[Aavso-photometry] Filters

[arne]

Thomas Rutherford wrote:
> Gentlemen:
> 
> I have a few questions that are a little more fundamental than most of 
> the postings that I have been reading on this list so please bear with me.
> 
> How does a modern "V" filter for CCD work compare to a "V" filter used 
> for photometry back in the 60's and 70's?  I ask this because an old 
> photometry filter is what I have.  While researching this question, I am 
> finding contradictory answers.  Some sources indicate that the old 
> filters do not stop the red and IR end of the spectrum since the old 
> photometers were not very sensitive at those wavelengths.  Other sources 
> indicate that for a "V" filter, this doesn't matter-- the old filters 
> are the equivalent of the newer ones.  Also, the camera has an 
> IR-blocking filter already-- should I keep this in place or remove it?   
> In addition, my filter is yellowish, while I always see V filters 
> referred to as being green.  Should I order a new "V" filter or use the 
> old one?
> Also, if I were to add one additional filter (a modern one), what should 
> it be?  I am using a Meade DSI camera and a Meade ETX 70 telescope (a 
> refractor).  This is for a high school astronomy class project that I 
> started last year and those are the items that we have to work with, 
> although I know now that they are not the best choices and if I were 
> starting over I would select different equipment (I do like the wide 
> field of the ETX, though).
> 
I have no evidence that anyone ever answered you. :-)

V filters for PEP and CCD are quite different.  The typical PEP V-band
filter is formed from 2mm of GG14, which looks yellow to the human eye.
A CCD V-band filter is made from 2mm of GG495 (blue edge) plus
3mm BG39 (red edge), which looks green to the human eye.  The difference
between these two filters is due to the QE response of the detector.
A photomultiplier tube has a *very* blue response (which is why it
does so well at U-band), and so you need a "redder" filter to shape
the transmitted light into a visual-like response.  In addition, I have
not looked at the wavelength response of GG14, but would not be
surprised to find that it has a red leak.

I would order a new V-band filter.  Some V filters are fairly inexpensive
(Schuler/Astrodon is about $60 for example) and work well.  For a second
filter, I'd suggest a B-band filter for several reasons.  First, the CCD used
in the DSI is a Sony HAD, which has excellent blue response.  Second,
most refractors are reasonably well corrected at B,V,Rc but poorly corrected
at Ic.  Finally, with a small telescope, you can get reasonable
magnitudes for your comparison stars from the Tycho2 catalog values,
which are essentially B and V.

Arne