Chromatic Aberration in SW 80ED or something else?

[jjosefsen]

Hello,

I have noticed what I think is chromatic aberration in my images, and I have a few questions regarding it.

Lets assume my guess is correct, and it is CA..

1. Is this level of CA acceptable for a SW ED80 ds-Pro, on the description from FLO it says it should be almost eliminated due to the FPL-53 glass.
2. Could something in my imaging train be inducing this? (No filter was used for this image)
3. Camera problem?
4. Bad seeing?

Attached is a heavily cropped image from slightly above and to the left of the center of my image.

No filter, Nikon D7100 DSLR, SW 80ED ds-Pro with matching corrector/reducer.

I have debayered one of the raw files and it is also an issue there, so not a calibration or stacking issue.

 

If this is the performance I can expect from my scope and camera combination, then fine, I will try and reduce the effect in processing, but it seems like the root cause is found in either equipment or during image acquisition.

Hoping for help.

 

[Thalestris24]

Hi

If you mean the mottled background, that's not CA, it's colour noise which you can reduce with dithering. Also you want to stack a large number of lights and calibration frames. Cooling is a help and a cooled mono camera is even better!

Louise

ps if you mean the slightly blue edge then that probably is CA and is inherent to the scope (though I've never owned an ED scope) Maybe a violet filter could help

[vlaiv]

It is a bit hard to tell from this image.

But here is what you can do to try to understand the source of the problem more.

It can be lateral chromatism due to focal reducer - examine rest of the image and observe where blue halo is "pointing" - in this section it looks like there is stronger blue halo on top than the rest parts around the star. If halo is not same everywhere and offset to halo is pointing towards the center then it is probably due to focal reducer - a sort of lateral chromatism.

See image for better understanding:

Second source of problem can be atmospheric refraction - examine images taken when object was in different part of the sky - orientation should be different (usually pointing in "vertical" direction - or AZ if you look in Alt/az coordinates). Also effect should be more pronounced when objects is closer to horizon than when it is closer to zenith.

If it is present in part of the image - it can indicate focal plane tilt, especially with ED doublet - some parts of image will have it, some will not, and there will be like linear gradient detectable in image.

Overall, I would say that such level of CA would not surprise me in ED doublet telescope even if none of the above is the case.

[jjosefsen]

Thanks for the feedback, exactly what I was looking for. ?

[jjosefsen]

2 hours ago, Thalestris24 said:

Hi

If you mean the mottled background, that's not CA, it's colour noise which you can reduce with dithering. Also you want to stack a large number of lights and calibration frames. Cooling is a help and a cooled mono camera is even better!

Louise

ps if you mean the slightly blue edge then that probably is CA and is inherent to the scope (though I've never owned an ED scope) Maybe a violet filter could help

I was referring to the blue fringes on the stars and not the background mottle. I am already dithering, and while it helps, it far from eliminates it.

Thanks for replying though. ?

[Uranium235]

Thats not CA, it looks more like a colour balance issue (ie: in processing), as there is a lot if green in it.

Having said that, you will get some (as you would with any glass) if its not properly in focus. Either by not being in focus overall - or defocusing caused by image train tilt (usually towards the edge/corner of thhe frame).

[Thalestris24]

Looking again, with my glasses on :), it's definitely to one side i.e. maybe to the 'north east' edge of the stars. Wonder what the whole frame is like?

Louise

[jjosefsen]

2 hours ago, Uranium235 said:

Thats not CA, it looks more like a colour balance issue (ie: in processing), as there is a lot if green in it.

Having said that, you will get some (as you would with any glass) if its not properly in focus. Either by not being in focus overall - or defocusing caused by image train tilt (usually towards the edge/corner of thhe frame).

Oh god I just hope it's not the scope, no idea how to correct that.

I wonder if PI has a tool to analyze something like this..

[Uranium235]

It wont be the telescope 80EDs are about as good as it gets for a doublet in regard to colour correction. Instead, looks at the colour weightings when you convert the raw images to RGB.

[jjosefsen]

19 hours ago, Uranium235 said:

Thats not CA, it looks more like a colour balance issue (ie: in processing), as there is a lot if green in it.

Having said that, you will get some (as you would with any glass) if its not properly in focus. Either by not being in focus overall - or defocusing caused by image train tilt (usually towards the edge/corner of thhe frame).

Hmm it seems to be pretty even across the whole frame, and in the same direction.

So probably not CA as you suggest.

Since it is present in the raw subs before calibration and any kind of processing, it must be something with either gear or acquisition.

Could it be a focuser that is sagging under the weight of the camera?

 

Attached is a pricture of all 4 corners + center.

[alan4908]

Well to me this looks like a small amount of camera tilt rather than CA. I suspect either the focuser may be slightly mis-collimated or you may have a non rigid scope to camera coupling. To find out which one, I'd carefully focus the scope and then take two test shots with the scope at a low elevation to the horizontal and then one with it pointing straight up to the zenith. If you see a significant difference between the results then it suggests a non-rigid coupling issue, if they are the similar, then the focuser is probably very slightly out of collimation. 

Alan

[Uranium235]

5 hours ago, jjosefsen said:

Hmm it seems to be pretty even across the whole frame, and in the same direction.

So probably not CA as you suggest.

Since it is present in the raw subs before calibration and any kind of processing, it must be something with either gear or acquisition.

Could it be a focuser that is sagging under the weight of the camera?

 

Attached is a pricture of all 4 corners + center.

With an 80ED, it really helps to do 2/3rds focusing.

ie: Instead of a centrally placed star for focusing, move it 2/3rds out towards the edge of the frame - then focus. That will balance out the field. But from the look of it, its probably better to move your focus star 2/3rds towards the bottom left corner before focusing. I had to do it with my 80ED, and that was with a slightly smalled chipped CCD (Atik 383L+, which is just smaller than APS-C).

Also before starting out, just grab a hold of the imaging train and give it a little wiggle... not too much force, that might reveal any slop in the focuser. I can safely assume your camera connection is solid because you are using the 0.85x reducer (screw on connection).

[jjosefsen]

2 hours ago, alan4908 said:

Well to me this looks like a small amount of camera tilt rather than CA. I suspect either the focuser may be slightly mis-collimated or you may have a non rigid scope to camera coupling. To find out which one, I'd carefully focus the scope and then take two test shots with the scope at a low elevation to the horizontal and then one with it pointing straight up to the zenith. If you see a significant difference between the results then it suggests a non-rigid coupling issue, if they are the similar, then the focuser is probably very slightly out of collimation. 

Alan

Hi,

I ran some subs through something called MaxViewer which can analyze all sorts of  things, like collimation and tilt.

It suggested that my collimation was off by 6,13% So you might be on to something there.

[jjosefsen]

39 minutes ago, Uranium235 said:

With an 80ED, it really helps to do 2/3rds focusing.

ie: Instead of a centrally placed star for focusing, move it 2/3rds out towards the edge of the frame - then focus. That will balance out the field. But from the look of it, its probably better to move your focus star 2/3rds towards the bottom left corner before focusing. I had to do it with my 80ED, and that was with a slightly smalled chipped CCD (Atik 383L+, which is just smaller than APS-C).

Also before starting out, just grab a hold of the imaging train and give it a little wiggle... not too much force, that might reveal any slop in the focuser. I can safely assume your camera connection is solid because you are using the 0.85x reducer (screw on connection).

I will try that, thank you.

I am using an all screwed on connection between everything, as I wasn't using a filter in this instant.

The focuser seems really solid though, prone to a little slipping when not locked in, but I can't find any slack in it.

Might "just" be a collimation issue..