Really big doughnuts - help needed

[almcl]

I've got some doughnut style artefacts in my images.  Not the mini dust doughnuts, but whole image light and dark circles.  They show up in individual lights and also in the stacked images, although different stacking programs react differently.  

Unfortunately they don't show up in the flats, which are also over correcting.

I'll  post separately about over correcting flats, but has anyone encountered anything like this?  Unfortunately, it's not a one-off phenomenon.  

The equipment is a Skywatcher 190 MN attached via an ASI OAG to an ASI 2600 MC.  The 2600 is running at -10°C, gain 100, offset 50 with the Anti-dew heater on.  APT version 3.87.9 is managing the captures at 180 seconds. There is an Astrozap dew shield protecting the corrector plate.

Here's a single, stretched, contrast enhanced sub to show the issue:

 

Here's what the whole DSS stack looked like, again stretched and contrast enhanced but otherwise unmolested:

Edited March 24, 2021 by almcl

[kirkster501]

No I haven't seen that before personally.  Looks like reflections occurring inside the optical tube.

Have you ever imaged with this scope before?

[almcl]

I have had the scope since last summer and initially used it with a Canon 700d, which doesn't/didn't display this phenomenon (at least as far as I can see).  I've tried the ASI 2600 with a 200 mm Canon lens and that doesn't do it, either.  Which has left me a bit stumped: the scope doesn't do it with a different camera and the camera doesn't do it with a different 'scope'.  

Perhaps, as you say, it's a reflection, although the 190 MN is quite well baffled, or some weird combination of the long dew shield/corrector plate/mirror is creating extra reflections around the 2ndry mirror?

Thanks for the thought!

[ampleamp]

Any chance it still might be dew? I know you said you have a heater and a shield, but I have had some weird artefacts on subs before which sorted once i realised i had messed up the dew control 

[tomato]

I note you have StarTools, would that be version 1.7? There is a synthetic flats tool in the Wipe module which might sort these, but that is I admit, dealing with the symptom, not the root cause.

I thought about vignetting but your DSLR has a bigger sensor than the ASI 2600 and that didn’t have the effect so I can’t see it being that. Have you always had the OAG or is that a recent addition?

Good call re possible dew, always worth a look at the plate and check it’s clear, I presume your dewshield is shielding the plate from any stray light?

[almcl]

1 hour ago, ampleamp said:

Any chance it still might be dew? I know you said you have a heater and a shield, but I have had some weird artefacts on subs before which sorted once i realised i had messed up the dew control 

Certainly a possibility, not at all sure how well the APT interface with the camera works - it is certainly at odds with ASI Image (the ZWO camera control software) over sensor temperature, so perhaps the anti dew device is mis-reported as well?

 

1 hour ago, tomato said:

I note you have StarTools, would that be version 1.7? There is a synthetic flats tool in the Wipe module which might sort these, but that is I admit, dealing with the symptom, not the root cause.

I thought about vignetting but your DSLR has a bigger sensor than the ASI 2600 and that didn’t have the effect so I can’t see it being that. Have you always had the OAG or is that a recent addition?

Good call re possible dew, always worth a look at the plate and check it’s clear, I presume your dewshield is shielding the plate from any stray light?

Yes, indeed StarTools 1.7.458 - the synthetic flats in inverse vignetting mode doesn't help, but that may be because of a problem with the flats.  I'll start a separate thread on that (!) but the two problems may well be linked.

The OAG is quite new, it's the ASI one - do you think something in that could be causing an issue?  The prism is fairly far out of the way 

The dew shield is doing quite a good job on stray light and the images were shot quite close to the zenith, but there is one of those pesky LED street lamps close by.  It had a nice shield on until the council came and replaced it with an almost completely ineffectual one.

At the moment I can just about tame things in StarTools but probably at the expense of some lost detail, and as you say dealing with the symptoms rather than the cause.

Edited March 24, 2021 by almcl

[Astro Waves]

I'm not gonna lie, when I saw that title and it wasn't about actual doughnuts I was pretty disappointed. Glad everyone else can help though 😎

[Davey-T]

Is this repeatable over different sessions ? looks similar to the effect of condensation somewhere in the imaging train.

Dave

[almcl]

It is unfortunately, Dave.  Looking at shots from early February I can see slightly less pronounced signs of it , but over the last few sessions it's getting quite pronounced.

I'll try a bench cooling test today (the %RH has dropped to an unseasonally low 53%) and see if I can find any misting on the sensor or its window...

[Davey-T]

56 minutes ago, almcl said:

It is unfortunately, Dave.  Looking at shots from early February I can see slightly less pronounced signs of it , but over the last few sessions it's getting quite pronounced.

I'll try a bench cooling test today (the %RH has dropped to an unseasonally low 53%) and see if I can find any misting on the sensor or its window...

If there's moisture trapped somewhere it will appear as soon as conditions change such as starting the camera.

Dave

[almcl]

Ran the camera at -15°C for an hour with the window uncovered and couldn't see any signs of condensation or ice anywhere.  Will have a close look at the rest of the imaging train tonight (corrector, secondary and main mirror).

Just wondering now if the offset may have any effect.  Using 50 at present but earlier on when the effect is less apparent it might have been lower. Not sure if the FITS header from APT records this. Must check.

[Froobyone]

I'm having almost exactly the same problem with a 127mm Apo triplet.

I've got dew heaters on the main objective and on the reducer and yet whatever I have in the optical train, .7 reducer/2" starlight filter wheel/269C Pro or .8 reducer/ZWO 1.25" filter wheel/183C Pro, I get the same really obvious rings in my images. It's driving me mad. I'll be following this to see if someone has any ideas. 

[almcl]

I tried reducing the offset from 50 to 10 and  I think it has reduced the rings a bit. It hasn't got rid of them and the test target (M42) is pretty forgiving, but I am going to experiment a bit more.