Re: GSC 748-1618

[Chris Lloyd <cl@astro1.bnsc.rl.ac.uk>]

Yes, what I call the raw magnitudes are are from the database. And yes,
it's basically an offset so that the magnitudes and the differences can be
compared easily. Part of the increased scatter is due to that stars being
fainter, but I would have expected the differences to be better because a
significant part of the TASS errors are correlated. As this seems to have
broken down here I suspect cloud, possibly only thin cloud, or something
else has compromised the measurements. At some point these correlated
errors will become dominated by statistical noise and the argument will
break down, but I haven't looked to see where that might occur. I would
have thought that it was fainter than this but I need to check.

Chris


On Tue, 18 Feb 2003, Michael Koppelman wrote:

> What do you mean by "raw TASS magnitudes"? From the ensemble
> photometry? Also, you say the second plot is the difference -- I'm just
> curious why it's plotted in the V=11 range when it should be <1? Is
> that just a label or relative offset issue? That seems like quite a lot
> of scatter. Is it tighter within a night and we are just seeing
> night-to-night difference? The scatter on the brighter TASS stars I've
> worked on is much less than this. Is the (what seems to me) increased
> scatter just due to the dimmer nature of this batch of stars?
>
> Thanks Chris!
>
> Michael Koppelman
>
> On Tuesday, February 18, 2003, at 03:25 PM, Chris Lloyd wrote:
>
> > Michael,
> >
> > I normally use the differential magnitudes when working with TASS data
> > but
> > in this case they are certainly no better and probably worse. I
> > enclose two
> > plots. The first shows the light curve of GSC748 relative to number
> > 89632 in
> > the big list.
> >
> > The filled symbols are the raw TASS magnitudes while the open symbols
> > are
> > the differential magnitudes plus the mean of the comparison. In reality
> > there is very little to choose between them but to me the differential
> > mags
> > have more scatter.
> >
> > The second plot is the difference between two comparisons, 89632 and
> > 89725
> > using the same phasing. The symbols are the same as before. Apart from
> > the
> > few outliers I can see no obvious difference but the standard
> > deviations are
> > 0.023 and 0.029 for the raw and differential mags respectively. There
> > is
> > clearly something afoot here because my experience is that the
> > differential
> > magnitudes are usually significantly more reliable than the raw mags.
> > I have
> > most of a technical note on this which I really must finish off. I
> > suspect
> > that there is something up with the TASS data which generally
> > increases the
> > noise but that doesn't explain why parts of the curve are
> > systematically
> > different.
> >
> > GSC 748-1618
> >  89720  135 103.5564   7.9413  2286.72869 11.406 0.130 0 10.645 0.074 0
> >  89632  194 103.4339   7.9736  2286.70740 10.477 0.017 0  9.050 0.027 0
> >  89725  143 103.5627   7.9019  2286.74822 10.878 0.023 0  9.665 0.014 0
> >
> > Cheers,
> >
> > Chris
> >
> > <gsc748_f1.png><gsc748_f2.png>
>