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Taichi Kato wrote: > I know a number of professional packages which can perform to a similar, > or even better, photometry. Although I don't know if their source codes > are publicly available, DAOPHOT is no longer the unique solution. > DAOPHOT was originally designed for extremely crowded (e.g. globular > clusters) fields, but this is not always the best approach if the field > is not so crowded. Have you ever seen the DAOPHOT source code? At a first > look (if you are familiar with software technology), you can soon guess > what factors could have prevented third-party improvement of this > package. > DAOPHOT II (I presume you are talking about the second version) is pretty typical Fortran code IMHO, with considerable effort to add logic for input errors and handling cases like saddle points in solutions. It is well documented and has extensive in-line comments. I have seen far worse code in my career. Regarding crowded fields, this is true if you are using the psf-fitting, but remember that DAOPHOT also includes a full aperture photometry section. I'm not advocating that everyone convert to using DAOPHOT for their photometric reductions; I'm just making the point that reliable, extremely well tested software like DAOPHOT does not happen very often and should not be denigrated just because it may not be written with the current compiler-de-jour. Bernard Heathcote wrote: >The comments (below) regarding the obvious difficulties and dangers of >dealing with low S/N data leads me to raise the question as to what you >consider to be the S/N value below which an expert's input is advisable? >As a relative newcomer in CCD photometry I have followed this discussion >with great interest, so all expert advise is most appreciated. Certainly by the time you get down to S/N=3 (a Poisson error of 0.3mag) as Kato-san suggests, you should be very careful of how you reduce your data for optimal results. I generally use a Poisson error of 0.1mag (S/N=10) as my personal dividing point between automatic processing and hands-on processing of important data, and try very hard to never reach that decision point! If possible, I stack frames to increase the signal/noise before extraction just to avoid systematic effects. This is similar to how Tycho2 improved its photometry over Tycho1. Kato-san has a different regime of interest, often looking for rapidly evolving features while using low signal/noise data from small telescopes. It can be done, as he has admirably shown, but I'd rather use my observing and processing time looking at objects better suited for my aperture. Arne
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