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

[arne at aavso.org]

On Thu, Feb 16, 2006 at 06:52:16PM -0500, Roland Roberts wrote:
> 
> arne said:
> > Roland Roberts wrote:
> [...]
> >> Why does one "need" to be clear?
> >>
> > There are times when you want to collect as many photons as possible.
> > There are more of those times than times where you need Rc measures.
> 
> I'm sorty for being slow, but if I'm doing photometry, when would I "need" a
> clear filter?  Getting more photons is well and good, but if I can't assign
> them to a color band, I've got a completely uncalibrated result.
> 
> What am I missing?
> 
Sorry to be so distant.  If you look at the CBA site:
http://cba.phys.columbia.edu
you will see that there are many projects where filtered photometry
is not absolutely essential.  Just finding periods, checking for
outbursts, etc. are examples.  It is always nice to do filtered
work when possible, but sometimes the systems are just too faint
for your particular telescope.  I usually suggest that if it is
too faint, you work on brighter systems.  This is not always
possible.  Even with the 1.0m telescope at USNO, I often do
time series on a 20th magnitude object; impossible with filters.
Sure, I should be using a 4m telescope and filters, but you
can't get the access on a bigger telescope.  So you will find
more occasion to go "unfiltered" than you will for using an
Rc filter.  That is why "clear" is the 4th slot.  Be careful
with the unfiltered option and understand its limitations.
Arne