Chromatic Aberration, CA, what, why, how? (1 Viewer)

[Binastro]

CA was very well known in the early 1600s, because telescope objectives were made of one piece of glass and had to be very long to be useful.

Newton officially said only reflectors were CA free, although he actually knew otherwise.

It was only when Dollond took to making doublets with crown and flint glass that were invented but not patented earlier that telescopes became shorter.

It wouldn't surprise me if eye glass makers in the 1200s or even the ancient Greeks were aware of CA.

Our eyes have chromatic aberration.

Late 1800s flint glasses greatly improved telescopes but unfortunately had to be repolished every 7 to 10 years because the glass was not stable.

Even today calcium fluoride crystal degrades rapidly unless serious precautions are taken.

Exotic glass has to be immediately coated otherwise it tarnishes when made.

Regards,
B.

 

[Pinewood]

Correct, my text here was mainly about binoculars where it's just the colour fringing, with high magnification scopes it's a different story.

Hello,

I think that colour fringing is an example of extreme CA. I noticed minor CA, black building against a white sky, in 6.5x binocular. The edge of the building lacked sharpness.

Stay safe,
Arthur

 

[Thrupenney Bit]

Also have a look at point light sources in the periphery. There are other types of image-degrading aberrations like coma, where points grow "hair" (coma means hair in Greek), astigmatism, spherical aberration and so on.

Thanks, will do.

 

[Thrupenney Bit]

Just going through these posts this morning.
The historical viewpoint is fascinating, and I'm aware it continues through to now.
I remember going to a field day by the same retailer that I'll be visiting today. It was in the early 1990's. Kowa had just introduced their new ED glass scopes. Soon people were trading in their old scopes to get theae new ones, and I picked up my scope as a second hand non ED one then.

I've been trying to get back up to speed these last few weeks, reading lots on this forum, getting great advice from people on here and enjoying re-immersing myself in it all.
I'm aware that reading reviews of scopes in particular, the message I'm getting is the continual striving to make even better glass, better coatings on lenses... just keeping on going to try and make perfect kit. As ever history keeps going!

Thanks for your comments, it a helps!
Cheers
Q

 

[TheBlackGrouse]

Hello,

I think that colour fringing is an example of extreme CA. I noticed minor CA, black building against a white sky, in 6.5x binocular. The edge of the building lacked sharpness.

Stay safe,
Arthur

Since I'm not a CA expert, I'll be careful, just curious, but when testing spotting scopes (not cheap ones) the first thing for me was sharpness and still there was colour fringing in one of the two options. Too much, even for me. They both had the same sharpness. Is that possible, different materials, construction leading to more CA with same sharpness? Or is there some independence between the two?

 

[Thrupenney Bit]

Just returned from a demonstration field day.
My wife was buying a new pair of binoculars and whilst she gave them a good test, I nosed around the telescopes on offer.

I had the chance to look at a Swarovski ATX85 ( can't remember what eyepiece ), and Vortex Viper and Razor, both 65mm.

The Swarovski was very nice but I'm not prepared to spend that much I'm afraid.
I was focussing on some objects against a blue sky and could not see any CA even at maximum zoom. It was sharp and the focussing and zoom rings were butter smooth. A very nice telescope.

The Vortex's were a surprise. The Viper being second hand and spotless.
I focussed on a post in the field and both gave good images, I could not detect CA... I don't think.
At maximum zoom the Viper gave an odd impression the flat top of the post had a slight green hue, not visible with the Razor but the Razor's eyepiece gave a slightly higher magnification and was in a slightly different position viewing the object so it might just have been that. Also it might simply be the top of the post, flat, had a green layer on it!
The post had holes in ( Steel Blue Woodwasp? ) it and both gave clear views of them, possibly a touch clearer in the more expensive Razor.

With all scopes I could not see any CA.
The viewing was outside, scanning around at 'objects in the field' either against a blue sky or a backdrop of trees. Not a hard test of any other them, but equally I'd be happy to take any back home. Especially the Swarovski!

 

[tenex]

... when testing spotting scopes (not cheap ones) the first thing for me was sharpness and still there was colour fringing in one of the two options. Too much, even for me. They both had the same sharpness. Is that possible, different materials, construction leading to more CA with same sharpness?

