[vsnet-chat 4175] R: SAO 109441

["Toni Scarmato" <toniscarmato@hotmail.com>]

Thanks at all for your replay and excuse me to have post an image too large.


Regards,

Toni Scarmato
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----- Original Message ----- 
From: Fraser Farrell <fraser@trilobytes.com.au>
To: Toni Scarmato <toniscarmato@hotmail.com>
Cc: <vsnet-chat@kusastro.kyoto-u.ac.jp>
Sent: Wednesday, April 04, 2001 7:28 PM
Subject: Re: [vsnet-chat 4171] SAO 109441


> 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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