[vsnet-chat 1031] Re: Some remarks

[Taichi Kato <tkato>]

Timo Kinnunen wrote:

> The problems are: extinction (which I believe is very colour
> sensitive), twilight, and everchanging atmospheric conditions.

   Though I believe more experienced photometrists have comment on the
"secondary color term", from my experience with CCDs, they are mostly
negligible when we are discussing to a level of 0.1 mag.  At Ouda Observatory,
the typical extiction coefficient in V is around 0.4 mag/airmass, in B around
0.7, in I around 0.2.  Assuming a "broad" BV-band (B+V band), a simple
calculation with B-V=0 and B-V=1.0 at airmass=1 (elevation=30 deg) yields
a secondary (differential) color term of an order of ~0.06 mag/(color
difference in B-V)/airmass.  The eye is not so broadly sensitive, so the
effect may be usually negligible.  In CCD and photoelectric photometry, the
effect usually should be considered, when a broad band filter (even more
for a clear filter) is used, and when a large change in elevation occurs
during observation.  On that occasion of the memorarable superoutburst of
UZ Boo, I persuited until the field was obscured by the trees.  The comparison
stars being moderately red, the effect of this differential color extinction
in the V band amounted nearly 0.1 mag.  On such extreme conditions, this
effect should be properly taken into consideration when analyzing (without
this, I could not even detect superhumps).

   Your observations of RX And near solar conjuction were indeed extremely
precious.  Considering the subsequent drop from the standstill, I presume
there may indeed have been temporal fadings as you reported.  Challenging
observations, and also challenging to theorists!

Regards,
Taichi Kato

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