Chromatic Aberration in Achromats

[John]

Chromatic Aberration (CA) or false colour is a fact of life with achromatic refractors but the degree to which it is present is linked to the aperture and focal ratio of the scope. That's my understanding at least !. I also understand that some people are more sensitive to CA than others so what bothers one observer might not bother another at all. All this relates to visual observing as I'm not an imager.

Browsing another forum I came across a rule of thumb that can be used to express the level of CA that a particular scope might be expected to show. Divide the focal ratio of the scope by the aperture as measured in inches, and this is termed the Chromatic Ratio (CR). A low CR will show more CA than a higher one. The example given was a 3 inch F/5 telescope which has a CR of 1.7 which is the same as a 6 inch F/10 so one should expect similar levels of chromatic aberration but the next part tries to quantify this.

To assess the practical impact of the Chromatic Ratio, two different thresholds are suggested, a 3X rule and a 5x rule. The 3x rule is says that a scope having a Chromatic Ratio of 3 ( eg: a 4 inch F/12) will have an acceptable level of false color, visible but not obtrusive even on the majority of the brightest targets. The 5x rule says that a CR of 5 means the CA will be really minimal and will not materially affect what can be seen at all. This would equate to a 4 inch F/20 (). In another example, a 60mm F/11 with a CR of 5 and therefore very little, if any, visible CA.

If you access the above thresholds and the CR concept, the popular 4 inch F/10s (CR=2.5) will therefore show some chromatic aberration and, in comparison to a color free scope, the views are not as crisp and clean, but they still show nice planetary images.

This is the first time I've come across this concept and I'm still mulling over whether it rings true to my own experiences with achromatic refractors or not. For example, the 6" F/8 achromats don't come out of this too well with a CR of 1.3 although I've had decent performance on the moon and planets with one of those which suggests that the ratio / thresholds might be flawed or at least too harsh

The poor old 6" F/5's get a CR of just .83

I'd be interested in other views on this - I've just posted it as a discussion topic, I'm not advocating it

[johninderby]

Interesting theory but it doesn't seem to take into account the quality of the optics. I've compared achros that on paper should have similar levels of CA as they were similar apertures and fl, but one had virtually no CA and the other had heavy purple fringing around everything. And that was using the same diag and eyepieces.

The theory would probably hold up though if comparing equal quality optics.

John

[MjrTom]

I am pretty new to the performance of the Achromats but what I can say about my 102mm f/10 Achro is that the CA is very well controlled considering.

In daylight when a harsh transition like a metal roof aerial against a white sky the CA is very obvious. Intra-focus yellow fringing, extra-focus purple fringing.

In real life conditions under the stars all the DSO viewing the CA is absent, its only when pointed toward to Jupiter for instance the CA becomes noticable but not objectionable. Far better than the f/5 Achros I have viewed before.

[RikM]

CA in my 127mm f/9.4 has never really bothered me too much. Using the above ratio I get CR = 1.9 so I guess my eyes must be pretty tolerant or like John M. says, maybe the thresholds are a little harsh?

[Peter Drew]

Pretty much my own experience. There is doubtless a specific professional optical equation that will accurately predict the amount of CA but in general, the "faster" the achromat, the greater the CA. At the end of the night(?) the tolerable amount of CA is up to the observer, if you can't stand it, buy a reflector. "the poor old 6" f5 with .83 CA ratio" will still beat a 4" APO hands down on DSO's that require light grasp visually and at a fraction of the cost.

[Doc]

I totally agree with what has been said, I've only had lmited experience of achromats but in my TAL 100r I saw very little false colour but being F10 this is what I had imagined. My other achromat even though it's a petzval design still shows way more CA then my TAL but this is F5 so pretty fast.

[John]

Interesting feedback folks - thanks

My general conclusion about this is that the chromatic ratio is an indicator but no more than that. I tend to agree that it's a rather harsh judge if used as I've described. It does serve to illustrate that the challenge of controlling CA, for a given focal ratio, increases with aperture I think.

I had some interesting results a while back when I was playing around with the Aries Chromacor correction lens and 6" F/8 refractors (I used a 6" Synta and a 6" Meade acrhomats with it, both F/8's). The Chromacor i) reduces CA quite considerably and ii) corrects spherical aberration, a common issue with mass produced objectve lenses I believe. Basically, with the Chromacor in place, both scopes displayed very low levels of CA on bright objects, similar to that which I see now with my ED120 F/7.5 refractor and handled high magnification much more readilly with 300+ becoming practical and often useful compared to 200x or so which was the useful limit with those scopes, uncorrected.

