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Photometry
Are you observing stars? Catalogue magnitude is notoriously inaccurate for photometry -- be careful with the source catalog, as many (most?) are not accurate better than 0.5 mag anyway.
A more important question in some ways is what is the variation in your measurements? If your stdev is 0.1 or 0.5 mag, then you need to improve your equipment or technique. I assume you are processing carefully with darks and flats? Also beware of "twinkling" when using a small aperture scope -- you will need longer exposures, which means better guiding.
From what I understand, using a Johnson-Cousins filter, along with some careful measurement and accounting for atmospheric extinction will allow you to get accurate absolute magnitude measurements. That is a reasonable amount of effort, but necessary if you really want absolute magnitude measurements.
As an alternative, you might consider differential photometry? I have had good results with this technique, even without filters. If I can use a comparison star with similar colour index (B-V), I can get around 0.02 mag error using my Canon DSLR. In fact, I have managed to capture an exoplanet transit light curve (Delta mag around 0.025) this way.
Ivan
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