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Single spike on some stars - Askar Refractor

Askar 80PHQ Equipment Refractors Acquisition Help Needed
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Marco Montella avatar
Marco Montella avatar

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.pngScreenshot 2026-01-08 at 14.46.23.png📷 Screenshot 2025-11-24 at 20.16.53.pngScreenshot 2025-11-24 at 20.16.53.png📷 Screenshot 2026-01-08 at 12.49.50.pngScreenshot 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 avatar
andrea tasselli avatar
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 avatar
Marco Montella avatar

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 avatar
Richard Carande avatar

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 avatar
Dan Brown avatar

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 avatar
John Hayes avatar

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 avatar
Marco Montella avatar

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:

Screenshot 2026-01-08 at 19.13.36.pngINNER REDUCER SURFACEScreenshot 2026-01-08 at 19.13.29.pngOUTER REDUCER SURFACEScreenshot 2026-01-08 at 19.13.23.png

Rick Veregin avatar
Rick Veregin avatar

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 avatar
andrea tasselli avatar
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 avatar
AlexFilothodoros avatar

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 avatar
John Hayes avatar

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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