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Be careful here Paul. You could be talking about two different things. Most image processing tools that work with FITs files perform two types of stretches – permanent and screen.
What you are most likely experiencing is a screen stretch. A screen stretch does not manipulate the raw data pixel values. Instead it’s just changing your screen brightness to show the details. This is great if you want to quickly analyse your data quality or blink images. A permanent stretch will actually change pixel values – typically performed by DDP, Curves (of different types such as spline or gama) and levels to name a few.
Why the hell would I want to stretch my data on the screen and not change the real pixel values? A very good reason – photometry. If you’re measuring a star’s brightness, the last thing you want to do is change the values of your pixels as all data accuracy is lost!
I don’t have intimate knowledge of AIP4WIN as I use MaximDL. However to turn off auto stretching in AIP4WIN. There is an option within the Display Control dialogue box. If you click on the stretch button, there is a check box to enable/disable auto stretch when opening a file. Note - this is only doing a screen stretch, not changing pixel values.
Personally, I’d leave it 'on' unless you have a valid reason to turn it 'off'. Having said that, I still manually screen stretch to analyse the data - it’s a matter of preference.
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