Yes. It's longitudinal CA that could degrade overall sharpness, and should be well controlled by a good ED objective. Fringing on high-contrast edges (lateral CA) is said to be mostly related to eyepiece design. For what it's worth, my bins have some variation in susceptibility to CA (with the most in Leicas) but my S2 scope has never shown any at all.

 

[jring]

Hi,

a blue sky or greenery as a background will not really show lateral CA (blue/yellow fringing) well. Hard contrast like black birds or branches against an overcast sky is the usual benchmark.

As for longitudinal CA, since all scopes use ED glass (assuming the current Viper HD and not some older non HD variant), the expected result for all would be no visible effect of longitudinal CA and thus a crisp image with an easy to find point of best focus at 60x.

For current ED scopes, sample variation is the limiting factor at 60x. So testing in person like you did is a very good idea... and then buying exactly the scope you tested and liked - bonus points for getting a discount for taking the demo unit.
When buying online, make sure that it's from a place with a no questions asked return policy and that you have the knowledge and time to test it inside the return period.

There is only one ATX eyepiece module with a 25-60x zoom range for the 65 and 80mm objectives and 30-70x for the 95 and 115mm modules.

Joachim

 

[Thrupenney Bit]

Hi,

a blue sky or greenery as a background will not really show lateral CA (blue/yellow fringing) well. Hard contrast like black birds or branches against an overcast sky is the usual benchmark.

As for longitudinal CA, since all scopes use ED glass (assuming the current Viper HD and not some older non HD variant), the expected result for all would be no visible effect of longitudinal CA and thus a crisp image with an easy to find point of best focus at 60x.

For current ED scopes, sample variation is the limiting factor at 60x. So testing in person like you did is a very good idea... and then buying exactly the scope you tested and liked - bonus points for getting a discount for taking the demo unit.
When buying online, make sure that it's from a place with a no questions asked return policy and that you have the knowledge and time to test it inside the return period.

There is only one ATX eyepiece module with a 25-60x zoom range for the 65 and 80mm objectives and 30-70x for the 95 and 115mm modules.

Joachim

Thank you Joachim.
The post I mentioned was against trees and a dark sky so perhaps that was a better test.
Yes the Vortex were all current models, HD marked on them, and thanks for the info on the ATX. It was lovely to use.

 

[fazalmajid]

For current ED scopes, sample variation is the limiting factor at 60x.

And atmospheric conditions. e.g. heat haze, although the latter seems unlikely to be a major concern in the environments Thrupenney Bit has mentioned he is going to use his scope in.

 

[Thrupenney Bit]

And atmospheric conditions. e.g. heat haze, although the latter seems unlikely to be a major concern in the environments Thrupenney Bit has mentioned he is going to use his scope in.

As I've currently experienced the wettest February for years, I wish I did have a heat haze to worry about.
... though we do have hot summers occasionally!!!

 

[Binastro]

History of optics Wikipedia says lenses were developed by the ancient Egyptians and others.

There are very old lenses in museums about 3,000 years old.

Visby lenses have aspheric magnifiers from the 11th century.

It is probable lens aberrations were known long ago.

B.

 

[Thrupenney Bit]

History of optics Wikipedia says lenses were developed by the ancient Egyptians and others.

There are very old lenses in museums about 3,000 years old.

Visby lenses have aspheric magnifiers from the 11th century.

It is probable lens aberrations were known long ago.

B.

Fascinating!
Thanks.
Q

 

[Rg548]

The Swarovski was very nice but I'm not prepared to spend that much I'm afraid.

But eventually you will.
My advice, is either go warranted second hand, or get the best you can afford, because you will want to upgrade in time.... it gets us all

 

[Thrupenney Bit]

But eventually you will.
My advice, is either go warranted second hand, or get the best you can afford, because you will want to upgrade in time.... it gets us all

.... oh no.
I think you must be psychic, it is on my mind for exactly those reasons.
Plus talking to the person running the event I was at yesterday and overlapping with comments on this forum, the backup and service from them sounds like second to none. That matters to me.
I'm not sure about the ATX series, but the ATS sounds more my thing. Plus the second hand values for the 65mm series seems to be much closer to the new price making it debatable not to head for new.
Who knows.... but!!!!

