26 replies to this topic

[jlausuch]

After acquiring an Epsilon 160ed and being very motivated by using it, I found myself in a rabbit hole trying to figure out why my lights have a strange pattern in the middle of the frame.

 

Equipment:

- Takahashi Epsilon 160ed

- ZWO ASI 2600mm

- ZWO Filter wheel 7x2" and LRGBSHO Antlia filters

- ZWO OAG L + 220mm mini

- ZWO ASIAIR

This is the master Light:

 

If you take a look at the center, you will see some sort of crater-like artifact. After removing the stars it becomes more apparent:

 

This is the masterFlat:

 

which of course, does not show anything, but I learnt a trick to reveal all imperfections from the optics, by removing the gradient using DBE + Normalization:

And this is the result, after STF:

 

As expected, there are some dust spots out of focus, which are not appearing in the masterLight (that's why it's so important to make always flats), but there is a white circle in the middle with unknown origin.

 

When I stack all the lights WITHOUT flats, the problem becomes also visible. 

 

Which makes me wonder why the flats are not correcting this pattern. Well, they try, but they create some sort of crater, maybe due to variability of the size and shape of that white circle. 

 

 

By the way, this does not happen when there is high SNR, like Ha nebula, or rich nebula like Orion. This is mainly visible on low SNR frames, like sky background. But this makes the tube unusable for IFN, LDN objects or galaxies...

 

After a bit of troubleshooting, these are my findings:

======================================

1. Attaching the same camera to other 2 scopes (FLT91 and ACL200) -> the problem disappears, the flats only show dust spots, no white circle. 
2. Removing the camera from the tube and making flats. -> no problem here, the camera is fine (no oil leak or anything).
3. Attaching other cameras to the Epsilon (2600mc duo and 294mm) -> the problem persists, same white circle appears. I can discard it's a camera problem.
4. Removing the filter wheel and the OAG. The 2600mm is directly connected to the Corrector via M54 extensors to achieve backfocus -> the problem persist. I can discard filter or OAG reflections.
5. Covering the focuser and the back of the tube with opaque pieces of cloth -> the problem persist. I can discard any light leak from any component.
6. Changing focus and taking some flats -> the circle grows or shrinks accordingly.
7. Trying with bad collimation -> the white circle moves from the center.
8. After good collimation -> the white circle is centered

After isolating the problem, I am still not sure where the culprit is. For sure, something related to the telescope.

 

My final guess is that this white circle is actually the black mark in the secondary mirror. But why would Takahashi do such design?

 

This is how the collimation looks like (photo taken from the phone through the Takahashi specific Chesire eye piece):

 

 

Any thoughts?

 

 

 
 

Edited by jlausuch, 22 May 2024 - 11:14 AM.

[sharkmelley]

You've done some useful tests but I've no idea what could be causing that bright central ring.  Out of interest, are you using a dewshield/light shield?

[Erosenberger]

Shouldn't you be able to measure the size of the ring and calculate the distance to the source? Similar to calculating the distance of dust motes so you can determine if they are on the camera lens or a filter etc... This might help you determine if it is that black ring or not.

 

Below is a calculator on Astronomy.tools

 

https://astronomy.to...tion_calculator

[andysea]

I second Mark's suspicion that it may be related to ambient light. A dew shield is imperative with this scope.

 

What is the light pollution level that these images were taken in?

 

If the white circle were the black mark on the primary, the fact that it appears white would suggest that the flat is over correcting your light. 

I don't know if this applies to you but In my experience I can't use my epsilons at my light polluted home in Seattle. I get uncorrectable circular artifacts.

[jlausuch]

You've done some useful tests but I've no idea what could be causing that bright central ring.  Out of interest, are you using a dewshield/light shield?

Yes, I am using a dew shield which has the perfect length not to affect vignetting.

