[Aavso-photometry] Differential photometry for dummies

[Michael Newberry]

I totally agree with Michael. And I would add one more critical point:

Do not allow resampling of the pixel values for the combined image. If you 
do, then the magnitude error estimates will be wrong, Not noly does 
resampling decrease the noise by square root of N image that are combined, 
but it also will smooth or otherwise corrupt the noise structure of the 
combined image. The result is that the magnitude uncertainties determined 
from the noise in the image will be underestimated. For that reason, we 
included in Mira a "nearest neighbor" option for the pixel resampling. The 
software computes and applies the transformation to register the images but 
then combines the "whole" pixel values from the image set that are nearest 
each pixel location. In other words, there is no splitting of pixel values. 
The result is that the gross adjustments are applied but there remains about 
a 0.3 pixel "jitter" in the pixels that are combined at each location. For 
photometry, this inflates the FWHM by a quarter of a pixel or so, on 
average, across the frame. But the benefit is that you perfectly preserve 
the ransdom noise structure of the images you combined, hence your 
photometric errors are good.

Michael Newberry

----- Original Message ----- 
From: "Michael Koppelman" <lolife at bitstream.net>
To: "R. Brian Potter" <potterrb at comcast.net>
Cc: <aavso-photometry at aavso.org>
Sent: Sunday, June 17, 2007 9:44 AM
Subject: Re: [Aavso-photometry] Differential photometry for dummies


> Most people would say you should not align or combine the images.
> Like Tom said, just do photometry on the individual images and then
> combine those mathematically. The exception is if the object is so
> dim that useful measures are not obtained from individual images. In
> that case, you could align and combine, but you should make sure your
> alignment routine is "flux conserving".
>
> M.
>
>
> On Jun 16, 2007, at 11:33 PM, R. Brian Potter wrote:
>
>> 3) Align individual images after dark subtraction
>> 4) Median combine images to create a single processed image of the
>> field
>
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