What is the deal with .fits conversion? It seems much shakier than it needs to be. I took a couple of not very good, newbie level captures of the lunar eclipse last night using CCDCiel, which saves them as .fits files. CCDCiel displays them decently. They don't seem to be displayable by any other program. My thought was to convert them to .jpeg or .png for sharing purposes, but none of the several tools I Googled for .fits to .png conversion worked. A couple were online, they produced a black rectangle instead of the image that CCDCiel was showing them. Can soimeone explain to me what's up with that? If I can't solve this issue, I'm reduced to taking screenshots for sharing, not ideal.
I'm not a CCDCiel user, but I'd guess that it is auto scaling the displayed image to as to make the image visible. Most of the time, astro images need to be stretched to see anything. Perhaps your other display apps don't auto stretch?
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“The PNG and TIFF picture will be saved as 16 bit linear images.”
What is a 16-bit linear image? Does this explain why a PNG file created by CCDCiel cannot be viewed by common graphic-viewer apps, while the 8-bit JPG created by CCDCiel is visible? CCDCiel seems to have been designed that way. Why?
Most of images have 8 bit per channel. That means colour values are in 0-255 range. 16 bit images have this in 0-65536 range. Problem is that lot of cameras produce images with only 0-4096 range or smaller range. So brightest pixels in image have value 4096. Most of image viewer doesn't stretch displayed image so you end up with 4096/65536=6% brightness which is almost black.
You can try load these 16bit images in GIMP and use levels tools to stretch it. Actually GIMP can load FITS images directly.
Astronomical images are not easily viewed on standard image processing / showing software. You need to perform an initial stretch of the data. All the capture software makes a "virtual stretch" while they are capturing images, so you can see what you are shooting at. But they store the original (linear, non stretched) data.
The best approach will be to use some viewer specific to handle astronomical image. For example, you can use SiriL (siril.org/download/). It's open source and it can help you to align / stack / colour correct and save as stretched TIFF or PNG image