Blue Halos.

[MattJenko]

After seeing this so-so image of Pleiades ( http://stargazerslounge.com/topic/230109-in-the-wake-of-the-pleiades/ ) and what can be done on a target like this, I had a chance to compare it to my effort, which contains the most data I have gathered for an object.

Mine contains significant halos, and it appears that Olly's version simply did not have halos in the data itself, it was not processed out after the fact. I have done a little internet research and found an interesting Astrodon article about research into these kinds of artefacts :

http://www.astrodon.com/articles_faq/articles_faq/press_release:391,355,49

This strongly implies that these halos are a result of internal reflections in the camera, which is something Earl has also mentioned in replies to my images in the past.

My question is: can anything be done about this, other than some mechanical hackery to try and fix the small reflection bloom that is causing these? If not - how would I even go about trying to process a halo which is the same colour as the nebula around it!

Thanks

Matt

[ronin]

If you used the ED80 then it will give CA, usually as a halo around bright objects (stars).

I am guessing Olly used an apo or a reflector but no diffraction spikes on his so presumably an apo refractor.

[tingting44]

Astronomy tools for PS has an action for removing small and large blue halo's, ive never used it but sounds up your street!

http://www.prodigitalsoftware.com/Astronomy_Tools_For_Full_Version.html

[Macavity]

Randomly: Have been playing around with my "typical" graphics program recently... Just trying out the "Magic Wand" selection tool! With a bit of random clicking, in different places, it seems possible to (sometimes!) preferentially select things like halos and then subject them to *whatever* post-processing you fancy? Or maybe magnify these... artifacts and "nibble" away at them or something... I'm certainly no expert!

[Gina]

Noel's Actions, an addon for Photoshop has an action for removing blue halos.  It also has many other very useful actions - recommended

[ollypenrice]

Excellent though they are Noel's Actions won't handle these effectively and they are not from the scope but from the camera or filters, I'm pretty sure. Poor (non apo) correction just gives a soft glow, not this kind of hard edged halo which shouts 'internal reflection.'

You could try something in Curves. Here I was looking at just the top two stars. All the big halo stars will need their own tweak. Alt-click on the good background sky or nebulosity just outside the halo. If you can, put a pin below it, a mark on the curve below to fix it in place. Here there wasn't room. Then drag the curve down as shown to kill the halo. Save the result and use it as a layer under the main image and erase, subtly (!!) the halo.

In truth this will be difficult and I chickened out and picked the easy ones not set in nebulosity! 

Olly

[MattJenko]

I have Noel's actions, and very good they are too, but they are no match for these halos. I also think these are reflections, so the next question is to ascertain where the reflection is coming from. From the Astrodon link I sent there are some formulas.

The halos look a fairly uniform 170 pixels wide. My Canon 450D has pixels which are 5.4 microns. This means the image size is 0.884mm across on the sensor. The reflection distance is given by the size of the reflection artifact * the focal ratio, which in this case is (600 * 0.85) / 80- as I am using the Skywatcher flattener/reducer on my ED80. This gives me an F ratio of 6.375. Putting the maths together, unless I am mistaken, this is a reflection at a distance of 5.6mm from the sensor. When processing this, I found that red was the predominant colour in the halo, and seeing as this is a modded camera, this all points to the reflection being on a surface inside the camera itself and probably the replacement IR filter. I did not do the modding myself, I bought it second hand already modded, so if I want to see if I am right, then I am looking at taking a peek at the replacement filter in this - gulp.

I know I shouldn't show Canon DIY fear, given that Gina has commented on this thread

As for the image itself, I have re-processed and think I have done a better job of masking and processing the halos individually so I could give the rest a better stretch without the halos going bananas. You are all right, it is tough and getting rid of the completely would simply involve artistic license, so they will remain .

Making great overall progress though with all the advice from here so thanks - thoroughly enjoying myself.

[ollypenrice]

That's a good job but you now have red inner rings to the modified stars. That can easily be rectified in colour balance with just the right part of the stars selected. This is a good image with the faint deep reds beginning to show.

Olly