Chromatic aberration in refractors

[Ags]

Interesting read:

http://www.astrotest.it/equipment-optics/laberrazione-cromatica-nei-rifrattori/

Google Translate recommended for the language challenged 

[John]

Interesting link.

The table below is quite a useful guide to the relative amounts of chromatic aberration that achromats of different specifications should show. It's a normal feature of the optical design though, in the same way that coma is to the newtonian design so we should not be overly concerned by it. Spherical aberration is present in many refractors and has more of an impact on performance than CA does.

 

 

[jabeoo1]

Thats why F15Rules

[Adam J]

56 minutes ago, John said:

Interesting link.

The table below is quite a useful guide to the relative amounts of chromatic aberration that achromats of different specifications should show. It's a normal feature of the optical design though, in the same way that coma is to the newtonian design so we should not be overly concerned by it. Spherical aberration is present in many refractors and has more of an impact on performance than CA does.

 

 

I never understood why it would be harder to correct C-A in larger apertures does the whole thing not just scale as the scope gets larger at a fixed f-ratio? Is it something to do with the size of the star resolved at the image plain?  

[Saganite]

1 hour ago, jabeoo1 said:

Thats why F15Rules

Sure does ! 

CA is Almost non existent  in Andromeda, shown in all her beauty....again ! 

In Latinized Greek Andromeda means ' Ruler of men' apparently, which is rather appropriate.

Dave ( F15 Rules) named her before I took ownership. 

[mikeDnight]

CA is usually an obvious abberation, but graphs and tables can be a little misleading, as they can imply that a short F ratio scope is useless, being crippled by false colour. Often, in practice, a well made short/fast achromat can still render a very pleasing image, and still have a well controlled level of CA. A 6" F8 achromat for example, will show CA, but is still an amazing rich field scope / Comet Seeker. Far more damaging than chromatic aberration is spherical abberation, which will cripple definition, rendering the scope next to useless as a lunar and planetary instrument, as well as harming stellar and even nebulous images. Many SW achromats suffer from significant SA, which is by far the greater of the two evils, yet they are nearly always criticised for their level of CA. SA is nearly always overlooked!

[Ags]

Just to be clear, the article I linked actally makes the point that the great refractors of the 19th century all had extreme CA, but yielded amazing breakthroughs in planetary science. Most of the big fracs have a worse Sidgwick ratio than an ST120. of course SA must be well controlled for high magnification work.

[John]

32 minutes ago, mikeDnight said:

CA is usually an obvious abberation, but graphs and tables can be a little misleading, as they can imply that a short F ratio scope is useless, being crippled by false colour. Often, in practice, a well made short/fast achromat can still render a very pleasing image, and still have a well controlled level of CA. A 6" F8 achromat for example, will show CA, but is still an amazing rich field scope / Comet Seeker. Far more damaging than chromatic aberration is spherical abberation, which will cripple definition, rendering the scope next to useless as a lunar and planetary instrument, as well as harming stellar and even nebulous images. Many SW achromats suffer from significant SA, which is by far the greater of the two evils, yet they are nearly always criticised for their level of CA. SA is nearly always overlooked!

Many chinese achromats have a noticable degree of SA as I discovered when I was playing around with Chromacor's a few years back. I think the chart is accurate from my experiences with various achromats but I agree that terming the CA as "unnacceptable" is not helpful. Thats down to personal preference I think. Personally I prefer as little CA and SA as possible which is why I was exploring Chromacor's back then. The difference in performance, particularly at higher magnifications, of a chinese 6" F/8, when the CA and SA were reduced around 80% was quite dramatic !

 

 

[martinl]

One thing about equivalent chromatic aberration is that the Sidgwick and similar criteria assume the same relative magnification, ie the same exit pupil. In practice larger scopes are used at lower relative magnifications (limited by seeing etc) so the perceived CA is often not as bad as the criteria would suggest.

A 4” f/10 should show the same CA at 200x as an 8” f/20 does at 400x, but if the 8” only operates at 300x the CA would be less intrusive than in the smaller scope. 

At least, that is how I understand it. 

[Ags]

One thing about CA is "you only see it around bright objects" and of course if you have more aperture there are more bright objects in any given field of view!

[Stub Mandrel]

Is there a similar chart for ED achromats?

[John]

5 minutes ago, Stub Mandrel said:

Is there a similar chart for ED achromats?

I've not seen one. It would have to take into account the various ED glass types, mating elements glass types, and combinations thereof, so lots and lots of options and a very complex chart !

The achromat one just needs to take account of the standard flint and crown glass achromat doublet and apply the Dawes and Conrady standard formula to the various apertures and focal ratios.

I've seen figures that indcate the potential CA reduction that certain types ED glass elements can deliver but there are a number of other factors that influence the end result that the observer actually sees.

I'm sure there are threads on other forums such as Cloudynights on this topic - many of them !

[TSRobot]

I have a CA full ST120. The CA is an indicator of where to look. Look at the blue highlight, the moon is the thing I'm meant to be looking at to the right of the blue highlight. CA flags up where to look. Can be quite useful.

When i put my cardboard home made 80mm/f8 aperture reduction device on, the CA disappears  and i can't find the moon anymore.

