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

[Michael Newberry]

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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