C11 Unexpected Diffraction Spikes - Forums

Topic starter

Jul 7, 2025

Greetings!

I'm getting geared up for a run on M57 and was reviewing some subs from my first night of images. I noticed that I've got some diffraction spikes on the stars and I'm not sure where those are coming from. I don't see anything obstructing the corrector plate, so I'm a little confused. I was expecting round stars and instead I'm seeing vertical spikes.

My imaging train is nothing too complicated. C11---> Starizona LF Reducer---> Starizona 91mm imaging spacer---> Antlia RGB Ultra tri-band filter---> ZWO ASI2600 MC Duo pro

I tried a quick pass with blur/noise exterminator, no luck. I also tried verified that the spikes exist in the un-stretched subs, so there's something in the optical path I would think. I also used ASTAP to measure tilt just in case something was out of whack in the imaging path, but the report shows no tilt. Could this be a collimation issue, or should I assume with no tilt that collimation is also good?

What am I missing?

Jeramie

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andrea tasselli

Frequent contributor

Jul 7, 2025

I had something akin to that a long time ago when I had a large Cass. It turned out it was a heat plume from the baffle tube. To be sure I'd need to see an unprocessed but calibrated frame.

Tobiasz

Jul 7, 2025

Are you using the celestron dew heater ring? I read somewhere that too much heat can warp your schmidt plate and produce abberations.

Regards

Jeramie

Topic starter

Jul 7, 2025

Tobiasz:
ch heat can warp your schmidt plate and produce abberations.

I am! I'm using the Celestron Dew Heater controller for it. It supposedly measures dew point and applies the correct amount of power to keep dew off. Maybe it's sending too much power.

andrea tasselli:
I had something akin to that a long time ago when I had a large Cass. It turned out it was a heat plume from the baffle tube. To be sure I'd need to see an unprocessed but calibrated frame.

I went to grab the frame and noticed in my file hierarchy that I didn't load up any flats for this one. Let me throw some flats on this real quick and see if that takes care of it. The corrector plate is rather clean but let me rule that out really quick and get back to you.

Supportive

GalacticRAVE

Jul 7, 2025

try it with the dew heater ring switched off and see whether you get better stars. For some additional insight, the following video may be interesting

https://www.youtube.com/watch?v=kR04U28iiZM

CS

Matthias

Jan Erik Vallestad

Jul 7, 2025

I have something along these lines as well. I have written it off as a result of dew heating too. A compromise I've gladly accepted after some even worse experiences.

Jeramie

Topic starter

Jul 7, 2025

I'm going to see how I can lower the temp to an acceptable level and monitor the effects. I also run the C11 dew shield so I should be able to lower the temp considerably.

Hoping to get some clear skies in the next couple of days and test more.

Michael Regouski

Jul 7, 2025

I had that same thing happen to my 11" edge. It seemed to be wind gusts and guiding problems as in too much slop in the mount especially if it close to it's weight limit.

Brian Valente

Jul 7, 2025

IMO the most likely candidate is a small obstruction

blink through your subs and see if they are limited to specific subs. I would also inspect the ota for spider webs, errant accessories, things protruding into the path, etc.

The other 'smoking gun' is the spike is on one side and then the other when doing a meridian flip

Brian

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Willem Jan Drijfhout

Jul 7, 2025

Brian Valente:
IMO the most likely candidate is a small obstruction

blink through your subs and see if they are limited to specific subs. I would also inspect the ota for spider webs, errant accessories, things protruding into the path, etc.

The other 'smoking gun' is the spike is on one side and then the other when doing a meridian flip

Brian

That was also my first association. Could even be obstructions outside the scope, such as a lamp-post, or a twig of a tree. But then only very few subs would be affected.

Brian Valente

Jul 7, 2025

Willem Jan Drijfhout:
Could even be obstructions outside the scope, such as a lamp-post, or a twig of a tree.

Something that far from the ota would not even show up as a spike, probably just a dimming of the target. These seem small enough to be within the dew shield or closer

B

Willem Jan Drijfhout

Jul 7, 2025

Brian Valente:
Willem Jan Drijfhout:
Could even be obstructions outside the scope, such as a lamp-post, or a twig of a tree.

Something that far from the ota would not even show up as a spike, probably just a dimming of the target. These seem small enough to be within the dew shield or closer

B

Perhaps it is different with a big reflector, but on my 130mm refractor such spikes always show up when I hit some parts of a nearby tree. A clear indication to stop, because after a few more frames, most signal will be gone.

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Brian Valente

Jul 7, 2025

>>>on my 130mm refractor such spikes always show up when I hit some parts of a nearby tree.

yeah i can see that. The OP description and image looks like it's more consistent and less a transient thing, but if he finds it imaging into the trees and fading out, and he included a bunch of those subs, that's a possibility. I think it's more likely an obstruction. I've seen tons of this kind of thing.

just an example, this was an errant cable in a hyperstar setup. it was only on one side, but the flip produced a similar 'saturn-style' artifact

https://app.astrobin.com/i/r0g2mv

Jeramie

Topic starter

Jul 7, 2025

I’ll need to see which part of the sky the scope was pointed to, but generally there are no trees or anything nearby. I also run a very clean setup wire-wise, nothing hanging in front of the corrector at all.

