6 replies to this topic

[YKQian]

Astronomik UHC just arrived. Tested it by covering the lens front with a white screen. I know I should expect some color casting, but I found that it appears like vignetting, and it does not go away with small aperture or long focal lengths. Is this normal? How should I remove this in post-processing? Thanks in advance.

 

70mm f/4.0:

 

200mm f/4.0:

 

400mm f/6.3:

 

600mm f/6.3:

Edited by YKQian, 14 August 2020 - 04:31 AM.

[sharkmelley]

Is the filter a clip-in filter or a filter screwed into a lens adaptor or is it a filter attached to the front of the lens?

 

Mark

Edited by sharkmelley, 14 August 2020 - 05:34 AM.

[YKQian]

Is the filter a clip-in filter or a filter screwed into a lens adaptor or is it a filter attached to the front of the lens?

 

Mark

It's a clip-in filter.

[sharkmelley]

The filter is an interference filter which means its spectrum characteristics will change with the angle of incoming light.  With clip-in filters this colour cast at the edges is a common problem for lens designs where the rays of light arriving at the edge of the sensor pass through the filter at an angle and not orthogonally.  You might be able to make some correction for it in post processing but it won't be easy.

 

The filter manufacturer IDAS specifically mentions this type of spectral issue when used with lenses here:

http://icas.to/space.../heuib-ii-e.htm

 

But other filter manufacturers are less open about the issue.  Here's one of my own images that shows the same issue with an Astronomik CLS filter:

 

 

Mark

Edited by sharkmelley, 14 August 2020 - 06:16 AM.

[YKQian]

The filter is an interference filter which means its spectrum characteristics will change with the angle of incoming light.  With clip-in filters this colour cast at the edges is a common problem for lens designs where the rays of light arriving at the edge of the sensor pass through the filter at an angle and not orthogonally.  You might be able to make some correction for it in post processing but it won't be easy.

 

The filter manufacturer IDAS specifically mentions this type of issue with lenses here:

http://icas.to/space.../heuib-ii-e.htm

 

But other filter manufacturers are less open about the issue.  Here's one of my own images that shows the same issue with an Astronomik CLS filter:

 

AstronomikClsTranmissionBandShift.jpg

 

Mark

Thanks, Mark! I have been trying to use blend mode Divide to combine a layer of flat frame like one of the four photos I've posted with a layer of the image to be processed. I only got some success with that. Wondering if there are better ways of doing it.

[asanmax]

Yes, as Mark says, it is normal unfortunately. This effect is especially visible when using shorter FL lenses or scopes.

I noticed that it almost disappears at 1000mm with a clip-in filter but you can't always shoot at that FL.

 

I moved to a 2" CLS-CCD filter that I put before the compressor in my imaging train AT6RC + CCDT67 compressor.

The FL is 920mm and I see zero filter color vignetting at the APS-C sensor.

[YKQian]

Yes, as Mark says, it is normal unfortunately. This effect is especially visible when using shorter FL lenses or scopes.

I noticed that it almost disappears at 1000mm with a clip-in filter but you can't always shoot at that FL.

 

I moved to a 2" CLS-CCD filter that I put before the compressor in my imaging train AT6RC + CCDT67 compressor.

The FL is 920mm and I see zero filter color vignetting at the APS-C sensor.

Thanks for your input! I already knew before buying the filter that the light has to be pretty orthogonal to the filter to avoid color cast. I didn't expect the requirement for orthogonality to be this strict. Unfortunately my lenses don't have the focal length reach of a telescope. Looks like I'm stuck with fixing it in post-processing... Oh well at least I can shoot flat frames.