From sean.dougherty@nrc.ca Tue Aug 6 19:27:56 2002 Date: Tue, 30 Jul 2002 12:07:46 -0700 From: Sean Dougherty To: Chris Carilli , Steve Rawlings Cc: Dave Wilner Subject: Comments on concepts for Stars SWG Chris and Steve, Find attached my comments related to the concepts and the stars SWG level 1 science goals. A BIG, BIG concern is the fact that many of the concepts have upper frequency limits that are far from OK with our science goals - even 15 GHz will not allow imaging of dusty envelopes around proto-planetaries (though the case of using ALMA for this will undoubtedly come up in the Groningen discussion) since photo-evaporation of the disk has to have emission that is visible at some point - where, who knows but it will be somewhere in the cm wavelength range. Until the issue of upper frequency is resolved (Ha, ha!) I figure we are all just shooting the breeze. Not sure I have provided enough info - I only included comments on those points that are really clear in each of the concepts. Eg. I don't mention anything about DNR. I question any one who just states they are going to get 1,000,000 DNR. What about side-lobes, and bright sources out in those sidelobes? I couldn't find any substantive discussion of this in any of the documents. One question that I thought should have been asked is "What fraction of the fov do you want at maximum resolution?" Will people want all of 1 degree at mas resolution? If so, they will demand one hell of a correlator/processing machine to handle that amount of data! I really doubt (hope?) that anyone would require this. Let me know if you were expecting something deeper out of me!!! Sean -- ------------------------------------------------------------------------------ Sean M. Dougherty www.drao.nrc.ca/~smd Dominion Radio Astrophysical Observatory 250 490 4329 (direct) PO Box 248, Penticton BC. V2A 6K3 250 493 2277 (switchboard) CANADA 250 493 7767 (fax) ------------------------------------------------------------------------------ [ Part 2: "Attached Text" ] ------------------------------------------------------------------------------ Level 1 science program: Sub-AU imaging of proto-planetary systems Special requirements: High frequency operation - at least 22 GHz. The biggest drawback in many of these designs are the low upper frequency limits. Need 10K brightness temp sensitivity at a few mas resolution at the highest frequencies LargeN-smallD Spec Requirement Design fail meet exceed comments ---- ----------- ------- ---- ---- ------ -------- A/T m^2/K 20000 Needs to hold at the highest frequency Freq range GHz <35 What is A/T at 35 GHz? Configuration: max baseline km 50% area baseline T_b sens K 1 Need 10K at 4mas resolution ----------------- Major problem(s): outline the major design problem(s) wrt science goal None - high frequency capability is great! Hopefully A/T will hold at 35 GHz!! Possible Compromises: ------------------------------------------------------------------------------ LAR Spec Requirement Design fail meet exceed comments ---- ----------- ------- ---- ---- ------ -------- A/T m^2/K 23000-30000 (for tsys=30-40K and in zenith) FoV sq.deg 1 Even the 0.5 deg fov suggested would be OK Freq range GHz <22 Has the capability for higher frequency operation Configuration: max baseline km 50% area baseline T_b sens K Need 10K at 4mas resolution ----------------- Major problem(s): outline the major design problem(s) wrt science goal - the compromise of a smaller fov suggested would be OK - this design can be pushed to even higher frequencies (say 35 GHz) - good! - configurations are not discussed in the white paper, though the ~1000-3000 km baselines with good sensitivity are possible. Possible Compromises: ------------------------------------------------------------------------------ Luneberg Lenses Spec Requirement Design fail meet exceed comments ---- ----------- ------- ---- ---- ------ -------- A/T m^2/K 13000 FoV sq.deg 1 Freq range GHz <5 Low upper frequency limit Configuration: max baseline km OK 50% area baseline T_b sens K Need 10K at 4mas resolution ----------------- Major problem(s): outline the major design problem(s) wrt science goal - Low upper frequency limit. Possible Compromises: - to enable this level 1 science it may be possibel to go down to 15 GHz, with longer baselines to maintain resolution of a few mas, though it is not clear if emission from dust or photo-evaporation will dominate at this frequency. ------------------------------------------------------------------------------ Cylindrical reflectors Spec Requirement Design fail meet exceed comments ---- ----------- ------- ---- ---- ------ -------- A/T m^2/K 20000 FoV sq.deg 1 Freq range GHz <9 Low upper frequency limit Configuration: max baseline km OK 50% area baseline T_b sens K Need 10K at 4mas resolution ----------------- Major problem(s): outline the major design problem(s) wrt science goal - Low upper frequency limit. Possible Compromises: - see comment under Luneberg lenses. ------------------------------------------------------------------------------ Apeture Array Tiles Spec Requirement Design fail meet exceed comments ---- ----------- ------- ---- ---- ------ -------- A/T m^2/K 20000 FoV sq.deg 1 Freq range GHz <1.5 Low upper frequency limit Configuration: max baseline km OK 50% area baseline T_b sens K ----------------- Major problem(s): outline the major design problem(s) wrt science goal - Low upper frequency limit. Possible Compromises: - see comment under Luneberg lenses. ------------------------------------------------------------------------------ KARST Spec Requirement Design fail meet exceed comments ---- ----------- ------- ---- ---- ------ -------- A/T m^2/K 20000 FoV sq.deg ~1 arcmin@5Ghz Imaging clusters of stars maybe restricted with fov Freq range GHz <8 - initially <2GHz Low upper frequency limit Low efficiency at the upper limit Configuration: max baseline 400km A little short! 50% area baseline T_b sens K ----------------- Major problem(s): outline the major design problem(s) wrt science goal - Low upper frequency limit. Possible Compromises: - see comment under Luneberg lenses. ------------------------------------------------------------------------------ Preloaded parabolic dishes Spec Requirement Design fail meet exceed comments ---- ----------- ------- ---- ---- ------ -------- A/T m^2/K ? FoV sq.deg ? Freq range GHz <10 Low upper frequency limit and low efficieny at the high end Configuration Presumably they could be placed max baseline km with baselines ~1000-3000km 50% area baseline T_b sens K Need 10K at 4mas resolution ----------------- Major problem(s): outline the major design problem(s) wrt science goal - Low upper frequency limit. Possible Compromises: - see comment under Luneberg lenses. ------------------------------------------------------------------------------