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517Re: [vsx-dis] Suitable filters to use

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  • arne
    Jun 10, 2013
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      On 6/9/13 9:07 AM, David Kennedy wrote:
      > Hello everyone,
      > I’m a new member to the group, joining after noticing a possible variable star
      > in my CCD images of the nebula NGC6559. Through the VSX website I’ve found
      > out the star is a suspected variable listed as NSV 10242. I’ve made
      > observations of the star on 3 nights over the past 3.5 weeks and using MaxIm DL
      > have measured that it’s decreasing in brightness. However, I’ve been
      > collecting data using the filters I use for imaging (LRGB and Ha Astronomik
      > filters) and I would like to know if useful photometric data can been gained
      > with these or do I need to use different filters instead? Can anyone help?
      > Thanks,
      > David Kennedy
      Hi David,
      NSV 10242 is an M-star, so very red. The tricolor G filter can normally be used
      for obtaining photometry that is very similar to what would be obtained using a
      true photometric Johnson V filter. However, my one concern is that some
      tricolor filters tend to have "red leaks"; that is, they permit some red light
      in the 800-1000nm range to pass through the filter. With an extreme red star
      like this one (16th magnitude at V/550nm, but 3rd magnitude at K/2200nm), any
      kind of an out-of-bandpass transmission will screw up the results. So I'd see
      if the results come out about 16th magnitude; if they are, say, 10th magnitude
      instead, then you may have such a leak.

      This is only true with the red variables, of course. Stars like eclipsing
      binaries, cataclysmic variables, or RR Lyr pulsators, can be measured using the
      tricolor filters and can be compared favorably with the standard photometric
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