[jlausuch]

Shouldn't you be able to measure the size of the ring and calculate the distance to the source? Similar to calculating the distance of dust motes so you can determine if they are on the camera lens or a filter etc... This might help you determine if it is that black ring or not.

 

Below is a calculator on Astronomy.tools

 

https://astronomy.to...tion_calculator

I will. 

However, there are some tests I did to discard a camera or filters problem:

1) Remove filter wheel and OAG, connect only camera to the corrector (with extenders for proper backfocus) --> same issue

2) Connect the same camera to another telescope --> no issue

3) Connect another camera to the corrector --> same issue 

[jlausuch]

I second Mark's suspicion that it may be related to ambient light. A dew shield is imperative with this scope.

 

What is the light pollution level that these images were taken in?

 

If the white circle were the black mark on the primary, the fact that it appears white would suggest that the flat is over correcting your light. 

I don't know if this applies to you but In my experience I can't use my epsilons at my light polluted home in Seattle. I get uncorrectable circular artifacts.

I have tested it on bortle4 and bortle9. Both show the same pattern when shooting at "empty" piece of sky with low light signal (surroundings of a galaxy). When I shoot bright objects, (Orion in LRGB, or narrowband nebulas), the artifact seems to be masked by the high amount of signal.

One of the tests I have done is taking flats in a completely dark room and covering the tube with opaque cloths. The doughnut-shape halo was still present on the flats.

 

 

Anyway, if that artifact were caused due to the secondary dot, I don't understand why shows brighter area and not darker, as it should block some light.. 

[guanzz]

My e180 is often tested on the optical axis  on my balcony（bortle 7）, and I have never encountered this problem. I also did not use any lens hood. This problem is indeed strange. If it is determined that it is caused by the secondary dot, use alcohol solution to erase it. However, this method will make it impossible to use the tak method to recalibrate the optical axis later.

Edited by guanzz, 27 May 2024 - 06:09 AM.

[Shameless Stinkhorn]

I had problems with light leaks with my E160. What I did was take a dark exposure in a perfectly dark room. I taped over all the led lights and made sure it was really dark. Next, I turned the lights on and made the room very bright and took a second exposure. I then opened the images in Pixinsight and performed some pixel math. 2nd exposure/ first exposure. If the resulting image was not flat and I had leaks. I found the leaks by darkening the room and pointing a light at different parts of the ota and taking more exposures. 

I had to make a cover for the primary cell and my focuser needed a lot of attention. I notice you have the stock focuser. I went and looked at my stock focuser and can see a giant leak between the housing and the focusing tube. 

How are taking your flats?

BTW nice dew shield, is it flocked?

Dan

https://www.astrobin...ers/Hellbender/

[jlausuch]

I had problems with light leaks with my E160. What I did was take a dark exposure in a perfectly dark room. I taped over all the led lights and made sure it was really dark. Next, I turned the lights on and made the room very bright and took a second exposure. I then opened the images in Pixinsight and performed some pixel math. 2nd exposure/ first exposure. If the resulting image was not flat and I had leaks. I found the leaks by darkening the room and pointing a light at different parts of the ota and taking more exposures. 

I had to make a cover for the primary cell and my focuser needed a lot of attention. I notice you have the stock focuser. I went and looked at my stock focuser and can see a giant leak between the housing and the focusing tube. 

How are taking your flats?

BTW nice dew shield, is it flocked?

Dan

https://www.astrobin...ers/Hellbender/

 

One of the first things I checked is actually that, finding light leaks. I covered all the possible places where the tube could leak some light from outside and took some flats. The doughnut was still there.

 

The dew shield is actually from AliExpress, carbon fiber. https://es.aliexpres...5699808439.html
Very nice, but I need to do something, the screws are scratching the external blue piece. 

Edited by jlausuch, 27 May 2024 - 12:44 PM.

[sharkmelley]

The dew shield looks nice and shiny on the outside. What steps are taken to avoid glancing reflections on the internal surfaces?

