Re: Blue-blocking filter useful for some CBA targets?

From: Joe Patterson <*jop_at_astro.columbia.edu*> Date: Mon, 08 Mar 2010 08:56:08 -0500 · jop_at_astro.columbia.edu

Here's my take on this blue-blocking filter issue.  Until 6-10 years 
ago, the affordable CCDs were somewhat blue-blind - hence not a problem. 
  Today, it's an issue.

1. The reason we like unfiltered observations is count rate.  For most 
of our stars and projects - mainly period-finding - this is best.

2. However, compared to an average field star (say a K), CVs, which are 
all blue, will show a differential extinction of ~0.06 mag/airmass.  So 
long time series, which may extend from 2.0 to 1.0 to 2.0 airmasses, 
will show an artificial curvature of about this amount (0.06 mag).  For 
the very bluest of CVs, this is more like 0.09 airmasses.  UGH.  This 
introduces false periodic signals of 1.0, 2.0, 3.0, and even 4.0 cycles 
per sidereal day.

3. It's BLUE/UV light which primarily causes this, because CVs are blue 
and extinction increases sharply in the blue (hence the color of the 
sky).  So a "minus blue" filter helps with this problem.  If you have 
such a filter and it has good throughput, try it out.  Especially if you 
take a long AM CVn time series - that's the perfect star to try this on, 
because it's *really* blue.

joe

Jerry Foote wrote:
> Tom & Lew,
> 
> This is a filter that Bruce Gary uses extensively when working exoplanets.
> In the second edition of his book "Exoplanet Observing for Amateurs" he
> discusses its use.
> 
> 
> Jerry Foote
> Center for Backyard Astrophysics-Utah
> jfoote@scopecraft.com
> 
> 
> 