What could cause this bizarre diffraction spike effect? - Forums

I was doing some testing tonight to get a target framed(it’s too windy to actually image tonight), when I noticed in my test shots, that the brights star, Ain, has a very bizarre appearance, with multiple spikes coming off in many directions.

I have not seen this before.
My image train is as follows:

150PDS Newtonian(with backyard universe CNC spider & primary mirror mask) - TS Optics GPU coma corrector- ZWO filter drawer(no filter installed)-2600MC pro.

The only change I made tonight from previous nights was to rotate the camera 50 degrees clockwise from its previous position.

Would anyone have any ideas as what the cause of this might be?

Many thanks in advance

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What happens, when you rotate the camera? Do the spikes rotate as well? If so, it may be a problem with the sensor or humidity. If they don't rotate, the cause must be somewhere else in the image train.

My guess would be primary mirror clips.

John Esser · Dec 9, 2025, 10:16 PM
What happens, when you rotate the camera? Do the spikes rotate as well? If so, it may be a problem with the sensor or humidity. If they don't rotate, the cause must be somewhere else in the image train.

I will try and see. Currently I am positioning the star in various parts of the frame to see what effect that has

Jon Main · Dec 9, 2025, 10:19 PM
My guess would be primary mirror clips.

The clips are completely hidden by a mirror mask though

Do you observe this problem with all frames?

Is there anything in the path, what could cause the spikes?

Is there anything in the filter drawer?

Armin Lukas · Dec 9, 2025, 10:33 PM
Do you observe this problem with all frames?
Is there anything in the path, what could cause the spikes?
Is there anything in the filter drawer?

It is happening tonight in all frames alright with varying degrees of annoyance depending on where the star is in the frame. There is nothing in the filter drawer and i dont see anything in the path that could cause it. I will upload some photos in a few mins of the effect in various parts of the FOV

I took test shots in various parts of the FOV. The wind is really bad so the tracking and guiding is abysmal(hence the elongated stars) but i think it seems as though the effect is far less in some parts of the FOV than others.

Pretty much non-existent on the left hand side of the frame for example. What do you guys think this could mean?

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John Walsh · Dec 9, 2025 at 10:23 PM
John Esser · Dec 9, 2025, 10:16 PM
What happens, when you rotate the camera? Do the spikes rotate as well? If so, it may be a problem with the sensor or humidity. If they don't rotate, the cause must be somewhere else in the image train.
I will try and see. Currently I am positioning the star in various parts of the frame to see what effect that has

📷 IMG_1631.png

It’s diffraction, not anything to do with the sensor. The pattern is very symmetrical which should be a hint. Are you sure the mask is still doing the job? There are clearly 4 symmetrically placed objects that are in the light path and the pattern tells exactly where to look.

Tony Gondola · Dec 9, 2025, 11:26 PM
It’s diffraction, not anything to do with the sensor. The pattern is very symmetrical which should be a hint. Are you sure the mask is still doing the job? There are clearly 4 symmetrically placed objects that are in the light path and the pattern tells exactly where to look.

Well that’s a relief at least. It only occurs to this extent when the star is in that upper right hand side position.

I’ve looked down the OTA. The mirror mask is totally covering the clips. I can’t see anything sticking out anywhere. The only thing i can possibly see is a hair or piece or web or something stuck to the flocking about 1/3 the way down the OTA

The target has passed the meridian now so the star is positioned in the opposite corner and displays none of the aberrant diffraction. I’m not quite sure what steps to take to try to get to the bottom of it to be honest

Have you tried looking down the tube from the edge of the tube to see if you can see the clips at a high angle? Anything maybe with the secondary? Any nuts holding anything that may be in the path? Does the Corrector go into the light path at all? Just the usual suspects.

I’ve seen diffractions from a power line that was near where the scope was placed, although never as symmetrical as this. But try to target stars in different parts of the sky, but placed on the same position of the sensor. That would at least rule out external sources of diffraction.

Cheers,

Wim

Rick Krejci · Dec 10, 2025, 03:27 AM
Have you tried looking down the tube from the edge of the tube to see if you can see the clips at a high angle? Anything maybe with the secondary? Any nuts holding the spider that may be in the path? Does the Corrector go into the light path at all? Just the usual suspects.

Hi Rick,

Yes I tried that and can’t see any of the clips from any angle at all.

