extreme offset vignetting in flats and lights - Forums

Topic starter

17 days ago

This issue has persisted for a while now through at least 3 set ups and I honestly cannot for the life of me figure out what is causing this.

Here is an image of one of my flats which displays the extreme offset in vignetting in my imaging train which I originally thought was due to the filter wheel not centering filters over the sensor. The filter wheel seems spot on when I look at it move and stop and I’m using 2” unmounted chromas with a 2600mm pro.

I have tried moving various adapters to no avail and I my stars are round edge to edge so it cannot be tilt?? if it were tilt this bad, I would think I would notice instantly.

My imaging train at the moment is all m54 and m68 using an Optec sagitta OAG, planewave 0.66 reducer and ZWO filter wheel.

here is the flat in question giving me trouble📷 annoyingflat.pngUsually, they calibrate just well enough that my images are mostly flat but lately I keep getting over correction with them and honestly, I would rather just find the root of the issue.

scope is a planewave cdk that was spot on with collimation from factory according to skywave.

Only thing I can think of still is the filter wheel or something I’m my imaging train is way off center and I’m leaning toward the camera/filter wheel combo.

Any insight would be greatly appreciated as usual!!!

Paul Larkin

16 days ago

Hi, Anthony.

I am not an expert in resolving this, but I do not see in your post how you take your flats. Whether fabric over the front; flat panel; etc. If fabric or something similar, what do use to take them, e.g. NINA FlatWizard etc.?

I imagine those who could better help would probably want to know this, because it’s another set of variables in the flat making process and could therefore be another contributor to your problem.

Perhaps one aspect of checking might be to see if your stretched subs (maybe stacked) also show a similar degree of vignetting. If not, I would imagine it may be something other than just the image train, which from what you describe, is not so narrow as to be a likely culprit.

And finally, if this is a new problem (i.e. did not exist with this scope etc. before) then the question might be, what have you changed? Of course if you have had this problem since day 1, that is possibly a moot question.

All the best, Anthony. There will be some gurus here who will likely be able to help more.

Cheers.

Paul

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Brian Puhl

IOTD Staff

16 days ago

I know you said the filter wheel looks like it’s stopping centered, but it might be due to a unidirectional setting. I’m not familiar with ZWO, but if there’s an option for unidirectional, I would turn it off. It was an issue with QHY filter wheels for a bit, their fix was flashing new firmware that got rid of uni.

I’m almost willing to put money on it, almost. I just dealt with the same exact issue on a CDK14 with QHY wheel.

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pfile

16 days ago

what is the actual difference in pixel values between the dark part and the light part? it may look much worse than it actually is just due to the STF.

and on the topic of unidirectional vs. bidirectional filter wheel motion, at least with ZWO the advice has always been to run in unidirectional mode in order to assure repeatable filter registration. is this no longer the reccomendation?

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Anthony Grillo

Topic starter

16 days ago

Paul Larkin · Nov 9, 2025, 02:54 AM
Hi, Anthony.
I am not an expert in resolving this, but I do not see in your post how you take your flats. Whether fabric over the front; flat panel; etc. If fabric or something similar, what do use to take them, e.g. NINA FlatWizard etc.?
I imagine those who could better help would probably want to know this, because it’s another set of variables in the flat making process and could therefore be another contributor to your problem.
Perhaps one aspect of checking might be to see if your stretched subs (maybe stacked) also show a similar degree of vignetting. If not, I would imagine it may be something other than just the image train, which from what you describe, is not so narrow as to be a likely culprit.
And finally, if this is a new problem (i.e. did not exist with this scope etc. before) then the question might be, what have you changed? Of course if you have had this problem since day 1, that is possibly a moot question.
All the best, Anthony. There will be some gurus here who will likely be able to help more.
Cheers.
Paul

I have had this issue with multiple different setups but all with zwo cameras and filter wheels, I use and EL panel and Nina flat wizard to take my flats.

