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My purple haze has turned green/blue

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I recently acquired a Bresser AR-102S, a 4", F5.9 achromat.

I was expecting to see colour fringing on brighter objects and that's what I did see, although not as badly as I'd anticipated. It was the usual purple colour extending past the limb of Jupiter, the moon and the very brightest stars.

Those observations were done with a Tak 1.25" prism star diagonal, replacing the stock diagonal. Last night I used for the first time a 2" quartz-dielectric mirror diagonal. On the brightest stars I could still see a little violet, but on the moon I was surprised to see instead that the fringe around the edge was now a "fluorescent" green, or sometimes a green/yellow (I tried with several different eyepieces). Also, the fringe was narrower than before, almost a line around the moon's edge, rather than a band extending into the surrounding space.

A quick search on here suggests that this green/yellow colouration isn't uncommon, but it leaves me with a question.

I had been thinking about getting a filter to tame the CA a bit, possibly one of the milder Baader versions (and if I got one with the neodymium substrate it would also double as a contrast enhancer on the planets). I've tried a cheap yellow filter but I didn't like colour cast. But now that my CA seems to be green/yellow rather than violet, I'm thinking that the usual minus-violet filters aren't going to make any difference, are they?

I'm also thinking that the switch from prism to mirror must be instrumental in the change that I see. I know that prisms can introduce some CA in faster scopes, so perhaps some of the fringing I was seeing previously was down to the diagonal and not the scope?

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I've tried #8 light yellow, 495 Longpass, WO VR-1, Fringe Killer, Lumicon MV, Contrast Booster and Semi-Apo. They all work to a degree and with differing amounts of yellow tinting. The Contrast Booster is the one I've kept and I really like it. It is the most aggressive MV but also has neodymuim and makes a great planetary/lunar contrast enhancing filter. Not just in achromatics but in all telescopes. I also tried a Burgess CED ( contrast enhancing diagonal ) which also worked well at cleaning up the image. It was the special coatings on the mirror which made the difference, so it goes to show that different coatings can and do have an effect on the views.

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+1 Baader Contrast Booster - it does work and has neutral tone compared to other filters (maybe a bit warm image).

Green/Yellow is companion to violet / blue as far as refractors go.

It all has to do with spherochromatism and best focus position. Prism influences spherical aberration of the scope (at least I think so), and much of fringing is not down to defocus but also due to spherochromatism.

There are two important graphs that you need to look at in order to understand what is going on and how prism / mirror swap changes levels and color of CA.

First is of course this diagram:


This shows what colors will be out of focus depending where you put your focus point.

If you focus at 550nm - and we usually do that because green part of spectrum carries the most luminance information - and we are most sensitive to sharpness in luminance (so that means green), then it can clearly be seen that both red part and blue part of spectrum (700nm and 400nm) will be out of focus. Violet part will be the most out of focus at 400nm.

This is "standard" mode of achromat telescope - and what it is usually optimized for.

If you change focus position - you will bring red and blue in focus - but in that case - central part of spectrum will be defocused:


Now look at spectrum:


That central part is green and yellow light.

If you shift your focus - color of chromatic aberration will change. We often don't see this as sharpest image is when we focus on green light mid spectrum at about 550nm.

Now let's look at one more graph (it is also present in above graph in top left corner - but it is not as significant in that model - we need to look at fast achromat for that):


In perfect achromat telescope - this graph would look like just bunch of vertical lines. Each line here represents one wavelength. Stronger the curve in the line (deviation from vertical) - more spherical aberration there is at that wavelength (spherochromatism).

If you look first diagram / top left corner - you'll see that for F/10 lines are much straighter. That is why we say that these short achromats often suffer from spherical aberration (while long ones don't) - it is really spherochromatism.

You might have significant spherical aberration in green part of spectrum and prism can correct that a bit. With prism - you will want to focus on on green line and then you get nice violet fringing. When you remove prism - you remove that correction and now green has spherical aberration again - and with spherical aberration - you can't find good focus - it's all blurry.

Then you instinctively focus to different line - one that is straight and will produce sharper overall image - and this means that green is now a bit out of focus (see that focal plane position) and you see green / yellow fringing.

You can test this by using mirror diagonal and then refocusing even if image is a bit blurry (don't look for perfect focus - look for different type of fringing as you refocus).

There is a way to correct this, but it is advanced technique. It involves changing distance between doublet lens. There doublets are usually separated by small bits of metal or similar little shims. You can replace those with slightly thinner version (or simply file down thickness of existing ones). It is trial and error method and you need some way to asses spherical aberration - best would be some sort of narrower filter (maybe using CCD green would be enough) and Ronchi EP and star testing telescope.

You can see results for example here:


There is good comparison of 3 different short achromats and results of testing for spherical aberration.


Here are shims that I mentioned for example. You can see that doublet lenses are real close as there are Newtonian rings showing.


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Vlav, thanks for that very full explanation. I will read up some more on the interplay between spherical and chromatic aberrations.

I didn't notice changes in colour with the focusing, but then I wasn't looking for it either. Next time, I will experiment a bit more.

I certainly won't be tinkering with the objective lenses though. The damage I might do would be far worse than the current CA 🥴

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