Single spike on some stars - Askar Refractor - Forums

Topic starter

Jan 8, 2026

Over the last few months I have been struggling with puzzling spikes on some stars during RGB (no filter) shooting on my refactor.

Here’s what I have observed so far about these spikes:

Consistent over multiple nights
Spikes DO NOT appear with priority on the brightest stars, but on stars positioned in given regions of the field of view (see attached)
Spikes are not parallel to each other
Spikes are not visible when using narrowband filters

I am attaching three pictures to provide visual reference:

📷 Screenshot 2026-01-08 at 14.46.23.png📷 Screenshot 2025-11-24 at 20.16.53.png📷 Screenshot 2026-01-08 at 12.49.50.pngDo you know what could be causing this? I inspected the front element of my refractor, and while it’s quite dirty, I’m not sure dirt is enough to justify this result.

andrea tasselli

Frequent contributor

Jan 8, 2026

Normally in my experience it is caused by strained glass, either because of faults in relieving them after casting or because of cold temperatures.

Marco Montella

Topic starter

Jan 8, 2026

andrea tasselli · Jan 8, 2026 at 02:22 PM
Normally in my experience it is caused by strained glass, either because of faults in relieving them after casting or because of cold temperatures.

Thank you! The temperature of operation was nothing special, ranging from 5° to 10° depending on the shot. I hope this is fixable, and not some kind of factory defect.

Richard Carande

Jan 8, 2026

I’ve seen similar things when my FOV passes by a thin suspended power line. However, that tends to affect all the stars, proportional to their brightness. I’ve never seen what you are showing.

Well Written

Dan Brown

Jan 8, 2026

I had the same issue last year. The stars in a particular part of the image showed the single spikes. After the meridian flip I the issue was not apparent. Subsequent outings did not show the spikes.

One hypothesis I had was that it may have been caused by frost or dew on a small portion of the sensor. I still don’t know for sure what caused it.

Dan

John Hayes

Frequent contributor

Exemplary contributor

Jan 8, 2026

First, this kind of long, linear diffraction spike occurs because of a line or straight edge oriented at right angles to the direction of the spike. It can be caused in either amplitude (by a visible blocking feature) or phase (by a refractive feature). It is interesting that the spike does not occur on all bright stars AND it appears to occur at different angles within the same image. That would imply that the problem is not coming from the objective and is being generated by a component very close to the image plane. Have you ever cleaned the camera window or the rear surface of a flattener or reducer? Cleaning residue (or condensation on cleaning residue) could conceivably cause this sort of problem. It could be caused by striations in a glass element but that’s pretty far fetched—and it would be a permanent effect.

I would suggest pulling the camera and carefully inspecting everything using a very bright flashlight. Look for spider webs or debris on the elements. The diffraction pattern looks pretty strong so I suspect that there’s something fairly big (at least big enough to see) causing the problem.

John

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Marco Montella

Topic starter

Jan 8, 2026

Hi John, thanks a lot for your extensive diagnosis attempt! I just inspected the 3 closest surfaces to the sensor, and despite looking fairly dirty when lit by harsh light, I didn’t see anything immediately jumping to the eye as a possible culprit.

But I’m not an expert of optical path obstruction, I’ll let you be the judge of that:

INNER REDUCER SURFACEOUTER REDUCER SURFACE

Rick Veregin

Jan 8, 2026

Hi Marco,

This looks really dirty to me, everything needs a very careful cleaning I would say. But as well that scratch could very well be one culprit. I had a small scratch toward the edge of a filter once and all the bright stars in one part of a quadrant of my image had a little spike like you are seeing in your images. Where you see the spike in the image depends on where the scratch is. It is surprising how even a very tiny scratch can cause this, I certainly was surprised.

The ZWO 2600 MC Pro has a protect window IR/UV/filter so presuming that scratch is on the protect window, you can buy the replacement from ZWO and change the window yourself. This camera (same model I have) uses the D60 UV IR-CUT protect window—you can find it on the ZWO site and on most astro equipment sites.

Rick

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andrea tasselli

Frequent contributor

Jan 8, 2026

Simple test: Rotate the camera by 90 degrees and see whether the spikes follow the camera (use a dense star field for this test)

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AlexFilothodoros

Jan 9, 2026

I see the same with my Askar PHQ. It can happen randomly in my case.
For now, I have changed the adapters to the camera, hoping that it’s internal reflection. BTW, I see it with narrowband filters.

John Hayes

Frequent contributor

Exemplary contributor

Jan 10, 2026

That looks pretty dirty to me. My suggestion is to first carefully clean up those surfaces and then try it again. Dust and debris could certainly be the source of the problem. Andrea’s suggestion to rotate the camera is a good one as well. I suspect that if you clean those surfaces, the problem will go away.

John

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SonnyE

Frequent contributor

Jan 12, 2026

I get similar spires occasionally.

📷 Spires.jpgI’ve experienced this anomaly for many years, with various cameras, and two different refractors, and two different mounts.

