[Aavso-photometry] Re: Questions on Henden sequences

[Alessandro Maitan]

I have the opportunity to partecipate to a programme to monitor symbiotic star magnitude variation. I am using Henden sequences to determine the transformation coefficients (I'm using only V & I since my camera is based on "old" KAF 400 chip) and therefore calculate the actual magnitude of the variable star. I'm finding very big differences in my photometry reduction, therefore I would like to get a piece of adivce on which could be the best way to face this situation considering that several star in the sequence are very red and the field I am worknig on (in Aql) is quite "croweded".
For my reduction I'm using IRAF and the tasks of both the apphot and photcal packages.
I understand that my question is too much generic, but I can't find a better way to ask unless writing a long amil detailing all the steps I did (obviously I can explain everything if necessary).
Apologizing for my poor English and thanking everybody in advance, I remain.

Alessandro Maitan