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Chromatic Aberration Chart

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#1 Remy Bosio

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Posted 22 December 2011 - 02:31 PM

Barry Simon sent me this chart. He said he got it off CN.

It shows the amount of CA levels in a given objective. Pretty neat! It looks like my Omni XLT 120 is in the "tan" range which is good!

Incidentally, my scope will be coming in tomorrow! :jump:

Weather forcast says rain and clouds till next Wednesday! :foreheadslap:

Attached Thumbnails

  • 4979783-Chromatic Chart.jpg


#2 Al Miller

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Posted 22 December 2011 - 02:37 PM

Ahh... what chart? I'm not seeing anything.

#3 caheaton

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Posted 22 December 2011 - 02:53 PM

Chart displays fine for me. I saved the chart onto my pc from when it appeared previously...it's quite handy for making estimates of CA in different scopes.

#4 Vondragonnoggin

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Posted 22 December 2011 - 02:57 PM

Since I have an F/6.5, I'm not even on the chart. Excellent. I thought I was getting some pristine views, but this confirms it!

#5 JIMZ7

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Posted 22 December 2011 - 03:01 PM

Post deleted by JIMZ7

#6 Scott Beith

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Posted 22 December 2011 - 03:07 PM

Thanks Remy.
I want pics of your new scope! :waytogo:

#7 Jon Isaacs

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Posted 22 December 2011 - 03:14 PM

I can see the chart but only if I use a minus violet filter. :)

I know that the Chromatic Ratio which is plotted here is not the be all and end all, there are more accurate calculations. But in my experience, they are quite accurate, an 80mm F/5 with a CR of 1.5 is pretty drastic, a 80mm F/11 at 3.54 is quite apparent not too distracting and a 60mm F/13.3 at 5.63 is almost free of chromatic aberration.

Jon

#8 Scott Beith

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Posted 22 December 2011 - 03:18 PM

If I look at the chart through a Semi-Apo filter, everything shifts towards the green. ;)

#9 Mr Onions

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Posted 22 December 2011 - 03:41 PM

Hey Remy.

#10 rwiederrich

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Posted 22 December 2011 - 04:05 PM

I can see the chart but only if I use a minus violet filter. :)

I know that the Chromatic Ratio which is plotted here is not the be all and end all, there are more accurate calculations. But in my experience, they are quite accurate, an 80mm F/5 with a CR of 1.5 is pretty drastic, a 80mm F/11 at 3.54 is quite apparent and a 60mm F/13.3 at 5.63 is almost free of chromatic aberration.

Jon


Yeah but why doesn't the chart go up to 254mm or 381mm apertures?

It stops prematurely IMV

Rob

#11 Scott Beith

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Posted 22 December 2011 - 04:16 PM

I was hoping for 8", 10" and 11" slots on the chart. :)

#12 Jon Isaacs

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Posted 22 December 2011 - 04:27 PM

Yeah but why doesn't the chart go up to 254mm or 381mm apertures?

It stops prematurely IMV

Rob


The math is simple:

To be in the green with a 10 inch it simply needs to be F/30, t0 be in the dotted green, F/50.

For the 15 inch, the green is F/45 and the dotted green is F/75.

Even the 10 inch F/30 with it's 25 foot focal length is somewhat impractical... Most birdwatchers would pass on it.

I have heard there are ways around this, good color correction in larger apertures at amazingly fast focal ratios. But I think it's magic, they do it all with mirrors.

:)

Jon

#13 Scott Beith

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Posted 22 December 2011 - 04:32 PM

Yep 3X for good. 5X for great. :)

#14 Mike Clemens

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Posted 22 December 2011 - 04:54 PM

I always thought my 60m,f/11 has obnoxious chromatic abberation during terrestrial viewing. Not sure I've ever used it at night.

