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PixInsight - Master flat file creation


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Hi,

I havd Canon 450d, and I've created 40 x 600 second darks at ISO800, 180 x Bias frames at ISO800 and have 120 x flats at ISO800.

I've been following the Master Calibration tutorial by Vincent Peris from the PixInsight web site and run in a an issue when calibrating the flat files.

For each flat processed I get the following message, (for some files I may also get this message for channel 1 and 2).

** Warning: No correlation between the master dark and target frames (channel 0).

Does anyone know how I can stop this from happening?

Thanks

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It is just a warning that the data in the master dark frame is not well matched to the light frame, which in your case is a flat.  What was the exposure time of the flats?  They are normally very short, so you would not expect the master dark to match.  So this is quite normal, it fact there is no need to use the master dark for calibrating the flat.

If you see the same message when you are calibrating your 600sec lights to your 600sec darks then you should be more worried because you would expect a good match.

Mark

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Hi,

I havd Canon 450d, and I've created 40 x 600 second darks at ISO800, 180 x Bias frames at ISO800 and have 120 x flats at ISO800.

I've been following the Master Calibration tutorial by Vincent Peris from the PixInsight web site and run in a an issue when calibrating the flat files.

For each flat processed I get the following message, (for some files I may also get this message for channel 1 and 2).

** Warning: No correlation between the master dark and target frames (channel 0).

Does anyone know how I can stop this from happening?

Thanks

Did you load up any Bias ? I wonder why you are using darks to calibrate the flats with?  This is normally done by either a master Bias or at times a master dark Flat if the Flat sub frame exposure is long enough to induce noise.

A.G

Edited by lensman57
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I followed the instructions on the tutorial

Can you kindly post a link to this tutorial please?

Regards,

A.G

PS: if there is an option to optimise dark frames then do not enable it, might just get rid of the error message.

Edited by lensman57
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I will not go to court with this but I am guessing that the Flats in the tutorials are " Sky Flats " with long exposure durations therefore the need of Dark subtraction as this contains both the Bias and the Dark current noise due to long exposure. With  light panel Flat framing with typically very short exposures, 0.025s~ 1s a master Bias made up of a large number of Bias frames will effectively calibrate the Flat Frames without introducing noise in the Master Flat.

Give it a try and see what you get.

Regards,

A.G

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My flats were 1/30th of a second.  So I should not be too worried about this message then and carry on following the tutorial to create my master flat?

Effectively it is taking a dark for a 600s shot and scaling it down to 1/30 of a second shot (i.e. scaling the dark noise down by 18000). As the scaling factor is so close to zero it is declaring thtat it cannot apply the dark to the flat.

As other people have said, do not worry about it, just make sure that you are using bias frames to take into account the read noise (which should be constant regardless of the exposure time)

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Ah cool, thanks for the advice guys.

Effectively it is taking a dark for a 600s shot and scaling it down to 1/30 of a second shot (i.e. scaling the dark noise down by 18000). As the scaling factor is so close to zero it is declaring thtat it cannot apply the dark to the flat.

As other people have said, do not worry about it, just make sure that you are using bias frames to take into account the read noise (which should be constant regardless of the exposure time)

Oh right, so yes that makes sense regarding the dark frame scaling (optimisation) - Thanks

All good advice above, no need to apply dark frames to your flats, just use bias instead.

I also have a quick how to on my site for an easy alternative tutorial http://cloudedout.squarespace.com/blog/2014/3/25/creating-a-master-flat-image-in-pixinsight

Thanks, I'll follow this when I get home later to create my master flat :)

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PS: if there is an option to optimise dark frames then do not enable it, might just get rid of the error message.

Don't disable optimisation unless you have a very good reason to do so. The warning message is telling you that using the dark frame results in a noisier image than not using it at all. If you see the warning that means the dark hasn't been applied to the image, which is what you want to happen in this case.

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Don't disable optimisation unless you have a very good reason to do so. The warning message is telling you that using the dark frame results in a noisier image than not using it at all. If you see the warning that means the dark hasn't been applied to the image, which is what you want to happen in this case.

Thank you Ian, sorry for the late reply BTW. I was thinking of my own CCD which is a Sony ICX and therefore no Darks or optimisation are needed. Thanks for pointing this out to me.

Regards,

A.G

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