 

[Peregrine Took]

I'm not sure about the ATX series, but the ATS sounds more my thing. Plus the second hand values for the 65mm series seems to be much closer to the new price making it debatable not to head for new.
Who knows.... but!!!!

Shamelessly gratuitous post alert!

I have an ATS for sale at nowhere near the price of a new one, yet it is barely two years old and still under warranty. PM if you're interested and we'll take it from there. PS. It's pretty much free from any CA.
.

 

[4John]

...... I'd be happy to take any back home. Especially the Swarovski!

Why ?

IIRC you started this journey on another thread asking whether today's scopes were better than yesterdays. Again IIRC, you have an Optolyth (?) scope with a 30X fixed mag eyepiece. Were you able to do a side-by side comparison ? I would be interested to hear what was so much better about the Vortex or Swarovski scopes.

 

[Thrupenney Bit]

Why ?

IIRC you started this journey on another thread asking whether today's scopes were better than yesterdays. Again IIRC, you have an Optolyth (?) scope with a 30X fixed mag eyepiece. Were you able to do a side-by side comparison ? I would be interested to hear what was so much better about the Vortex or Swarovski scopes.

Yes you are quite right. It's an Optolyth straight with a x20 eyepiece.
I've learnt quite a few things on the way since my first post, and have now got a Mulepack now which I find a very comfortable way to carry my scope leaving hands free for bins, notes etc.
I've taken to walking round with 2 legs extended and an angled scope could be used with just the centre column extended, I'm nearly there with my straight scope but not quite.

One thing I've realised is that I'm now starting to use my driving glasses walking round the wetland as my eyesight is deteriorating. I have no eye relief on my Optolyth, and juggling glasses with everything else is a nuisance. Having eye relief, as I have on my binoculars would certainly help as I know I miss things walking round now without wearing my glasses.

The images on both Vortex and Swarovski were very bright and clear, plus using a zoom for the first time was great.
It is lots of little things that are drawing me to feel like a change...

 

[wllmspd]

Zoom…. I started with the zoom on my mm60 at the weekend, then swapped to the wide angle fixed 23x and left it in place. Seemed to give a nicer view and less chromatic aberration (the zoom has some round the edges). Slightly lower power gave more contrast as well. Of course the little voice in your head wants “more magnification”, so I’ll probably pop the zoom back on for the next trip. Probably like the 8vs10x bino debate… sometimes stuff is just too far off and a dim zoomed view or a brighter slightly lower power aren’t such a huge deal different in the detail you can see. If you want more power you really need a bigger scope to keep the brightness. The most engaging views of the day were with my binos, but wanting a two eyed view and high powers is a recipe for trouble!

Peter

PS still not seen any spotting scope with a finder attached…. The cable tie trick did great service (as always)!

 

[Thrupenney Bit]

Zoom…. I started with the zoom on my mm60 at the weekend, then swapped to the wide angle fixed 23x and left it in place. Seemed to give a nicer view and less chromatic aberration (the zoom has some round the edges). Slightly lower power gave more contrast as well. Of course the little voice in your head wants “more magnification”, so I’ll probably pop the zoom back on for the next trip. Probably like the 8vs10x bino debate… sometimes stuff is just too far off and a dim zoomed view or a brighter slightly lower power aren’t such a huge deal different in the detail you can see. If you want more power you really need a bigger scope to keep the brightness. The most engaging views of the day were with my binos, but wanting a two eyed view and high powers is a recipe for trouble!

Peter

PS still not seen any spotting scope with a finder attached…. The cable tie trick did great service (as always)!

I helped at a pop up event the other weekend.
The ranger had his scope focussed on some Avocets on the distant island, and as visitors came along many wanted to see them as they were the new attractions jetting in from a close estuary.
I could see the island with x8 bins, but not a lot else. The ability to increase the magnification when needed was a revelation to me. I'm sure a fixed wide angle gives a nicer view, but as ever, there seems to be those times when you need a bit more.
bit like life really