Diffuse halo around stars - Not Filter related - Forums

Hey All,

I pretty much always have this issue when imaging with my 150PDS Newtonian reflector. Around any brightish star, I get a fuzzy diffuse halo, regardless of whether I use a filter or not and it happens all the time.

Just wondering what the cause of this could be. It happens even when there are no high clouds anywhere. The halo is not a crisply defined halo that I have seen in posts about filter-related halos. I don’t remember it being so bad before I got the Coma Corrector but I would have to go back over the old files to see and do not have access to them right now.

I normally deal with it in post-processing but am curious as that what could potentially be causing this.
Sample autostretched starless image below for reference. The halos are visible on the right hand side

Many thanks in advance

Equipment:

150PDS

2600MC-Pro

TS-Optics GPU Coma Corrector

📷 Screenshot 2026-05-10 at 14.36.46.jpg

Can you post the started image too? Unprocessed

I had those with the Epsilon 160ED too. I have seen it in most fast newtonians images on astrobin with some bright stars in the field. It is one of the reasons I didn’t like post processing the Epsilon’s data. Just never comes out clean.

It might come out better if you shorten your subs, but the remote data significantly increases and I just never did it.

Ashraf AbuSara · May 10, 2026, 02:01 PM
I had those with the Epsilon 160ED too. I have seen it in most fast newtonians images on astrobin with some bright stars in the field. It is one of the reasons I didn’t like post processing the Epsilon’s data. Just never comes out clean.
It might come out better if you shorten your subs, but the remote data significantly increases and I just never did it.

That’s interesting Ashraf. With your scope being of much higher quality, I’m surprised to hear that you’ve seen the same. Thanks for the input!

Craig Towell · May 10, 2026, 01:59 PM
Can you post the started image too? Unprocessed

Here you go Craig. If you mean the actual FITS file let me know and I’ll get it to you somehow

📷 Screenshot 2026-05-10 at 15.15.30.jpg

It’s scattering due to the coatings applied to the mirrors surfaces. There’s nothing you can do about it, it’s a by-product of the coating technologies used and the diffraction from central obstruction and spiders.

andrea tasselli · May 10, 2026, 02:27 PM
It’s scattering due to the coatings applied to the mirrors surfaces. There’s nothing you can do about it, it’s a by-product of the coating technologies used and the diffraction from central obstruction and spiders.

Ah! Thanks Andrea. Is this the quality of the mirror coating on my scope so or do all Newt imagers deal with this?

There’s going to be a degree of scattered light in any optical system. Every glass to air surface scatters a little bit of light and think about how many there are in a typical F/4 Newt. There’s also the effects of diffraction from edges and of course even dust causes scatter. Add all that up and it can become significant. Then you take a highly sensitive camera and stretch an image to the wall. Yes, things become noticeable. It’s one of the reasons why I prefer slower Newtonians with small sensors that don’t require the extra glass of a coma corrector.

These are leftover after star removal. I get them with my refractor as well and I’ ve seen people discussing about them in various fora.

John Walsh · May 10, 2026, 03:18 PM
andrea tasselli · May 10, 2026, 02:27 PM
It’s scattering due to the coatings applied to the mirrors surfaces. There’s nothing you can do about it, it’s a by-product of the coating technologies used and the diffraction from central obstruction and spiders.
Ah! Thanks Andrea. Is this the quality of the mirror coating on my scope so or do all Newt imagers deal with this?

I had them in all my reflectors/cats. Sensitivity comes at a price, alas. Faster systems show them more obviously than slower systems and always dust adds its own scattering too.

Since we do not discuss about atmospheric effects, and the issues “coatings” and “dust” have already been mentioned, i want to add another one: Scratches.
Wiping dust off an optical surface can leave fine scratches behind. These scatches produce stray light. Depending on the coating of the surface, these scratches do not necessarily have to result from the dust, they can also be due to the wipe itself. Most of the time, it ist the dust collected by the wipe that causes it.

I dont think proper HR coatings do produce stray light when mint. Maybe some types of AR coatings could, i wouldnt know of but we do a lot of stretching in astronomy. I’ll have a look at some data next week.

HR_Maurer · May 10, 2026, 08:27 PM
I dont think proper HR coatings do produce stray light when mint.

My understanding of this is, sort of. No AR coating is 100% efficient, especially through a wide range of wavelengths. Each air to glass surface with a premium AR coating is going to reflect anything from 0.2% to 1.5% of the light hitting it. This isn’t diffuse light however but rather it follows the standard laws of reflection.

Tony Gondola · May 10, 2026, 08:53 PM
HR_Maurer · May 10, 2026, 08:27 PM
I dont think proper HR coatings do produce stray light when mint.
My understanding of this is, sort of. No AR coating is 100% efficient, especially through a wide range of wavelengths. Each air to glass surface with a premium AR coating is going to reflect anything from 0.2% to 1.5% of the light hitting it. This isn’t diffuse light however but rather it follows the standard laws of reflection.

That’s true, especially when the AR is designed for a wider range of wavelengths - they tend to reflect quite some amont at the band edge. However, reflection isnt stray light.

HR_Maurer · May 10, 2026, 09:31 PM
Tony Gondola · May 10, 2026, 08:53 PM
HR_Maurer · May 10, 2026, 08:27 PM
I dont think proper HR coatings do produce stray light when mint.
My understanding of this is, sort of. No AR coating is 100% efficient, especially through a wide range of wavelengths. Each air to glass surface with a premium AR coating is going to reflect anything from 0.2% to 1.5% of the light hitting it. This isn’t diffuse light however but rather it follows the standard laws of reflection.
That’s true, especially when the AR is designed for a wider range of wavelengths - they tend to reflect quite some amont at the band edge. However, reflection isnt stray light.

