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72894Re: [ccd-newastro] Re: Calculating average brightness in an area of a fits image

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  • Pete Su
    Jun 2, 2013
    • 0 Attachment
      I would think that if the imbalance is mostly from LP gradients then
      rebalancing each picture would be the right way to go.

      If your sky is like my sky, the gradients are very different in different
      areas.

      Pete


      On Sun, Jun 2, 2013 at 3:17 AM, Mark Striebeck <mark.striebeck@...>wrote:

      > After spending many weeks focusing on my mount and guiding accuracy, I
      > could finally take some images.
      >
      > eXcalibrator gave me the following values:
      >
      > Green: 1.662
      > Blue: 1.970
      > Red: 1
      >
      > Which sounds extreme.
      >
      > The last 2 nights, I took images of M51:
      > Lum: 10x10min (binned 1x1)
      > RGB: 10x7.5min (binned 2x2)
      >
      > When I combined these images with the same weights, I got this image:
      >
      > https://lh6.googleusercontent.com/cS88vNQCDLaXKJhogYIuB0KEo0CMReLIjTfJnrzVekIw=w737-h590-no
      >
      > When I used the weights from above, I get this image:
      >
      > https://lh6.googleusercontent.com/E749SZJEL3JepyckXufBrDiQX-if9elWPdE-1NMBKSHc=w737-h590-no
      >
      > Seems as if eXcalibrator was pretty accurate after all! It just shows how
      > much it sucks to live in such a light polluted area :-(
      >
      > If you are interested in the individual images and histograms, check
      >
      > http://mstriebeck-astrophotography.blogspot.com/2013/06/measuring-white-balance.html
      >
      > So, now where I know these values, is it better to adjust the exposure
      > times accordingly? Or to keep the individual exposures the same and apply
      > these weights when combining the images?
      >
      > MarkS
      >
      >
      > On Sun, Apr 14, 2013 at 8:54 AM, Mark Striebeck <mark.striebeck@...
      > >wrote:
      >
      > > Hi Bob,
      > >
      > > On Sun, Apr 14, 2013 at 1:07 AM, Bob Franke <bfranke@...>
      > wrote:
      > >
      > >> **
      > >>
      > >>
      > >> Hi Mark,
      > >>
      > >> With a (b-v) magnitude of 0.676, 20LMi is a good star for color
      > >> calibration.
      > >>
      > > Thanks - I didn't have the time yet to figure out how to do the
      > > measurement with CCDStack.
      > >
      > >>
      > >> M44 is covered by the Sloan Digital Sky Survey. eXcalibrator should get
      > >> excellent results with the SDSS-DR7 or DR9 stars. However, the numbers
      > you
      > >> stated are a bit extreme. What camera and filters are you using?
      > >>
      > >
      > > I'm using an H694 from Starlight Xpress and Baader filters. I am imaging
      > > from a very light polluted area, so I expected that red will be quite
      > > dominant. Just not as much as it did.
      > >
      > > The last couple of nights, I imaged the Leo triple and wanted to give it
      > > another try with XCalibrator (just downloaded the 4.01 version). But
      > every
      > > time I use one of the images as for the WCS file, I get a "function
      > > loadFitsKeyWords failed for WCS File." error. I took the images with the
      > > camera add-on from TheSkyX, i.e. the WCS headers should be filled in.
      > >
      > > Thanks
      > > MarkS
      > >
      > >>
      > >> Cheers,
      > >>
      > >> Bob
      > >>
      > >> --- In ccd-newastro@yahoogroups.com, Mark Striebeck <mark.striebeck@
      > ...>
      > >> wrote:
      > >> >
      > >> > Thanks for the reply Bob,
      > >> >
      > >> > I have this particular star from this page
      > >> > http://starizona.com/acb/ccd/advtheorycolor.aspx (and I've seen this
      > >> same
      > >> > table in a couple of other places). Is there somewhere a better list
      > for
      > >> > suitable stars?
      > >> >
      > >> > To be honest: I tried XCalibrator for this a few days ago with an
      > image
      > >> of
      > >> > M44. But the values that I got out were pretty extreme (I got a factor
      > >> of
      > >> > more then 2 for both green and blue). So, I wanted to do a manual
      > >> > verification.
      > >> >
      > >> > MarkS
      > >> >
      > >> >
      > >> > On Tue, Apr 9, 2013 at 11:15 AM, Bob Franke <bfranke@...> wrote:
      > >> >
      > >> > > **
      > >>
      > >> > >
      > >> > >
      > >> > > Hello Mark,
      > >> > >
      > >> > > You are on the right track, you just need proper data and tools.
      > When
      > >> you
      > >> > > said G3 star, I assume that was a typo. You need to use G2V stars.
      > >> However,
      > >> > > all G2V stars are not equal. Some are slightly yellow and others
      > >> slightly
      > >> > > blue. Use stars that have a (b-v) magnitude close to 0.65.
      > >> > >
      > >> > > Here are a couple of the freeware options for determining the color
      > >> ratios
      > >> > > for your image train.
      > >> > >
      > >> > > Christian Buil's IRIS program as an aperture photometry tool. This
      > is
      > >> > > excellent for comparing star flux to determine color balance. It
      > gets
      > >> the
      > >> > > same results as Source Extractor.
      > >> > >
      > >> > > Then, of course, there's my freeware eXcalibrator. eXcalibrator can
      > >> use
      > >> > > the Sloan Digital Sky Survey database to identify white stars in an
      > >> image.
      > >> > > The program then uses Source Extractor to measure the star flux in
      > >> the red,
      > >> > > green and blue images. Finally, eXcalibrator presents RGB color
      > >> ratios to
      > >> > > make the identified stars white.
      > >> > >
      > >> > > Here's a webpage to determine if a specific area of the sky is
      > >> covered by
      > >> > > the Sloan survey.
      > >> > > http://www.sdss3.org/dr9/index.php#coverage
      > >> > >
      > >> > > Here's a link to my eXcalibrator page
      > >> > > http://bf-astro.com/eXcalibrator/excalibrator.htm
      > >> > >
      > >> > > Hope this helps,
      > >> > > Bob
      > >> > >
      > >> > >
      > >> > > --- In ccd-newastro@yahoogroups.com, Mark Striebeck
      > <mark.striebeck@>
      > >> > > wrote:
      > >> > > >
      > >> > > > Hi,
      > >> > > >
      > >> > > > To determine the white balance of my setup, I took a red, green
      > and
      > >> blue
      > >> > > > image of 20LMi (a G3 star). But I am struggling to find a way to
      > >> measure
      > >> > > > the average brightness in the star. The Fits Liberator gives me
      > the
      > >> mean
      > >> > > > for the entire image, but not for a smaller area. Anybody knows a
      > >> way to
      > >> > > > get this?
      > >> > > >
      > >> > > > Thanks
      > >> > > > MarkS
      > >> > > >
      > >> > > >
      > >> > > > [Non-text portions of this message have been removed]
      > >> > > >
      > >> > >
      > >> > >
      > >> > >
      > >> >
      > >> >
      > >> > [Non-text portions of this message have been removed]
      > >> >
      > >>
      > >>
      > >>
      > >
      > >
      >
      >
      > [Non-text portions of this message have been removed]
      >
      >
      >
      > ------------------------------------
      >
      >
      >
      >
      >


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