[vsnet-chat 5861] Re: GSC vs Tyc2

["Stan Walker" <astroman@xtra.co.nz>]

Greetings,

I spent considerable time recently measuring the fields of V4742 Sgr and AE
Ara. These measures were based upon CCD photometry using V and R filters and
standards from the E Regions with appropriate system transformations. I got
the whole system working to my satisfaction (errors of less than 1%) and
will use this again once all the holiday visitors have departed. But one
thing I noticed was that values derived in this manner were quite different
from those of V and V-R derived using the transformations from Bt and Vt
mentioned by people such as Aaron Price.

The field of V4742 might not have been a fair test as it is heavily reddened
in parts. But there were similar discrepancies in Ara as well. I get my Bt
and Vt magnitudes from Bill Gray's Guide 6 so later versions might have
different values - I don't know whether these are Tyc2 or not. But the
colours and V magnitude were often 0.05 out and occasionally 0.1 different
from my values using calibrated photometry. The range in question was from
around 8.8 to 10.5. My saturation level was about 7.8 so this should have
been a good test.

My conclusion was that the Tycho values were good enough for visual
observing but of no value at all for any precision CCD or pe photometry.
This wasn't unexpected as the Tycho system has the defects of the old
photovisual and photographic magnitude systems. A third bandpass in the U
region might have helped resolve some of the problems.
Basically, if you are trying to determine V, V-R or V-I there is no
substitude for standard stars measured with these filters.

I notice that a number of people use CCDs with a V filter and publish (and
presumably submit these somewhere) without transformation. The V filter I
use has a 7.5% slope against V-R and I presume that others are similar. So
values determined without transformation corrections are probably 3-10% out.
Add to this the uncertainties introduced by using poor standards and it's
doubtrful that there is any value in the measures. Bad observations are
extremely frustrating and any method which encourages these is not good.

As an extra test I checked 11 stars in E Region 7 and 10 in E Region 2. The
first is a somewhat reddened field, the second is away from the Milky Way a
bit. Errors in E7 values determined from Bt and Vt were fairly similar to
V4742 Sgr. But the E2 region was much better. The U-B colours could not be
matched - the SD was 0.226 and 0.139 but the other colours, as expected,
were not bad. The V SD in E7 over 11 stars was 0.073, in E2 0.018.  The
errors were larger with stars around magnitudes 9 and 10 as was suggested.
V-I also showed larger errors than the central colours. Maybe some people
are happy with this standard of photometry but it doesn't appeal to me much.

Regards,
Stan


----- Original Message -----
From: "Taichi Kato" <tkato@kusastro.kyoto-u.ac.jp>
To: <vsnet-chat@ooruri.kusastro.kyoto-u.ac.jp>
Sent: Tuesday, January 07, 2003 5:21 PM
Subject: [vsnet-chat 5855] Re: GSC vs Tyc2


> Re: [vsnet-chat 5847] GSC vs Tyc2
>
> > Its response wasn't exactly the standard
> > shape for the B, V passbands, but the people who created Tycho-2
> > worked hard to transform the measurements to the standard
> > system.
>
>    One comment.  This sentence might sound that one can transform (any)
> non-standard passbands to standard bands with *hard work*, but this it
> not always true.  However hard one might try, there may be cases in which
> acceptable conversions are impossible.  When one tries to use non-standard
> passbands, one should have to design the bands carefully to enable such
> a conversion.
>
> Regards,
> Taichi Kato
>

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