Let me take a look at a few things and come back to this. My money is on the “auto” setting for the dew heater over-heating the corrector.

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Rick Veregin

Jul 7, 2025

One other thought. I have a filter with a small scratch on it–it produces a linear spike like this especially on bright stars, and since the scratch was toward the edge of the filter, the spikes only showed up on that side of the image, not across the whole image.
Rick

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Tommy Mastro

Jul 7, 2025 · Edited: Jul 7, 2025

C11.jpg

My C11 has an almost identical spike as well (Bright star on right side of attached image). It was worse before I collimated and centered the OTA. Then the spike wasn't "as bad" but was still obviously present on bright stars.

It affects all my subs.

Any chance this is a manufacturer's defect? My OTA is at least 15 years old (deforked). Carbo Fiber OTA.

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Brian Valente

Jul 7, 2025

Tommy Mastro:
C11.jpg

My C11 has an almost identical spike as well (Bright star on right side of attached image). It was worse before I collimated and centered the OTA. Then the spike wasn't "as bad" but was still obviously present on bright stars.

It affects all my subs.

Any chance this is a manufacturer's defect? My OTA is at least 15 years old (deforked). Carbo Fiber OTA.

I don't really see the identical spikes on this image. These are almost always obstructions, scratches, etc. in the optical path. There are more SCTs without this kind of thing than with, so imo a 15+ year old undiagnosed optical defect across multiple versions of the ota seems unlikely to say the least.

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Tommy Mastro

Jul 7, 2025

You're probably right.

But the spike is there partially covered by the platesolving. It was more prominent before I had the OTA serviced.

MaksPower

Jul 8, 2025

I do see it esp on the briught stars - they all have a spike at 2 o-clock.
My suspicion is that this is due to a thermal plume.

Tommy Mastro

Jul 8, 2025

Yeah but then it wouldn’t be consistently present for each session. And I don’t use a heater with it. Just a unpowered dew shield.

Michael von Berner-Purgstall

Jul 8, 2025

Jeramie:
Greetings!

I'm getting geared up for a run on M57 and was reviewing some subs from my first night of images. I noticed that I've got some diffraction spikes on the stars and I'm not sure where those are coming from. I don't see anything obstructing the corrector plate, so I'm a little confused. I was expecting round stars and instead I'm seeing vertical spikes.

My imaging train is nothing too complicated. C11---> Starizona LF Reducer---> Starizona 91mm imaging spacer---> Antlia RGB Ultra tri-band filter---> ZWO ASI2600 MC Duo pro

I tried a quick pass with blur/noise exterminator, no luck. I also tried verified that the spikes exist in the un-stretched subs, so there's something in the optical path I would think. I also used ASTAP to measure tilt just in case something was out of whack in the imaging path, but the report shows no tilt. Could this be a collimation issue, or should I assume with no tilt that collimation is also good?

What am I missing?

Jeramie

Good Morning!

Thank you for that post...as I've recently started Deep Sky imaging with my C14 Edge HD that problem occured to me too - of course I do have bought that Celestron dew heater ring too. Now it does make sense why sometimes everything is fine and why in the same night, same object that spikes happen.

Jul 8, 2025 · Edited: Jul 8, 2025

That's the dew heater ring uneven heating effect, the glass gets too hot, expands, and causes that. I'm currently investigating a seemingly permenant abberation in my system that I believe could possibly be caused by warping in the glass from this dew heater ring. You should not run this ring over 25-30%.

I'm going to test out different optical incidence angles to see if it's tilt, induced abberation, or the plate itself over the next few days and will likely update this with my findings.

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Blue

Jul 8, 2025

I saw some odd spikes at one point when I used to shoot with a C925, they turned out to be from a spot of lint that had found it's way on to the secondary mirror. Impossible to spot without removing the mirror. Obviously that is easier on later scopes like mine was which have the removable secondary for a Fastar/Hyperstar setup.

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MaksPower

Jul 8, 2025 · Edited: Jul 8, 2025

Even without a heater, your whole OTA is warm during the day. in particular the primary mirror.

When you set up at night the topside of the OTA is radiating to the night sky, unless you use an insulated wrap.

The primary is supported only on the central baffle (aluminium) and hence heat flows from the mirror to the baffle, and a convection current (plume) transfers heat from the that to the OTA metal.

A while back someone used a thermally insulating wrap imaging camera on an SCT to confirm the source of the heat is the mirror annd baffle, and that the top of the OTA was quite cold.

To reduce the plume apply a thermally insulating wrap (Reflectix or similar). Slowing the loss of heat also prevents dew for longer.

Also there is no need to cool an SCT.

There are many threads about this on CN going back 20 years.

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Tony Gondola

Frequent contributor

Jul 8, 2025

Brian Valente:
Willem Jan Drijfhout:
Could even be obstructions outside the scope, such as a lamp-post, or a twig of a tree.

Something that far from the ota would not even show up as a spike, probably just a dimming of the target. These seem small enough to be within the dew shield or closer

B

Actually, it can. I see all kinds of odd diffraction spikes when I accidently get into a branch on my tree infested site.