Department of Physics University of Durham Level One

6. Measurement of the Proper Motion of Barnard's star: Part 2

To illustrate how the position of a star can be determined from an uncalibrated image we now consider a CCD image of Barnard's star that was taken by Level-3 students as part of an Astrolab experiment. The image was taken on our 12-inch telescope with an ST9 CCD on 2000 November 01.

This Durham image, barn_st9.fit, covers a sky area of about 12 x 12 arcminutes.

To accompany this image we have a calibrated image of the same area of sky. This image, b_dss.fit, has been obtained from the on-line STScI Digitized Sky Survey and was taken in 1950. It covers an area of 15 x 15 arcminutes. Note this image shows stars of a fainter magnitude than barn_st9.fit.

Digitized Sky Survey image, b_dss.fit, from a photograph plate taken in 1950. This covers the area where Barnard's star was in 2000. 

In order to measure the position of Barnard's star on the uncalibrated image we must interpolate from stars with known RA/Dec positions. This can be achieved as follows:

  1. Identify a few (at least 3) background stars that are common to both images and that bracket Barnard's star position on barn_st9.fit
     
  2. Display calibrated image, b_dss.fit, with FITSview.
     
  3. Record the RAs and Decs in b_dss.fit of the common stars.
     
  4. Display the uncalibrated image, barn_st9.fit, with FITSview.
     
  5. Record the x and y positions in barn_st9.fit of the common stars. Also measure the x, y position of Barnard's star.
     
  6. By hand, plot a graph of the RAs (from b_dss.fit) against the measured x positions (from barn_st9.fit) of the common stars. Draw a line through the points. Now transform the measured x position of Barnard's star into an RA.
     
  7. Make a similar graph of Dec verses y and estimate the Dec of Barnard's star.
     
  8. Check whether this position is consistent, with your previously measured proper motion, i.e. is the observed position on 2000 November 01 in agreement with that predicted from your first measurement.

This page is maintained by jrl.     Last updated:  2000-Nov-01