[Aavso-photometry] [AAVSO-DIS] Software comparison

[Wolfgang Renz]

It might be even simpler.

Tom didn't state anything about the actually used radii of the star
aperture and the sky annulus. If they are choosen differently it
will also cause a mag offset even in the same software package.

Clear skies
 Wolfgang

-- 
Wolfgang Renz, Karlsruhe, Germany
Rz.BAV = WRe.vsnet = RWG.AAVSO


Michael Newberry wrote:
> I can comment more on the Mira side of this thread later today.
> But for now, let me say that an average magnitude *difference*
> for two data sets results from two things:
> 1) how the background is measured and
> 2) how the partial pixels are handled.
> As a first guess at the cause, Tom should be sure he is using the
> same background method (mean, median, mode) in the two
> software packages he is comparing.
> 
> Michael Newberry


Bill Dillon wrote:
> Actually, I'm impressed they only differ by ~0.01 mag (my random
> errors are often bigger than that, depending on the target brightness,
> exposure time, etc.).
>
> You could do a test with an artificial star, but the key is how the
> software handles noise: background stars, merged stars, hot pixels,
> partial pixels in circular apertures, etc.  Perhaps better would be to
> take a field with good comps and, one by one, pretend the comps
> are a variable -- see how close your estimates are to the given value,
> using the same image and different software packages.
>
> If you want algorithm transparency, then maybe IRAF is a way to go.
>
> --Bill


Peter Nelson wrote:
> I seem to recall a post somewhere about some software that could
> produce an artificial field with known magnitudes.
> If this existed, could we then test reduction software against this?
>
> Regards
> Peter


Michael Linnolt wrote:
> Not surprised at all! I had expressed concerns about the CCD
> software packages before, specifically you really dont know
> exactly what they are doing "inside". What precise algorithms,
> and how they are implemented in each package is something
> known to the design and coding teams alone. This stuff is not
> usually revealed to end users, and "reverse engineering" it is
> out of the question too. So, trying to figure out the source of this
> .01 mag difference is going to be hard.
> 
> Mike


Tom Richards wrote:
> Hi all
> 
> I just completed the first photometry run on a backlog of GW Lib data.
> I used MaxIm on 944 B-filter observations from April 13 and found a
> fascinating plot with some intriguing scatter around some of the 2.09
> h peaks and a plunge right on meridian flip (phooey). Worried that
> these might be instrumental, I redid all observations in Mira, which
> gives far more data that might help the question of spuriousness.
> Comparing the results I found
> (a) all the worrisome points were still there in Mira and related data
> such as SNR was quite normal,
> (b) Mira gave consistently fainter results by 0.0096 mag, and the
> standard deviation of that difference was 0.00316 mag. 
> The original data has according to Mira an error of ~0.0015 mag,
> though I'd put it higher.
> 
> I'd be interested if anyone else has done similar comparisons between
> packages.
> 
> Tom Richards
> Woodridge Observatory,  CBA Melbourne
> <http://www.woodridgeobsy.org/>
> 
> Carpe noctem!