Does anyone know what this aberration is from?

Equipment Artifacts Troubleshooting
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Terri avatar
Terri avatar
Hi Everyone, I've been dealing with this aberration  for some time now and can't seem to figure it out.    This is taken with an ASI6200MM with OAG, filter wheel and Chroma filters.   I get the same aberration for a particular target regardless of what filter, so doesn't seem to be the filter wheel or filters.  I don't see anything shiny inside of the camera or OAG area    It looks like a some kind of reflection but can't figure out what it's from.   I've flat blackened any screw heads.   It's not a simple star spike from off image as it has significant width and in this particular case, there is no bright star just outside the field.  This is just a snipped of the SE corner taken out of a 60s stretched  image of the witchhead in luminance, but I see the same thing in other images with different filters.        I can get it in any of the corners on the short side and different targets give me different locations and differing amounts of this aberration.   I don't see it on the long sides of the frames.      I've been looking all over the web but have not found a similar looking aberration.    

Any help or direction to help me locate this issue would be hugely appreciated.
Terri


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technoir avatar
technoir avatar
Is this with an SCT? SE or maybe NW? Rigel is very close to the witchhead. I have the exact same spikes when imaging the Horsehead with an SCT because of Alnitak.

Nvm, did not see you see it on other targets too.



Anyway here is my image. On first glance those spikes looked similar to me
John Hayes avatar
John Hayes avatar
That isn't an aberration.  It is stray light.  The source is an out of field bright star--or (less likely) a planet or the moon.  It is caused by the off-axis rays bouncing from surface that has a direct path to the sensor.  This is most commonly due to a baffling issue but it could also be due to a reflection from an inner connector surface as well.  I recently had a similar problem with my ASA600 and I wrote about how I dealt with it in processing here:  https://www.astrobin.com/poht3o/K/.

John
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Terri avatar
Terri avatar
John, thank you so much for the reply.    The moon was up, but not really very close in the field so moon light shouldn't have been entering the objective at that time.  I didn't mention the scope but it's a TEC 140 with AP QTCC reducer/flattener.       I guess it could have been Rigel, just to the east of the frame.    When you say a baffling issue, do you mean baffling in the scope itself, or baffling in the camera/filter wheel+OAG assembly?     The size of this stray light in my image is huge compared to that small mark in the frame in your image.    It's hard to compare them.     So is the likelihood that this is a large reflecting surface somewhere?      I have been using the QTCC for years and this just all of a sudden popped up.It's so odd as it looks like it's coming almost orthogonal to the path of light into the sensor and brightest on the edge of the sensor.      I've tried editing it out on various images but it's quite large and a bugger to get smooth and for the most part I only get it on broadband images, when I use narrowband filters I don't see it.      Best,Terri
Terri avatar
Terri avatar
technoir:
Is this with an SCT? SE or maybe NW? Rigel is very close to the witchhead. I have the exact same spikes when imaging the Horsehead with an SCT because of Alnitak.

Nvm, did not see you see it on other targets too.



Anyway here is my image. On first glance those spikes looked similar to me

Hi, it's very difficult to see the spikes you're referring to due to the image compression.    Are they the blue streaks to the left of the horsehead?    That does look similar, and even larger than my spikes.   Did you figure out how to fix it?

I am using  TEC140 with AP QTCC Flattener/reducer.


Terri
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technoir avatar
technoir avatar
Yes exactly. Sorry about the compression. That's an old image I coincidentally reprocessed just the other day, that's why I commented.
It was taken with a C5 and probably with the reducer. No OAG but guide scope instead.

Not sure if this is helpful advice, but what I would do in such a case is to deliberatly point near the brightest star you can get and then add one component by one to see if that makes a difference. Perhaps start with the simplest setup with just camera only and no guiding for testing purposes, and then build up? If we can have bets - mine is on the OAG : )

edit: I forgot to add, I never fixed the issue with the C5, it's probably just the way it is, so close to a bright star. I've switched scopes since then. In your case where the issue suddenly popped up it should be a different and fixable situation
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John Hayes avatar
John Hayes avatar
Terri:
John, thank you so much for the reply.    The moon was up, but not really very close in the field so moon light shouldn't have been entering the objective at that time.  I didn't mention the scope but it's a TEC 140 with AP QTCC reducer/flattener.       I guess it could have been Rigel, just to the east of the frame.    When you say a baffling issue, do you mean baffling in the scope itself, or baffling in the camera/filter wheel+OAG assembly?     The size of this stray light in my image is huge compared to that small mark in the frame in your image.    It's hard to compare them.     So is the likelihood that this is a large reflecting surface somewhere?      I have been using the QTCC for years and this just all of a sudden popped up.It's so odd as it looks like it's coming almost orthogonal to the path of light into the sensor and brightest on the edge of the sensor.      I've tried editing it out on various images but it's quite large and a bugger to get smooth and for the most part I only get it on broadband images, when I use narrowband filters I don't see it.      Best,Terri