[F15Rules]

10 hours ago, jabeoo1 said:

Thats why F15Rules

Why, thank you, kind sir??..

Dave

[F15Rules]

8 hours ago, Saganite said:

Sure does ! 

CA is Almost non existent  in Andromeda, shown in all her beauty....again ! 

In Latinized Greek Andromeda means ' Ruler of men' apparently, which is rather appropriate.

Dave ( F15 Rules) named her before I took ownership. 

Looking superb, Steve. 

I have to say, she looks fabulous on that pier and with your EQ6 under her.

I've never seen better views of the moon through any scope, than with my then Baader Maxbrights in Andromeda..just jawdropping. I think it was really worth having the tube shortened to allow native magnifications without needing an OCS?...

Dave

[F15Rules]

11 hours ago, John said:

Interesting link.

The table below is quite a useful guide to the relative amounts of chromatic aberration that achromats of different specifications should show. It's a normal feature of the optical design though, in the same way that coma is to the newtonian design so we should not be overly concerned by it. Spherical aberration is present in many refractors and has more of an impact on performance than CA does.

 

 

I agree, John.

When I was a lad, virtually all the available refractors for amateurs were achromats. Most of them were F15, so in a long tube. Optically, many of the Japanese ones were very good (within the limits of their 60-80mm aperture), but they were nearly always under mounted, and so didn't perform well at high magnifications due to the poor mount stability and also the poor design of many of the then 0.965" eyepieces.

But chromatic aberration wasn't a big deal then. I actually grew up thinking Vega was a blue white star (partly because it was so described in Sir Patrick's "Observer's Book of Astronomy"), and partly because that's how it looked through my 60mm F15 refractor! But the important thing was that the image was super sharp and very pleasing, and below about magnitude 2 you just didn't see it.

In Andromeda, referenced above, (a USA built D&G 5" F15 achromat), the CA is almost invisible, even on Vega, and below mag 1.0 or so I would defy most Observer's to notice it. But what you would notice is a perfect Airy disk, with single diffraction ring and a real snap to focus. It really does deliver a Tak sharp image, but of course at the cost of an almost 2m long tube that needs a large and heavy mount.

But of its design type, it's pretty much perfect, and about 20-25% the cost of an equivalent apochromat.

It's no wonder that a 4" or 5" F15 design was the weapon of choice for many of the 19th century pioneer observers?.

And a 5" achromat was a favourite tool of our own late lamented Sir Patrick Moore as well?..

Dave

Edited January 26, 2018 by F15Rules

[John]

I think a 6" achromat needs to be f/15 at least to reduce the CA to levels that are not really visible in the eyepiece. My Istar 6" F/12 had a very nice objective (ie: nice, smooth figure) but the CA was quite obvious around the brightest targets I thought.

The 6" F/8 achromats I've owned showed more, until the Chromacor was installed and then it was controlled to ED doublet levels.

Sir Patrick Moore's 5" Cooke refractor is one of my very favourite telescopes

It was part of the motivation behind my decision to buy the 5.1" TMB / LZOS F/9.2 - the latter being a touch easier on the mount than an F/12 but having roughly similar lines and proportions perhaps ?:

 

[TSRobot]

Need a new sub forum. "CA- nuff said" 

[F15Rules]

2 hours ago, John said:

I think a 6" achromat needs to be f/15 at least to reduce the CA to levels that are not really visible in the eyepiece. My Istar 6" F/12 had a very nice objective (ie: nice, smooth figure) but the CA was quite obvious around the brightest targets I thought.

The 6" F/8 achromats I've owned showed more, until the Chromacor was installed and then it was controlled to ED doublet levels.

Sir Patrick Moore's 5" Cooke refractor is one of my very favourite telescopes

It was part of the motivation behind my decision to buy the 5.1" TMB / LZOS F/9.2 - the latter being a touch easier on the mount than an F/12 but having roughly similar lines and proportions perhaps ?:

 

Yes, I can see that John..Andromeda kind of reminded me of Sir P's Cooke too☺

Dave

[VNA]

Chromatic Aberration does not necessarily bother everyone, I have a Celestron 6" refractor aka "yard cannon" and don't see any CA, but I am not a spring chicken! However young people and well seing people are definitely annoyed by it looking through my scope!

With a fringe killer it helps a great deal and in addition you can reduce the aperture--I can't see myself spending ten times the amount for a scope without CA.

[geophilip06]

On 26/01/2018 at 05:32, Adam J said:

I never understood why it would be harder to correct C-A in larger apertures does the whole thing not just scale as the scope gets larger at a fixed f-ratio? Is it something to do with the size of the star resolved at the image plain?  

I know this is an old post, but this explanation may help:   

https://www.telescope-optics.net/polychromatic_psf.htm

"Since the RMS wavefront error is proportional to the P-V error, the actual wavefront error for an achromat of given aperture changes approximately in inverse proportion to the square root of its focal ratio. Actual chromatic error in an f/10 achromat is only 0.71 of that in an f/5, but in the latter it is also only 1.4 times larger than in the former. This is what the advanced optical design software programs, using diffraction calculation, imply (note that the value of PPDI in the visual range doesn't change with scaling doublet achromat while keeping the focal-ratio-to-aperture ratio F/D constant: 100mm f/12 has identical PPDI as 200mm f/24)."