[vsnet-chat 3651] Re: Overobserving

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

>     I wonder whether analysts apply corrections for systematic differences
> between observers.  My experience with the VSOLJ data has quite contrary
> impression.  After correcting systematic differences (by an analytic way),
> the resultant light curve is often far better than a combination of
> a selected prolific observers with less frequent observations.
> Even low-amplitude multiperiodic light modulations in R CrB stars can
> be detectable (R CrB itself has a far less amplitude than that of RY Sgr
> or V854 Cen), using an ensemble of densely sampled (ideally one per night
> for each observer) data.

Is it possible to correct for various observers' errors? The word systematic
is frequently used but I don't know that this is a fair description. We're
talking about Miras and so forth so the subjective errors which are
associated with CVs and RCrBs don't come into it. I've checked a lot of
visual measures against pep and the differences are not systematic. Colour
response probably comes into it but the easiest solution is to take one
observer's measures and use these uncorrected. In any case I'm not trying to
get a pretty light curve - I'm trying to establish epochs of maximum to plot
on an O-C diagram.

What are you achieving by analysing visual measures of stars like RY Sgr,
etc? It's easy enough to derive a period using DFT or something but this
period is not particularly accurate. Whereas you can get a good seasonal
sample from pep the visual measures give a value about 20-50 times less
reliable. Take a Cepheid such as R Crucis - the pep measures will give an
epoch +/- 10 minutes, the visual ones about 10 hours. So the error bars are
much greater than the suspected period change.

The only place in these stars I see the visual measures being useful is with
longer period Cepheids where there are just no pep or CCD measures.

> Not all CV outbursts are recorded by PEP or CCD.  Actually only few of
> them are observed in sufficient manner.  [Even in the presence of early
> warnings of BF Ara outbursts, I have not yet heard of CCD/PEP observations
> which are sufficient to conclude that dense observations are no longer
> necessary.]  Every CV outburst may have its own variety, as best
experienced
> in the well-known SS Cyg or U Gem, in some of whose outbursts "dips" were
> observed during outbursts.  Unfortunately there were NO simultaneous
> CCD/PEP observations, and analysts have had to rely on visual
observations.
> I wonder whether this is already the feature overdocumented using CCD/PEP
> observations.

I'm not quite certain what you're saying here. I'd like to look at BF Arae
but the weather hasn't been helpful. Other times the stars are not well
placed. The CCD observers aren't going to waste time on doing a run on a
star you can only observe for 2 hours before it sets, or the sun rises. I
note CU Vel is bright at the moment but it's unobservable with CCD. Ara
isn't quite in such a bad position at present but then there are often other
stars in outburst which appear more attractive. I look in the GCVS and it
tells me that BF Arae is a UGSS which generally have a longish period, no
superhumps or orbital signatures and relatively uninteresting outbursts. So
if something else is bright then it probably comes first.

The time and accuracy problem comes up again with CV eclipses. Oddly enough,
I
began my CV observations in 1967 looking visually for eclipses on VW Hydri,
BV Centauri and Z Chamaeleontis. We tried looking at EX Hydrae for practice.
It's certainly possible to time the orbital humps on VW Hydri visually, but
not with great accuracy. But I've compared simultaneous photometry and
visual measures of VW Hydri during superoutbursts and wondered if we were
looking at the same star as the visual people. Perhaps it's significant that
until superhumps were recognised about 1974  from pep measures the visual
people, in spite of thousands of measures, didn't notice these. There was
then an abortive attempt to prove that these could be tracked visually.

I'm unclear about the 'dips'. What was the time scale? Are we talking about
a decrease covering several orbits or just something lasting part of a
cycle? This brings up something which I've been thinking about for a time.
Everyone's efforts in the CV field would be improved f there was some sort
of dossier on each of the stars. How many observers know that VW Hydri rises
to about V=12 and then pauses for a time before continuing the outburst.
This pause is related to the eventual brightness of the outburst. Is this a
common feature of SU UMa stars? What does someone else know about CU Velorum
that I don't?

Maybe what I'm saying here is that the dense observing of CV stars is
unplanned and consequently doesn't prove much. It's easy enough to find
periodicities in visual data if the pep and CCD people have already
established that these exist. But there are few good examples of the visual
people making the discovery.

During the next few years all the southern CVs will, no doubt, be looked at
with CCDs. I've already spent a night or two on BB Vel and a few others with
no result. In the interim, I'm trying to arrange for good sequences down to
about V=16.5 for as many fields as possible.

In conclusion, I'm still not convinced that all this visual attention to CVs
during outbursts produces anything of value. And since many Miras are so
underobserved it would be great to see more attention devoted to these.
There's no chance that we're going to get the 50-100 measures a night that
some CVs attract so the overobserving question doesn't arise.

Regards,
Stan
----- Original Message -----
From: Taichi Kato <tkato@kusastro.kyoto-u.ac.jp>
To: <vsnet-chat@kusastro.kyoto-u.ac.jp>
Sent: Friday, September 29, 2000 7:20 PM
Subject: [vsnet-chat 3643] Re: Overobserving

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