Ring pattern in images with Newtonian telescope - Forums

Alright, I'll try and post here as well as a last resort before I throw my scope out.

I am having issues with rings in my broadband images using my 200PDS. I have spent close to 5 months trying to diagnose and fix this issue and I've now run out of ideas.

Things I have done to my scope:

Dew shield
Flocked the tube and drawtube
Flocked the CC both top and bottom
Aperture Mask installed (also flocked the face of this)
Painted secondary black (along with white rim of the mirror)
Placed cork between the primary and the clips to stop it from moving
3D printed a light shield for the focuser so that no stray light can enter the drawtube (this was also flocked)
Bottom of the scope and the mirror cell is covered up with a 3d printed cover and a cap.
All screws and cracks are taped up using electrical tape

Also:

Entire imaging train is covered with aluminum and a neoprene sock
I take flats mid run to make sure everything is in the same position as when imaging the target.

This is my latest image taken last night with all of the above improvements. It's a stack of 80x20s subs + calibration files using the 200PDS and 294MM-Pro.

What I have concluded:

It is NOT a light leak (trust me I've gone through the entire tube multiple times and I can take darks by just covering the top.)
It is NOT the CC (I've gone through 3 CC's and they all more or less give the same results. MPCC MkIII, TSGPU and TSGPU 2.0)
It must be a reflection somewhere but I can't for the life of me figure out where... I use a 3d printed light shield that covers the drawtube from getting hit by stray light and I've flocked EVERYTHING possible including all adapters and extenders in my imaging train.

More images can be provided for anything that I've described here.

This is a last plea for help before I sell this scope and mount and downgrade to widefield setup.

just to check do all filters produce the same pattern/issue?

Sean van Drogen:
just to check do all filters produce the same pattern/issue?

Yes, LRGB all produce this pattern although to some varying degree of strength. Oiii also has issues but Ha and Sii are fine I suspect this is just due to the wavelength they let in. Lum is by far the worst. Can also add that I've tried without filter as well and it's still there.

What's your flat field procedure ? Ive got rings and it turned out that the flat was taken at different focus than the light. To get a handle on this you could take two flats with different focus and divide them with pixelmath. If you then get such patterns, this will likely be the cause.

Götz Golla:
What's your flat field procedure ? Ive got rings and it turned out that the flat was taken at different focus than the light. To get a handle on this you could take two flats with different focus and divide them with pixelmath. If you then get such patterns, this will likely be the cause.

As I mentioned in the post, I take my flats mid run. So everything is in perfect focus and all the mirrors are in the same position as if they were taking lights. I use a light pad to take flats in the middle of the night.

OK, sorry for not reading your post to the end.

Very interesting problem and I wish I had a good answer for you. I'm wondering if it's a property of the mirrors. Does rotating the primary mirror change the ring in any way?

Does the flat reproduce the same ring pattern (it seems they are multiple, which is a kind of record I guess) that of your lights? If so then you should be able to correct it once you get the flat procedure nailed. If, otherwise, the flat does NOT reproduce the same pattern then the thing is related to the position of the object, in our case at infinity. It could be a number of things but let's look at the flats first.

This is my latest image taken last night with all of the above improvements. It's a stack of 80x20s subs + calibration files using the 200PDS and 294MM-Pro.

Hi, can you post a picture WITHOUT calibration (to rule out the role of the flats in getting taht problem and not 'mask' the potential cause as well) ?

Have you tried with another sensor as well (a DSLR or other planetray camera you would have) ? What is the effect of rotation the sensor ?

Jérémie:
This is my latest image taken last night with all of the above improvements. It's a stack of 80x20s subs + calibration files using the 200PDS and 294MM-Pro.

Hi, can you post a picture WITHOUT calibration (to rule out the role of the flats in getting taht problem and not 'mask' the potential cause as well) ?

Have you tried with another sensor as well (a DSLR or other planetray camera you would have) ? What is the effect of rotation the sensor ?

Uncalibrated image (after stretch):

andrea tasselli:
Does the flat reproduce the same ring pattern (it seems they are multiple, which is a kind of record I guess) that of your lights? If so then you should be able to correct it once you get the flat procedure nailed. If, otherwise, the flat does NOT reproduce the same pattern then the thing is related to the position of the object, in our case at infinity. It could be a number of things but let's look at the flats first.

The flat also shows a dark spot in the middle, however it seems to be almost inverted.

Image of master flat (stretched and DBE):

And a single uncalibrated sub (uncalibrated straight from camera):

OK. The issue is that:

1. Your flat calibration isn't good enough. It overcorrects the light frames.
2. You should definitely clean your imaging train, less shadows to deal with. This means sensor front window, filters and CC surfaces.

andrea tasselli:
OK. The issue is that:

1. Your flat calibration isn't good enough. It overcorrects the light frames.
2. You should definitely clean your imaging train, less shadows to deal with. This means sensor front window, filters and CC surfaces.

This is not the issue. The central spot is not a dust mote. It changes size and location depending on focus and collimation. I KNOW that my flat calibration is good enough but I think something, somehow is moving between the 10 minutes that I take the flat and when the stack finishes.