I take John's point about the quality of the figuring and polish of the optics reducing CA - I guess what you can get with a top quality lens is the maximum possible control of CA for the given aperture / focal ratio - something that the majority of mass produced objectives would not quite achieve perhaps.

The points about CA not being an obvious issue when observing deep sky objects are of course perfectly true but there must be some impact, even on deep sky objects, I would have thought.

[MjrTom]

I suspect that the quality of the objective in my 10 year old Japanese built vixen 102mm f/10 Achro is probably better figured than say the more recent Chinese equivalents.

No firm evidence to prove this but that seems to be the case

I was pushing all the way upto 333x magnification last night on Epsilon Lyrae and not a hint of CA present with a beautiful diffraction rings when defocussed.

Quite a surprise really

[John]

I suspect that the quality of the objective in my 10 year old Japanese built vixen 102mm f/10 Achro ....

The Japanese made Vixen objective lenses have an excellent reputation. My Vixen ED102SS, although an F/6.5 ED doublet, is a superb optic

[johninderby]

A problem with any theory predicting how much CA an achro will have is in actually defining exactly what an achro is in the first place. If you have a rigid definition that anything less than an APO is an achro that leaves such a wide range of optical quality that any theory based on just aperture and fl is not going to be accurate enough to be of any real use.

With my Lyra Optic 102mm f/11 achro CA just isn't a problem. Can't wait to see how the Carton 100mm f/13 achro performs when the project's finished.

BTW remember the semi-APOs and the arguments over that definition.

John

[ronin]

As CA is caused by only having 2 wavelengths at the same plane the effect can be better or worse by the choice of the 2 wavelwngths selected.

Often Red and Blue are the 2, say 400 and 700 nM, this would make 550nM the "worst". If you focussed at the 550 then you get a red+blue (purple) halo, alternatively if you focussed at the 400/750 position you get a green halo.

If the lens is designed to have say 500 and 600 at the same plane then you get a different criteria and in this case possibly better as 400, 700 and 550 are all off the ideal plane but not by much. So less apparent CA to most eyes.

Add in that an achro is basically Crown and Flint glass but there are several of each. One manufacturer may use one that has a lower dispersion (more like ED glass) then another manufacturer does, in which case that lens will be a little better (hopefully).

The maths for an achro (especially a cemented one) are pretty easy and there are a few free packages out there. Have a laugh and give it a go. As a start have the forst lens the positive and have radaii is the ratio of about 1:6 (minimum spherical aberation).

[MjrTom]

Very true John,

Its a very fluid definition and manufacturing quality varies considerably.

If the objectives cannot focus all wavelengths to the same focal point its not an Apochromatic lens. Hence I refer to my ED80 as an Achro with a ED element not an APO.

[John]

BTW remember the semi-APOs and the arguments over that definition.

John

Oh yes, I can recall very heated debates on that

The info that Ronin has posted causes me to wonder if a smart manufacturer could design an objective that is corrected in a way that appears largely apochromatic when used visually but in reality is not when used photographically ?.

Still, when alls said and done perhaps "CA is in the eye of the beholder"

[RikM]

Strange, but the Phenix I look after is a real cheapo-scope, but optically superb. Did we just get lucky and get a good one? It is only the sloppy focuser that lets it down. Other than that, it is a truly wonderful lunar / planetary / double star scope.

[RikM]

Just read your last post John and I think you have it there. My Phenix is visually great, but when you stick a camera on it you get basically a triple image red / yellow / blue.

[Double Kick Drum]

Interesting.

I have a 5" F5 (CR of 1, though principally bought to look at DSOs) and suffer CA on anything brighter than magnitude zero.

Is there anything that can be done to compensate?

It is nice having the occaisional squint at Jupiter etc.

[nightfisher]

dont think the theory holds up, i have a 6" f5 as mentioned in john`s first opening post and to be honest it gives really good views, yes it does show false colour, but no where near as bad as i would have expected.

My tal 100 f10 has very little c/a, as you would expect

[A McEwan]

Many years ago, I side-by-side tested a friend's 102mm f9.8 Helios achro against my 8" Dob. Observing the Moon in perfect seeing at high powers. The Helios quickly became unusable at more than 200x while I was able to employ 480x or so in the Dob. The CA in that 102mm was so bad it put me off achros for a long time.

I turned to ED scopes for my "refractor fixes".

