[vsnet-chat 1353] Magnitude estimates from unfiltered CCD images

[Gianluca Masi <gianmasi@fr.flashnet.it>]

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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