Thank you both.

Zocky - not sure I can say much more than I've said already. The PI part is easy - simply apply the FWHMEstimator script to R, G, B (aligned) master frames separately and export as CSV files.

The rest is manipulation in Excel.

Steps I took:

1) Combined each of the frames into one file - one tab, per filter.

2) Created a "Flux column" for each filter equal to A * Sigma_X * Sigma_Y*PI()/3. A, Sigma_X and Sigma_Y are the amplitude and star sizes from the Moffat4 fit.

3) Created an index column for each star equal to INT(xstar+0.5)+INT(ystar+0.5)*NX. The INT(xstar+0.5) calculates the nearest integer. Multiplying the y co-ord by NX and adding the x co-ord creates a unique index for each pixel on the chip and allows you to then cross reference star locations between filters.

4) Choose the filter with the most stars. Use VLOOKUP to find the flux for the other filters using the index column - you need to place the index column on the left to make this work - see the documentation for VLOOKUP. Use FALSE as the final argument to ensure that only exact matches are reported. Don't worry about the N/A values where there is no match.

5) Find a named star using your favourite technique (I used the Annotate script, but there are other ways e.g. Aladin). Use Vizier or similar to look up its magnitudes.

6) Find the star in your tables. Calculate the magnitude = - 2.5 * LOG10(Flux). Calculate the offset needed to match the Vizier magnitude. Say you calculate - 0.3 and the magnitude is 12.4, then you need to add 12.7 to the magnitudes. Calculate for both G and B. G is the closest filter for the old V band, so use that as V.

7) Now you can calculate the apparent magnitudes - simply calculate -2.5*LOG10(Flux)+Offet for each start for the Blue and Green filters.

Now you can calculate the colour of the star by working out B-V.

9) FInally, create a scatter plot of V vs B-V, with B-V as the X-Axis. You'll need to play about with the axis definitions to reverse the Y axis (highest values at top), and to set the range for the data. The plot will ignore the N/A data.

I've posted a slightly cut down version of my spreadsheet below (removed some stars to get below 500kb).

I haven't written a script before, but if someone who has written a script is interested, I'm happy to work with them to automate this as much as possible!

Colin