#15 Scott Beith

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Posted 22 December 2011 - 05:05 PM

daytime is tougher

#16 BarrySimon615

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Posted 22 December 2011 - 05:10 PM

I used the Chromatic Aberration (CA) Ratio to take a look at how some of the largest observatory refractors in the United States "stack up". The formula for finding the CA Ratio is f/ratio divided by diameter of the objective (in inches). Here is how they stack up:

Yerkes 40" (1020 mm) with a 19.4 meter focal length (f/19) has a CA of .475

The Lick 36" ( 910 mm) with a 15 meter focal length (f/16.5) has a CA of .458

The Allegheny Obs. 30" (760 mm) with a 14.1 meter focal length (f/18.55) has a CA of .618

The U.S. Naval Obs. 26" (660 mm) with a 9.9 meter focal length (f/15) has a CA of .576

The Lowell Obs. 24" (610 mm) with a 6.7 meter focal length (f/11) has a CA of .458

The Sproul Obs. (at Swarthmore College) 24" (610 mm) with a 11 meter focal length (f/18) has a CA of .75

So all of these scopes, at least by calculation should have horrible chromatic aberration, off the chart to the left and lot's worse than my 6" f/5 Jaegers with a CA of .83

Can anyone comment about the chromatic aberration that they have observed thru the historic large telescopes?

Barry Simon

#17 Al Miller

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Posted 22 December 2011 - 05:43 PM

I can see the chart but only if I use a minus violet filter.



:lol:

This time it shows up after I've been out for a while. Yes this chart has been posted a few times before.

#18 rwiederrich

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Posted 22 December 2011 - 05:45 PM

I used the Chromatic Aberration (CA) Ratio to take a look at how some of the largest observatory refractors in the United States "stack up". The formula for finding the CA Ratio is f/ratio divided by diameter of the objective (in inches). Here is how they stack up:

Yerkes 40" (1020 mm) with a 19.4 meter focal length (f/19) has a CA of .475

The Lick 36" ( 910 mm) with a 15 meter focal length (f/16.5) has a CA of .458

The Allegheny Obs. 30" (760 mm) with a 14.1 meter focal length (f/18.55) has a CA of .618

The U.S. Naval Obs. 26" (660 mm) with a 9.9 meter focal length (f/15) has a CA of .576

The Lowell Obs. 24" (610 mm) with a 6.7 meter focal length (f/11) has a CA of .458

The Sproul Obs. (at Swarthmore College) 24" (610 mm) with a 11 meter focal length (f/18) has a CA of .75

So all of these scopes, at least by calculation should have horrible chromatic aberration, off the chart to the left and lot's worse than my 6" f/5 Jaegers with a CA of .83

Can anyone comment about the chromatic aberration that they have observed thru the historic large telescopes?

Barry Simon


I have never looked through one, but I can't imagine how they ever discovered anything with such CA levels... :roflmao: Real world has a lot to do with it.

Rob

#19 Jon Isaacs

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Posted 22 December 2011 - 05:48 PM

Can anyone comment about the chromatic aberration that they have observed thru the historic large telescopes?

Barry Simon



I have never looked through any of these scopes but I do remember reading a comment by Roland that with the Yerkes scope, you moved the focuser something like a quarter of an inch to focus the different colors.

One fact to remember though is this (that is if I remember it correctly)

Chromatic aberration is evaluated in comparison to the size of the airy disk. The size of the chromatic blur is the same in a 3 inch F/10 as it is in a 6 inch F/10. However since in the 6 inch, the size of the airy disk is half that of the 3 inch, the chromatic aberration is doubled and to bring back to the same relationship, the focal length and therefore the focal ratio must be doubled.

In smaller scopes, I think this is a reasonable way to evaluate chromatic aberration but as telescopes become larger the chances of resolving the airy disk become less and less and so it is no longer such a useful tool. The airy disk in a 10 inch is about 1/2 arc-second, certainly half arc-second seeing is common enough that it one would resolve the airy disk and therefore any chromatic aberration. But anything larger, you are getting into range where the the resolution will be limited by the seeing rather than by the optics.

I think this is basically why Rob can get very good views with his 10 inch F/15's and stuff when the simple analysis suggests that CA would be a big problem.

Jon

#20 rwiederrich

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Posted 22 December 2011 - 05:59 PM

My 10"f/15 charts out at a 1.5 CA ratio(filterable)
Stopped down to:
8" f/18 I get a 2.25 CA ratio, (filterable)
7" f/21 I get a 3.0 CA ratio, (minimum to no CA)
6" f/25 I get a 4.1 CA ratio, (no CA to Apochromatic)
5" f/30 I get a 6.0 CA ratio, (Apochromatic)

These numbers are reflected on bright objects..so in reality CA is less visible on most objects at less aperture reduction. Personally, I don't see any CA on DSO's at full aperture.