At first, I thought it was focuser sag or something like that because I take my flats with the scope horizontal with the roof closed facing the wall, but I was able to verify that not the case just by aiming the scope at zenith, plus I can see the vignetting slightly in my lights also

Anthony Grillo

Topic starter

16 days ago

Brian Puhl · Nov 9, 2025, 04:00 AM
I know you said the filter wheel looks like it’s stopping centered, but it might be due to a unidirectional setting. I’m not familiar with ZWO, but if there’s an option for unidirectional, I would turn it off. It was an issue with QHY filter wheels for a bit, their fix was flashing new firmware that got rid of uni.
I’m almost willing to put money on it, almost. I just dealt with the same exact issue on a CDK14 with QHY wheel.

did not even consider this!! I have a qhy268m with filter I was going to test also but I will go and uncheck that right now.

thanks

Anthony Grillo

Topic starter

16 days ago

pfile · Nov 9, 2025, 04:43 AM
what is the actual difference in pixel values between the dark part and the light part? it may look much worse than it actually is just due to the STF.
and on the topic of unidirectional vs. bidirectional filter wheel motion, at least with ZWO the advice has always been to run in unidirectional mode in order to assure repeatable filter registration. is this no longer the reccomendation?

dark side is 0.40 and middle of the flat registers around 0.46. I am going to do some more research on the filter wheel issue, but it seems it is something related to settings and I’m getting conflicting answers between bidirectional and unidirectional. I will probably just recalibrate and then uncheck or check whatever option I have for direction

Brian Puhl

IOTD Staff

16 days ago

pfile · Nov 9, 2025, 04:43 AM
and on the topic of unidirectional vs. bidirectional filter wheel motion, at least with ZWO the advice has always been to run in unidirectional mode in order to assure repeatable filter registration. is this no longer the reccomendation?

As I understand it, it’s kinda like a backlash thing. If you run it in the same direction always, it’s not an issue.

Willem Jan Drijfhout

16 days ago

Have you tried calibrating the ZWO filter wheel? Sometimes they can develop a little bit of an offset, and running the calibration routine can sort that out. If you do, don't worry about the strange noises coming from the filter wheel. It is quite a noisy process, but should come out fine.

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Geert Vanhauwaert

16 days ago

Hello

Have you checked that the cental baffle tube is correctly placed centered and perpendicular to the primary mirror? For me, il looks as of the light path is partially obstructed by the tube.

Also, even when the telescope is collimated, it can be that the primary and/or secondary mirrors are not centered in the tube or trust, so that the light path is partially obstructed by the top ring.

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pfile

16 days ago

Brian Puhl · Nov 9, 2025, 06:14 AM
pfile · Nov 9, 2025, 04:43 AM
and on the topic of unidirectional vs. bidirectional filter wheel motion, at least with ZWO the advice has always been to run in unidirectional mode in order to assure repeatable filter registration. is this no longer the reccomendation?
As I understand it, it’s kinda like a backlash thing. If you run it in the same direction always, it’s not an issue.

ok. but if i understand your original post you’re saying that QHY has a bug with unidirectional motion, so they removed the option entirely from the firmware but the driver still lets you set the unidirectional bit, leading to some kind of undefined state in the filter wheel?

Matias Svenningsson

15 days ago

Hi,

Same issue and found this post and started to investigate on my scope, a SW 150 PDS.

I have collimated it to perfection (i thought).

I always use a laser and stars and gotten very good results with sharp stars but I have had this bad vignetting from one side and always figured it was a street lamp at the corner of my garden.

BUT when googling i made a DYI collimation cap and then I saw my secondary mirror was not in center of the focuser tube. Re-aligned it and collimated and now the vignetting is more centered.

It is not perfect but much better and Im getting there eventually. I made an aggressive stretch to make it more vivible in the pictures below.

Hope this helps.

📷 image.png

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