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Physics 134: Lecture 13 - Efficient Data Processing in Observational Astrophysics, Assignments of Physics

University of California - Santa BarbaraPhysics

The steps for reducing ccd data in observational astrophysics using iraf, including image alignment, flatfielding, and flux calibration. Announcements regarding assignments and project selection are also included.

Typology: Assignments

Pre 2010

Uploaded on 09/17/2009

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Physics 134:
Observational
Astrophysics
Lecture 13; May 13, 2008
Efficient Data Processing
Announcements:
• Report from second group of observers (Todd, Jared,
Marc R.) and advice to third group (Tim, Erin, Steve,
Matthew)
• HW#7 on galaxy photometry is now due May 22nd
• Accepting HW#6 today with 5% penalty
• Accepting HW#6 Thursday with 10% penalty
• Project selection
• Computer accounts (with password) coming soon
• $ cp important_stuff /backup/YourAccount
pf3
pf4

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Download Physics 134: Lecture 13 - Efficient Data Processing in Observational Astrophysics and more Assignments Physics in PDF only on Docsity!

Physics 134:

Observational

Astrophysics

Lecture 13; May 13, 2008 Efficient Data Processing

Announcements:

  • Report from second group of observers (Todd, Jared, Marc R.) and advice to third group (Tim, Erin, Steve, Matthew)
  • HW#7 on galaxy photometry is now due May 22nd
  • Accepting HW#6 today with 5% penalty
  • Accepting HW#6 Thursday with 10% penalty
  • Project selection
  • Computer accounts (with password) coming soon
  • $ cp important_stuff /backup/YourAccount

Outline:

  • Overview of CCD Reduction Steps
  • IRAF Scripts: An example that will speed up

image alignment and shifting

  • IRAF.hsel -- A faster way to make image lists

for processing

  • How to do the flux calibration

Basic CCD Data Reduction Steps

A Users Guide to CCD Reductions with IRAF (Massey)

  • Process all frames to fit and subtract the bias

overscan and trim the image [OT][ccdproc]

  • Construct the average bias [imcombine]
  • Zero-correct all frames [Z][ccdproc]
  • Construct the average flatfield [imcombine]
  • Flatfield all frames [F][ccdproc]
  • (optional) Illumination correction [I][ccdproc]

will take out large-scale gradients in the sky

background level

Flux Calibration

  1. Measure the count rate from the standard, C(cnts/s), in one band
  2. Correct the standard to zero airmass, C’ (cnt/s)
  3. Look up the star’s magnitude and write down the magnitude equation
  4. Or, convert star’s magnitude to flux units, and write down the inverse sensitivity. Your values for several stars can be averaged together.
  5. Measure galaxy or supernova counts and correct for airmass
  6. Compute galaxy or supernova magnitude

Summary

  • The goal is to make one or more astrophysical measurements including both statistical and systematic uncertainties.
  • Calibrating the raw images is a necessary step.
  • We correct the count rate from the galaxy to the effective count rate above the atmosphere and then multiply by the inverse sensitivity to obtain the flux (or flux density) of the galaxy.