[Aavso-photometry] ASAS182612 Campaign & CIs of Comparisons

[arne]

The standardized photometry for the ASAS182612 comparison stars is
online with the new VSP (enter ASAS182612 and click find charts; then
click on the "field photometry" box at the bottom to get the full
BVRI dataset).  For instance, the 109 star is there with V=10.892
and (B-V) = 0.665.

Regarding saturation - you can certainly saturate a star in the field
that is not the target nor a comparison star.  Many fields, especially
faint objects, will have bright stars that you just have to ignore.
Saying that, you do need to be a little careful.  Non-ABG chips will
"bloom", so if you saturate a star that is in the same column as the
target or a comp, you can contaminate the important star.  Most
front-illuminated CCDs will also show ghost images (called residual
bulk image) if you saturate a star while observing unfiltered or with
a red filter, and the ghost image will hang around for several subsequent
images.  If you changed fields, or did a GEM flip, the residual image
might affect an important star.  So be careful, think ahead, but don't
be afraid of saturating a field star.
Arne

-------------------
R. Brian Potter wrote:
>  
> Just another quick question from a CCD newbie...
> 
> Can I let a bright star in my FOV saturate in order to get a better signal
> on my target, comp & check stars?
> 
Greg Crawford wrote:
> I am attempting to arrive at standardised measurements of ASAS182612, as per the
> campaign advised in Alert Notice 351.
> 
>  
> 
> Using the general formula:
> 
> M = m-X(k'+ k"CI) + (Tm*CI) + ZP
> 
> where Tm = transform for the single colour (not colour index), and
> 
>            CI = standard colour index of the object.
> 
>  
> 
> and the reduction technique in my software, it is necessary to know the CIs of
> the comparisons in order to work out the CIs of the target. So with transform
> coefficients, first order extinction and nightly zero points now worked out, it
> seems I need CIs for the comparisons, which will lead to CIs for the target, and
> thus the ability to convert m to M. 
> 
>  
> 
> Alert Notice 351 gave colour indices for one comparison star, namely the comp at
> 18:26:09.44 +12:17:37.2 (no. 119 in the AAVSO star chart). My questions are,
> should I use just this one comparison star, or use all four provided in the
> chart (numbered 109, 119, 133, 134)?  If the latter; where do I get the colour
> indices for the other three comparison stars from?