[jlausuch]

The dew shield looks nice and shiny on the outside. What steps are taken to avoid glancing reflections on the internal surfaces?

It is matt in the inside, some sort of black velvet.. Anyway, I have tried to shoot without it and the issue is there still. I also shoot the flats without the dew shield, and it appears in the flats too... 

[jlausuch]

I have run a test with different exposures to get my lights in different ADUs, to see if the flats can properly correct the doughnut (I would prefer that it does not appear at all).

 

So have taken the following exposures with the Luminance filter:

200 x 10s

100 x 20s

66 x 30s

Then, I took flats with 2 different ADU levels, one at ~15k and the other at ~25k which is more or less the ADU I chose for my flats in any filter and camera.

As you can see, the doughnut is there. 

 

This is the Stack with 15k ADU flats:

 

 

This is the Stack with 25k ADU flats:

 

I don't really see any difference between any of those stacks... I still can see the artifact in all of them

 

In this example it does't seem very critical but it makes processing complicated when there is need to stretch the background for certain regions of the sky (LDNs or IFN).

 

The fact that this white halo appears there is what I would like to avoid in the first place, but I really don't find the root cause.

 

If there was an internal reflection o similar, is it possible to calculate the distance of that artifact given the diameter of that white doughnut?

[andysea]

Why is the donut in the flat white? The center dot on my epsilon is black.

[jlausuch]

Why is the donut in the flat white? The center dot on my epsilon is black.

I am not sure. I am tempted to think that it causes a defocused reflection from somewhere. If I take flats with different focus points, the doughnut eventually becomes a spot without a hole...  

[jlausuch]

I ran some more tests.

 

To check on artefacts on the flats, same process as before, DBE with normalization and 10 tolerance to remove the vignetting. 

 

1) 2600MM ---> Filter Wheel ---> Corrector only

 

The flats don't show the doughnut pattern.

 

Then, I used the same camera connected to different Telescopes I own.

 

2) 2600MM ---> William Optics FLT91

 

3) 2600MM ---> Askar ACL200

 

 

At this point I can conclude it's not a camera issue.

 

So, I went ahead and tried other cameras on the Epsilon:

 

4) 2600MC DUO ---> Epsilon

 

5) 294MM ---> Epsilon

 

 

So, all the cameras seem to show the same pattern. 

[jlausuch]

I checked the master Flats when I had the 2600mm connected to my C9.25 HD. 

 

 

They show a similar doughnut pattern!

BTW, the band you see at the bottom is due to the OAG, that's expected.

 

I have the 2600MC Duo now connected to the C9, and same:

 

 

So, now I start to think if this is actually an expected effect in telescopes with secondary mirrors....  maybe due to the secondary obstruction?

 

I would appreciate if someone with a Newton or SCT could check the flats from any session to see if they are similar... 

 

 

In any case, if this is expected, why the heck the pattern is not properly corrected by the flats? It creates some sort of bevel effect crater-like... 

 

 

 

 

[Dan_I]

I ran some more tests.

 

To check on artefacts on the flats, same process as before, DBE with normalization and 10 tolerance to remove the vignetting. 

 

1) 2600MM ---> Filter Wheel ---> Corrector only

 

 

The flats don't show the doughnut pattern.

 

Then, I used the same camera connected to different Telescopes I own.

 

2) 2600MM ---> William Optics FLT91

 

 

3) 2600MM ---> Askar ACL200

 

 

 

At this point I can conclude it's not a camera issue.

 

So, I went ahead and tried other cameras on the Epsilon:

 

4) 2600MC DUO ---> Epsilon

 

 

5) 294MM ---> Epsilon

 

 

 

So, all the cameras seem to show the same pattern. 

The 2600mc and 294 give donuts of different sizes... was the backfocus the same? Is the sensor to glass cover the same in both cameras?

 

I suspect there's a reflection somewhere in the imaging train (sensor, camera glass cover, corrector lenses ?)