Rob

#21 rwiederrich

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Posted 22 December 2011 - 06:10 PM

Can anyone comment about the chromatic aberration that they have observed thru the historic large telescopes?

Barry Simon



I have never looked through any of these scopes but I do remember reading a comment by Roland that with the Yerkes scope, you moved the focuser something like a quarter of an inch to focus the different colors.

One fact to remember though is this (that is if I remember it correctly)

Chromatic aberration is evaluated in comparison to the size of the airy disk. The size of the chromatic blur is the same in a 3 inch F/10 as it is in a 6 inch F/10. However since in the 6 inch, the size of the airy disk is half that of the 3 inch, the chromatic aberration is doubled and to bring back to the same relationship, the focal length and therefore the focal ratio must be doubled.

In smaller scopes, I think this is a reasonable way to evaluate chromatic aberration but as telescopes become larger the chances of resolving the airy disk become less and less and so it is no longer such a useful tool. The airy disk in a 10 inch is about 1/2 arc-second, certainly half arc-second seeing is common enough that it one would resolve the airy disk and therefore any chromatic aberration. But anything larger, you are getting into range where the the resolution will be limited by the seeing rather than by the optics.

I think this is basically why Rob can get very good views with his 10 inch F/15's and stuff when the simple analysis suggests that CA would be a big problem.

Jon


Jon..I tend to agree. There have been times when seeing was nasty and the views were nasty..almost a wash..The moisture in the atmosphere causes the larger objective to act like a CA magnifyer...while on exceptionally good seeing nights the airy disk is not bloated..hens not appearing to contribute to the CA level. Images are crisp and CA is greatly reduced.

Personally I don't see it but on extremely bright objects. And why am I looking at those..... :foreheadslap:..I'm not.

:lol: :lol:

Rob

#22 dannyg

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Posted 22 December 2011 - 06:12 PM

I have the omni xlt 120 you will love it!!!

#23 Cotts

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Posted 22 December 2011 - 10:35 PM

Charts like this always leave out the magnitude of the object being viewed. There is also no differentiation in the chart between visual and the far more noticeable photographic CA levels.

My experience recently suggests that, for a six inch f/5.9 achromat visually, there is no CA visible on stars fainter than magnitude 3.5. At magnitude 5, 7 or 9 there is no CA difference between a $900 achromat and a $9 000 APO.

The above is most certainly not true for any kind of long exposure photography where star bloat and purple fringes ruin the image of almost every star in the field of view. My six inch f/5.9 put ridiculous purple around stars as faint as magnitude 8 and noticeable bloat on stars down to mag 12 in a 4 minute exposure.

Visually, there are three factors to consider; aperture, focal ratio and the magnitude of the object being viewed.

For photography there are four factors; aperture, focal ratio, the magnitude of the object being photographed and the length of the exposure.

I'm not sure how to make three and four variable charts in two dimensions but that's what we really need for this topic.

Dave

#24 BarrySimon615

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Posted 23 December 2011 - 08:35 AM

Here are two photos taken some years ago thru my Jaegers 6" f/5. The lower photo of the North American and Pelican Nebulae is unfiltered, notice the blue chromatic bloat. The upper photo of the same area has been shot thru a Deep Sky Filter. This brings out the red nebulosity and helps suppress the chromatic aberration. Photography of non-nebular areas can be shot thru a Deep Sky Filter and the background can be improved in post processing.

The upper photo is rotated slighly more in a clockwise direction.

Barry Simon

Attached Thumbnails

  • 4980865-North American, filtered and not.jpg


#25 Vondragonnoggin

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Posted 23 December 2011 - 08:51 AM

Now I'm confused as you sent the chart to Remy, but the chart says a 6" F/5 has unacceptable levels of CA and the next category is filterable levels of CA which a 6" F/5 does not fall in, yet you have taken a filtered shot of those Nebula and it turned out beautifully. Quite acceptable levels after the deep sky was on there.

That doesn't make sense. Maybe just forget about the chart and use what you find acceptable? Is that chart supposed to be some guide for photographers? It says "visual" on it. Just curious.

I find both shots beautiful actually, but the top shot with filter has stars that are more true color.


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