[vsnet-chat 705] Re: Comparison sequences

[W S G Walker <astroman@voyager.co.nz>]

Greetings,

I've been away for a while and one of my computers (with most of the 
discussions about this topic) is off the air at the moment. I might be 
one of the amateurs that Brian mentions and I still get some joy out of 
doing conventional photometry - but will be also able to use an ST6B with 
Bessell's BVRI filters from about mid-March onward. My interest is that 
the Photometry Section of the RASNZ might be able to help in some cases.

All sky photometry is not difficult even from cities. We've done this in 
Auckland on good nights and manage about 1% to 2% results on the brighter 
stars (V = <9) in the E Regions using the Cape values. I've also tried 
the same at Kaitaia (right in the north of New Zealand) where the weather
is much better - particularly this year with El Nino producing 
semi-drought conditions and clear skies.

Many of the values of southern comparison stars for the VSS were measured 
at Auckland by Barry Menzies and his group. I've done a few but mainly in 
CV fields. We also tried comparing pe and visual measures and concluded 
that there was a lot of subjectivity in the latter. More recently I have 
been analysing visual observations of Mira stars and note quite a spread 
from observer to observer. Since all these people are doing their best 
one presumes that there is quite a difference in individual eye response 
- or is it the telescopes? We concluded in Auckland at the time - and the 
more recent work on Mira stars supports this - that visual measaures are 
ideal for determining maxima and minima of longer period stars - or CV 
outbursts - but in other areas some people are trying to get too much out 
of them. I have also tried to get epochs of some 7 day cepheids but the 
results were +/-10% of the cycle lengths which didn't help much. Which 
poses the question - is there any value in getting better magnitudes for 
visual comparisons than +/- 0.1 to 0.2 magnitudes? The colours of 
comparison stars are also important at low altitudes - which seems to be 
ingnorerd by the VS people.

I'm not quite sure what is needed. Taichi Kato quotes values around 
magnitude 13 to 15 which is really only accessible with CCD equipment and 
a reasonable telescope. If these are meant to be comparisons for faint 
variables then it's a simple matter to take four to nine interlocking 
frames to cover an area of say half a degree centred on the variable. A 
standard star can then be calibrated using a nearby E Region. Rudolf 
Novac's Munidos programme would allow quick reductions of the images. 
Calibration using the Cape sequences would not be hard. I imagine that 
we're looking at 5% photometry at the fainter limits. If you're only 
preparing VS sequences then why is all sky photometry involved? 

A project which I have in mind and will try to encourage some CCD people 
to adopt is aimed at getting better information about the periods of 
southern Mira and semi-regular stars. To study the alternate 
periodicities of these stars maxima need to be determined to about 1-2%, 
but the visual data seems limited to about 2-4% of the period. This is 
ideal for all sky photometry, preferably using B and V filters. The ST6 
is quite blue sensitive (the ST7 and ST8 are not so good at that end) 
which allows good calibration and measures Miras where the amplitude is 
greatest. Along the way this will obviously find some good faint 
comparisons, even if it is not the intention to take too much notice of 
the minima of these stars.

We could probably help with stars from 0 to 30S if anyone is interested. 

Regards,
Stan Walker


Brian Skiff wrote:
>   Brian Skiff wrote:

>      There is a lot of work to be done.  Much of the problem could be
> solved by even a few amateurs willing to do "all-sky" photometry, but it
> seems the number able/willing to do this is inexplicably close to zero.

  Not to disagree with you, Brian, but I'm a professional astronomer,
and the number of times during which I was

            a. willing to do all-sky photometry
      AND
            b. able to do all-sky photometry

has, in my career so far, turned out to be a total of about 3 nights.

  At my current position, in Rochester, NY, the number of photometric
nights per year is probably about 10, if that :-(  I suspect that the
same is true -- if not more so -- for most amateurs.  So even if the
spirit is willing, the sky may not...

  It's just a LOT easier to do differential photometry, especially for
those of us stuck far from Arizona.

                        Michael Richmond




>      The USNO-A1.0 comparison looks promising for temporary use, but I do
> worry about the occasional "ringers" that are 0.3-0.5 mag. in error compared
> to the Henden photometry.
>      Michael Richmond will chime in here I'm sure about the TASS data,
> but at present they are running with V and I filters (a good combination
> from an astrophysical standpoint), but may add others in future systems.
> They've already got reasonably good photometry for over 100,000 stars along
> the Equator down to about V mag. 12-13.
> 
> \Brian

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