Yes, it’s not diffuse light but depending on how it reflects I suppose it can still end up where you don’t want it.

Ashraf AbuSara · May 10, 2026, 02:01 PM
I had those with the Epsilon 160ED too. I have seen it in most fast newtonians images on astrobin with some bright stars in the field. It is one of the reasons I didn’t like post processing the Epsilon’s data. Just never comes out clean.
It might come out better if you shorten your subs, but the remote data significantly increases and I just never did it.

I would like to pause here and not let this go. The idea of shorter exposures somehow solving issues of halos or scattered light is I think not a productive way to think about this issue. I am happy to be corrected.

How would shorter exposures help with halos or scattered light? When you stack (integrate) your images for some period of time the differences in how you divide up the total exposure time will not significantly affect this result. This is a real signal- it “adds up” just like any other faint signal in your images. You see, how could a shorter exposure help here and not harm the faint nebula/galaxy…etc?

-adam

Adam Block · May 11, 2026, 05:13 AM
Ashraf AbuSara · May 10, 2026, 02:01 PM
I had those with the Epsilon 160ED too. I have seen it in most fast newtonians images on astrobin with some bright stars in the field. It is one of the reasons I didn’t like post processing the Epsilon’s data. Just never comes out clean.
It might come out better if you shorten your subs, but the remote data significantly increases and I just never did it.
I would like to pause here and not let this go. The idea of shorter exposures somehow solving issues of halos or scattered light is I think not a productive way to think about this issue. I am happy to be corrected.
How would shorter exposures help with halos or scattered light? When you stack (integrate) your images for some period of time the differences in how you divide up the total exposure time will not significantly affect this result. This is a real signal- it “adds up” just like any other faint signal in your images. You see, how could a shorter exposure help here and not harm the faint nebula/galaxy…etc?
-adam

Indeed. Because the halos are signal, technically speaking.

Thais everyone for the replies. I wonder would cleaning both mirrors lead to a marginal improvement?

Unless they are really filthy marginal would be the right word (if at all).

andrea tasselli · May 11, 2026, 08:27 AM
Unless they are really filthy marginal would be the right word (if at all).

Good to know Andrea, thanks. I guess I will just live with it

John Walsh · May 11, 2026 at 08:26 AM
Thais everyone for the replies. I wonder would cleaning both mirrors lead to a marginal improvement?

Here is my own test of a dirty vs clean mirror, very bright star (Regulus). You can clearly see the extra light scatter due to the dirty mirror - though it would likely be less noticeable of less bright stars.📷 Dirty Vs Clean.png

Craig Towell · May 11, 2026 at 09:31 AM
John Walsh · May 11, 2026 at 08:26 AM
Thais everyone for the replies. I wonder would cleaning both mirrors lead to a marginal improvement?
Here is my own test of a dirty vs clean mirror, very bright star (Regulus). You can clearly see the extra light scatter due to the dirty mirror - though it would likely be less noticeable of less bright stars.📷 Dirty Vs Clean.png

A quite apparent difference Craig! Was it just the primary you cleaned or both?

John Walsh · May 11, 2026 at 09:57 AM
Craig Towell · May 11, 2026 at 09:31 AM
John Walsh · May 11, 2026 at 08:26 AM
Thais everyone for the replies. I wonder would cleaning both mirrors lead to a marginal improvement?
Here is my own test of a dirty vs clean mirror, very bright star (Regulus). You can clearly see the extra light scatter due to the dirty mirror - though it would likely be less noticeable of less bright stars.📷 Dirty Vs Clean.png
A quite apparent difference Craig! Was it just the primary you cleaned or both?

No just the primary, secondary never seems to get dirty which makes sense as it never points to the sky.

Adam Block · May 11, 2026, 05:13 AM
Ashraf AbuSara · May 10, 2026, 02:01 PM
I had those with the Epsilon 160ED too. I have seen it in most fast newtonians images on astrobin with some bright stars in the field. It is one of the reasons I didn’t like post processing the Epsilon’s data. Just never comes out clean.
It might come out better if you shorten your subs, but the remote data significantly increases and I just never did it.
I would like to pause here and not let this go. The idea of shorter exposures somehow solving issues of halos or scattered light is I think not a productive way to think about this issue. I am happy to be corrected.
How would shorter exposures help with halos or scattered light? When you stack (integrate) your images for some period of time the differences in how you divide up the total exposure time will not significantly affect this result. This is a real signal- it “adds up” just like any other faint signal in your images. You see, how could a shorter exposure help here and not harm the faint nebula/galaxy…etc?
-adam

That’s a great point Adam. Thanks for pointing it out.

John Walsh · May 11, 2026, 08:26 AM
Thais everyone for the replies. I wonder would cleaning both mirrors lead to a marginal improvement?

It won’t hurt…

Craig Towell · May 11, 2026, 09:31 AM
John Walsh · May 11, 2026 at 08:26 AM
Thais everyone for the replies. I wonder would cleaning both mirrors lead to a marginal improvement?
Here is my own test of a dirty vs clean mirror, very bright star (Regulus). You can clearly see the extra light scatter due to the dirty mirror - though it would likely be less noticeable of less bright stars.📷 Dirty Vs Clean.png

Wonderful, a real world example.