RASC Hamilton In camera darks

[Andy Blanchard]

I get it, we are talking about two different things. Terry and Alan are referring to an immediate dark in camera is more closely related to the prior image. Not to be confused with the actual math. 

Most in camera darks are instantly taken by the camera before the chip has had sufficient time to cool down. About 5 seconds. This creates a hot chip getting hotter. So if your taking a 5 minute exposure, and instantly start a dark the chip is still warm and continues to get hotter, creating a false dark that does not truly reflect the image you are trying to calibrate. 

When a dark is taken, there is a picture taken of the noise the camera produces for a given temperature and air pressure. Yes air pressure. Here is where things get complicated. The camera needs to be able dark subtract a raw image within its internal processor at the original bit rate. I have yet to find a camera processor that can do this math. All camera processing so far, is done at a bit rat far below the cameras optimal bit rate. Basically throwing away good and necessary data.  

So unless you can control the delay on in camera darks and the processing rate on the dark subtraction. In camera processing is a very bad thing for astrophotography. 

There is one caveat to this, if you shoot in Monochrome raw the camera won't toss out any data.  


Have a great day

Andy Blanchard
289-337-6766H
905-847-7777B
905-330-5626C

> On Nov 30, 2013, at 8:38 AM, "Mark Kaye" <mark.kaye at sympatico.ca> wrote:
> 
>> On 13-11-30 8:28, David Galbraith wrote:
>> There's compression and then there's compression. "lossless compression" changes file size without altering pixel-by-pixel raster data. Lossy compression (like JPG) alters the bitmap data to save file space. Some cameras can store data straight to TIFF files. No loss.
> 
>    I do not declare to be an expert on digital imaging.  I can only guess that the way a dark raw is compressed and data frames are compressed is enough different that an external dark subtraction is not quite as good as an internal.  I am only the messenger here.  I concede that doing your own dark subtraction is attractive given that we have limited actual dark sky time and that doing internal darks cuts one's imaging time in half.
>    It could also be possibly because the dark frame done in the camera is done at pretty much exactly the same conditions, while a dark library is slightly different.  As originally stated, both Terry and Alan conclude that in camera dark subtraction is marginally better.  They do have the experience in this sort of thing to be taken seriously.  But imaging a moving object like a comet, where you have a very limited window for imaging, you are going to be better served by using a dark library.
> -- 
>    Clear Skies!
>                MK
> 
> http://www3.sympatico.ca/mark.kaye/index.htm
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