CfAO Summer Internship

Michelle DarrahSmithsonian SubMillimeter Array

Supervisor: Alison Peck

•Dual Track Internship:

•Observing Assistant - learned how to prime and operate the array for observations

•Research

Observing and Priming

Quasar Calibration at Submillimeter Wavelengths

SMA

Michelle Darrah

SMA Calibration• Optical telescopes adjust for bad seeing due to thermal

problems, radio interferometer telescopes adjust for phase differences due to H2O in the atmosphere and turbulence.

• Creating an image of an astronomical source, we must compensate for phase changes by observing bright point-like source.

Path Length and Phase Shift

• The signals from each telescope is going to arrive slightly out of phase from intermittent and unsmooth atmospheric interference.

The signal at each antenna is going to arrive slightly out of phase due to different path through the atmosphere.

Precise Calibration Benefits

• Better Image Fidelity• Increased Data Accuracy

• Quality Assessment of Data• Ability to Take Data From Dimmer or More

Distant Objects

Calibrator Requirements

• Stability

• Comprehensive Documentation

• Minimal Weather Dependence

• Bright at Multiple Wavelengths

Quasars

• Intrinsically brighter than their surroundings at multiple wavelengths

Calibration Publication

• At each new wavelength a list of calibrators must be established.

• A published list of calibrators and their accepted flux values gives future telescopes a step ahead in their calibrations and data collection.

Steps

• Observe quasar candidates with the observations spanning at least six months

• Use Miriad or MIR software to reduce data

• Plot data and check for stability• Narrow the list to those quasars that there is sufficient data

on, and observe more data for those that do not make that list.

Observing

•Observing script nflux.pl

•Long list of quasars calibrated against Callisto or Ganymede.

•Edited daily (using VI), to update priority rating for each quasar.

Data• Data from observing gets reduced using Miriad or

MIR and then plotted; observed time (MJD) vs. flux (Jy)

• Plots analyzed for stability and brightness

• Sorted into stable objects, useful for calibrations, and unstable objects that are interesting for their own intrinsic scientific value.

• Cross referenced to VLA data to look for correlation in time of flux change to frequency.

• Repeatedly observed to ensure continued stability

Plots

0402-014

Flux Variability 62% at 850m

Unstable for Calibrations

3c273

Flux Variability 11% at 850m

Stable

Continued Observations

3c454.3

Was a stable calibrator until the end of 2004.

Good example of why to continually observe and re-evaluate.

From more than 330 calibrators, the list was analyzed for comprehensive data over 6 months or more, and then separated into 2 lists, stable and unstable, at both 1mm and 850m.

Results

• Maximum flux variation 30% for stability• 16 stable calibrators at 850 m

• 18 stable calibrators at 1mm

• Not enough correlation in timing between the VLA data and SMA data to draw conclusions.

Acknowledgements

• Alison Peck, Shelbi Hostler, Ant Schinckel• SMA Staff• CfAO

• University of Hawaii at Hilo• Funding provided through the Center for Adaptive

Optics, a NSF Science and Technology Center (STC) AST-987683