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I have a set of observations of two spherical galaxies, from which I wish to investigate the presence of variable stars and plot mainly luminosity vs period for any that I might find. I have data from six nights of observations, some of which is mostly single central field data from the respective galaxies, while in some cases I have images for 2x2 or 3x3 mosaics.What I'd like to ask about it mainly some general advice on how to do this in an efficient manner. I'm well at terms with reducing the images (imred, ccdred) as well as carrying out crowded field photometry (apphot, daophot) up to the point where I have a set of output data from the allstar task.From there it seems to me I ought to implement a world coordinate system to make it possible to identify stars between observations and generally (have read the guide by Frank Valdes on creating mosaic wcs), and also that I should complete the data treatment as per the photcal package (currently gone with mkapfile, mkimsets, mkobsfile and heading towards fitparams and evaluation, however it seems the wcs needs to be done for that to go well).To solidify my query somewhat:1. I do not presently have access to the mscred package, although I do have imcoords. Is this sufficient to make a good wcs?2. Would it be a reasonable plan to now first sort out the wcs, and then complete the photcal work in order to get some sort of final data?3. What could be a reasonable method to go from photcal output with wcs data available, to contruct light curves or even more directly identify (possible) variable stars in the fields?4. Any other advice that springs to mind, I'd be grateful. I'm not very experienced with this type of work and my possibilities of getting help at my department are somewhat limited at this point in time.
If there is anything else I can add to help answering my questions, please let me know. Many thanks for any help.

Hi,I think you have a pretty good handle on things. It is true a WCS can ease matching the photometry for the same star in different exposures though it is not absolutely necessary. It could be done tediously with imcoords or doing one source at a time.I am not very experience with the range of photometry tools so I don't know off-hand how to take the photometry output and convert it into a form for light curve analysis.The only obscure potential thing that might be useful is the PDM package for finding periods. But I'm sure that is what you are after.Sorry I can't be more informative and maybe someone else will step in with some experience.Yours,
Frank Valdes