All telescopes are equipped with stray light baffles of some sort to prevent off-axis light from reaching the sensor.  The most effective baffles in a refractor are glare stops.  At the rear of the scope, within your guider/filter/camera assembly, it's important to also have anti-reflection measures implemented.  Interior threads, black paint, and/or flocking paper are common methods.  This kind of problem is very specific to the location of the bright off-axis star and that's almost certainly why you've never seen it before. The level of the stray in my image is actually about a third of the frame but it was considerably fainter than what you have.  I used to have a stray pretty similar in brightness and size to yours when I shot M109 with my former C14 Edge system.  There just wasn't any way to solve the problem so I gave up on trying to image M109 with that scope.  This kind of problem can be very difficult to fix.

John
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Habib Sekha avatar
Habib Sekha avatar
technoir:
Is this with an SCT? SE or maybe NW? Rigel is very close to the witchhead. I have the exact same spikes when imaging the Horsehead with an SCT because of Alnitak.

Nvm, did not see you see it on other targets too.



Anyway here is my image. On first glance those spikes looked similar to me

My image looks like a copy of yours. Wanted to ask here and on a local Dutch forum about the stray light / reflections.


In my case I have a suspicion that the not so dull carbon fibre trusses on my CDK cause reflections, possibly enhanced by dew drops on them.

Going to follow the link John Hayes gave and read it,
Terri avatar
Terri avatar
John, Thank you again, I will inspect the inside of the tube and focuser.   There are numerous baffles inside of the TEC140 tube.    I'll try to see if any have bare metal or unpainted surfaces.    I'll also go through the camera assembly again, the spacers, OAG and the QTCC (which is a huge hunk of metal and glass).      On Cloudy Nights someone suggested it is a 6.6 mag star just outside the north east side of the sensor.     I would not have guessed a 6-7mag star would cause such an issue.  I would have guessed Rigel or something much brighter.   But the star pointed out is in a direct path to the stray light path signature on the sensor.  

Terri
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TiffsAndAstro avatar
TiffsAndAstro avatar
Is this amp glow?
John Hayes avatar
John Hayes avatar
TiffsAndAstro:
Is this amp glow?

No it is not.
TiffsAndAstro avatar
TiffsAndAstro avatar
John Hayes:
TiffsAndAstro:
Is this amp glow?

No it is not.

looks like 294 sensor amp glow.
Is it in the darks?
technoir avatar
technoir avatar
The newer sensors don't have amp glow, such as the sensors used in the 533, 2600 or the 6200 camera in question here
John Hayes avatar
John Hayes avatar
TiffsAndAstro:
John Hayes:
TiffsAndAstro:
Is this amp glow?

No it is not.

looks like 294 sensor amp glow.
Is it in the darks?

IMX455 based cameras don't have amp glow.  This is what a stray from an off-axis bright star looks like.
TiffsAndAstro avatar
TiffsAndAstro avatar
John Hayes:
TiffsAndAstro:
John Hayes:
TiffsAndAstro:
Is this amp glow?

No it is not.

looks like 294 sensor amp glow.
Is it in the darks?

IMX455 based cameras don't have amp glow.  This is what a stray from an off-axis bright star looks like.


Apologies I didn't see  mention of the specific camera.
Terri avatar
Terri avatar
John, With yours and help from Francesco on Cloudy Nights who suggested the same phenomenon and to find it suggested that I look in the focuser end for the reflection, I found the attached.     Previously I had always been looking for light leaks around the photon cage/OAG/Filter wheel as well as looking from the front of the scope with a flashlight.    
Some data I didn't share is that the issue was coincident with having a new optical tube and cell made for my TEC140 because I had some optical plane tilt that I could not fix and I have one of the very early TEC140s, number 53, which had no means of collimation.    Yuri offered to make a new tube and place my optics in a collimatable cell for a reasonable price.  
Unfortunately, this also coincided with using an ASG photon cage which I have thought for the last year the source of my issue.    I talked to Yuri initially about these issues and from that discussion didn't think it was the scope, but with the photon cage now off, and seeing this specular highlight, like the sun reflecting off of a chrome bumper,  I believe it's the new lens cell causing the issue.   I get specular highlights reflecting off the very edge of the lens cell.  In this case it's the sun, but suspect other point sources would do the same, just a lower level.       I took some images (which don't come out very well) and sent them to TEC and looks like I'm sending my scope back to Colorado.     I tried an astro-physics scope with the same test, and there is no reflection seen anywhere in the scope or lens cell no matter how close I get to the sun (obviously not towards the sun).    It's as dark as can be and no reflective surfaces or reflections seen.
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Philippe Barraud avatar
Philippe Barraud avatar
Technoir, I think that the stray light could come from Alnitak, a very bright star close to the Horsehead.
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technoir avatar
technoir avatar
Thank you, yes you are correct. I posted my image as an example of a nearby star reflection in response to the initial post. I understand the issue is about to be resolved too.
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Brian Valente avatar
Brian Valente avatar
@Terri  this is quite interesting and mirrors a number of other similar quests to find reflections in the imaging train

Is your resolution to have TEC examine and address whatever is the reflective surface in the ota?

