I am looking at the OGLE IV catalog of target: http://ogle.astrouw.edu.pl/ogle4/ews/blg-0004.html
They provide a color-magnitude diagram of the given field.
The microlensing (ML) event is marked with a cross-hair.
If I wanted to place the ML event on the color-magnitude diagram using a suitable telescope with standard filters would the following do the job:
Step 01: take an image of the field in I filter
Step 02: take an image of the field in V filter
Step 03: for each image perform aperture photometry around the ML event and other stars. You now have two numbers for the instrumental magnitude in the two filters for each star.
Step 04: form the difference V-I for each star. You now have measured the color for each star.
Step 05: you now can plot I (or V) versus color for each star.
While the color should be the same for my telescope and the OGLE telescope (we form the difference) the instrumental magnitude should be different and depending on the exposure time, right?
How to do the above automatically? Given two images as input data? What software is best?