1. (sub)mm VLBI Science with ALMA
MBH – paradigm, useful but dangerous, need to really push techniques to make MBH case solid.
Shep Doeleman
MIT Haystack Observatory
Charlottesville, August

2. Haystack Observatory

3. sub(mm)VLBI Science No scattering through ISM
High Angular resolution (20micro arcsec)
No scattering through ISM
Enables observations with few Schwarzschild radius
resolution for nearby AGN.
Focus on SgrA*/M87: can observe strong-field GR effects.
free fall
rotating
non-
rotating
orbiting
GR Code
0.6mm VLBI
1.3mm VLBI

4. sub(mm)VLBI Progress Disk based Recorders: Mark5A, Mark5B, Mark5C
Wideband VLBI Instrumentation:
Disk based Recorders: Mark5A, Mark5B, Mark5C
Digital Backends: DBE, DBE2/RDBE
IF Converters
Stable Frequency Standards:
Modification of Hydrogen masers
Exploration of alternative standards: Sapphire osc.
Collaborations to bring new sub(mm) VLBI sites on line.
JCMT, CSO, SMA, CARMA, ARO/SMT, IRAM, PdeB.
ASTE, APEX, ALMA, S. Pole, LMT
new sites (ATF).
Phased array processor work: SAO.
Logistics, Scheduling, Correlation, Analysis Techniques

5. mm/submm VLBI Collaboration
MIT Haystack: Alan Rogers, Alan Whitney, Mike Titus,
Dan Smythe, Brian Corey, Roger Cappallo, Vincent Fish
U. Arizona Steward Obs: Lucy Ziurys, Robert Freund
CARMA: Dick Plambeck, Douglas Bock, Geoff Bower
Harvard Smithsonian CfA: Jonathan Weintroub, Jim Moran,
Ken Young, Dan Marrone, David Phillips, Ed Mattison, Bob Vessot, Irwin Shapiro, Mark Gurwell, Ray Blundell, Bob Wilson
James Clerk Maxwell Telescope: Remo Tilanus, Per Friberg
UC Berkeley SSL: Dan Werthimer
Caltech Submillimeter Observatory: Richard Chamberlain
MPIfR: Thomas Krichbaum
NRAO, NAOJ, ASIAA …

6. 1.3mm Observations of SgrA*
908km
4030km
4630km
Carried out 230GHz VLBI. 2. High resolution See through scattering 4. Design of experiment simple (3 baselines) and aimed at determining if there was structure on scales implicated by all light curve, vlbi, proper motion evidence already shown. These high freq obs make use of non-vlbi-facility instruments: have to create arrays using separate mm/submm facilities, but it can be done.
days out of 5 possible (weather), 4Gb/s at each site, new
Hydrogen maser at Mauna Kea and CARMA.

7. Determining the size of SgrA*
OBS = 43as (+14, -8)
SMT-CARMA
INT = 37as (+16, -10)
JCMT-CARMA
SMT-JCMT
1 Rsch = 10as
Doeleman et al 2008

8. Caveat: Very Interesting Structures
SMT-CARMA
14 Rsch (140as)
JCMT-CARMA
SMT-JCMT
Gammie et al

9. April 2009: M87

10. sub(mm) VLBI System

11. IF Conversion

12. Polarizer Needed to convert LCP/RCP to Linear Polarization.
Can use grooved dielectric or x-cut quartz (sapphire too).
Anti reflection coating required for quartz.

14. Observing Frequency
Must avoid CO absorption feature near CO(2-1) line!!

15. Media 4Gb/s = 44TByte/24hours 22TBytes if 50% duty cycle.
$0.10/Gbyte: ~$1300 for 8TByte Module ($500/chassis)
High Altitude can be an issue.
VLBA uses pressurized container.

16. Correlation Currently use Mark4 correlator capable of 1Gb/s processing
can handle 2Gb/s at half-speed.
limited to 16 frequency channels on input: could
possible modify DBE to provide 64MHz channels.
Number of stations?
Software correlation
DiFX
To correlate 7 antennas at 16Gb/s data rate in real
time requires ~100 similar nodes.
Issues: data input and data writing off to disk.

17. ALMA VLBI Top Level Block Diagram
LO Gen
Antenna
Electronics
Central
Electronics
H Maser
Or CSO
CVR
ALMA
Correlator
Digital
Backend 
ALMA
Monitor/Control
Post Proc.
Software
VLBI
Recorder
VLBI
Mode
Phase/Delay
Algorithms