[Aavso-photometry] Re: [AAVSO-DIS] SBIG's New Low-Cost Photometric CCD Camera

[arne]

Vello Tabur wrote:
> arne wrote:
> 
>> Petriew, Vance wrote:
>>
>>>  
>>> Well to pleasant surprise, SBIG is now offering an internal filter 
>>> wheel with BVIC filters (as recommended by Arne).   
>>> http://www.sbig.com/sbwhtmls/402bvi.htm
>>>  
>>
>> Yes; Alan Holmes and I discussed this in several emails, as well as in
>> person at the SAS meeting last May.  I think their special ST402
>> interference BVI filters will work fine. 
> 
> 
> I was just about to purchase a set of BVR filters and see that you 
> recommend I instead or R. I'm wondering what the rationale is for this? 
> Is it because the V-I allows a more accurate color-temperature 
> determination than V-R? The advantage of R is greater S/N (due to higher 
> QE). Just curious about the trade-offs...
> 
The tradeoffs are interesting.
(B-V) and (V-Ic) are the two color indices that most professionals use.
They have about the same size, and give the most range as a function
of star temperature.  (V-Rc) is has a much smaller size (about half of
the other two indices), so transformations are less accurate.
V and Ic have completely separate passbands, so transforming to
other systems is cleaner.  Ic is less affected by dust and atmospheric
extinction than is Rc.  Ic is easier to measure for red stars.

On the other hand, Rc is near the peak QE of most CCDs, so you get more
throughput.  Rc measures Halpha, and BVRc is closer to the RGB tricolor
imaging set, so that you can make pretty pictures.  There are some
spectral features, such as the cyclotron hump in magnetic CVs, that
are better seen in Rc than other passbands.

The constraint in this case was only 4 filter holes.  One of those
holes needed to be "clear" or unfiltered, leaving only 3 filter positions.
My preferance is BVRcIc, but if you have to leave one filter out, Rc
gives the least amount of science.
Arne