[vsnet-chat 4054] Re: Visual estimates accuracy

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

Dear vsnet observers:
                                 I disagree with most of the comments
expressed. But it's very interesting. I respect all the opinions and I think
that if you get accurate observations everything is okay. But all things you
have mentioned don't work for me. And through my method I get +- 0.1
mag. accuracy.


From: reinder j bouma

> > If stars are not the same color and if it's not made for detection at
the
> > limitting magnitude, I always try to avoid averted vision for all those
> > problems.
>
> This last remark is really amazing to me!
> If an observer wants to take full advantage of the capabilities of his
> instrument,  and at the same time wants to produce consistant data, he
should use a
> SINGLE method.
> Well, that can ONLY be achieved by using averted vision, because that
> allows you to still see the faintest stars.

You are as amazed as I am finding out that nobody makes a difference between
bright and faint stars and the different types of vision.
Consistent with what?? This word is used everywhere but almost always means
"using the same (poor) charts", "making the same mistakes" and the like.
An example:
I am observing a red variable. Some of the comparison stars are red and this
is good so there is no different response from a star to another. BUT in
some part of the sequence there is only yellow, white or blue stars. So the
star was rising at a constant rate but when I comapare it with the bluer
stars, these appear brighter because I use averted vision and my variable
stops its rising.
If I had used near-direct vision  this wouldn't have happened. This example
is
for a telescopic faint star. If it is a naked eye star, things are
different.
My observations can only be CONSISTENT if I use different types of vision,
if not, I will not be seeing a lightcurve of the star's behaviour but one of
my eyes' behaviour.

About my technics, this is a part of my article:

"...This effect is stressed when the stars are too bright (naked eye) or too

faint (near the limitting magnitude of our telescope).
--A naked eye red star will be always TOO bright if we use direct vision and
compare it with a bluer star.
--A very faint blue star at the limit of our scope will be always too bright
if we use averted vision and compare it with a redder star.

My approach is to use an intermediate type of vision when we
are at an intermediate brightness (most of the cases). This way nor the red
star neither the blue one will "win" over each other.

--Near the limitting magnitude, given the sharp brightening of the blue
stars
(rods being color-blind and very sensitive to faint stimulus) and the
dissapearance of the red stars (cones being color-sensitive but not working
when the stimulus is not strong enough), I recommend using direct vision.

--And for very bright stars, the opposite approach is recommended: using
near-averted vision to compensate the overestimulated sensation that bright
red stars produce in our cones.

So pure averted vision is only recommended for detections near the limit of
our capabilities, when an accurate magnitude is not as important as the
detection of the star (CV's in outburst).
Or in the case that we are observing all blue faint stars ."

The different approachs are necessary for that reason: instruments magnify
the problems along with the star's brightness. They also change the
background so it is another factor: it may sound a bit crazy but I prefer my
city skies for estimating variables because in a very dark sky the different
responses between cones and rods are stressed and the estimate is more
dependent on the type of vision you use. You can deduce from this that it is
also applied to changing magnification (= using different eyepieces): If you
use more power, you'll have a darker background and you'll need to be more
careful.


> To avoid 'color' effects of
> bright stars, one should defocus the stars until no color is noticible
> anymore. Only that way one is certain (more or less) that one is
> observing in the same spectral bandpass (of the rods).
> Rods are most sensitve at 520nm, while cones (as used in direct vision)
> have their peak slightly redward near 560nm.

The defocusing method doesn't work for me: in my experience, a
red star disappears much sooner than a blue star of the same magnitude
(Actually color is not noticeable anymore... and also the star ). It is
understandable if we know that we are taking the brightness of the star near
the limitting magnitude (by defocusing it) where averted vision domains,
which means blue stars will be seen and red ones not.

> So mixing observations with both direct and averted vision is a bit like
> mixing CCD-V with CCD-unfiltered data, and not saying so.

It's not a random mix. You must know stars' colors and then you act on that
basis. It's the only way our data are consistent with V. V is the standard
and we can achieve it easily if we want it. If you want to use that "v" and
only use averted vision whatever the color, then all charts should have "v"
(or "averted vision magnitudes"). These charts don't exist. Magnitudes are a
disaster. This is an example:

R Lep AAVSO chart:

AAVSO / "v" mag. /  V mag

85    /     8.39   /   8.38
86    /     8.75   /   8.63
90    /     9.09   /   9.01
91    /     9.08   /   9.06
95    /     9.69   /   9.45
100  /   10.48   / 10.18
102  /   10.48   / 10.26
104  /   10.40   / 10.18
106  /   10.73   / 10.48

It speaks for itself.
For instance if I use averted vision and the star is between 85 and 86, I'll
be seing a 0.3 mag. change but I will report it as 0.1.
When it comes to 100 and 102 I'll be imagining a 0.2 change because in v
both comparisons are the same.

