[vsnet-chat 3652] Re: re Overobserving

["Stan Walker" <astroman@voyager.co.nz>]

Hi All,

I note John's comments which describe the situation in detail. Actually, so
long as the bad measures are more or less evenly distributed it doesn't
affect the result. There are other problems with the measures - a lot of K
stars seem to have low amplitude variations of up to 5% over a period of a
few years (UBV photometry with adequate checks) which I thought might be due
to spottiness but one of the people at Mt John was finding low amplitude
pulsations which might be the answer.

Our programme st Auckland on Miras and SRs enabled direct comparison of
visual and pe measures. You get offsets with one observer consistently
bright (or faint) or stretching or compression of the light curve. Some of
this was done with BH Crucis, a rather red object, but others with
'ordinary' Miras.

So, overall, our conclusion was that attempting to correct the data was a
waste of time. And, once again, the visual errors are NOT SYSTEMATIC as the
various comments suggest. You almost need a 'personal equation' for each
night and each star. And then finally, what is being proven? After dredging
out all the faulty measures and correcting the rest the epoch of maximum
changes by less than 2% of the cycle length and is still +/- a lot!

None of this matters much for Miras but is bad with a 20% superhump and a
period of 90 minutes!

By the way, John, I found a consistent period of about 63 days in 8000
visual measures of R CrA (or one of those stars). Probably an orbital
signature but when I mentioned this the visual people wouldn't believe it.
Quite puzzling. Why do they observe the star if they don't credit the
analyses?

Regards,
Stan

----- Original Message -----
From: <crawl@zoom.co.uk>
To: <vsnet-chat@kusastro.kyoto-u.ac.jp>
Sent: Saturday, September 30, 2000 4:56 AM
Subject: [vsnet-chat 3649] re Overobserving


> Taichi Kato wrote:
>
> >             Why not implelement the (known) correction algorithm(s) to
the
> >             analysis package? My strategy was quite simple -- to add a
> constant
> >             value for  each observer, and minimize the high-frequency
> power (expressed
> >             in other numerical form, however). This can be implemented
by
> using a
> >             simple linear least-squares equation, and can be fully
> automatically done.
> >             Other algorithms, if exist, could be implemented likewise.
This
> >             approach, however, reduces one degree of freedom from each
> observers, so
> >             observers who made only one observation is automatically
> given zero
> >             weight...
>
> Okay, I'm going to have to answer that via three more or less specific
> examples, which highlight [in my view] the point that it is only really
> safe to think of [or even _hope_] that problems with raw data can be
> considered as self cancelling, or at least incoherent non-signal creating,
> noise.  As soon as any level of pre-processing is done on the raw visual
> obs, this assumption is compromised further.  Some of the following uses
> values from memory, so don't take them as rigorous periods etc!!!
>
> 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.
>
> 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.
>
> 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.
>
> I just crop that observers observations out and throw them away nowadays.
> Before I noticed this happened, they probably just got lost in the noise
> when I did an analysis.  Granted that correction factors for this
> particular observer for each particular variable could be derived and
> applied, but that's a special set of algorithms for one person, not a
> global route as described above.
>
> 2nd, a year or so ago I was lucky enough to get hold of some data from
> AAVSO, which was a good trick cos in those days getting hold of aavso data
> was like pulling teeth, but less fun.
>
> The 1961 to more or less present data is quoted by AAVSO as being
> "validated".  AAVSO observers ostensibly use AAVSO stars.  Yet it was
> possible to find two observers who observed within 0.01 day of each other
> who had observations disparate by a whole magnitude.
>
> 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!!!!!
>
> Of course, that'd be fixed if you _banned_ overobservers, extending the
> guilt to all for the behaviour of a few, but these are decisions beyond
> algorithms, and beyond straightforward analysis and global corrections
> and/or global weightings etc.
>
> 3rd, the BAA VSS's turn.  Before a now updated sequence came into force,
> the BAAVSS data for UU Aurigae was seriously compromised due to comparison
> star E being very blue and star F being very red [or was it the other way
> round?], and the variable being quite red.
>
> 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!
>
> Interestingly there are two sidepoints here.  First, the BAAVSS practice
of
> recording the actual _estimate_ as well as the reduced observation allowed
> me to reverse engineer this problem, and second the farce probably carried
> on as long as it did because people were probably _not_ looking at their
> previous observations in order to avoid bias!!!!  Everybody is warned off
> about this bias effect, but if just a few people had thought to look at
> their own observations they'd have thought "hey, this star is constant",
> noted the point to someone, investigations would have ensued and it would
> have been found not to be constant, and the problem would have been solved
> earlier.
>
> There's too many dogmatic "mustn't do this" statements in visual variable
> star observing that would be a good project for some group out there to
> double check... ...you'd need enough folk to be able to split into a two
> teams, one being a control group.
>
> Anyway, the point of the UU Aur ramblings?  Well, the BAAVSS data on it is
> seriously compromised because of this, yet if you do "simple" DFT on it
you
> can still readily recover the 400+ and 200+ day periods of this doubly
> periodic semiregular from the data, with results very little different
> period-wise than if you analyse the independent AFOEV data for this star
> over the same time interval, the AFOEV data being problem free.  The
> amplitude of the peaks is of course seriously reduced in the result from
> the BAAVSS data, but they're still readily distinguished.
>
> In other words, if you just accept that every non-valid observation of a
> variable star is only going to add to the _noise_, without creating any
> spurious periodicities, then even with very bad data for a variable, you
> can recover its period[s].  [Over and above other problems like annual
> "pseudo-aliasing", and even some good old fashioned beat aliasing for some
> low amplitude objects].
>
> 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.
>
>
> Of course, for very low amplitude objects the aforementioned noise can
mean
> some data is hidden, but global fixes are only going to move the noise
> around, not remove it.
>
> You see, the basic assumption appears to be that all we need to do is fix
> scatter due to different instruments [I include the eye-brain combination
> as an "instrument"] and conditions, and if you allow weightings or
> de-weightings due to observer frequency or skill or what have you, the
> noise will magically go away.
>
> This neglects the fact that a not insignificant proportion of all visual
> archives are so much crap, and irrecoverable no matter how clever the
> analytical technique used.
>
> And I thoroughly expect to be strung up for the last sentence.
>
> Cheers
>
> John
>
> JG, UK
>
>

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