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Part I: The Distance To M12
Note: It is highly recommended that your read the Introduction
before you start at this exercise.
Observations, data reduction and analysis
The globular cluster M12 was observed on June
18th, 1999 using the FORS1 instrument on
ANTU (UT1) of the VLT at the ESO Paranal Observatory (Chile).
For this exercise we have chosen images of the outer parts of the cluster
where there are slightly fewer stars. The exposures
were taken through a blue filter (B-band) and through a green filter (V-band for Visual).
To observe and to reduce data (the process of
removing instrumental and other artefacts from
the data) is a job requiring large telescopes and
sophisticated computer programs. The really interesting
part for astronomers — the data analysis
— starts afterwards.
In this exercise the data has already been collected
and reduced. We have simplified the analysis
a little by selecting a set of stars that can
be considered as representative of the population
of the whole cluster.
Hints for analysing the images
To analyse the images, the B and V magnitudes
of each star have to be measured carefully. Errors
made in the first parts of this exercise will
affect the later results.
The 45 stars are split into six sections:
- 1-2 Nine stars nos. 1 to 9 — ‘training stars’
- A,B,C,D Thirty-six stars nos. 10-45 are split into four
groups (A, B, C and D).
Make sure your measurements are consistent.
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