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Flat frame exposure trouble

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#1 Binsky3434

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Posted 07 December 2018 - 03:29 AM

Hello, I use a small refractor SV70t with a canon 60Da camera as well as a 55mm-250mm zoom lens with a canon 450d xsi. I use the t shirt and I pad method that as worked fine in the past but for some reason I am gettin some issues. The issue is that when I get the max pixel count and start making my exposures shorter and shorter till the mean pixel count is half that of the max I start decreasing the max before I get near the mean. I tried changing the brightness on the iPad and double wrapping the t shirt but didn’t have much luck. I guess a good question is how important is it to get the max pixel count to the maximum of the cameras? Because I can get a lower pixel count for the max and be able the get the mean half of that. Or is it more important to the the max correct and the mean off or the ratio correct between the max and mean? Or is there a compleatly different way/ method I don’t know about or should try? Any help would be awesome. Clear sky’s :)

#2 jgraham

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Posted 07 December 2018 - 09:04 AM

I have not tried this method, but you want to make sure that all of your flat is within the linear range of your sensor. Modern sensors tend to be linear over about 90% of their dynamic range. To be safe I keep the brightest part of my flats no higher than 80% of saturation; below about 52k.I also avoid taking flats with a bright source to avoid odd reflections inside the tube.

Food for thought.

#3 bill5wjw

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Posted 07 December 2018 - 03:44 PM

I thought pixel count was an issue for CCD cameras. For my DSLR, getting flats to the proper exposure is just a matter of setting the camera to aperature mode-let the camera do the work.

 

Bill



#4 whwang

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Posted 07 December 2018 - 11:05 PM

CCD or CMOS becomes nonlinear when the count exceeds about 50% of the full well potential.  This is why we want to keep the counts lower than 50% saturation everywhere in a flat image.

 

However, on DSLRs, because we often use high ISOs (400 to 3200).  The linear counts we read from the raw images were further amplified by the readout system.  Even if you read a saturated count from a pixel (16383, if the sensor has a 14 bit output), it's far from real saturation.  For example, a count of 16000 at ISO 400 is equivalent to 4000 at ISO 100.  Although 16000 at ISO 400 looks very close to saturation (16383), it is actually very safe.

 

So, unless you are using an extremely low ISO (which is almost never recommended for astrophotography), all you need to do is to make sure every pixel is below the saturation value.  You don't want to get too close to 16383, since there can be fluctuation in exposure between your flat frames.  If you get extremely close to 16383 in your first test shot and use the same exposure in all subsequent shots, there is a chance that some pixels exceed 16383 in those subsequent exposures.

 

Cheers,

Wei-Hao




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