StarTools

Hi,

I noticed that my Canon 1100D produces streaks coming out of very bright stars (like in M45), an example is attached. Is there a way to fix this
in StarTools? At the moment what I do is after using StarTools I got to Gimp and use resynthesize, but there should be a better way...

Thanks,
Epicycle

Hi Epicycle,

First off, that's very nice data you have there!
While StarTools has a dedicated function for deblooming (in the repair module), 'negative' blooming like this where some data is still visible, just offset in brightness, is a little trickier to deal with - there is actual data that you'd want to preserve (e.g. stars). Luckily, StarTools comes with its own/similar implementation of 'resynthesize', with some extra features and controls for astronomical images.

To fix blooming like this, you can use a similar technique to getting rid of dust donuts.
Key to this technique is the creation of a mask. I uploaded a new version of StarTools (1.4 alpha - see download section) that will allow you to create a suitable mask easier for your particular situation.

In the mask editor, clear your mask, set the brush mode to 'Line Toggle (new in 1.4). For each dark column you see, click and drag to create a thin line on top of the dark column.
When you've done every dark column like this, simply click the 'Grow' button a few times to grow the thin lines into thicker lines. Make sure the thick lines cover the columns completely.
Cool! We now have a mask that points StarTools to the columns.
Next, launch the Heal module and StarTools will start beavering away. When it's done it will have 'resynthesised' a plausible substitute.
Chances are, tough, that it will have synthesized some stars or 'light patches' as well. This is probably where you would stop in The GIMP.

Not to worry - we'll restore as much as possible. Keep the result, relaunch the Heal module again. It will come up with yet another plausible rendition (different from the one before). Keep this one too.

Now for a cool trick - launch the Layer module and click the 'Undo->Bg' button. This will copy whatever was in the undo buffer into the background buffer. Next, set Blend Mode to 'Darken'. It will now when creation the composite, ST will now choose whichever pixel is darker and use that as the final result. You should notice any synthesized stars and light patches disappear. You can repeat this trick a couple more times if you want (heal, layer, heal, layer).

To finish off, and to restore any stars that were healed out out of the original, launch the Layer module once more, click the 'Open' button to load you original image into the foreground Layer. Now set Blend Mode to 'lighten' and voilà - you should have a natural looking image with minimal artifacts!

Before: After: Hope this helps!

Hi Ivo,

that's very useful thanks. Having better ways to create mask seems an excellent way to improve StarTools. For instance I find
the isolate feature in Life great to pull out detail but it takes me ages to set up a suitable mask. Perhaps something like a lasso tool/rectangle to create masks could be implemented?

Anyway, returning to the issue at hand, I did try exactly your suggestions, but my end result doesn't seem that great (it's attached).
Did you iterate many times? I also got some additional artifacts, which may have to do with the fact that my mask wasn't quite right.
At the moment the Gimp fix seems quicker and smoother... Any additional tips? Thanks!

By the way, do you know what could be causing this negative blooming in the first place? I find it very annoying...

best
Epicycle

Epicycle wrote:Hi Ivo,

that's very useful thanks. Having better ways to create mask seems an excellent way to improve StarTools. For instance I find
the isolate feature in Life great to pull out detail but it takes me ages to set up a suitable mask. Perhaps something like a lasso tool/rectangle to create masks could be implemented?

Way ahead of you! (that's what I did yesterday)

Anyway, returning to the issue at hand, I did try exactly your suggestions, but my end result doesn't seem that great (it's attached).
Did you iterate many times? I also got some additional artifacts, which may have to do with the fact that my mask wasn't quite right.
At the moment the Gimp fix seems quicker and smoother... Any additional tips? Thanks!

The mask should be covering the full column width (otherwise ST will try to reconstruct the bad column!). If you find it's not getting rid of the column properly, try growing your mask a little (you can use the Grow Mask parameter in the Heal module).

I just iterated twice. In addition to using the 'Darken' trick to get rid of introduced stars/light patches, you could also make use an alternative trick, using Layer's Brightness Mask (luminance mask); with the original image as the background and the healed image in the foreground, select 'Where Composite is light, use bg' and play with the Brightness Mask Power. This should see Layer switch seamlessly between the original for the lighter parts of your image and the healed image for the darker part of your image. When you're happy with the result, click 'Copy' to copy the result to a temporary buffer. Click 'Reset' to reset everything and click 'Paste->Fg' to paste the result we just attained as the new foreground. Then set Layer Mode to 'Lighten' and you should see any stars that were healed out be put back in place.

You should be able to get very good results and successfully be able to reconstruct your data.
In your case, the trick is to retain data as much as possible (it's there, just darkened). It will be slightly more laborious but the alternative - blatant inpainting & making up data (which is what resynthesizer does) - is considered a cardinal sin by many (it's also where I draw the line personally - especially when it's not needed).

By the way, do you know what could be causing this negative blooming in the first place? I find it very annoying...
best
Epicycle

I have to be honest - I've never seen a 1100D do this. How do your flats/bias/darks look? Can you see any streaks in there?

Way ahead of you! (that's what I did yesterday)

Greatest
News
Ever

Many thanks. I'll give this other trick a go. Glad to hear that new tools for creating masks are coming up!

No, flats and darks don't show the streaks. Apparently some Canon users have reported this, but not everyone gets it (there are threads
on Cloudy Nights about this), but I don't really have a convincing explanation,

best
E.

I have heard for some people that this happens only at certain ISOs. If you are using 800, maybe try 1600, etc.

Otherwise I might think about selling the camera or talking to Canon about warranty repair/replacement.