[vsnet-chat 3655] Re: Overobserving

[Taichi Kato <tkato>]

Re: Overobserving

> 1st, absolutely rubbish observations.  Possibly it would be kinder to call
> these "miscorrect" observations.  I invariably give visual observation
> datasets, no matter how massive the dataset, a quick look through with an
> ascii reader and many, many presses of the "page down" key.

   This is not an exclusive matter of visual observations.  Even in CCD/PEP
observations, one have to disregard some observations siginificantly
deviating from the rest, probably caused by istrumental noise or transient
change in the atmospheric condition.  It's simply a matter of degree.
You could simply reject some observations using the conventional n-sigma
rejection program, or by detecting jumps in the sorted light curve.
The latter algorithm is being automatically used when I incorporate
observations reported to VSNET into my personal database; this can at least
help eliminating a part of clearly incorrect (likely mis-identifications)
observations [but I haven't always delete these observations, even if
the program makes a warning, which may introduce some errors in the
VSNET WWW light curves].  Well, you seem to be very much inclined to
character-based data examination ;-)  Almost the same thing can be more
easily performed using a GUI light curve viewer/editor.  Just a few
mouse clickes are usually necessary.

> If you do this with quite a bit of AFOEV archived data you will find one
> particular observer, who will remain fully unidentified here, who has a
> strange habit.  Little if any observing seems to be done for longish
> periods, and then an LPV may be observed half a dozen times or more over a
> night or two.

   There can be.  We usually regard it is some sort of responsibility
of the organization or the database manager to ask such an observer to
check the cause (there could be a lot of sources: the usage of an obsolete
or adequate chart, typographical error in chart magnitude etc.), or
at least try to find the cause of peculiarity.  We have a lot of experiences
such peculiar observations are corrected via network communication
and/or real-time comparision between observers.  Often the cause has
truned out to be trivial, such as a typograhpical error in some astronomy
magazine chart.  The recent case of V1062 Cyg in the VSNET is another
good example.

> You may think this is the "overobserving" case again, probably cured by the
> above linear regression route or some such.  However, this observer is
> _INVARIABLY_ quoting observations from half a magnitude up to two
> magnitudes brighter than anybody else who observed on that night.

   If the observer observed by a constant above the rest of observers,
the difference can be easily corrected using a constant correction
(self-evident).  Otherwise, the cause of deviation needed to be more
rigorously examined.

> Okay, let's say one of the observers was more prolific than the other, well
> then, unless it can be rigidly proven that someone who observes a lot is
> more likely to be more accurate, how would you choose between these two
> observers?  If more prolific did equal more accurate then, fair enough, the
> more prolific observer's data would receive more weight.  But the AFOEV
> example above had a seriously "prolific" observer who always reports a lot
> brighter than everybody else!!!!!

   In the usual sense, an observer called as "prolific" needs to be at least
as accurate as other observers...   I may have been "prolific" in the
number of observations made in the past, but I'm not sure I could have
been a prolific observer [it's just like that the number of e-mails doesn't
need to mean the e-mail sender is prolific...]

> John Howarth and I analysed this beast amongst other stars in the BAAVSS
> archives, as part of a general look-see, and I decided the data was
> abysmal, and stubbornly refused to publish anything on it.
> 
> But I did check out what had happened.  Fortunately 2 of the more prolific
> observers of this star were experienced observers.  One is head of a UK
> variable star group, the other does the charts for the other UK variable
> star group.  And as luck would have it, one _invariably_ used stars E and F
> in comparisons, whilst the other very rarely used these stars.
> 
> So, I analysed their data separately, and found that if people used star E
> and/or F to compare UU Aur they barely saw it vary, whilst if they did not,
> something akin to the real situation could be found.  This had already been
> more generally evident in the folded lightcurve, which showed a scattered
> sinusoid superposed by thick horizontal "tramlines".  The horizontal
> tramlines invariably were the result of people using stars E &/or F to
> estimate UU Aur... ...interestingly these themselves clustered at separate
> values a few tenths of a magnitude apart dependant on how particular sets
> of observers' eyes worked!

   This is an example of a caveat to non-Pogson-type (I don't know the
exact terminology) visual estimates.  If observers use "steps" against
fixed comparison stars, this could be mostly avoidable.  We had the
same experience in W Ori, in which "classical" magnitude of comparison
stars (likely from the HD catalog) were very noisy.  The same is true
for R Cnc and some other such examples, which introduce much confusion
between observers.  However, the number of such cases is limited,
and in the presence of modern Hipparcos/Tycho, CCD photometry and
with careful selection of comparision stars, we probably don't need
to be bothered by such problems.  [And I wish the database manager of
the existing variable star observations would as quickly as possible
to correct old observations based on wrong comparison magnitudes.]

> John Howarth, who has serious pieces of paper for maths and statistics from
> _top_ UK universities, went on to correct this data on his own using
> methodologies and routines beyond my understanding, and presented a paper
> to the BAA which'll get in the JBAA eventually.  The results he got weren't
> much different than the ones I got without cleaning up the data, and
> despite weightings and corrections and slights of hand, still weren't as
> good as those gleaned from the AFOEV data.  Yet all these datasets/routes
> presented periods within a day of each other.

   Mathematically speaking, constant corrections to individual observers
will not change the power spectrum, as long as observations are evenly
or randomly spaced.  This doesn't necessarily mean that corrections to
non-evenly spaced data have the same little effect.  If observations with
a large personal bias cluters in some observation gaps, corrections would
certainly reduce "noise" in the power spectrum.

   However, the dramatic effect of corrections appear is to the human
eye on the corrected light curve, rather than the power spectrum [and is
quite natural.]  Without corrections of personal biases, I found it hard
to detect visually modulations (before that fading) in the combined light
curve of FG Sge, but they dramatically appeared after simple corrections!

   This suggest that the most of noise (what looks like noise to human
eyes) is caused by personal biases, presumably caused by different comparison
stars or personal preference.

Regards,
Taichi Kato

VSNET Home Page

Return to Daisaku Nogami

---

vsnet-adm@kusastro.kyoto-u.ac.jp