Prospects for High-Frequency Calibration with the SMA Dual-IF/Receiver System Todd R. Hunter, Jun-Hui Zhao (CfA) Sheng-Yuan Liu, Yu-Nung Su, Vivien Chen (ASIAA) Based on data from February 2005 “690 GHz Campaign”

Lack of Strong Gain Calibrators SMA sensitivity: Tsys ~ 100 K at 230 GHz (10 mJy in 5 min) Tsys ~ 2500 K at 690 GHz (250 mJy in 5 min) For good phase solutions, we need S/N ~ 10 per baseline 0.5 Jy at 230 GHz (70 quasars with F > 1Jy) 10 Jy at 690 GHz (maybe 1 or 2 quasars) This requires { Quasars are inadequate for the SMA at 690GHz

Flux density Typical 230 GHz 690 GHz Diameter Callisto 6 Jy 45 Jy 1.3” Ganymede ” Ceres ” Titan ” Pallas ” Minor planets as calibrators These objects work adequately if one of them is available Otherwise, need lower frequency “phase transfer” with the SMA dual-IF system synthesized beam in compact configuration 1.1”

Fundamental components of SMA dual-IF system 10 MHz 1. Common reference frequency LO 1 Receiver Feed GHz Receiver Feed GHz LO 2 IF 1 IF 2 Correlator 2 Correlator 1 2. Co-aligned receiver feeds ( < 1/6 beam) 3. Duplicate paths for simultaneous down-conversion and correlator processing YIG, DDS Antenna

January 28, 2005: First dual-IF fringes SiO J=5-4, v=1 at 215 GHzH 2 O 1 1,0 -1 0,1 v=1 at 658 GHz Simultaneous maser lines from W Hydra These screens show only 2% of the total correlator data product.

First Dual-IF Phase vs. Time solutions 215 GHz maser in LSB658 GHz maser in USB Ant 1 Ant 3 About 3x larger phase change and opposite sign (as expected) Ant 1 Ant 3 2 hours 360 o

1. Strong, compact source (e.g. Ceres) to compare 230 and 690 phases 2. Stronger source (possibly resolved) for passband information (e.g. Callisto, Ganymede) 3. Science target 4. Quasar near the science target Investigation of “Phase transfer” Part I: Observation strategy Observe four sources: We had 7 nights with low opacity during the recent 690 GHz Campaign in February

Linear fit of Ceres 690 phase vs 230 phase Antenna Correlation Slope reference antenna theory Antenna Correlation Slope reference antenna theory USB data LSB data

Investigation of “Phase transfer” Part II. Search for phase relationships 1.Do passband calibration of calibrator and target 2. Do phase-only selfcal on calibrator at 230 & Examine correlations of 690 vs 230 phase solutions 4. Flag any phase jumps or unstable periods that degrade the correlation 5. Compute slope and offset relating 230 and 690 phases on each antenna

Ceres 690 Selfcal Example #0: phase transfer using Ceres (on itself) Ceres phase transfer image Ceres 690 uncalibrated data Derive coefficients and 690 gain table Apply 690 gain table Ceres 230 Selfcal (rms = 50 mJy, S/N = 260) (rms = 70 mJy, S/N = 193) +

Investigation of “Phase transfer” Part III: Imaging tests 1.Selfcal the test target at 230 GHz 2.Apply slope & offset from the phase transfer calibrator to create a new gain table appropriate for 690 GHz 2. Image the test target at 690 GHz using the new gain table 3. Compare with image from “normal” calibration

Example #1: phase transfer on a point source 690 GHz phase transfer (S/N=5) Apply coefficients from Ceres to make 690 gain table Quasar GHz selfcal solution Direct 690 GHz calibration (S/N=8) Apply 690 gain table 690 GHz uncalibrated data

The phase transfer analyses in the previous slides were done in Miriad. Here is an example done in MIR / IDL (see poster 4.69 by Su & Liu). In this case, the frequency ratio (rather than the fit) was used in the scaling. Phase transfer from quasar Example #2: phase transfer on IRAS Direct 690GHz calibration (Ceres)

Summary of 690 GHz calibration schemes in the “compact” configuration (1” to 5”) MethodAdvantagesDisadvantages 1. Direct 690 calibration using minor planet Most direct approach, does not require dual-IF phase stability Calibrator often quite distant from target (over 40 degrees) 2. Phase transfer using minor planet plus nearby quasar Can use quasar close to target – better positions Requires dual-IF phase stability and measurement of phase slope and offset 3. Phase transfer using 215 / 658 GHz masers Additional pool of compact sources close to target. Might be only solution for extended arrays. Arbitrary spectral line setups not feasible (or frequent re-tuning required).

Q: What limits this method? A: instrument problems 1.Phase jumps and drift 90 o jump 230GHz Sometimes seen in one IF only, sometimes both. Under investigation. Also, slow changes in phase offset with time between the two IFs may require frequent measurement of phase transfer coefficients. We see 60 degrees of phase change at 690 GHz per o C of antenna cabin temperature change. Sensitive to any thermal imbalance between the IFs. 2. Phase vs. temperature No jump 217 GHz 679 GHz

1. Passband measurements in extended configs No point sources (< 0.4”) bright enough Can we use Lunar limb? (works for measuring delays) Noise source for passband phases? Autocorrelation on the ambient load for amplitudes? 2. Weather? So far, best dual-freq. phase correlations occur on nights with little wind (< 10 mph, January 28), or constant wind direction (February 18, March 02). Coincidence? More experience may tell us. Other limitations for 690 GHz calibration

Summary and future work We have demonstrated our first attempt at phase transfer at submm frequencies Improvements are possible: Cabin temperature control (antenna 6 now < 0.1 o C RMS) Phase jump investigation (and elimination) New receiver band coming ( GHz) Will allow more frequent dual-band observations (due to less stringent weather requirements) and higher S/N testing of the phase transfer technique. Conclusion: We remain open-minded and hopeful to realize the full potential of the SMA.