[Aavso-photometry] Alternate V filters

[Arne Henden]

BailyHill at aol.com wrote:
> Did an unfiltered run on BZ Uma and really like the added thruput.
> 
Yes.  Depending on the CCD and filter, you can get a magnitude or two
increase in throughput by going unfiltered.  By doing so, though, you
lose wavelength information, so it is not always wise.  In the case of
CVs, where they are basically A0-like and have neutral colors, you
can often get away with unfiltered photometry.

> This got me thinking that possibly an interferrence filter with high 
> transmissibility might gain a magnitude back.  It seems that V filterered costs about 
> 2 magnitudes or more.  How much would a V interference filter gain over the 
> classic Green glass type?  I know it would be expensive, but when I compare that 
> to the cost of increasing aperture for the same gain, it seems like it would 
> be very cost effective.
> 
You *can* make an interference-V, but actually a colored-glass, AR-coated
V filter has pretty good transmission.  Going interference would
pick up at most 10-20percent in throughput at significant cost.
Interference filters also typically do not have flat bandpasses (wiggles
and bumps)and can be more prone to ghosts.  Remember, the 2mag throughput
difference is primarily because the V filter is restricting the
wavelength range of the CCD, not because it lacks transmission inside
of its bandpass.
   The best way to pick up the lost photons is to build a dual-channel
system with a dichroic beam splitter, sending blue photons to one
camera and red photons to another, such as was done with the MACHO
project camera.
Arne