[vsnet-chat 1357] Re: Magnitude estimates from unfiltered CCD images

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

Greetings,

A similar problem has occurred in doing time series photometry of CVs using
unfiltered ST6 cameras. It's not too bad in some fields but the problem was
solved by getting Bruce Sumner to produce charts (or to send the USNO
details of the area) in the R version. On these the fields can readily be
identified. A great help when you're using fork mounted SC telescopes and
don't have room for a conventional field inspection eyepiece. 

The magnitudes have not much in common with V, particularly in regions
where red and blue stars dominate, such as the Magellanic Clouds or some
regions of the Milky Way. It seems rather pointless to compare them with
any sort of V magnitude sequence - I was working in a field in Sagittarius
where there appeared to be a difference of about two magnitudes between
several unfiltered stars and the V chart I was using. If the colour of the
variable stays the same and you only use the one comparison star the
problems are minimised - only a zero point correction is needed - and a
good light curve is obtained. And it doesn't affect short term
periodicities. This is the basis of the type of photometry we do.

Regards,
Stan

----------
> From: Gianluca Masi <gianmasi@fr.flashnet.it>
> To: vsnet-chat@kusastro.kyoto-u.ac.jp
> Subject: [vsnet-chat 1353] Magnitude estimates from unfiltered CCD images
> Date: Monday, October 05, 1998 5:12 AM
> 
> Dear Colleagues,
> I've been dealing with this question over the last days and finally I
> decided to submit it to this list.
> 
> During my observing sessions, I take unfiltered CCD images and then I
> calculate differential magnitudes between my target and some reference
> stars in the same field.
> Sometimes, I need to find an _estimate_ of the magnitude of the
> variable, so, considering that my CCD is mainly sensitive in the red
> part of the spectrum, I assume, for the reference stars, their R mags
> from USNO A1.0. Neglecting any problem with this catalogue magnitudes
> (their accuracy), I was wondering if it is better to consider R mags or
> V mags (from the GSC, or the same USNO A1.0) when working with sistems
> with a quite red sensitivity. The origin of this question (and this can
> sound as an off topic) is that in minor planet astrometry observers use
> mainly the GSC and their mags estimates, obtained with unfilterd CCDs
> (often with a QE peaking in the red part of the spectrum) are evaluated
> from V GSC mags. This is also true in variable star photometry, where
> quite often observers report GSC, V based magnitudes with unfiltered
> CCDs, even if their detector is more sensitive to the R band.
> 
> Do you have any comment bout this matter?
> 
> Have a very nice day/night (very bad weather here in Italy :(( )
> 
> Best Regards,
> Gianluca Masi
> 
> 
> -- 
> ***********************************************************************
> * Gianluca Masi                           "Two things fill the soul   *
> * Via Madonna de Loco, 47               with awe and reverence [...]: *
> * 03023 Ceccano (FR)                     the starry sky above me and  *
> * ITALY                                    the moral law within me"   * 
> * email: gianmasi@fr.flashnet.it                                      *
> * http://vsnet.eurolink.it/comets/                        I. Kant       *
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