[Aavso-photometry] newbie filter question

[arne]

Ian Bacon wrote:
> Hello,
> 
> I have been pestering people on the SBIG discussion group with a few  
> questions, and was directed here for follow up questions.
> 
> By way of background, I have been carrying out visual variable  
> observing for years (BIX), but have now decided to go ccd. My scope  
> is a 10" LX200GPS. I plan on buying a SBIG ST-7, a 10 position filter  
> wheel, a F5 focal reducer (NextGen), and Optec focuser.
> 
> My specific question is this, what are the best filters to buy for  
> variable use? I would like to get a complete set (UBVR, maybe even  
> I). I realise that in the beginning I will not use this full set, but  
> I plan to expand my observing as my experience grows.
> 
Always get V - that passband has the highest priority.  The next
two filters should be B and Ic, as they give both the widest
color indices as well as the widest spectral response.  Rc is
in the next tier - most of the time, V and Ic give you most of
the information you would get from an Rc measurement, but since
the CCD has greater effeciency around Rc, and some projects (such
as cyclotron emission from CVs) can effectively use Rc, it should
be considered.  I'd put U-band as your lowest priority.  The
CCD is less sensitive at U, the focal reducer may not work well
at U, the LX200 will have fairly low transmission a U, the
atmosphere has the highest extinction at U, and there are fewer
stars that you will be able to measure.  It doesn't hurt to have a
U filter, just if you are having to decide, Ic is probably a better choice.

Most filter vendors are ok, though if you have the funds, I'd go
with Custom Scientific (or the equivalent SBIG).  If funding is
tight, the Astrodon/Schuler filter set is perfectly adequate, just
slightly lower in quality (no AR coating, not quite as flat, etc.).

Good luck!  Keep us posted on your progress and raise any questions
you might have.
Arne