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 6 44 1.5”Ceres 2.6 14 0.5”Titan 1.1 9 0.7”Pallas 0.6 5 0.3”

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 MHz1. Commonreference frequency

LO 1

ReceiverFeed 2

690 GHz

ReceiverFeed 1

230 GHz

LO 2

IF 1

IF 2 Correlator 2

Correlator 1

2. Co-alignedreceiver feeds ( < 1/6 beam)

3. Duplicate paths for simultaneous down-conversionand correlator processing

YIG,DDS

Antenna

January 28, 2005: First dual-IF fringes

SiO J=5-4, v=1 at 215 GHz H2O 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 LSB 658 GHz maser in USB

Ant 1

Ant 3

About 3x larger phase change and opposite sign (as expected)

Ant 1

Ant 3

2 hours

360o360o

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 1 0.97 3.0 2 0.85 2.3 3 0.95 2.4 4 0.88 2.3 5 0.94 3.2 6 reference antenna theory 1.00 3.0

Antenna Correlation Slope 1 0.94 3.1 2 0.72 2.0 3 0.85 2.0 4 0.83 2.2 5 0.85 3.3 6 reference antenna theory 1.00 3.1

USB data

LSBdata

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 & 690

3. 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 imageCeres 690 uncalibrated data

Derivecoefficients

and 690 gain table

Apply690 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 coefficientsfrom Ceres

to make690 gain

table

Quasar 1743-038230 GHz selfcal solution

Direct 690 GHz calibration (S/N=8)

Apply690 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 1743-038

Example #2: phase transfer on IRAS 16293-2422

Direct 690GHz calibration (Ceres)

Summary of 690 GHz calibration schemes in the “compact” configuration (1” to 5”)

Method Advantages Disadvantages

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 drift90o 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 oC 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 oC RMS) Phase jump investigation (and elimination)

• New receiver band coming (320-420 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 hopefulto realize the full potential of the SMA.