Chromatic Abberation in Filters

[glend (Glen)]

I just wanted to get some idea of why I am getting chromatic abberation when using my DGM Optics NPB filter? This appears with every type of EP used (it's a 1.25" filter). It shows up as a red blue shift (red to the left and blue to the right) of stars in a particular FOV. It's not a focal related shift, ie it doesn't move from red to blue as focus is changed, but appears as left and right shifted sub-images to the star in focus. For example, looking at the Orion Nebula Trapezium stars, each has a red and blue shifted component with the filter. Of course this appears when looking at other stars in other nebula as well. No chromatic aberration occurs when the EPs are used without the filter, so I have to assume it is due to the filter coating. I realise that Nebula filters are not really made for stellar observation so I accept that this is 'normal' for this quality filter but i wanted to know why it is happening.
I realise chromatic abberation is something that refractor guys seem to deal with but seems to rarely appear in Newt type discussions.

[cometcatcher (Kevin)]

It's not CA. It's something to do with the pass and stop bands of the filter that make it look like that.

[Merlin66 (Ken)]

I agree!
A plain parallel glass plate (ie a filter) can't introduce CA.
Must be as a result, as said, of the bandwidth(s) and focal ratio....

[barx1963 (Malcolm)]

It is not technically CA. I did have a PM discussion I think with John B a while back because I noticed the same thing in my old Lumicon filters.
I cannot remember the exact details but it is to do with the method of making the cheaper filters.

Leave it with me and I will try and find the info for you!

Malcolm

[cometcatcher (Kevin)]

Yeah my old lumicon Deep Sky does it. It has the appearance of the red and blue being out of register with each other.

I can't remember if the Astronomik CLS does it or not. I don't use it much visually. When it's clear next I'll have to check.

[Don Pensack]

When filters have non-parallel surfaces and/or some of their layers (and some nebula filters can have up to 75 layers (!)) are not parallel, a lateral shift can occur with wavelength, resulting in the lateral offset of the red passband versus the blue-green/blue.
This could be due to a "sandwich construction" (two pieces of glass with layers between) which was common on early Lumicon filters and several others.
And it can simple be due to a filter that is tipped in the housing (call it 'wedge"). You can try loosening the retaining ring and tapping the housing to seat the glass, then lightly re-tighten the retaining ring.
Or, replace the filter with one that has no visible red transmission (example: Orion Ultrablock).
One way to test the tilted glass hypothesis is to look at a bright star with the filter held in front of the eyepiece. Try tipping the filter to see if the colors can be "stacked". If it requires quite a bit of tilt to do so, it is likely the two sides of the filter glass are not parallel, and the filter should be replaced.
Normally, your eye won't focus the red and blue green at the same time, so some stars will appear as a blue-green point with a bit of red fuzz around them. But the colors should be coincident.

[cometcatcher (Kevin)]

That's excellent information. Thanks Don.