[Aavso-photometry] [AAVSO-DIS] Software for variable star work

[Brian D. Warner]

Cross posting to photometry group

Hi Mike,

We can go tit-for-tat forever on points and I'm sure you believe that Mira is
better (probably perfect) in all regards. At some point, I'm sure list members
will grow weary of "anything you can do I can do better."

FWIW, I'm using the simpler Howell formula.

I've conducted tests regarding the changing size of the aperture and do recommend
using smaller sky annulus but also that one take into consideration the density
of the surrounding star field. Using too few pixels can be just as bad as using
too many, since it borders on undersampling - especially if the annulus has
several stars. I don't claim perfection, not by any means, but the program
produces consistent results on the order of 0.01m and better. For those in search
of consistent precision on the order of a couple millimags, go for it. We'll all
benefit to one degree or another but for the vast majority of cases, 0.01m is
certainly an acceptable level of precision.

Your algorithms may take more factors into consideration but there is a point of
diminishing returns and including every factor known to man may look impressive
and provide bragging rights but just how much does it really affect the final
result, especially when one often "pounds" a target with a plethora of data? No
doubt, you will say that even the most miniscule possibility matters, and it
does - in that wonderland called "theory." Therein lies the rub. Sometimes it's
possible to get so wrapped up in theory that practical results suffer. Yes, yes,
I know the arguments about understanding the source of errors, etc. and one
should make that effort but not to the detriment of providing highly usable data
in pursuit of perfection. As the saying goes, "Better is the enemy of good
enough."

We've both provided essential background on our programs and I'll be glad to
discuss any questions off-line with anyone who asks. If my program can be
improved such that the effort required justifies the gains, I'll be the first to
admit it and try to do so as time allows.

In the meantime, I don't want to get into a prolonged discussion that is nothing
more than thinly veiled commercial advertising involving subjective claims.
That's what our web sites are for. Let each reader decide after looking at those
or posing off-line questions which program best suits his needs and budget.



Clear Skies,
Brian D. Warner
Palmer Divide Observatory (716)
http://www.MinorPlanetObserver.com

Collaborative Asteroid Lightcurve Link
http://www.MinorPlanetObserver.com/astlc/default.htm



----- Original Message ----- 
From: "Michael Newberry" <mnewberry at mirametrics.com>
To: "AAVSO Discussion group" <aavso-discussion at aavso.org>
Sent: Thursday, December 15, 2005 12:24
Subject: Re: [AAVSO-DIS] Software for variable star work


> Hi Brian,
>
> I have embedded some followup reposnses to some of the items you mention.
>
> ----- Original Message ----- 
> From: "Brian D. Warner" <brian at minorplanetobserver.com>
> To: "AAVSO Discussion group" <aavso-discussion at aavso.org>
> Sent: Thursday, December 15, 2005 1:18 AM
> Subject: [AAVSO-DIS] Software for variable star work
>
>
> > Since it was the mention of my software that got this thread going and
> > both Mike
> > and Richard have commented, I thought I would throw in my nickel's worth
> > to the
> > discussion.
> >
> > Regarding background subtraction,
> [ with a star contaminating the sky annulus...]
> > Using Mike's standard of how the instrumental magnitude is
> > affected, I measured several stars where another star that was about 0.5m
> > brighter was and was not included in the sky annulus. The difference in IM
> > of the
> > target was 0.001-0.003m. It was not until I included a star with an IM
> > about 1.2m
> > brighter that the target IM changed by 0.012m.
> >
>
>      The larger the number of pixels
> inside the sky annulus, the better can be the rejection of contaminating
> star(s) and thus the lower the magnitude bias in your star's magnitude. This
> makes the appearance of increasing the accuracy of the magnitude. However,
> there is another type of systematic error that plagues photometry. Remember:
> you want to estimate the background underneath the object, not underneath
> somewhere else, and the background is neither "perfectly" flat nor free of a
> diffuse background and lumpy distribution of fainter things. The more pixels
> in the sky annulus, the further away it samples pixels. That tends to reduce
> accuracy of the background correction by measuring it less locally to the
> star in question. These are competing issues---which magnitude bias is
> worse...
>
> > Canopus has used ensemble photometry - in the sense of multiple
> > comparisons -
> > since it was first written by allowing up to five comparisons to be
> > averaged for
> > a single reference. For now, it's a simple average, with no weighting,
> [...]
>
>     Of all the programs available, Mira has maximum versatility in this
> regard---always has. It handles any number of target objects and any number
> of standards (with or without ensemble weighting), in any number of images.
> When doing time series (or otherwise measuring an image set), the numbers of
> objects and standards do not have to be the same for each image.
>
> [...]
>
> > As for errors, Canopus handles only one of the general types mentioned by
> > Mike.
> > Specifically, errors are computed based on SNR converted to magnitudes
> > using
> > Howell's formula and adding the errors of each comp in quadrature as well
> > as that
> > of the target. The complete formula is provided in two of the output file
> > headers
> > so that there is no doubt how the error is computed.
>
>     Which "Howell's formula" are you talking about? Is it the sigma(m) =
> 1.0857 / S/N or the complete magnitude error formula I published in
> Astronomical Journal, January 1991?
>
> Regards,
>
> Michael Newberry
>
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