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ASIAIR master flats not correctly imported


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Hi

Recently experimenting with using Affinity Photo for my astro photo processing. Loving the macros (thanks) and thought I'd give stacking a go too to try and keep everything together in one application (Affinity).

Previously, I've successfully stacked in DSS, APP and most recently, ASI's own DeepSkyStacking as part of their 'studio' application on Mac OSX. These all work well and provide very similar results +/- a bit.

In Affinity though, I can happily add BIAS, DARK and LIGHT frames and these app appear as normal. However, my FLAT files all come in at 100% green. By this, I mean that if I create a file group, select "flat" as the group type and add my (master precompiled flat) it results in the first attached image (Stack Green.jpg). This results in the stacking failing to produce an image suitable for further processing.

If I exit the Astrophotography persona, and load the flat file normally, it displays the same (Load Green.jpg). In both cases you can see the histogram pinned to the right, and the INFO inspector showing 100% green. I do note though that on the normal load, this is auto-stretched, if I turn this off things look a little better. (Flat Green.jpg). Does this mean that the flat file is being auto-stretched when being loaded via the Astrophotography persona?

Additionally, the histogram (for unstretched) shows quite the spread of R,G,B values. However, if I load the flat file in ASI's own viewer, I see this unstretched (Fits View.jpg). This both appears to be closer to what I would expect, but also shows a more colour neutral histogram.

I have attached all of the image examples noted above, as well as the master flat that I was testing with.

I welcome any suggestions, rectifications or other - and am more than happy/willing to try/test/tinker or even beta-test if required.

Thanks in advance

Greg

Stack Green.jpg

Load Green.jpg

Flat Green.jpg

Fits View.jpg

f_EXTREME.fit

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8 hours ago, talktogreg said:

If I exit the Astrophotography persona, and load the flat file normally, it displays the same (Load Green.jpg). In both cases you can see the histogram pinned to the right, and the INFO inspector showing 100% green. I do note though that on the normal load, this is auto-stretched, if I turn this off things look a little better. (Flat Green.jpg). Does this mean that the flat file is being auto-stretched when being loaded via the Astrophotography persona?

Additionally, the histogram (for unstretched) shows quite the spread of R,G,B values. However, if I load the flat file in ASI's own viewer, I see this unstretched (Fits View.jpg). This both appears to be closer to what I would expect, but also shows a more colour neutral histogram.

Hi @talktogreg, thanks for posting and hope you're finding the macros useful.

There are two things Affinity Photo does that will account for the result on-screen looking stretched:

  • Internally, within the Astrophotography Stack persona, it adds Curves and Levels nodes to the invisible layer 'stack' for the FIT file preview. This is just to help boost the tones so you can inspect the data.
  • A non-linear gamma transform is performed during the view presentation. Everything is composited internally in 32-bit float linear, but by default the view is colour managed (based on your display profile) with a gamma transform so that you're seeing the result you would get when exporting to a gamma-encoded format such as 8-bit JPEG, 16-bit TIFF etc.

The actual data itself is always treated linearly. To test this, you could try the following:

  • Load your f_EXTREME.fit file up individually, and you will see the Levels and Curves layers added. Turn these off.
  • Go to View>Studio>32-bit Preview to bring out the 32-bit Preview panel if it isn't already active. Switch from ICC Display Transform to Unmanaged: you are now seeing the pixel values in linear light with no colour management. The result should hopefully be in-line with what you're seeing with other software.

Be aware, however, that you should keep ICC Display Transform on. This ensures that your view when compositing in 32-bit linear will match the exported result when you finally save out a JPEG or other interchange format.

It's also worth noting that the histogram will always display the linear values, even though the view presentation is being managed with the non-linear transform. We should perhaps have a toggle for this...

Apologies for the slight technical ramble, but it's difficult to clarify without venturing into linear/non-linear and colour management discussions.

I think the bottom line is that your data files are being treated as linear internally, so there shouldn't be anything to worry about. Hopefully your master flat should just calibrate as expected with the light frames and everything just works!

Hope that helps,
James

Product Expert (Affinity Photo) & Product Expert Team Leader

@JamesR_Affinity for tutorial sneak peeks and more
Official Affinity Photo tutorials

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Hi James,

Thankyou for you time and detailed explanations. I don't mind technical ramble (I'm an Engineer and I do this to people all of the time LOL).

Between what you outlined above, and the very recent Live stream you published - I found my error. I'm falling on my sword here on the slim chance it helps anyone else.

What I did *wrong*, was put each of the calibration (master) files, as well as the light frames, in seperate file groups!!
Didn't dawn on me at the time that the file groups were for different filters, or exposure times, or gains etc - where you'd put all the light and calibration files that matched each other together.

Affinity gave me no warning that I'd done something odd, but not its fault really. Maybe a dialog to indicate when a file group has no light frames would be useful, as that would be a pointless file group and probably indicate a user (me) error?

Also thanks for detailing what the Astro Persona is doing re: previewing the calibration files. I still do find it a bit distracting/misleading that the flat frame is shown 100% green - but in the end, I measure by the results and given that I'm now doing the right things, results are what I'm getting - and very quickly too.

Kind Regards
Greg

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