Then a while back I stumbled across a Pentax J60 which was, for all intents and purposes to my eyes colour-free. Foolishly I sold it on, but I have now been struck by the long achro bug. I now own a 60mm f13.5, 90mm f14.4 and 6" f8. The CA in the 6" f8 is quite bad without a correcting filter, but using the Baader Semi-Apo filter corrects it very well indeed.

The 90mm exhibits "some" false colour but only in very small quantities, and it is very easily overlooked. I find myself looking "past" or "through" the colour and mentally shunting it aside. The view does not suffer for it. The 60mm is essentially volour-free except on Vega and the lunar limb.

Having used ED and APO refractors for many years now I'm actually finding it quite refreshing to adjust my observation technique and perception to allow for the "colour" in achromatic views. It's actually a lot easier than I thought it was going to be...

Ant

[MjrTom]

The CA in the 6" f8 is quite bad without a correcting filter, but using the Baader Semi-Apo filter corrects it very well indeed.

interesting Info Ant.

I had been toying with the idea of one of these filters for my 102mm f/10. CA is really only visible in lunar and planetary viewing.

Might try and pick one up soonish

[philj]

Interesting thread John. As an Apo and Achro user this is something Ive noticed over the years.

I used to have an ST120 F5 achro and the colour was really bad in that but for deep sky its was great, I just didnt turn it to lunar or planets very often:)

Now my only achros are F15 because of the CA and FL effect. I was watching a transit of Io through my 40 year old 102mm F15 the other night and after the scope had cooled I just couldnt detect any false colour at all, the view was just as good colour-free wise as my 127mm F7.5 triplet apo.

My 1960s Towa 80mm is the same, this gives superb colour free views and Im able to push the mag to silly levels on lunar when conditions are right without much breakdown.

I once had a ZS80 semi apo, this was a pretty fast scope at f6.5 but displayed far more CA than any of the F15s. I even tried the minus violet filter trick and although it did reduce CA it gave a very yellow colour which was distracting for me at any rate. Also the filtered view interfered with the planetary filters I use quite alot so eventually that scope went.

Philj

[Merlin]

Even if some observers say they can live with CA, it's still wasted light. CA might not appear to be evident on DSOs, but it's still there as a fringe of light that in a good achro would be where it belongs, in the image itself.

I have the BAADER Semi-Apo filter, which is very good and colour-neutral, but I'm still aware that the filter doesn't put the CA back into the image where it's needed, but just filters it out.

[michael.h.f.wilkinson]

Many years ago, I side-by-side tested a friend's 102mm f9.8 Helios achro against my 8" Dob. Observing the Moon in perfect seeing at high powers. The Helios quickly became unusable at more than 200x while I was able to employ 480x or so in the Dob. The CA in that 102mm was so bad it put me off achros for a long time.

...

Ant

The fact a 102mm scope quickly becomes unusable above 200x may have more to do with aperture than CA. My 80mm APM triplet APO quickly becomes unusable above 160x in my view, aven in skies that allow me to go over 300x in my C8. The APM 80 is still a cracking scope though.

[damnut]

An interesting thread John. I suspect that most (including me) when buying a scope get the best that their budget allows, and then deal with any CA later if it is an issue. I have two refractors 120 & 127mm which have a CA calc of 2 & 1. I use the 120 as my grab & go so if I did not have it i could not observe when on jollies.

So I accept the CA for the cost of the scope, at some point I may be tempted by a UHC or Semi apo filter , but that is 50% the cost of the 120 scope. So I'm not tempted that much.

My main scope is a 10"sct that I bought second hand for the same cost as the 127mm. So I am not in the same market where most of you have ed/apo/triplets, as if i put one of those in the boot I would care if the dogs slobbered on it or got sand / snow on it! :-)

Cheers

[michael.h.f.wilkinson]

Don't knock your equipment, Damian; your SCT has my C8 beaten . It is true triplets are expensive, even the comparatively cheap 80mm APM I have is about the same price as a 6" F/5 achromat from Bresser. The latter will show more DSOs but the APM is more easily transported (in a compact case, so nobody gets to slobber of it), and can be used in the future for DSO imaging.

Let's not forget that many important discoveries were done with single lens refractors. All this newfangled achromatic stuff would make the likes of Christiaan Huygens, not to mention Galileo drool (NOT ON MY SCOPE PLEASE)

[MjrTom]

We are definitely spoilt these days with the quality of kit that can be picked up for very reasonable costs.

Its interesting so see how most Achromats do not sell very well and can be picked up for next to no cost on the used market unlike APO's and ED scopes which always command a high price relative to the original retail price.