Brian
Terri avatar
Terri avatar
Brian Valente:
@Terri  this is quite interesting and mirrors a number of other similar quests to find reflections in the imaging train

Is your resolution to have TEC examine and address whatever is the reflective surface in the ota?

Brian

Hi Brian, yes, I am in discussion with Yuri and have shared data.   I was hoping John Hayes would take a look at the specular highlight/reflection up at the lens cell edge and weigh in on whether it could cause the issue I'm seeing.    I know it isn't definitive but some of these guys, like John, are real optical experts.    I wouldn't ask Roland since it's not an AP scope, but when I tested my AP Stowaway the same way, there were zero reflections off the lens cell edge or anywhere in the optical tube for that matter.      The entire edge of the new lens cell seems to reflect light in a huge way on this rebuild.    Not sure if you read the whole thread or not, but the tube and cell were replaced with a version that can be collimated.  I still have my original glass in the new cell.   This was so the optics could be centered and made parallel to sensor plane.   As made originally, just for visual, it was slightly tilted.    This stray light popped up with the new tube, but I also started using that ASG Photon cage and my initial discussion with Yuri didn't make it seem like the scope tube could be the issue, so I've been dealing with it on broadband images (which are the minority of my imaging) for a year now.     It can really eat into the frame and significantly impact my FOV.   It's been difficult to edit out cleanly on many images.   Luckily, with my narrow band, I just use the RGB for stars and it doesn't really matter.   But when doing an RGB image, it's been a big issue.

best,
Terri
Brian Valente avatar
Brian Valente avatar
@Terri I had similar (and in my case much worse) reflections on an SVX80T. Vic/Stellarvue replaced the very early flattener with a newer (inhouse?) design, and it greatly improved things, but still have some of these issues. I suspect i will find similar types of reflective surface points like you have, but alas mine is in chile, so it's really difficult to troubleshoot that remotely.

Brian
Terri avatar
Terri avatar
Brian Valente:
@Terri I had similar (and in my case much worse) reflections on an SVX80T. Vic/Stellarvue replaced the very early flattener with a newer (inhouse?) design, and it greatly improved things, but still have some of these issues. I suspect i will find similar types of reflective surface points like you have, but alas mine is in chile, so it's really difficult to troubleshoot that remotely.

Brian

Well, the thing that helped is looking through the focuser end at the bright sky, which was recommended by a Francesco on Cloudy Nights, who also said it is stray light and likely from a baffle.  He also identified the star he suspects might be causing the stray light.  He did chime in after seeing the images and thought it could be the issue.    I'm hoping this is it!     I had been staring through the front, and trying to use a flashlight to identify any reflective surfaces which has been a total waste of time.   

Best,
Terri
John Hayes avatar
John Hayes avatar
Terri:
John, With yours and help from Francesco on Cloudy Nights who suggested the same phenomenon and to find it suggested that I look in the focuser end for the reflection, I found the attached.     Previously I had always been looking for light leaks around the photon cage/OAG/Filter wheel as well as looking from the front of the scope with a flashlight.    
Some data I didn't share is that the issue was coincident with having a new optical tube and cell made for my TEC140 because I had some optical plane tilt that I could not fix and I have one of the very early TEC140s, number 53, which had no means of collimation.    Yuri offered to make a new tube and place my optics in a collimatable cell for a reasonable price.  
Unfortunately, this also coincided with using an ASG photon cage which I have thought for the last year the source of my issue.    I talked to Yuri initially about these issues and from that discussion didn't think it was the scope, but with the photon cage now off, and seeing this specular highlight, like the sun reflecting off of a chrome bumper,  I believe it's the new lens cell causing the issue.   I get specular highlights reflecting off the very edge of the lens cell.  In this case it's the sun, but suspect other point sources would do the same, just a lower level.       I took some images (which don't come out very well) and sent them to TEC and looks like I'm sending my scope back to Colorado.     I tried an astro-physics scope with the same test, and there is no reflection seen anywhere in the scope or lens cell no matter how close I get to the sun (obviously not towards the sun).    It's as dark as can be and no reflective surfaces or reflections seen.

Terri,
Good job!  This is EXACTLY the right way to go about finding the source of the problem.  You've got to figure out how to blacken all that stuff to reduce stray light.  When you stack and stretch your data, it will amplify even the smallest amount of stray light--as you've seen.   You have to be very OCD about making sure that everything is as non-relective as possible within the entire image train.  Good luck with it!

John
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Terri avatar
Terri avatar
Thank you again John, I think the scope is going back to TEC to have them look at the lens cell.   Anything else I'd do myself, but not going to take chances on that.    Just waiting to hear back from Yuri.


Terri