Hello Harry,
There's no problem at all with your image; PixInsight opens and shows it correctly.
What happens is that your image has a limited data range stored as an integer 32-bit FITS file. Besides that, the software that created this image seems to have problems handling the entire range of 2^32 values.
Here is your image loaded in PixInsight:
Note the minimum and maximum values shown on the Statistics tool. As you probably know already, PixInsight uses a normalized real range from 0=black to 1=white for readouts. This range is used for all supported data types. This allows you to work uniformly regardless of the data type of a particular image.
These are the extreme pixel values for the red, green and blue channels in your image:
Minimum: 0.5000000192 0.5000000113 0.5000000162
Maximum: 0.5000123802 0.5000141963 0.5000152589
The actual ranges are even smaller because your image has very dark regions near the borders, due to partial stackings.
The range of the data in your image fits comfortably in the 32-bit floating point format, which provides around 10^7 discrete values.
Something that should *not* happen is that the lower half of the available dynamic range is not being used. You know this because the minimum values for the three channels are slightly above 0.5. This means that the main peak of the histogram is located near the center (for this reason you see the image as neutral gray when you open it). I've seen this before as a result of applications not being able to handle unsigned integer values internally. This is not a practical problem with your image because, as I've said, the integer 32-bit range is many orders of magnitude larger than the actual range of the existing data.
To deal with images like this, you can use the Rescale process (TransferCurves category). If you apply it to your image, you'll get a much better usage of the available numerical range, and all will be much easier, too. Here's an example:
Of course, Rescale is a linear operation, so it will not change the linearity (or non-linearity) of your data. As you see, after Rescale the extreme values cover a much wider range:
Minimum: 0.0005191790 0.0000000000 0.0003206695
Maximum: 0.8112020522 0.9303078425 1.0000000000
Note that this doesn't mean that you have more data now; it's simply that you have the *same* data much better distributed.
On the screenshot above, you can see how a simple histogram stretch shows your image. I defined a preview that doesn't include the dark regions due to partial stacks. These regions lead to a "false" histogram extended toward the shadows.
Hope this helps. The difference here is that PixInsight always shows you the truth <g>. Other applications show you the data with automatic screen stretchs applied by default. While I realize this can be handy, and I don't criticize it necessarily, PixInsight follows a different design principle: maximize the information and the control given to the user.
