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

Thu Dec 15 16:18:25 EST 2005

Brian,

I think you've missed the spirit of the discussion. Please don't take any of 
this personally---I don't think it has been anyone's intention to hurt 
anyone's feelings. We are trying to do science here, so objectivity and 
criticism is the name of the game---with respect and kindness, of course. 
Speaking for myself only, I think it has been a pretty good discussion that 
has gotten some people to start thinking about the details.

I speak both from theory and practical experience. If I tell you about the 
limitations of a data rejection method used in photometry, then that is 
information that I would think you would want to know. Please defend it if 
you think I am wrong. And if you have criticism about algorhtms we use in 
Mira, please tell me. I may give a response that we already looked into that 
and it isn't an issue, or I may say that it a valid point, or an interesting 
idea to try. We are all bettered by learning from what others have tried and 
from those people who have found better ways of doing things or have found 
problems with other solutions. We all have our trade secrets that allow us 
to do something well or to make a living. But I think there's a broad area 
in which everyone wins if we share ideas and criticism, don't you?

Regards,

Michael Newberry

----- Original Message ----- 
From: "Brian D. Warner" <brian at minorplanetobserver.com>
To: "AAVSO Discussion group" <aavso-discussion at aavso.org>; 
<aavso-photometry at mira.aavso.org>
Sent: Thursday, December 15, 2005 1:32 PM
Subject: [AAVSO-DIS] Software for variable star work

> 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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>>
>>
>>
>
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