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

From: Joe Patterson <*jop_at_astro.columbia.edu*> Date: Mon, 08 Mar 2010 09:24:15 -0500 · jop_at_astro.columbia.edu

BTW this is the reason why a column denoting airmass is useful to add in 
the time-series photometry.  It allows me to remove differential 
extinction to first order.  Not as good as a narrow-band filter, but a 
significant asset.

joe

Joe Patterson wrote:
> 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
>>
>>
>>
> 
> 