[jlausuch]

The 2600mc and 294 give donuts of different sizes... was the backfocus the same? Is the sensor to glass cover the same in both cameras?

 

I suspect there's a reflection somewhere in the imaging train (sensor, camera glass cover, corrector lenses ?)

I guess due to the different sensor sizes, the 294mm is quite smaller than the 2600 series, so there is some crop factor, which makes the doughnut look bigger.

I also bet for a reflection, but I don't know how to find it out.

[Dan_I]

I guess due to the different sensor sizes, the 294mm is quite smaller than the 2600 series, so there is some crop factor, which makes the doughnut look bigger.

I also bet for a reflection, but I don't know how to find it out.

The 2600 sensor is only 1.23x wider than the 294 sensor and it seems that the donut size difference is much more than that?

[sharkmelley]

If I perform a DBE with the same parameters as you on my Tak E180ED master flat, I see a similar bright donut.  So I guess it's perfectly normal but I don't entirely understand the cause.  However, it calibrates my lights perfectly. 

 

But, if I take the master flat and displace it by 20 pixels using the PixelMath expression "pixel($T,x()+20,y())" then I can calibrate the original flat with the displaced flat and see a very similar "bevel effect crater" to yours.  Try it on your own master flat and see.

 

This suggests a potential cause of the incomplete calibration you are seeing.  If there is any kind of mechanical flex in your optical system e.g. focuser, filter wheel, camera adapter etc. then the flats could be displaced relative to the lights and cause the "crater".  I'm talking about only 0.1mm of displacement.

 

Mark

[jlausuch]

If I perform a DBE with the same parameters as you on my Tak E180ED master flat, I see a similar bright donut.  So I guess it's perfectly normal but I don't entirely understand the cause.  However, it calibrates my lights perfectly. 

 

But, if I take the master flat and displace it by 20 pixels using the PixelMath expression "pixel($T,x()+20,y())" then I can calibrate the original flat with the displaced flat and see a very similar "bevel effect crater" to yours.  Try it on your own master flat and see.

 

This suggests a potential cause of the incomplete calibration you are seeing.  If there is any kind of mechanical flex in your optical system e.g. focuser, filter wheel, camera adapter etc. then the flats could be displaced relative to the lights and cause the "crater".  I'm talking about only 0.1mm of displacement.

 

Mark

That would explain it. I found this thread, which has a similar issue with a different scope, there is also a good explanation from a member: https://stargazerslo...comment-3408131

May I ask you to post here an example of your flat with DBE?

 

[sharkmelley]

May I ask you to post here an example of your flat with DBE?

Sure. I use a full-frame sensor on my Tak E180ED so here is a version cropped to the same size sensor as yours:

 

 

[Edit]  I've just noticed a "Bas Relief" dust donut in your original image.  I'm quite familiar with the cause of that but I didn't realise that it can also lead to that "bevel effect crater" in the centre of an image, even with no dust.

Edited by sharkmelley, 04 June 2024 - 04:44 AM.

[jlausuch]

Sure. I use a full-frame sensor on my Tak E180ED so here is a version cropped to the same size sensor as yours:

 

TakE180_masterflatDBE.jpg

 

[Edit]  I've just noticed a "Bas Relief" dust donut in your original image.  I'm quite familiar with the cause of that but I didn't realise that it can also lead to that "bevel effect crater" in the centre of an image, even with no dust.

Thanks for confirmation, it really helps. Now I know that I need to be accurate with taking flats and making sure nothing moves or flexes.

 

Anyway, out of curiosity, I would really like to understand the reason for that doughnut-shape halo    

[jlausuch]

Thank you all for your answers.
Interesting, I also found this one
https://www.cloudyni...unt/?p=12439794
 

I actually have the PE200 which I use with the TC-40, good I can re-use it. Anyway, I'm not sure if the rig would be too high with the PE200 on top of the pier, maybe the PE70 instead of the PE200 would make it the whole thing lower down a bit.