Is this the chaos that is Alnitak? Or do I have something else going on....

[mos]

Hello everyone,

The other night I decided to try and capture an image of the horse head with my ZWO 1600mm and matching 36mm filters. Being well aware of the this star and its halos, I was expecting some sort of halo, and imaging with ZWO filters I was expecting perhaps slightly worse halos than other filters. 

Attached is a very saturated and quickly processed image to demonstrate what I've ended up with. YOINKS. 

Obviously there is a massive halo around Alnitak...I'm not sure if this can simply be put down to ZWO RGB filters? Green and blue are plainly visible as separate halos but my luminance and red channels didn't generate a significantly visible halo. 

The lens diffraction pattern is there too, and while entirely expected, could be making the halos worse? 

My imaging train includes an MPCC MKiii and my scope is a 150PDS. 

The filter orientations are correct too to the best of my knowledge but I have flipped the blue and green to test next time I have a clear night. I'll also try removing my coma corrector to make sure it's not responsible for introducing and stray reflections. 

Is this just what I should expect if I go near the Horsehead with my current setup or is there something more going on here? I knew narrow band and more so Oiii could cause halos, but haven't come across many instances of such bad haloing with RGB. 

Once again, any help or insight is very much appreciated. 

Matt

 

 

[Demonperformer]

Yikes!

I don't have the expertise on RGB to be able to comment on your questions, but - on the plus side - you clearly succeeded in "captur[ing] an image of the horse head" and, with Alnitak out of the picture, quite a good one, I reckon.

[Tim]

Haloes are normally worse in RGB I find. Ha filters suffer least, and some filters out perform others.

Small point, but your diffraction pattern will improve if you can better line up the spider, it will get rid of the diverging lines on the horizontal pattern.

Pixinisight has a halo removal routine.

I haven't time to look too closely at your image, but I wonder if you would get a less harsh edge to the haloes with pattern on if you used a 1/4" edge mask around your mirror, either that or black the front ground edge if it has one VERY carefully. I would try a black cardboard mask first though.

[Tim]

Impressive amount of reflection nebulosity captured btw, don't often see that much detail and separation in it

[Tim]

Just re-read your post. Might be worth a try without the MPCC as well so you know where you are working from with a pure system. 

[vlaiv]

It's probably due to MPCC/filters combination, have a look here at the same problem (not the same star), but even worse in size:

When imaging very bright star - there is plenty of light that bounces around an will be registered as halo. This is due to reflection on components. Most components are anti reflection (AR) coated - but that does not mean that they are fully protected from reflections - you will often see 99.7% (or any other number close to 99%) transparent. This means that there is about 0.3% light reflecting back. So light gets to interference filters - reflects back (this is how interference filters work to block unwanted wavelengths) and hits coma corrector and it's curved surface. Small percent of that light is reflected back towards the sensor (mentioned 0.3% for example) but at a slightly different angle and then manages to pass thru filter as unfocused light (filters work their band pass best when light is perpendicular to it - if at angle band pass changes). If you take into account all percentages - for normal stars it's not an issue, because something like 1/10000th of the light behaves this way - but for very bright star you end up with enough light to be visible as reflection in long exposure.

What is to be done? Changing spacing often helps as it alters path that light is taking. Can even make it worse (wavelike behavior and interference can either "cancel out" or "intensify" light).

As for separation of colors - it is due to misalignment of image centers (optical axis) in each channel. Because MPCC is curved - it will bounce back light at an angle, and if you move star further off center - reflected light will move even more.

[JOC]

mind you it makes for a stunningly dramatic image if you want something for a wall!

[mos]

Thanks for the positive comments about the image! I guess without Alnitak, there is some nice detail there. I'll process it properly and see what I can do

Thank you very much Tim and Vlaiv - I'll look to experiment a bit with the MPCC and the spacing between the filters and look into a mask for the mirror edge if I can make no improvements 

[alacant]

Hi. Software solution? I wonder if you had a gently stretched layer beneath the fully stretched version as a mask; use a soft brush to remove the top layer around Alnitak?  Similar to getting the core in M42. Try with a pressure sensitive screen or one of the old Wacom tablet things? Easier said than done of course!

Just an idea but HTH anyway.