[vsnet-chat 5941] Chart revision and worldwide observers

["Sebastian Otero" <varsao@fullzero.com.ar>]

Dear all,

Janet sent a guide line of what should be done in a future chart revision.
All of the points mentioned are really important but these criteria can
already be met by people that work with responsability.
But since it takes time, and not all the observers want (and they have the
right to) or are able to use their time to make sequences or revise charts,
some people have to do it.


>     - In order for such a program to work, we need a database of all
> comparison-star-magnitudes on AAVSO charts. We are in the early stages
> of a project to prepare such a database.
>     - When it is completed, the online chart program will make it much
> easier to make changes to charts since all we need to do is update the
> comparison-star-magnitude database. It will also allow flexibility for
> the eventual inclusion of new photometric databases when they become
> available.

The charts are not the problem, sequences are.
Marc Biesmans couldn't have expressed it better:

> I think what we need are certainly not additional chartmakers.
> What would be the difference if we had only/all new computerized charts
> instead of the old Prelim. stuff.
> Nothing !
> For several years already I am absolutely desperate about good sequences.

Several people use AAVSO charts (old or new) with the V magnitudes of the
comparison stars written on it... It is not a cosmetic thing but a question
of numbers.
I think that this problem goes beyond AAVSO. It's related with a way of
working.
Not all solutons necessarily need to pass for AAVSO (this is not the only
organization in the world) and not all the blame has to be put on it too.

>     - The data transformation programs, presently being developed at
> HQ, must be optimized so that early and recent data can be transformed
> successfully.

This is a big issue...

Mike wrote:

> a few well placed bad comps can create
> scatter that can hide important aspects of a stars actual behavior, both
sho
> rt term and long term.
> Take for example, a well observed Z Cam type star like SY Cnc
> How do you fix that?

You don't.
Adding examples to that mentioned by Mike is the case of the mira star R
Aquilae.
Take the "b" R Aquilae chart.
This is a V mag. sequence (GCPD/Tycho1-2) ordered by AAVSO magnitude:

66             6.29 (1.62)
67             6.71 (0.4)
71             7.20 (0.27)
74             7.44 (0.35)
----------
76             7.49 (0.9)
78             8.44 (1.3)
----------
79             8.53 (0.35)
83             8.84 (0.13)
----------
85W          8.93 (2.0)
85SW       9.65 (0.7)
----------
88             9.29 (1.9)
89             9.32 (0.5)
91             9.36 (1.4)
95S           9.53 (0.6)
95SSW     9.73 (1.1)

When the star is around 8th magnitude, the lightcurve will show something
that isn't happening. A full amplitude fading will be reported as a 0.2 mag.
oscillation, that can be wrongly interpreted as a stansdstill.

> This thinking would lead one to believe that the errors in the charts are
> simply a matter of zero point adjustments

And this adjustment is impossible even when having the comparison stars
used.
Why?
Because 99% of the observers blame their own eyes for the differences and
try to see what the chart says and end up observing both "85" stars as if it
were the same or the 76 and 78 stars as similar when they are almost a full
magnitude different.
Look at the BX Mon charts (I don't know if they were fixed) that have a pair
of comp stars 90 and 95 with their magnitudes interchanged... If noone
noticed it is just because everybody struggled to see the 95 star brigther
than the 90.... And they seem to have been succesful trying..;-(
People really observed a white 6.85 mag. constant star vary during 47 years
until the T Columbae chart was fixed and T Caeli (7.5-8.0) took its actual
position...
So re-reducing observations based on new values is not as simple.... People
calibrated their eyes to the wrong values and there is no soultion once it
has already happened.
The visual observations (I am not referring to the CCD observations
mentioned by Gary Walker) are biased (at the moment they are being done) by
the numbers given on the charts and if the numbers are this wrong, results
would be impossible to correct.
BUT -related to my earlier posting-, the question is why do you need to be
so accurate if in most of the cases even if you correct for the new
comparison star values the results will be inside the error box you accept
for visual observations????
Your plan includes large amplitude variables. The worst cases such as those
mentioned by Mike and me are prone to a quick correction, but I don't see
any further improvement to come and beter things given the hetereogeneity of
the methods use in visual observations. The sactter will be there and will
be greater than the corrections undertaken.
Maybe some zero points adjustments in faint CV's... But why is the need of
accuracy there if the goal is to catch the outburst?
Well, yes, I would use the better sequence available too, but my point is:
if you use data of half a magnitude accuracy, why bother with corrections
that -in most of the cases- will be below that limit?

Small amplitude variables are the ones that would suffer the most from
comparison star magnitude changes, but these stars are not included in
AAVSO's program.

I think there are two kind of observers: those that only want to report
observations and those who only wish to know what is actually happening to
the observed star.
The latter don't need to be limitted by the perpetuation of errors until
something can be fixed. It is simple.
Well, they can choose (as I did a long time ago when I realized my needs and
varible star types were not in agreement with AAVSO parameters) whether to
report to some organization or the other. To choose the one that is useful
for oneself.
I respect AAVSO work and people, I enjoy the website, being part of this
discussion list and having this great feedback with people that share the
same hobby, but I have different targets and interests observatinally.

> As regards the impact of the "Simocharts" based on Henden photometry that
> are published on the internet,

Any new chart must use a V sequence, it makes no sense to use charts that
are worse than the available ones.
Using a V chart would ensure consistency if observers calibrate their eyes
to the V scale by applying direct vision instead of averted vision. That is
only good for detection at the limitting magnitude. Most of observers chase
after CV eruptions and it's perfect to use averted vision for that cases:
they simply don't need to be so acurate.
But a bright star (a positive) observation is not the same.
Cones= accuracy
Rods= detection.
There is a way to calibrate your eyes to the V scale and leave comparison
stars in peace for a while. Just try to improve the own observer's skills.
If there is a V scale that provides us with accurate values for our work, we
need to stick to it.
Once we do it, comparison star magnitudes won't be such a terrible issue.
Once a comparison star seems to be causing trouble, we could research the
cause of the anomaly, but using a V scale this will be 1 in a 100 cases, I
bet...
There are lot of photometric data on the internet for the brighter stars.
Variable star observing is not only about 15th magnitude stars.

I am afraid that any revision won't leave some of us happy because there are
points that seem to be left unchanged and making a difference with our way
of working.
The introduction of a comparison star database would be a good oportunity to
start using comparison star magnitudes to two decimal places.
Rounding off accurate V mag. values doesn't make sense.
It can even create unnecesary errors. The result is what would be rounded to
show the accuracy of the observation reported.

That's why I also think that charts are not the most important thing but
data are.
Of course if I use comparison stars to two decimal places but the convention
is to use it to one decimal plcae, I'll be always left outside of the
standards. Standards that don't improve anything, on the contrary.

Two decimal places and the inclusion of the star color are very important
issues that would certainly help reduce the scatter of visual data. I don't
think we need to have 1 million numbers about the comparison stars but only
those. Any other thing needed on a specific problematic star, can be
recoverd from internet or the original source. Nowadays this is possible.

I think all the observers would fit in a high quality standardization.
But some observers will be left outside (by own choice) if the
standardization doesn't give the best tools available to their work.

Best wishes,
Sebastian.




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