[vsnet-chat 5212] Red stars estimates

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

There is a lot ongoing about "deep minima" for several carbon stars.

W Orionis is being reported incredibly dim while I observe it as 6.5 - 6.6
magnitude, almost the same as in the last cycle.
R Leporis, that one we have discussed it the other day, I didn't see a very
deep minimum (fainter than 11.0) but a normal 9.5 magnitude.
UY Centauri is said to be at 10th magnitude. The star has been fainter than
ever (at least I didn't find any record in the literature) but it is near
8.5 now. (After fading to 9.0 - 9.4 last year)

I uploaded my lightcurve of W Ori for comparison.
http://ar.geocities.com/varsao/Curva_W_Ori.htm

There are charts for UY Cen and W Ori in my web page:
http://ar.geocities.com/varsao/Carta_W_Ori.htm
http://ar.geocities.com/varsao/Carta_UY_Cen.htm

This topic is also being discussed in AAVSO discussion group. The other day
I wrote something that may be of interest for vsnet readers:

"Always try the lower magnification to observe red stars because the
estimate
will be closer to the standard system this way.
The fact is that when you add power, you darken the sky. The darker the sky,
the fainter the red star will appear.
The way we observe a red star is dependent on its own brightness and on the
brightness of the sky.
While observing with the naked eye, you'll be a victim of the Purkinje
effect if you look straight at the star and stare.
That's because the stars are bright and the sky too ! You are working with
the brightest sky possible for a given place and time if you observe with
the naked eye. As soon as you use an instrument, this changes completely and
the red star starts losing its crown. It's like travelling from a polluted
city to a country site...;-) Country observers often do fainter estimates of
red stars than city victims...
Cones, the cells that perceive the colours, need a relatively strog
stimullus to work and when the star is too faint, it happens to almost
disappear beacuse is not bright enough to activate the cones.
That's why we observe fainter blue stars at our limitting magnitude (even 1
or 2 magnitude fainter than the red ones). It's beacuse the rods (peripheral
vision) are working and they are color-blind.

So when you observe red stars with the naked eye you shouldn't use direct
vision (the star would appear TOO BRIGHT).
But when you are observing with a telescope and specially at high
magnifications, try to stare at the star as much as possible and don't use
peripheral vision to compare it with other color stars (or the star would
appear TOO FAINT)."


As V has been adopted as the standard system it is important to try to stick
to it.
There's no way to turn "v" magnitudes into V because the results depend on
HOW you observe the star. They depend on the power you use and the type of
vision you use. So, "v to V" conversions would suffer the same scatter and
problems as raw v data if we don't take all these things in account.

Try this:
Observe three different red CONSTANT stars twice, one time with direct
vision and the other with peripheral vision. Make an estimate using whiter
stars as comparisons.

1) A bright 3rd magnitude NAKED EYE star (pi Puppis, epsilon Lep, bet Ara,
and the like)
2) A 6th magnitude BINOCULAR star.
3) A red star near the limit of your TELESCOPE.

Compare the results obtained with direct vision with those obtained with
averted vision.
CHECK these results against PEP(V) values. Please use reliable sources as
the GCPD.(http://obswww.unige.ch/gcpd/indexform.html)

Make this kind of experiment to try to stick to V. Otherwise all the efforts
for standardization will be useless.

Regards,
Sebastian.



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