10.5 Practical: Constructing an HR Diagram of an Open Cluster
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• Slew to the target. Take a short exposure image to confirm that you are on target
and still in focus.
• Turn on tracking and select the first filter. Take several images of the target at
different exposure times until you are happy that you have a good linear image.
• Undertake the same procedure for another filter.
• Take bias and flat frames.
• Save files to a portable medium and shut down the observatory.
10.5.3 Reduction
Once you have the images, you need to reduce them, perform photometry, and plot
the results.
• Produce science frames by subtracting the bias and dark frames from your light
frames and dividing by the flat.
• Open the first science frame in ATP (or a suitable alternative).
• Select a star and perform an aperture slice. Use the FWHM of the slice to set the
aperture and annulus radii. Set the sky algorithm to median sky subtraction.
• Select a source list and then create a new source list. Process the source list.
• Hitting the List results button will display your photometry results. Export this file
and then clear the contents.
• Load the second image and repeat the photometry process listed above.
• Load both photometric results files into a spreadsheet. Move all but the centroid
RA, centroid Dec, magnitude, and magnitude error columns.
• If you know the filter’s zero points, apply this information to the magnitude
columns. This is not required if you do not have them.
• Crossmatch the two source lists (for example in TOPCAT) so that you have both
magnitudes for most stars in the field (some will always be missing a magnitude,
in which case discard them).
• Plot magnitude in filter A minus magnitude in filter B versus magnitude in filter.
10.5.4 Analysis
Write up your report in the standard format. You should include all your plots and
observational logs as usual. You should discuss your errors and their implications.
You should identify the main sequence in your plot and the main sequence turn-off.
Discuss whether all the stars you imaged are cluster members and the implications
if they are not. What do you think is the benefit of imaging a cluster, rather than a
normal star field?
159
• Slew to the target. Take a short exposure image to confirm that you are on target
and still in focus.
• Turn on tracking and select the first filter. Take several images of the target at
different exposure times until you are happy that you have a good linear image.
• Undertake the same procedure for another filter.
• Take bias and flat frames.
• Save files to a portable medium and shut down the observatory.
10.5.3 Reduction
Once you have the images, you need to reduce them, perform photometry, and plot
the results.
• Produce science frames by subtracting the bias and dark frames from your light
frames and dividing by the flat.
• Open the first science frame in ATP (or a suitable alternative).
• Select a star and perform an aperture slice. Use the FWHM of the slice to set the
aperture and annulus radii. Set the sky algorithm to median sky subtraction.
• Select a source list and then create a new source list. Process the source list.
• Hitting the List results button will display your photometry results. Export this file
and then clear the contents.
• Load the second image and repeat the photometry process listed above.
• Load both photometric results files into a spreadsheet. Move all but the centroid
RA, centroid Dec, magnitude, and magnitude error columns.
• If you know the filter’s zero points, apply this information to the magnitude
columns. This is not required if you do not have them.
• Crossmatch the two source lists (for example in TOPCAT) so that you have both
magnitudes for most stars in the field (some will always be missing a magnitude,
in which case discard them).
• Plot magnitude in filter A minus magnitude in filter B versus magnitude in filter.
10.5.4 Analysis
Write up your report in the standard format. You should include all your plots and
observational logs as usual. You should discuss your errors and their implications.
You should identify the main sequence in your plot and the main sequence turn-off.
Discuss whether all the stars you imaged are cluster members and the implications
if they are not. What do you think is the benefit of imaging a cluster, rather than a
normal star field?