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Canon 5D MkII photometry

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ewiley's picture
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Embarrassing as it is, I am just now gearing up for some photometry. Has anyone determined the transforms on a Canon 5D Mark II? I am assuming it will be different than my 450D. I will be working with a Canon 100mm Macro and have a Kenko Sky Memo which allows tracking of 5 minutes without guiding. So--I am thinking I need to characterize this camera using Landolt fields. If skies during the Okie-Tex star party cooperate I should be able to get several nights worth of data. Any suggestions would be appreciated.


Hi Ed,
Sorry about the long reply (turns out commenting on your post was disabled, I've fixed that though).  With the method we've outlined in the tutorials you determine all of the coefficients (transformation, color, and offset) on a per-image or per-stacked-image basis.  These actually change depending on several conditions including the camera temperature and the moon phase!  
To get a good idea of what they "should" be snap an image of a field that has a wide range of color and magnitudes of stars near the zenith without the moon in the sky.  The coefficients often hover around these values, but do change.  For instance Tom Pearson had an image where the moon was nearby and the extinction coefficient adapted to be a "scattered moonlight" coefficient removing the extra brightness the aperture photometry method didn't catch.  His end results were right in line with PEP values even though the extinction coefficient was positive!
I found the coefficients changed dramatically from some images I took here in Denver (heavy light pollution, warm temps, 5280 ft elevation) to those I took up at Mt. Evans (almost no light pollution, very cold, 14,128 ft).
Hope this helps,
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Roger Pieri
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Hi Brian, Ed,

Here-after I attached an analysis of the atmosphere reddening in function of the extinction depending from both the extinction coefficient of the day and the observation height. As you can see the variability is huge ! Much higher than any difference between DSLR... This confirms the value of the proposed method.

I am somewhat surprised by the impact of the moon light, normally such background is subtracted from the foreground aperture and should not impact the result if properly done. But sometime the stratification of the atmosphere is such the extinction gradient goes the opposite way or in various directions, this happens often near the Zenith, above 70 deg height (moon or not... )

Clear Skies,


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ewiley's picture
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Many thanks for the responses. On the AIP4Win Photomety Yahoo site I met Marl Blackford from New Zealand. He and several others from NZ are very active in DSLR photometry. Check out articles in various issues of the newsletter, link here: Of special interest are two: Feb and March 2011. Also, they have a detailed DSLR-friendly map for Eta Aql which I will try next new moon at the Okie-Tex. This inspired me to see if I can paste together AAVSO maps to some of the other targets. Ed
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Here is a graph that map my transform coefficient against different airmass during the epsilon Aurigae campaign, This is for a Canon 450D and the RGB-channels 2x2 binned. As you can see there is a bit spreading but the trend is clearly visible. Regarding moon shine, I have not noticed any special change in the Tc because of the moon.

Thomas Karlsson

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Looks like these trends are very linear, that's quite good for DSLRs.  I've only seen this odd moon effect in one data set so I'm not convinced it is a common happening. 

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