  systems.

16
Implementation Descope?

• A suggestion has been made to reduce the number
  of EVLA-outfitted antennas.
• Savings limited in many areas we have already
  bought in bulk for the whole project.
• Very strong science impact
• Point-source sensitivity reduced by linear
  fraction.
• Imaging capability (particularly for complex
  fields) reduced by a greater fractions
  baselines rise as N2.
• This descope would affect all bands, all
  observers, all programs, all science.
• I think this idea is a non-starter. It should
  be dismissed at the highest level, now.

17
Software Descopes

• Software in three major areas
• MC
• E2E
• Post-Processing.
• We consider MC as sacrosanct. The group is of
  just-sufficient size.
• Post-processing group already minimal
  sub-critical. No fat here!
• We look at e2e for descope opportunities.

18
Descopes in e2e?

• Level 2 and Level 3 software e2e deliverables
  already descoped.
• Budget is currently unable to provide all Level
  1 deliverables.
• Three more staff needed to achieve Level 1
  deliverables.
• This does not include extra staff for imaging
  algorithm RD.
• Approximately 1M of contingency already utilized
  in addressing shortfalls in e2e.
• Could review Level 1, for further descopes.
• Probable course of action is to delay some
  deliverables.
• Effect would be to move some to operations.

19
E2E Descope Options

• (A list of Level 1 deliverables which would be
  deferred/removed, should we have to go this route
  )
• (Perhaps a reminder that e2e, and the One
  Observatory, is an NRAO/AUI obligation, and that
  the failure of the 2000 DMD model is not the
  fault of the EVLA Project).
• (Hence, more resources from NRAO/CV, and AUI, are
  needed).

20
Deferment Options

• Although not formally a descope, perhaps the
  best options are to defer planned capability.
• Hardware Already mentioned that costs could be
  offset into future operations by purchasing
  early, with implementation deferred until post
  2012.
• Not a great option, but doable, deferring xxxK
  to future generations.
• Risk Much better receivers might be available
  later? (I doubt this, given the existing
  capabilities).

21
Deferment Options

• Software.
• Clearly an area where deferment is possible
  even likely
• Real-time scheduling Can be done by a person
  indefinitely, until good software can be written.
• Automatic image generation Can be done by
  trained individuals until more automated systems
  can cover harder and harder configuration/frequenc
  y/weather combinations.
• Post-processing software. Although development
  of improved algorithms is non-deferrable, we can
  match scheduling of tough experiments
  (full-field, noise-limited, high-resolution
  polarimetric imaging at L-band, for example)
  until the software is ready.
• Nothing new in this done by the VLA.
• Other examples

22
Summary

• Advertised scientific productivity of EVLA
  requires all hardware and software deliverables
  to be met.
• It is unwise policy to remove frequency bands.
  History shows recovery will be at least a decade
  away.
• All software deliverables in e2e are key for full
  scientific usage especially by those not
  trained to be radio astronomers.
• The preferred solution is an increase in the
  budget sufficient to keep us on track and on
  time. Estimate of this 5M. ( only 6 of
  total EVLA1 budget).
• The acceptable, but not preferable, means is
  deferment of promised capability into the
  operational years.