PEP(V) magnitudes used with the proper technics are the best way of reaching
the 0.1 accuracy. Furthermore, it has become the only way to make all the
observations consistent (based on Johnson V).

> This is particularly obvious indeed in the R Lep data, but also in other
> red stars like U and CH Cyg, where the 'direct' observations really
> stand out as being far too bright.

Far too bright compared with what? What I've always read are magnitudes far
too faint....


> So, always use averted (either in or out of focus) vision as Berto
> already pointed out before, albeit in a more lenient way.
> I think the other 'monsters' will agree.

And my advice keeps saying: Don't use averted vision unless you need to
compare red and blue very bright naked eye stars or if you need to work at
your limitting magnitude.

It's true that most of the observers (unlike me) work at that limit and it
is natural to use averted vision in that cases. I do it too. But I don't
believe you can generalize its use to intermediate brightness stars.


From Georg Comello:

>>I was also astonished by the remarks of Sebastion Otero, using two
>different
>methods in variable star-observing. This is the best way, to create scatter
>I think.

This is the only way to avoid scatter according to my records. More than
this,
there are three methods: direct vision for red-blue comparisons at the
limit, averted vision for red-blue comparisons with the naked eye and a mix
of the two in most of the other cases (So nor the blue star neither the red
one will "win" over each other.) SOMETHING MUST BE CLEAR: The best thing to
do it is to compare stars of the same color. This way, it doesn't matter HOW
I look at the star. That would be great but it's just a dream.
And another thing is important: I use the standard V magnitudes and my
observations are 0.1 mag. accurate for it. That's why I use my methods. I'm
not interested in reporting "v" magnitudes out of the standard system where
all works can be compared.


>>I always use averted vision when observing variable stars to get most
>out of my telescope. Further I defocuse in the case of the brighter
>variables
>to avoid color-effects, though I am not very color-sensitive. It is easier
>and
>less critical to estimate an extended object than a point-source with the
>same
>part of the retina. So the result most probably will be more accurate.
>I fully agree with Reinder Bouma's comments


I think the question is: do you want to report averted vision  magnitudes or
V magnitudes? Because my intention is to demonstrate that we can report
magnitudes that agree with V.


From: "Berto Monard

>Direct vision is only possible if stars are bright enough, ie well within 1
>magnitude of the telescope limit at the observing time.
>But then, direct vision can result in staring, which will cause especially
>red stars to 'become' brighter.

I agree with Berto's comments. And I observe mostly stars more than 1
magnitude above that limit.
The "staring" Purkinje problem is apparent for (very) bright red stars and
that's why I do use averted vision for that cases.
And that's why I recommend the "intermediate type" (and not direct) of
vision for most of the cases.


>Observer's color (intensity) vision differ, which will cause more estimate
>scattering on stars with extreme colours, especially those red.
>Most often, differences in estimates are due to differences in the
sequences
>used, or even different stars of the same sequence. The latter even with
>differences in the angle position errors.
>There are so many factors as you have refered to. I want to read your
>article.

All those factors should be taken in account but let's suppose we've alredy
done it and our problem is just the colors.

>Yes, I use mostly averted vision since my targets are often faint and I
have
>a hard and impossible time to reach the magnitudes seen by the Aussies (but
>wait another two weeks!).
>For eta Car and other brighter stars I use out-of-focus comparison.

I always get more accurate when observing punctual sources. But again, each
observer should know what is better for him. What matters is the result.


>My opinion: it really would help visual observing if v (visual) magnitudes
>were tabled for a typical (30' +) darkadapted vision in stead of V
>(photopic). Extreme darkadaptation seems to render vision closer to B than
>V....

.... averted vision.... closer to B.
I don't think that standardizing out of V will help...


>>Mosters, he? What is that?

Sorry !!! It was "monsters" but with a lot of respect, people !!

Wait for more comments.

Best regards,
Sebastian.




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