[vsnet-chat 204] Re: V versus v Re to vsnet-chat 181

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

Roland BONINSEGNA wrote:
>
>>The discrepancies between v and V don't matter even for visual estimates of
>short
>period variable stars. A determination of the time of a minimum, made on an
>eclipsing star, from visual estimates, will be as valuable as a
>photoelectric one
>(especially if no photoelectric measures has been done!). The choice of correct
>comparison stars is the most important. The Hipparcos and Tycho catalogues will
>help a lot.

I think the degree of accuracy needed of the timing comes into this. I've
spent some time determining epochs of maximum of Cepheids from visual and pe
measures. To look for period changes in these you need to determine the
maximum to +/- 0.01 days or better. Even 500 visual measures of a short
period cepheid during an observing season cannot achieve this. But there are
many cepheids with periods over 20 days which are not observed with pe and
where a determination of 0.1 days would be acceptable. This is a good but
neglected field for visual observers. The amplitudes are also better - from
1 to 1.5 magnitudes. Cepheids have some phase jitter, so seasonal maxima are
usually more reliable than measuring a single peak.

I'm not sure how this affects eclipsing binaries and visual timings of
minima. Most I have studied would not be suited to visual measures. The CV
ones mentioned are usually ~0.5 deep and about 5-10 minutes in duration.
Thus visual measures have about 5 magnitude steps and 10-20 time steps. Very
poor resolution - even without the scatter in the estimates. And we're
looking for such small changes in period that ET (or whatever it's called
now) corrections of <100s are signficant.

Wouldn't the lack of colour and extinction corrections distort the light
curve? Most of the deep eclipses are of blue stars where extinction is
significant. They'd also last long enough for the star to move through a
significant air-mass. When all these effects are considered, do they make
the errors in the determination of minima by visual observers greater than
the expected period change?

Regards,
Stan Walker

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