[Aavso-photometry] Web tool for mag error vs aperture radius

[Jim Jones]

Michael

It was the SNR Calculator that I was playing with not the Mag Error vs 
Aperture Radius.

Jim Jones

Michael Newberry wrote:
> Hi Jim,
>
> I just saw your message. Thanks for trying the tool. The SNR and the Mag
> Error are functions of many variables, so it is possible for this to happen.
> It is not so simple that increasing the focal length automatically increases
> or decreases the SNR---it depends on the detailed parameters and where you
> start with the measuring radius. You changed just the focal length but did
> you also increase the measuring aperture to compensate for the change in
> image scale? Look at a curve for mag error versus radius and look to the
> right of the minimum error where the curve is rising to the right. Now
> decrease the measuring radius---see how the mag error drops? Decreasing the
> radius (in pixels) is akin to increasing the focal length without changing
> the radius in pixels. Of course, I am ignoring the small change in dark
> current and sky brightness under the star that occur when you change focal
> length.
>
> Michael
>
> ----- Original Message ----- 
> From: "Jim Jones" <nt7t at centurytel.net>
> To: "AAVSO Photometry" <aavso-photometry at mira.aavso.org>
> Sent: Sunday, August 10, 2008 12:11 PM
> Subject: Re: [Aavso-photometry] Web tool for mag error vs aperture radius
>
>
>   
>> Michael
>>
>> You have some very nice "tool" on your web site.  Thanks for putting
>> them up.
>>
>> I spent far more time than I should have fooling around with your SNR
>> calculator. According to the calculator, increasing the focal length
>> (with no other changes) results in an increase in SNR.  This seems
>> contrary to a number of authors, some of whom make  very strong and
>> definite statements that this isn't the case.
>>
>> Jim Jones
>>
>> Michael Newberry wrote:
>>     
>>> I've added a web tool for calculating (and graphing) the magnitude error 
>>> and SNR as a function of aperture radius. The graph helps you see the 
>>> effect of different instrumental parameters and observing scenarios and 
>>> how to choose an "optimal" aperture. You can see how all the various 
>>> parameters fold into this (for example, how does binning affect things?). 
>>> The optimal radius is a function of all the parameters, including the 
>>> star's magnitude. So the question arises about what to do for stars of 
>>> different magnitude in the same field. The best strategy, in my opinion 
>>> (and the one I've always used) is to choose the aperture size that works 
>>> for the target stars, even if it is not optimal for the brighter standard 
>>> stars. Then use multiple standards to beat down their errors, when 
>>> possible.
>>>
>>> You can use this tool by going to http://www.mirametrics.com. Select "Web 
>>> Tools" from the left menu and then select "aperture photometry optimizer" 
>>> from the list.
>>>
>>> Michael Newberry
>>> _______________________________________________
>>>
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>>>
>>>
>>>
>>>       
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>>     
>
>
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