TIFF vs. RAW: Problem of Stacking RAW in DSS Solved!

So, along time ago when I first started out using Deep Sky Stacker (DSS) to stack frames captured with my Canon T3i/600D, DSS would not read the CR2 images generated by my camera so I had to convert them to tiff format.  I read this as a workaround somewhere.  I used Canon's Digital Professional to convert the raw frames.  The Clarkvision website actually recommended converting to tiff on one of the threads and gave some technical explanation as to why but it went over my head. 

After a couple of years and updates, DSS was finally able to stack the raw images. Eureka! It will be much quicker to process, however, when I examined the the stacked image I noticed the stars looked like dim donuts.

For this experiment I used M92 as an example.
- 50 light frames, 90s, ISO 1600
- 10 dark frames, 90s, ISO 1600
- 12 flat frames, ISO 1600
- 10 bias frames, 1/4000s, ISO 1600

Image 1 is the autosave file generated by DSS with the default settings. Notice the poor quality stars.

Image 1 - autosave image stacking raw frames

As I normally do, I converted the raw frames to tiff and the stacked image with the same default settings. Image 2 was the result. Notice the more well defined stars.  This has been my normal procedure for years.

Image 2 - autosave image using converted tiff frames 

I posted the question of why stacking my raw frames gave such poor results on Cloudy Nights and got plenty of reasons why stacking raw frames is much better.  The check out the link to Cloudy Nights to see all of the responses.

Mark summarized it as follows:
"When you stack TIFF files, you are stacking files that have been processed by the raw converter. The data are no longer linear but have been transformed into a non-linear colour space (e.g. sRGB or AdobeRGB). The stacked image is therefore also non-linear. When you subtract light pollution (which is typically high in red and low in blue because it is dominated by sodium street lights) from non linear stacked image, the result tends to have increasing blue with decreasing scene intensity and it tends to have very oversaturated colours."

The original question, however, was answered by another responder.

Wei-Hao summarized it as follows:

"The donut-shaped stars are likely caused by the cosmetic correction or hot pixels removal in DSS. If the setting is too aggressive, DSS would take all stars as hot pixels and remove them, leaving the stacked images no stars or dim stars with funky shapes.

You can first disable all options related to these and see if it solves the problem. Once you confirm the problem is indeed caused by the cosmetic correction, then you can try various settings there to find the best combination for your images."

I did what Wei had suggested and finally, after many years, my stacked raw image looks much better! I wish I posted this a long time ago, I don't know why I did not...

Image 3 - autosave image using raw frames with hot pixel removal disabled

The stars on this image are well defined as they are in the single raw image frames. Although converting to tiff was a pain I don't think my final images were of poorer quality, I believe they were potentially just as good, but I think I had to to a heck of a lot of extra processing in order to accomplish a decent image.  

To end with I have attached final versions of both methods.

Image 4 - Final stacking with tiff

Image 5 - Final stacking with raw (disabled hot pixel)

Hope this helps anyone with similar problems, 

Cheers Kurt