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AIP has different routines for different purposes. The CVK independently method is good for asteroids that travel through the field or for rotating fields. The C-offset is great for stars that fade away rapidly in non-rotating fields. If your field rotates, C-offset will not work for long. The C-offset routine does *NOT* centroid the V and K stars except for the first frame. That is the idea - it follows the C star and offsets a certain number of pixels in the X and Y direction for both V and K stars. That way if the V star goes very faint, you do not have the problem of failing to centroid the V star. If you get a value that is just barely measurable, fine. If there are other stars near the V and K stars, when the V or stars get faint, the centroid will gravitate to the other nearby stars. Note the stars can get faint for a number of reason, such as clouds or obstrutions to the telescope/light path. If stars are so faint that you can't centroid them, your statistics are going to be bad, but why compound the problem by measuring the star poorly? Lew --- Michael Koppelman <lolife@bitstream.net> wrote: > I don't understand why this would be better. If you are > doing > photometry on a star too dim to centroid, you are in big > trouble. As > users of daofind know, you can find statistical stars at > levels that > are hardly visible by the eye. I can't imagine that > centroiding is the > problem here. > > The opposite problem seems much more likely to me: if you > use only the > C star to find your position, and the frame rotates under > you due to > bad polar alignment or alt/az-type issues. I'm not sure > if AIP > re-centroids these stars or not, but I assume it must, > which would make > this a non-issue as well. > > The aperture moving completely off the star, though, is > certainly > possible, and has happened to most of us. The crime there > is not > noticing. Sometimes you can't trust those programs to > find the star for > you, especially if it wiggles around a lot. In this case > it usually > does not slowly drift off the star, though, if gets way > off very fast. > > From what I've heard said so far, I can't imagine that > AIP is handling > any of these situations incorrectly. > > Michael koppelman > > On Tuesday, July 8, 2003, at 05:56 AM, Berto Monard > wrote: > > >> The centering error on faint stars that Taichi-san > mentions > >> in AIP4WIN can be avoided altogether by doing > photometry as > >> C-Offset where the comparison xtar (presumably bright > >> enough to be reliably centered on all images and the > >> variable and check are not centered except on the > first > >> image of the set and the apertures are rigidly placed > in > >> the same location relative to the comparison star. > > > > Thanks, Lew! Is everyone using this option? (In > precise > > photometry, > > I regard this option as a default -- we use multiple > stars to > > compensate > > a small-order deviation from a constant offset. With > an Alt-Az mount, > > one also need to incorporate image rotation). Use this > option whenever > > applicable. If everyone uses this option and still > gets unrealistic > > depressions in the light curve or other unusual > features, we should > > search > > for a different explanation -- in any case, this would > provide a > > critical > > test for the software. > ===== Regards, Lew CBA Concord http://vsnet.geocities.com/lcoo/eng29.htm The CBA Concord telescope is in the debugging process CBA Pahala http://vsnet.geocities.com/lcoo/pahala.htm
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