[vsnet-chat 4172] Re: SAO 109441

[Fraser Farrell <fraser@trilobytes.com.au>]

On Wed, 4 Apr 2001, Toni Scarmato wrote:
>What is your comment? It is possible to find variable star
>in the images of LASCO C3?

In principle it is possible to find variable stars in the
LASCO C3...

   ** but ** 

it is using an unfiltered CCD camera; so it will detect both
visible and near-infrared light. For the LASCO C3 this is
not a problem, because its primary tasks are to detect
objects near the Sun (such as suicidal comets), and to
detect eruptions of gas leaving the Sun (such as the one
that hit us last weekend). It was never intended to do
photometry; so adding a V filter or an infrared-cutoff
filter would just decrease the LASCO C3 sensitivity for no
good reason.

Because the Unfiltered CCD Sky looks so different from the
Visible Light Sky (and the V Band Sky), you cannot compare
magnitudes from an unfiltered CCD image with the V (or
visual) magnitudes listed in a catalogue. They are not the
same thing. 

To do believable CCD photometry you need images taken
through standard filters; or at least through a filter that
blocks infrared. Otherwise your "magnitudes" are going to be
something-resembling-R-magnitude-with-I-&-V-mixed-in. And
you may get unnecessarily excited by Very Red Stars or Stars
Within Dust Shells.

Unfiltered CCD images are useful when we don't need to know
a magnitude; such as detecting supernovae, timing eclipsing
variables, measuring precise positions of variables.  They
can also be used for comparison if you have a series of
images. For example, the LASCO C3 images clearly show that
Delta Sco was brighter than usual during November 2000 when
compared to Novembers 1999, 1998 and 1997. But I won't try
to derive a visual magnitude from these images.

What you need to do is examine LASCO's images of your
starfield from previous years. This may show you if your
"difference" is real variability or not. But don't quote
magnitudes to 3 decimal places until you have done
photometry through the appropriate standard filters.
Otherwise you're going to embarrass yourself.

And I'm sure the CCD gurus on this list have much more
advice for you.


cheers,
Fraser Farrell

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