[Aavso-photometry] Filters to minimize differential extinctioneffects

[RICHARD MILES]

Tom,

Further to Arne's comments, I would add that the type of CCD camera may 
matter when working unfiltered.

CCD response will be significantly different if you use a Sony chip (more 
blue-sensitive) compared to Kodak, Marconi, etc.  My feeling is that 
unfiltered photometry is best if the variable star has a colour index close 
to the 'mid-response' wavelength of your camera.  You can find this value in 
say B-V terms by imaging a range of red-blue pairs.  You will find working 
unfiltered that if you plot the transformation coefficient against the mean 
colour of the red-blue pair, its value changes sign around this point.

Here's an interesting study I did fairly recently:
Using three cameras each equipped with a Sony chip (SXV-H9 camera), I 
measured extinction using sets of Hipparcos stars selected to be in two 
different colour ranges (differing in colour on average by 0.80 mags).  For 
each extinction coefficient I measured between 4 and 10 Hipparcos stars at 
different altitudes up to X=4.  This was done simultaneously using a V 
filter, Ic filter and Unfiltered (cl) on each of six different nights.  This 
is what I obtained.

Red stars (0.80<(B-V)<0.85)
Night          k(i) k(cl) k(v)
Night 1:  0.075 0.183 0.204
Night 2:  0.108 0.221 0.253
Night 3:  0.117 0.254 0.276
Night 4:  0.121 0.243 0.298
Night 5:  0.151 0.251 0.269
Night 6:  0.158 0.303 0.339
Mean:    0.122 0.243 0.273

Blue stars (0.01<(B-V)<0.04)
k(i) k(cl) k(v)
Night 1:  0.081 0.220 0.205
Night 2:  0.121 0.265 0.251
Night 3:  0.139 0.291 0.268
Night 4:  0.146 0.316 0.320
Night 5:  0.139 0.302 0.274
Night 6:  0.162 0.372 0.369
Mean:    0.131 0.294 0.281

So from this you can see that stars having a B-V colour of close to +0.3 
will appear to dim at about the same rate when measured unfiltered as do 
stars measured with a V filter!  This is close to the mid-response of the 
CCD chip which is about +0.5 in B-V terms derived using red-blue pairs.

If stars have a 1.0 mag difference in B-V colour, then you can expect them 
to change by an enormous 0.06 mags for each airmass when measured 
unfiltered.  So you can see how critical it is matching the colour of the 
comparisons star to that of the variable.

I am not sure when working unfiltered whether a CCD camera fitted with a 
Sony chip has any advantage for measuring CVs (which are relatively blue in 
colour) when compared to a more red-sensitive chip.  If you follow the CV 
over a wide range of airmass will the Sony camera be less prone to 
differential extinction effects?  I suspect not as this second-order effect 
is >90% contributed by the properties of the atmosphere and not the sensor.

Hope you get on O.K. with the filters.

Cheers,
Richard Miles

----- Original Message ----- 
From: "Tom Krajci" <tom_krajci at tularosa.net>
To: <aavso-photometry at mira.aavso.org>
Sent: Thursday, June 26, 2008 2:57 PM
Subject: [Aavso-photometry] Filters to minimize differential 
extinctioneffects


> I'm slowly learning about filtered photometry, but for years my bread
> and butter work has been unfiltered.
>
> I know that my unfiltered data may capture lots of photons, compared to
> ye olde V filter bandpass (or other Johnson-Cousins filter bandpass):
> http://www.astrodon.com/custom/_2e2a/content/files/SchulerUVBRIYel.jpg
>
> But I can have significant problems with differential extinction for
> comp and target stars with poor color match.
>
> Is there a filter type (inexpensive, easy to obtain) that provides a
> larger bandpass than Johnson-Cousins, that is considered scientifically
> valid and useful in the amateur science/photometry community?  I want to
> try and strike a balance between gathering lots of photons, and reducing
> differential extinction problems.  (Some sort of minus-blue filter 
> perhaps?)
>
> Thanks in advance,
> Tom Krajci
> Cloudcroft, NM
>
> -- 
> -------------------------------------------
> Tom Krajci
> Cloudcroft, New Mexico
> http://picasaweb.google.com/tom.krajci
>
> http://overton2.tamu.edu/aset/krajci/
>
> Center for Backyard Astrophysics (CBA)
> http://cbastro.org/ CBA New Mexico
>
> American Association of Variable Star
> Observers (AAVSO): KTC http://www.aavso.org/
> -------------------------------------------
>
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