I spent 2 months doing nothing but flats to figure out the optimal settings and procedures for my camera. That means taking 1500 flats for each histogram value in different configurations with different methods (light box, t-shirt, sky flats. lum flats. etc).

If you look at the uncalibrated in my reply and the calibrated image in my OP the dust motes are perfectly calibrated out but the reflection is not. So this is not a straight forward calibration issue.

With my lack of Newtonian knowledge I can only look at the geometry of the issue. Ignoring the diagonal and the upper right corner for a moment, the main issue seems to be sort of a proper circle which is a bit offset from the centre of the image:

So anything in the imaging train that fits this specs and which you touched is the main suspect.

Edit: The fact that the issue is bright in the upper half and dark in the lower half may provide another hint.

Clear skies
Wolfgang

With my lack of Newtonian knowledge I can only look at the geometry of the issue. Ignoring the diagonal and the upper right corner for a moment, the main issue seems to be sort of a proper circle which is a bit offset from the centre of the image:

So anything in the imaging train that fits this specs and which you touched is the main suspect.

Clear skies
Wolfgang

Thanks for your input Wolfgang, This is as I suspect where the reflection is coming from. I first thought it was the inside wall of the CC which fits your diagram due to somewhat off collimation. But after flocking this it didn't improve. Then I thought it was the white outline of the secondary, same result. I settled on the "face" of the focuser and 3D printed a "light shield" inlay to sit on the CC which I flocked.

See images on how it looks when shining a bright light on the top of the protruding light shield (note the circular reflection in the reflection of the secondary on the main mirror):

The dust motes might have been removed (but not completely, see the one on the far right side) but if the flats were done properly you should be able to remove ALL non-uniformities. First, sky flats if done properly, are the best way around many issues (although not all, admitedly). Secondly, it appears the your flats overcorrects the top end and undercorrects the lower end, which suggests something is tilting. What that might be I can't say. You might try pointing the scope at the zenith at dusk, shoot a series of luminace flats and then point it away form the zenith and shoot another series of flats. Created the master flat with the first series. Then calibrate the first series with the master flat. You should get a series of blank frames. Then do it again for the second series. Does it perfectly calibrate out. Either way that should offer insight on the issue. Which brings the final question. How do you calibrate the master flat frame?

i know i have same thing, when some light falls on the bottom of primary mirror, even it is covered with reflecting materials, tit still let trough some light from bottom, in most cases it is laptop monitor, that is shining below telescope, when i close the lid everything goes back to normal.

One observation: that uncalibrated image with such a tight (small) dark centre doesn't look like any of mine. I also have the 200PDS with Skywatcher CC and filter wheel. Please would you post a photo of your imaging train (camera, filter wheel to focusser). FWIW: My 200PDS has none of the mods you have tried and does not behave like yours. I also have a 4/3rds mono sensor although mine is the older ASI1600MM Pro.

andrea tasselli:
The dust motes might have been removed (but not completely, see the one on the far right side) but if the flats were done properly you should be able to remove ALL non-uniformities. First, sky flats if done properly, are the best way around many issues (although not all, admitedly). Secondly, it appears the your flats overcorrects the top end and undercorrects the lower end, which suggests something is tilting. What that might be I can't say. You might try pointing the scope at the zenith at dusk, shoot a series of luminace flats and then point it away form the zenith and shoot another series of flats. Created the master flat with the first series. Then calibrate the first series with the master flat. You should get a series of blank frames. Then do it again for the second series. Does it perfectly calibrate out. Either way that should offer insight on the issue. Which brings the final question. How do you calibrate the master flat frame?

I would disagree. Flats do not remove light pollution or local light pollution which I believe is causing this reflection. If the reflection moves from taking the flat to taking the light no matter how good your flats are it will not correct it. This is why I see the "embossed" effect on the final calibrated stack. This also explains the far right dust mote since it is by far the most extreme of them all. Being in the far corner and being darker than the rest it would be more susceptible to inconsistencies in movement. As to speak to the tilt. Yes, you are correct. I had a tilt issue when running this test since I had to resort to using the skywatcher original two screw "clamp" mechanic to attach the camera. So the top/bottom inconstancies are due to this.

All my calibration is done through Pixinsights WBPP.

Although, suppose you are correct in your assumption that somehow my flats are not good enough. I do not see any other way to do them. I literally take them within minutes of taking the lights and still they do not correct, any suggestions?

i know i have same thing, when some light falls on the bottom of primary mirror, even it is covered with reflecting materials, tit still let trough some light from bottom, in most cases it is laptop monitor, that is shining below telescope, when i close the lid everything goes back to normal.

I can with certainty say that there is no light leaking from the mirror cell into the system. I put the scope on an industrial work light with my cap on and saw nothing when looking down the tube.

Andy Wray:
One observation: that uncalibrated image with such a tight (small) dark centre doesn't look like any of mine. I also have the 200PDS with Skywatcher CC and filter wheel. Please would you post a photo of your imaging train (camera wheel to focusser).

