Filter-induced halos? - Forums

Topic starter

9 days ago

Hi everyone,

Every since I have started mono imaging, I have been plagued with prominent and large halos around (very) bright stars. See for yourselves on these LRGB stacks of LEO I, next to Regulus: Reflection.jpg

It appears to be especially noticeable in the green channel. Please note I cannot rule out dew and/or high clouds for the less sharp halos.

The images were captured using a 200 mm f4 newtonian with a Paracorr II, bringing the focal length to around 920 mm. Astronomik L2 and Deep Sky RGB were used in front of a ASI2600MM camera.

In order to try and diagnose the source of the reflections, I came acress this archived article from Astrodon, which describes a method to locate the reflection in the imaging train: https://web.archive.org/web/20140729003031/http://www.astrodon.com/articles_faq/articles_faq/press_release:391,355,49

In the green channel, I measured the halo to be approximately 1340 pixels wide, which ultimately leads me to a reflection distance of ~23 mm from the sensor. As per the article, this number has to be divided by 2 to get the physical distance, so ~11.5 mm. Accounting for the measurement errors, we get dangerously close from the distance between the front of the camera (without the tilt plate, which I removed) and the sensor itself.

I’m not really sure my method is sound, but would I be right to suspect the AR window of the camera, or its interaction with the filters? And, most importantly, is there anything I can do to resolve this issue, without resorting to buying new filters?

I would be very interested in hearing your opinions and any advice you would have.

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andrea tasselli

Frequent contributor

9 days ago

Possibly but given the large number of ASI2600MM out there and not many reports of large halos in green but not in luminance I suspect the Astronomik are to blame (my older Astronomik also did produce prominent halos in green). Even sub-par AR coatings perform well at the central wavelength in my experience.

Tony Gondola

Frequent contributor

Exemplary contributor

8 days ago

I agree, filters would be your prime suspect followed by the corrector followed by internal reflections. That said, this field really is an extreme test of your optical train.

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sharkmelley

8 days ago

You seem to be assuming that the problem exists within the camera because half the additional light path length causing the halo broadly matches the distance between the sensor and the camera window. But what is the distance between the filter and the camera window ? Is this also a match and so could filter reflections be the problem? Also what happens if you remove the Paracorr II?

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l.roulin

Topic starter

7 days ago

Thank you all for your answers.

sharkmelley · Jul 15, 2026, 08:51 AM
You seem to be assuming that the problem exists within the camera because half the additional light path length causing the halo broadly matches the distance between the sensor and the camera window. But what is the distance between the filter and the camera window ? Is this also a match and so could filter reflections be the problem? Also what happens if you remove the Paracorr II?

Right, I didn’t think this distance could be found elsewhere and be a potential cause as well. I will need to take some measurements. I will also need to find a way to remove the Paracorr, as I am using a click-lock and need the 2” tube.