[Aavso-photometry] Differential photometry for dummies

[R. Brian Potter]

So I totally understand that I should not combine images, but rather analyze
the individual images after subtracting a master dark created from at least
7+ images.

My questions remaining are
1) How do I measure signal-to-noise (specifically in CCDSoft if someone is
doing this)
2) How do I provide an error estimate for my results? (Std deviation on
multiple measurements, 1/SNR, ???)
3) The CCD Observing Manual states that the observed K-C should be equal to
the known K-C.  Using the 9.8 mag star in the field for R Uma as my comp
star gives a value of 8.04 for my check star, vs a known value of 8.2.  How
close is close enough? If its not exactly equal to the known value, do I
make an adjustment to the magnitude of my target star to account for this?

Sorry for all the questions, but I want to make sure I do things correctly
for my first CCD submission.

Thanks,
Brian

-----Original Message-----
From: Michael Newberry [mailto:mnewberry at mirametrics.com] 
Sent: Sunday, June 17, 2007 12:58 PM
To: Michael Koppelman; R. Brian Potter; aavso-photometry at aavso.org
Subject: Re: [Aavso-photometry] Differential photometry for dummies

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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