(cba:news) when not to love your filters...

Joe Patterson jop at astro.columbia.edu
Thu Nov 12 23:53:05 EST 2009

Hi Terry et al.,

This is a good opportunity to comment on filters... though it's somewhat 
specialized to TT Ari.

Because of the star's extremely strange and unprecedented state - with 
cool secondary and hot WD dominating in the non-flaring phases - it 
really is true that multicolor data can be of enormous help in decoding 
the relative contributions to the light.  (And this is rare: most CVs, 
i.e. their disks, are so broadband that there is seldom much advantage 
in using filters.)  However, this presupposes that the multicolor data 
has high statistical accuracy... and there is practically no chance of 
achieving that with small telescopes on a flickering 16th magnitude star.

So one lesson is a general one: improve S/N by observing clear (or 
roughly clear; a little blue block can help with differential-extinction
issues).  This is almost always optimum for period-finding issues... and
shortening exposures to ~60 s will help with time-resolution and 
possibly data quality (if your drive is problematic).

But here's an important consideration that cuts the other way.  This is 
a landmark event in the history of humanity's acquaintance with TT Ari. 
  The light curves are something I've never even dreamt of; and here 
they are, rolling off our little telescopes.  Since TT Ari is the 
brightest CV in the sky, that's nothing to sneeze at.  So our 
observations have solid archival value - except for unfiltered data, 
which doesn't, since the bandpass is not defined.  Personally I tend to 
prefer data of high statistical quality (unfiltered), since that helps 
me get science from it in the short run; but I could imagine filtered 
data being equally or more productive for other studies down the road.

For brighter stars and/or bigger scopes, V has much to commend it, 
because it is excellent (ideal) for archival purposes, has low 
extinction, and you can often get decent S/N.  (Of course, if our stars 
weren't rapid variables, we could always get a desired S/N by just 
observing longer... but we don't have that luxury!)

Sorry for going on so long... but in my opinion, 300 sec is too long. 
When the star flares, significant changes take place in 300 sec.
Anything over about 120 s is difficult to recommend for CVs, unless you 
know - as we never do - that no rapid state changes are occurring.

Fortunately, some CBAers ignore my advice and observe in V.  That 
greatly helps in calibration.  If everyone observed unfiltered, our
calibration would be really pathetic.

For people who have been observing this star for a while now, I don't 
recommend changes in filters or comp stars.  I've measured the offsets 
between different observers; and while they change slightly (and 
somewhat inexplicably) from night to night, they are of great use in 
splicing light curves.  If you do change, announce it prominently in the 


Terry Bohlsen wrote:
> I took 2 hours of R filter data last night from the east of Australia before 
> it clouded in.
> Hopefully this will help a bit.
> What exposures are OK?
> I tried 120 secs and 180 secs but the SN is still a bit low. Is a 300sec 
> exposure too long?
> Cheers
> Terry Bohlsen
> Armidale NSW
> Australia

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