[Aavso-photometry] Filters to minimize differential extinction effects

[Donn Starkey]

Tom:

I know that a lot of your observations are of cataclysmic variable [CV's].
The main source of light from these objects comes from the accretion disk,
and not the stars themselves.  Therefore, there is a lot of UV and blue
energy [hot!] generated at the point where the in-falling mass from the
secondary contacts the disk.  

"Typical" visible spectra for CV's [if there are such things] are composed
of a blue component more so than the red component.  [See references for the
LAN90 CV and the OAGH CV below]  The red component is often the H alpha line
at 6563 A.  A majority of the area under the graph is form the "blue-er"
end, however: <5000 A.  There are lots of exceptions, though, particularly
in the polar type CV's.

If you look at the graph for the J-C Filter Characteristics in the
references below, you can see that the V filter gives a reasonable coverage
of wavelengths in question.  B filter also gives good coverage, but, because
of the CCD's low sensitivity to blue, the exposure times would be brutal for
even modestly 'dim' objects.

When making observations for Joe Patterson [CBA] I normally run unfiltered
if I need to keep the exposure time < 90 sec.  However, if the star is
bright, I tend to use the R filter to reduce extinction.  I may try a V
filter after reading Arne's comments.  Choosing a comparison star of similar
color also helps with extinction and is probably more important than filter
selection.

If you use TheSky when choosing comparison stars, the relatively dim stars
from the USNO A2.0 Survey, USNOB 1.0 surveys often have information on the B
and R magnitudes of the star.  Although the values from these surveys are
not particularly precise, this data can help in choosing a comp star of
similar color to your program star.  

Donn   

Ref:

J-C filter Charisterics:
http://kereszty.csillagaszat.hu/photom/UBVRI.gif

Lan90 Spectra
http://www.noao.edu/noao/staff/lanning/uvsurvey/lan90/lan90_fig3.png

Spectrum of a cataclysmic variable observed at OAGH
http://images.google.com/imgres?imgurl=http://www.aanda.org/articles/aa/full
/2004/05/aa4089/img44.gif&imgrefurl=http://www.aanda.org/articles/aa/full/20
04/05/aa4089/aa4089.fig.html&h=350&w=490&sz=11&hl=en&start=5&um=1&tbnid=LiiM
VHlGC5LRqM:&tbnh=93&tbnw=130&prev=/images%3Fq%3D%2527cataclysmic%2Bvariable%
2522%2Bspectra%26um%3D1%26hl%3Den




==============================================
Donn Starkey
starkey73 at mchsi.com
http://www.starkey.ws
AAVSO Observer SDB  -  CBA Indiana 
VSNET Observing Team  -  MPC Code H63
============================================== 


> 
> Message: 2
> Date: Thu, 26 Jun 2008 08:57:08 -0600
> From: Tom Krajci <tom_krajci at tularosa.net>
> Subject: [Aavso-photometry] Filters to minimize differential
> 	extinction	effects
> To: aavso-photometry at mira.aavso.org
> Message-ID: <4863AE44.2020602 at tularosa.net>
> Content-Type: text/plain; charset=ISO-8859-1; format=flowed
> 
> 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
>