A single bright star with a distorted halo in an image of M44

Processing Deep Sky NGC2632 M44 Messier Objects
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SouthWestAstro avatar
SouthWestAstro avatar
This is an image of M44 taken with my new Astro-Tech AT60EDP, an ASI294MC Pro and and L-Pro filter. The data is 2 hours of lights, plus darks, biases and flats.  The image was processed in Pixinsight

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I'm looking for an explanation for the distorted halo on the bright star at the top-left in what I think is an otherwise nice image. Any ideas ? 


Sean Mc avatar
Sean Mc avatar
Do you have the distorted halo on a single sub?  

Or…

After stacking. But before any other processing?
Abhijit Juvekar avatar
Abhijit Juvekar avatar
Looks like focuser/camera tilt issue here.
Check using tilt adjustment programs.
https://www.youtube.com/watch?v=NQUmcIOQT2A
andrea tasselli avatar
andrea tasselli avatar
That is normal behavior for halos.
SouthWestAstro avatar
SouthWestAstro avatar
Sean Mc:
Do you have the distorted halo on a single sub?  

Or…

After stacking. But before any other processing?



It seems to happening in the final integration step in Pixinsight. I did not see the halo when I blinked through the registers frames.
SouthWestAstro avatar
SouthWestAstro avatar
Abhijit Juvekar:
Looks like focuser/camera tilt issue here.
Check using tilt adjustment programs.
https://www.youtube.com/watch?v=NQUmcIOQT2A



If it was a tilt issue there would distortions in other stars.
SouthWestAstro avatar
SouthWestAstro avatar
andrea tasselli:
That is normal behavior for halos.



Thanks. That being the case I have a few additional questions:

Can you provide a link to an image with similar behavior ?

Is there a solution ? Is it due to the L-Pro filter ?
Wei-Hao Wang avatar
Wei-Hao Wang avatar
All stars in your image have such halos. It's only that it's easier to see on bright stars.  If you integrate 10x longer, you will see such halos on other stars.  Or, in this particular image, if you look hard enough, you will see a similar halo behind the bright blue star in the upper-right edge.  This must be caused by some internal reflection between optical surfaces in your optics/camera.  Usually the filter is the first candidate of causing such halos.
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SouthWestAstro avatar
SouthWestAstro avatar
Wei-Hao Wang:
All stars in your image have such halos. It's only that it's easier to see on bright stars.  If you integrate 10x longer, you will see such halos on other stars.  Or, in this particular image, if you look hard enough, you will see a similar halo behind the bright blue star in the upper-right edge.  This must be caused by some internal reflection between optical surfaces in your optics/camera.  Usually the filter is the first candidate of causing such halos.




Thank you. I see what you mean. Next chance I get I'll try taking the filter off.
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Dave Rust avatar
Dave Rust avatar
This can sometimes happen with BlurX, as it tries to correct star shapes but doesn't always grab the related halo. Easy to test: Just do your usual workflow without BX and see if the artifact is the same.
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SouthWestAstro avatar
SouthWestAstro avatar
Dave Rust:
This can sometimes happen with BlurX, as it tries to correct star shapes but doesn't always grab the related halo. Easy to test: Just do your usual workflow without BX and see if the artifact is the same.



Thanks. It’s already there in the stacked image before further processing.
andrea tasselli avatar
andrea tasselli avatar
An interference filter reflects back the light that it doesn't let through so the fault is in general with the optics for a broadband LP filter such as the L-Pro. I have used it time and time again with no issue whatsoever. Also, large FOVs are complicit in this as it becomes more apparent the larger the FOV is.

Note: the reflections depends on how good the off-band reflectivity values (which is a measure of the properties of the anti-reflection coatings) of the filters are and in general they are not great (OD~2/3) for the standard L-Pro because are aimed at reproducing natural color as well as possible while rejecting LP emission lines. A tighter filter might have less of similar issues but lower color fidelty reproduction.
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Jared Willson avatar
Jared Willson avatar
I'm afraid this is totally normal with interference filters (though some filters and filter brands do it more or less than others). The halo is a slightly out-of-focus reflection of the star itself. It is off-center because the star is in the corner of the frame. For stars in the middle of the frame, the halo would be centered on the star. It's not an artifact from processing, and it's not because you did anything wrong. It's not tilt, or any indication of an optical problem. 

For an open cluster like M44, I would recommend skipping the L-Pro filter. You're not trying to draw out any faint nebulosity that might be hidden underneath your light pollution. Rather, you are trying to show accurate colors on some very bright stars. While the filter may be cutting down on the overall light pollution, that isn't necessary for a subject like this one. Try without the filter. You may see a brighter light pollution gradient than you get without the L-Pro, that's easy enough to get rid of in processing, and you'll be left with the ability to get more accurate star colors without the halos. 

- Jared
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SouthWestAstro avatar
SouthWestAstro avatar
Jared Willson:
I'm afraid this is totally normal with interference filters (though some filters and filter brands do it more or less than others). The halo is a slightly out-of-focus reflection of the star itself. It is off-center because the star is in the corner of the frame. For stars in the middle of the frame, the halo would be centered on the star. It's not an artifact from processing, and it's not because you did anything wrong. It's not tilt, or any indication of an optical problem. 

For an open cluster like M44, I would recommend skipping the L-Pro filter. You're not trying to draw out any faint nebulosity that might be hidden underneath your light pollution. Rather, you are trying to show accurate colors on some very bright stars. While the filter may be cutting down on the overall light pollution, that isn't necessary for a subject like this one. Try without the filter. You may see a brighter light pollution gradient than you get without the L-Pro, that's easy enough to get rid of in processing, and you'll be left with the ability to get more accurate star colors without the halos. 

- Jared



Thank you very much.

I'll be trying it without the filter as soon as the weather permits.

Turns out that Arizona has a lot more cloudy nights than most people realize !
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