Are the Default 16-bit FITS Files Stretched?

Discussions of Electronically Assisted Astronomy using the Live Stacking feature.
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Lee_Dodge
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Are the Default 16-bit FITS Files Stretched?

Post by Lee_Dodge » Fri Jul 12, 2019 9:07 pm

I am using SharpCap 3.2 with a ZWO ASI1600mm Pro camera, and the software seems to be working great when taking Live Stack data. I saved the LRGB images to the default 16-bit FITS file. I now think that was a mistake, since I want to combine the LRGB data in PixInsight, and the files stored in the 16-bit format are rescaled (or stretched) according to SharpCap manual statements below. It appears that I should be using the Raw (32-bit) Stack instead. Is that correct? Is there any information in my stored (16-bit) that would allow me to "un-scale" the scaled data? (The 16-bit files seem to be combining OK, and maybe I already had a few saturated stars that were already at the 65535 level, which prevented the rescaling?)

From the SharpCap manual:
"• Save as 16 Bit Stack will rescale the stacked data linearly between the 0 and the maximum pixel value into the range 0 to 65535 and save this as a 16-bit FITS file. The 16-bit FITS option is the default as it gives a high bit depth image with the full range of the image used (i.e. brightest pixel is 65535).

• Save as Raw (32-bit) Stack will save the full 32-bit stack data without any scaling as a 32-bit FITS file. The maximum value in it will depend on the number of frames stacked, meaning more adjustments will be needed when viewing (without further adjustment this FITS file will probably appear black when opened into a FITS viewing application)."

SharpCap is a pleasure to use. Thanks for its development.

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admin
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Re: Are the Default 16-bit FITS Files Stretched?

Post by admin » Sun Jul 14, 2019 8:27 am

Hi,

If you use the save as 16-bit option at the top of the list in the life stacking then the stretching colour balances are not applied, all that happens for that save option is that the data is re-scaled to fit in the 16-bit range.

If you use the save with adjustments option lower down the list then you get an image that does have the adjustments applied such as stretch and colour balance.

Hope this helps, Robin

Lee_Dodge
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Joined: Fri Jul 12, 2019 8:39 pm

Re: Are the Default 16-bit FITS Files Stretched?

Post by Lee_Dodge » Sun Jul 14, 2019 4:09 pm

I am taking four sets of images through the following four filters: luminance, red, green, and blue. I would call each of the resulting images monochromatic, or just gray-scale images. I want to turn them into color images by combining them outside of SharpCap. If I store them as the 16-bit default option on the top of the list for storage options for Live Stack images, and they are re-scaled to fit the 16-bit range, then it seems that I do not have a good way to combine them into an LRGB image, since the relative scaling factors for the different colors have been lost. Is that an accurate interpretation?

Thanks,
Lee

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Re: Are the Default 16-bit FITS Files Stretched?

Post by admin » Mon Jul 15, 2019 9:25 pm

Hi,

In theory you are correct, but I suspect that in practice the scaling will be the same for all three channels because there will be some stars that will be saturated in all three channels in each subframe, meaning that the peak value used for the rescaling to 16 bit will be tied to the saturation value of those stars.

Cheers, Robin

Lee_Dodge
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Re: Are the Default 16-bit FITS Files Stretched?

Post by Lee_Dodge » Mon Jul 15, 2019 11:30 pm

Thanks for the help, Robin.

Lee

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