What is common is sharpest focuses. There are no other things in my experience’s other than the occasional sharper focuses.

I’ve pondered it. But in the end, they always come out in stacking. And they are sporadic at best.

Not worth worrying about to me.

AlexFilothodoros

Jan 12, 2026

Unfortunately, in my case they stay in the stacked image as well :(

Médéric Hébert

Jan 13, 2026

SonnyE · Jan 12, 2026, 07:41 PM
I get similar spires occasionally.
📷 Spires.jpgI’ve experienced this anomaly for many years, with various cameras, and two different refractors, and two different mounts.
What is common is sharpest focuses. There are no other things in my experience’s other than the occasional sharper focuses.
I’ve pondered it. But in the end, they always come out in stacking. And they are sporadic at best.
Not worth worrying about to me.

I sometimes get that very same artifact, for me it’s when the clothes line is in the light path

SonnyE

Frequent contributor

Jan 14, 2026

Médéric Hébert · Jan 13, 2026, 05:32 AM
SonnyE · Jan 12, 2026, 07:41 PM
I get similar spires occasionally.
📷 Spires.jpgI’ve experienced this anomaly for many years, with various cameras, and two different refractors, and two different mounts.
What is common is sharpest focuses. There are no other things in my experience’s other than the occasional sharper focuses.
I’ve pondered it. But in the end, they always come out in stacking. And they are sporadic at best.
Not worth worrying about to me.
I sometimes get that very same artifact, for me it’s when the clothes line is in the light path

Well, no clothes lines here. We use a Natural Gas fired electric dryer. 🤣

Marco Montella

Topic starter

Jan 14, 2026

@SonnyE I think your case is slightly different from mine: for starters, all your spikes are parallel, and it seems like every star is affected proportionally to their brightness, rather than the inconsistent pattern I see in my stacks.

Have you checked yourself for scratches/features near the sensor?

Pirate Mike

Jan 14, 2026

I had what appears to be the same issue once. The culprit was a very minor streak on the sensor window. Once cleaned up, the issue disappeared.

Well Written Concise

SonnyE

Frequent contributor

Jan 14, 2026

Marco Montella · Jan 14, 2026, 08:34 AM
@SonnyE I think your case is slightly different from mine: for starters, all your spikes are parallel, and it seems like every star is affected proportionally to their brightness, rather than the inconsistent pattern I see in my stacks.
Have you checked yourself for scratches/features near the sensor?

No doubt ours are different.

We all seem to use vastly different pieces of equipment. But in generality these spikes or spires do occasionally show up. What I’ve shown simply is a lately occurring example that I singularly grabbed out of a set of subs.

Why yours would be limited I do not know. But I do know I’ve seen this occasional spire appear and it is not due to any findable problem. It just occurs randomly in my case.

They do vanish in stacking for me. Good luck in finding a cure.

AlexFilothodoros

Jan 14, 2026

SonnyE · Jan 14, 2026 at 04:21 PM
Marco Montella · Jan 14, 2026, 08:34 AM
@SonnyE I think your case is slightly different from mine: for starters, all your spikes are parallel, and it seems like every star is affected proportionally to their brightness, rather than the inconsistent pattern I see in my stacks.
Have you checked yourself for scratches/features near the sensor?
No doubt ours are different.
We all seem to use vastly different pieces of equipment. But in generality these spikes or spires do occasionally show up. What I’ve shown simply is a lately occurring example that I singularly grabbed out of a set of subs.
Why yours would be limited I do not know. But I do know I’ve seen this occasional spire appear and it is not due to any findable problem. It just occurs randomly in my case.
They do vanish in stacking for me. Good luck in finding a cure.

Do you have any specific settings in stacking, that could help removing this issue?

SonnyE

Frequent contributor

Jan 15, 2026

AlexFilothodoros · Jan 14, 2026, 07:10 PM
SonnyE · Jan 14, 2026 at 04:21 PM
Marco Montella · Jan 14, 2026, 08:34 AM
@SonnyE I think your case is slightly different from mine: for starters, all your spikes are parallel, and it seems like every star is affected proportionally to their brightness, rather than the inconsistent pattern I see in my stacks.
Have you checked yourself for scratches/features near the sensor?
No doubt ours are different.
We all seem to use vastly different pieces of equipment. But in generality these spikes or spires do occasionally show up. What I’ve shown simply is a lately occurring example that I singularly grabbed out of a set of subs.
Why yours would be limited I do not know. But I do know I’ve seen this occasional spire appear and it is not due to any findable problem. It just occurs randomly in my case.
They do vanish in stacking for me. Good luck in finding a cure.
Do you have any specific settings in stacking, that could help removing this issue?

Not that I recall offhand. I use ZWO ASI cameras, (1600MM, 2600MC, and an ASI290MM for my guiding), and I got to using ASI Studio many years ago to view my fit files, and to do the stacking. So I imagine whatever is generally normal in that. (Default)

I’m not one to micro-manage too much. Once things are working I quietly back away and just watch. 😅 (If it works, don’t fix it.)