The scope is currently outside under my telegizmo cover and it's raining so I can't take a picture of it. This is the latest picture I have from last week when I had it in and was doing some modifications. The imaging train is the following: ASI294MM-Pro - 1mm spacer - EFW - 11mm spacer - OAG - TSGPU CC - focuser

Andy Wray:
One observation: that uncalibrated image with such a tight (small) dark centre doesn't look like any of mine. I also have the 200PDS with Skywatcher CC and filter wheel. Please would you post a photo of your imaging train (camera, filter wheel to focusser). FWIW: My 200PDS has none of the mods you have tried and does not behave like yours. I also have a 4/3rds mono sensor although mine is the older ASI1600MM Pro.

I have also never seen a *dark* spot in the center of uncalibrated images. But maybe it is due to your OAG. Have you already looked in detail at your OAG (stability, reflections etc.) ? Not using an OAG (but rather a finder scope) may be a relatively inexpensive option before dumping your telescope alltogether.

Götz Golla:
Andy Wray:
One observation: that uncalibrated image with such a tight (small) dark centre doesn't look like any of mine. I also have the 200PDS with Skywatcher CC and filter wheel. Please would you post a photo of your imaging train (camera, filter wheel to focusser). FWIW: My 200PDS has none of the mods you have tried and does not behave like yours. I also have a 4/3rds mono sensor although mine is the older ASI1600MM Pro.

I have also never seen a *dark* spot in the center of uncalibrated images. But maybe it is due to your OAG. Have you already looked in detail at your OAG (stability, reflections etc.) ? Not using an OAG (but rather a finder scope) may be a relatively inexpensive option before dumping your telescope alltogether.

It's not the OAG, I had issues with it showing before but it would only darken one corner and was easily fixable. The dot in the center is something else.

@Andy Wray Would it be possible to ask for some samples of your data? A light frame and a flat frame using Lum filter?

How severe was the effect with the MPCC?

This looks like a known issue with the long GPU corrector, especially in light pollution. The GPU front element will generate reflection artefacts when stray light or light pollution hits the front element. The result is a ring-shaped reflex that cannot be corrected with flats. That's why I suspect flocking of the corrector tube does not prevent the issue.

This topic on cloudynights shows the reflections very well https://www.cloudynights.com/topic/654342-two-years-and-counting-dealing-with-this-image-artifact-help/

ONTC telescopes have 3 different mounting positions for the primary mirror cell. Mine was designed with the native focus either 115, 145 or 170 mm above the tube wall. I previously used the GPU corrector on the top position (170 mm) to push the corrector all the way inside the focuser, which should reduce the reflections from the front element as much as possible. One downside of moving the primary all the way up, is that the camera hangs very far away from the tube and this increases the risk for tilt issues.

The scope is currently outside under my telegizmo cover and it's raining so I can't take a picture of it. This is the latest picture I have from last week when I had it in and was doing some modifications. The imaging train is the following: ASI294MM-Pro - 1mm spacer - EFW - 11mm spacer - OAG - TSGPU CC - focuser

It's probably worth comparing notes as it looks like our two setups are quite similar and I do not have that dark centre/light outside issue on my uncalibrated images. My setup is:

ASI1600MM Pro (very similar in terms of sensor size and resolution and the same 6.5mm distance to the sensor
Camera screwed directly into EFW7 Mini filter wheel (no spacer)
I'm using the ZWO 1.25" screwed in filters
I actually use a 9mm spacer and then a few others to get me to the required 56mm backfocus (I had to do this as the OAG prism was catching on the long thread of the coma corrector)
I use the Skywatcher coma corrector for F5 Newtonians
I also use the ZWO OAG V2 although I bought the helical focuser with it
Not important here, but I have the ZWO EAF 5V also having flipped the focusser by 180 degrees

What are the differences (apart from the obvious one of the coma corrector)?

Victor Van Puyenbroeck:
How severe was the effect with the MPCC?

This looks like a known issue with the long GPU corrector, especially in light pollution. The GPU front element will generate reflection artefacts when stray light or light pollution hits the front element. The result is a ring-shaped reflex that cannot be corrected with flats. That's why I suspect flocking of the corrector tube does not prevent the issue.

This topic on cloudynights shows the reflections very well https://www.cloudynights.com/topic/654342-two-years-and-counting-dealing-with-this-image-artifact-help/

ONTC telescopes have 3 different mounting positions for the primary mirror cell. Mine was designed with the native focus either 115, 145 or 170 mm above the tube wall. I previously used the GPU corrector on the top position (170 mm) to push the corrector all the way inside the focuser, which should reduce the reflections from the front element as much as possible. One downside of moving the primary all the way up, is that the camera hangs very far away from the tube and this increases the risk for tilt issues.

I've been following that post and many other on cloudynights with this exact issue. This however, doesn't seem to be my case I have the inside of the CC flocked, both back and front:

As well as a light shield: https://www.thingiverse.com/thing:5404022 that sits on top of the focuser tube and protrudes into the light path blocking any and all light from hitting the CC directly. My CC also sits very far in the focuser tube itself.