The focuser doesn’t really intrude by any appreciable amount and the effect is only when a star is in that small area of the FOV so far as I can see.

The only thing might be the 2 screws that hold the focuser reinforcement plate. (Upper left of the image below).

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Wim van Berlo · Dec 10, 2025, 06:19 AM
I’ve seen diffractions from a power line that was near where the scope was placed, although never as symmetrical as this. But try to target stars in different parts of the sky, but placed on the same position of the sensor. That would at least rule out external sources of diffraction.
Cheers,
Wim

Thanks Wim,

I placed a few bright stars in the same location of the sensor and they exhibited the same effect. A bit left or right, up or down from there and the effect is minimised..

Those spikes are likely from the bolts' heads in the picture above (up-left). Why do you need a reinforcement plate at all?

andrea tasselli · Dec 10, 2025, 10:10 AM
Those spikes are likely from the bolts' heads in the picture above (up-left). Why do you need a reinforcement plate at all?

Thanks Andrea,

Apologies, the plate and screws are from the findershoe, not the focuser.

Given that the protrude so little into the OTA, how could they cause diffraction? I can’t see a straight line path that would go past the screws and still hit the primary mirror, given that it is masked. I could be misunderstanding the mechanism at work though.

After the flip, I also got a glorious reflection from Aldeberan in the same problematic corner of the FOV, which was not there before the flip.
I might try flocking the threads at the end of the coma corrector to see if that helps. I made a very long temporary extension to the dew shield when i saw that but it did not eliminate the reflection.
Looks like last night was a a real ‘Nox Horribilis’!

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Remove them and you'll see. From the picture they look pretty conspicuous to me (and still, I'd question the need of a plate for findershoe even more). My sympathies, terrible night here too.

andrea tasselli · Dec 10, 2025 at 12:44 PM
Remove them and you'll see. From the picture they look pretty conspicuous to me (and still, I'd question the need of a plate for findershoe even more). My sympathies, terrible night here too.

Sorry to hear that Andrea. They are currently holding the finder show in place where my guide cam is located.

I wonder could I cover them and the plate in the same black flocking that I used in the OTA?

Flocking won't avoid diffraction. And the finder shoe is the worst place to put a guide-scope on. As a test you could put a cardboard aperture ring to cover those bolts and see what that might yield.

andrea tasselli · Dec 10, 2025 at 01:26 PM
Flocking won't avoid diffraction. And the finder shoe is the worst place to put a guide-scope on. As a test you could put a cardboard aperture ring to cover those bolts and see what that might yield.

Hopefully will have a chance to test tonight. I thought if the screws were underneath a layer of flocking, there would be no hard edges to give and noticeable diffraction from?

It will change the pattern but not eliminate it. Any non circularity in the aperture will lead to some diffraction effect.

And I agree that a guider there isn’t ideal due to differential flexure, even with the plate.

Hi John,

Nice focus! What I think is bothering you is you are seeing the spiders in a fine focused image. They cause the spires that radiate from objects in the view of a Newtonian telescope. I believe I’ve seen them in other types of telescope’s images.

I’ve seen bi spires in my Refractors on occasion as well, just two spires in my case. Usually caused by a very sharp focus. So, in my refractors they come and go sometimes. I learned to ignore then because they disappear in stacking.

But the spires are something typical in a Newtonian, and some folks actually use post-processing software to insert spires into their images because they like them so much.

Here is more about Newtonians in another You Tube video.

SonnyE · Dec 10, 2025, 03:03 PM
Hi John,
Nice focus! What I think is bothering you is you are seeing the spiders in a fine focused image. They cause the spires that radiate from objects in the view of a Newtonian telescope. I believe I’ve seen them in other types of telescope’s images.
I’ve seen bi spires in my Refractors on occasion as well, just two spires in my case. Usually caused by a very sharp focus. So, in my refractors they come and go sometimes. I learned to ignore then because they disappear in stacking.
But the spires are something typical in a Newtonian, and some folks actually use post-processing software to insert spires into their images because they like them so much.
Here is more about Newtonians in another You Tube video.

Thanks Sonny. I’m used to the fairly sharp diffraction spikes alright but confused as the appearance of multiple radial spikes that only appear when the star is in that particular part of the frame.
Hopefully there will be opportunity to test more tonight!