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Lee Clement PHYS 2070 Thursday, 7 April, 2011. Observation of the Crab Nebula. Image Credit: NASA (Hubble). M1: The Crab Nebula. Plerionic type remnant of Supernova SN 1054 Filled shell structure powered by pulsar wind (Green) Located in Taurus

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Lee clement phys 2070 thursday 7 april 2011

Lee Clement

PHYS 2070

Thursday, 7 April, 2011

Observation of the Crab Nebula

Image Credit: NASA (Hubble)

M1 the crab nebula
M1: The Crab Nebula

  • Plerionic type remnant of Supernova SN 1054

    • Filled shell structure powered by pulsar wind (Green)

  • Located in Taurus

    • RA: 05h 34m 31.95s Dec: +22° 58’ 54.4” (J2000) (Cutri et al.)

  • First observed by John Bevis (1731)

  • Independently rediscovered by Charles Messier (1758)

  • Distance: 1930 pc (Trimble, 1973)

  • Angular size: 6’ x 4’ (Dyer)

  • Magnitude: 8.4 (Dyer)

Image Credit: Stellarium

Gao data collection
GAO Data Collection

  • 20-21 January, 2011

    • Conditions

      • Cold! (-27 C)

      • Sunset: 17:03

      • Moonrise: 18:38

        • Waning gibbous with 99% of the visible disc illuminated

        • Hoped to get data before moon got too high

      • Cloud: Initially none

        • Worsened over the course of the evening

      • Sky glow: Initially some to the North (Winnipeg)

        • SQM: 19.61

        • Worsened as moon rose and clouds rolled in

          • SQM: 17.15 around 20:45

          • SQM: 15.63 around 21:30

Data collection challenges
Data Collection Challenges

  • Series of unfortunate events

    • Deadbolt on dome side door frozen shut

      • Possibly inadequate temperature equalization inside dome

    • Telescope would only focus in one direction

      • Had to wait for Jennifer

      • Wire had become disconnected in dome

      • Rheostat adjustment needed in fuse panel

    • Dome froze stuck

      • M1 had drifted past the edge of the aperture

      • Spent ~30 mins trying to assist dome motor to rotate dome

  • End result

    • Not able to start collecting data until 21:20

      • Bright moon

      • Partial cloud cover

Raw data
Raw Data

  • 30 x 60-sec exposures

  • Large variations in brightness

    • Variable clouds

    • Bright moon

Calibration frames master bias
Calibration Frames: Master Bias

  • To sample readout noise of the CCD chip

  • Average of 10 x 0-sec exposures

Calibration frames master dark
Calibration Frames: Master Dark

  • To correct for thermal noise in the CCD chip

  • 5 x 60-sec exposures with the shutter closed

  • Master bias subtracted then frames averaged

  • Interesting feature: periodic noise

    • Unknown source

Calibration frames master flat
Calibration Frames: Master Flat

  • To correct for illumination variations, dust, etc.

  • 30 x 60-sec exposures of sky regions near M1

  • Master bias and master dark subtracted

  • Normalized to mean value of 1

  • Median taken

  • Also exhibits periodic noise

    • Unknown source

Image registration
Image Registration

  • Master bias and master dark subtracted from raw data

  • Result divided by master flat

  • Registered images in 3 parts

    • Brightness variations

    • Discarded 5 frames

Reduced data
Reduced Data

  • Average of registered calibrated images

  • Problem:

    • Persistent gradient from northeast to southwest



Gradient removal
Gradient Removal

  • Used ImageJ’s ‘Subtract Background’ function

    • Radius: 300.0 px

    • Create background

    • Sliding paraboloid

  • Produced a model of the gradient

Gradient removal1
Gradient Removal

  • Subtracted gradient model from reduced data



Expansion rate
Expansion Rate

  • Nugent (1998) measured the expansion of the Crab Nebula

    • Scans of 4 high-resolution photographs

    • Measured the motion of several filaments of the Crab’s structure

  • Measuring filaments not feasible with GAO data

    • Too blurry

    • Needed to look at larger scale features instead

  • Compared GAO image to POSS I (1951) and POSS II (1991) red filter images

Expansion rate1
Expansion Rate




  • Took the square root of the gradient-subtracted image to enhance contrast around the edges

    • Vertically flipped to match orientation

  • Resized POSS I and POSS II images to be a similar scale to GAO image

    • Also stretched brightness so that only portions visible in GAO image were visible

      • Some guesswork




GAO (2011)

POSS I (1951)

POSS II (1991)

Calibration of image scale
Calibration of Image Scale


  • Made use of the bottom two stars of the southwest trapezoid formation

    • In Aladin, measured separation on POSS I and POSS II images

      • Distance: 1.09’



Measurement methodology
Measurement Methodology




  • Initially intended to measure distance from central pulsar to various features

    • Problem: Not obvious where the centre is




GAO (2011)



POSS I (1951)

POSS II (1991)

Measurement methodology1
Measurement Methodology

  • Alternative method

    • Measured distance between two sets of easily distinguishable features

    • Tried to approximate the major and minor axes

GAO (2011)


  • All measurements made using line segments in ImageJ


  • Plotted measured “axis” distances against Julian dates of observations

    • Calculated slopes of lines of best fit to approximate rate of expansion

      • Similar for both “axes”

        • Uniformly expanding


  • Rate of expansion was found to be 0.5 arcsec/yr

    • Inconsistent with published value of 0.15 arcsec/yr (Bietenholz)

  • Extrapolated to point of zero size to determine original date of supernova

    • Found CE 1405 ± 29

      • Inconsistent with result of Nugent (1998): CE 1130 ± 16

      • Inconsistent with result of Trimble (1968): CE 1140 ± 15

      • Closer to result of Bietenholz (1991): CE 1245 ± 92

        • Measured only the synchrotron component

    • However, known date of supernova is CE 1054

      • Confirms result that expansion of the Crab Nebula is accelerating

        • Exact expansion function unknown (Nugent)

Sources of error
Sources of Error

  • Very limited data set

    • Low detail of GAO data precluded measuring more than a few features

    • Random errors would average out with more measurements

  • Guesswork involved in determining what parts of POSS I and POSS II images were visible in GAO image

  • Guesswork involved in matching edges of features between images


  • Despite unfortunate observing conditions, managed to acquire data that shows some large-scale structure of the Crab Nebula

  • Data not so good for astrometry!

    • Calculated expansion rate inconsistent with previous results

    • Calculated date of supernova inconsistent with previous results

      • Still later than actual date

        • Confirms acceleration of expansion




Bietenholz, M. F., Kronberg, P. P., Hogg, D. E. and Wilson, A. S. The Expansion of the Crab Nebula 1991

Cutri, et al. 2MASS All-Sky Catalog of Point Sources 2003

Dyer, Alan. The Messier Catalogue 2010

Green, Dave. G184.6-5.8 2009

Trimble, Virginia. The Distance to the Crab Nebula and NP 0532 1973

Trimble, Virginia. Motions and Structure of the Filamentary Envelope of the Crab Nebula 1968