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Chromatic Aberration Chart

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#51 clintwhitman

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Posted 28 December 2011 - 07:18 AM

In My experience not all doublet objectives are cut from the same cloth. Many lens makers went to great lengths to find highly disperse glass blanks and a million other tricks to to reduce chromatic aberration. A bunch of makers like Clark, Nippon Kogaku, Tinsley, Ziess and others make really good very low color objectives when compared to the mass producers and just plain bad lenses that are out there.

So in my opinion a chart like this would be about as useful as a chart judging a cars performance based on the tire size or wheel base
:foreheadslap: :o

#52 Jon Isaacs

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Posted 28 December 2011 - 08:29 AM

Clint

Roland says if you look at the Abbe chart, those magic glasses just don't and did not exist. However almost all the scopes of the classic era are in the range where CA is not a major issue. No one was building 6 in f/5's and pointing them at Venus.

Jon

#53 galaxyman

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Posted 28 December 2011 - 09:21 AM

Cheers, Karl. You may know that I had an APM 8-inch f/9 several years ago and I can say that it was a real delight, although it overcame my bad back, eventually....

CA is not an issue below 3rd - 4th magnitude on virtually any amateur sized achromats. The vast majority of the stuff we look at is fainter than 4th magnitude.

But already know this, don't we?!?

Dave


Hi Dave

Yep, it was actually your review of the scope on APM's site that pushed me forward in getting it (8" f/9).

Anyway you mentioned the stuff we look at. For instance Ngc 404 (Mirach's ghost) looks terrific through the 8" and an Nagler 11mm T1 (164x) , with Mirach's nice golden color and the galaxy standing out nice and bold, with a bright nucleus and the round disc very mottled. Some CA, but far from distracting.

Most of my views of course do not include such a bright star in the field, but it's a view that I return to every Autumn night.


Karl
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#54 clintwhitman

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Posted 28 December 2011 - 09:45 AM

Clint

Roland says if you look at the Abbe chart, those magic glasses just don't and did not exist. However almost all the scopes of the classic era are in the range where CA is not a major issue. No one was building 6 in f/5's and pointing them at Venus.

Jon

Hi Jon,
Not all 6" F15s Achromatic refractors telescopes have the same amount of chromatic aberration in fact they all differ to some extent. I have used 100s of them and can say for certain some are much better than others when it comes to color. Jon Pons had 5,25" F15 that showed 0 color even when trained at Venus at high power the lens was a doublet prototype made in the 1950s. The He sold it for $7k due to its amazing properties. I have another 6" F15 that shows a small amount of color on Venus and have seen other 6" F15s that paint everything blue. The 6" and larger scopes that are at F10 or below I have just assumed would have color but even those vary. Saying that design and glass make no difference when it comes to the color produced by a refractor goes against everything Pons, Barry and several other telescope designers ever said to me. And if this was the case people would not spend $3500 for a 80mm AS Zeiss lens. Roland is a good guy and he I have an on going project with him. He lives in APO land...not ACHRO land. We have never spoke of doublets, and I get the feeling he might hang up If I brought the subject up.
:question:

Clint

#55 ValeryD

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Posted 28 December 2011 - 12:24 PM

Roland is a good guy and he I have an on going project with him. He lives in APO land...not ACHRO land. We have never spoke of doublets, and I get the feeling he might hang up If I brought the subject up.
:question:


I think your feeling was wrong. Many years back we spoke with Roland about apochromats and he told me that he ceased the doublets because _customers_ prefer to buy triplets and he agreed, that best visual APOs are doublets.

#56 clintwhitman

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Posted 28 December 2011 - 10:14 PM

Hi Valery
I miss spoke I have a 9" F10.6 refractor Roland built that he is going to help out on. It is a Triplet APO. I was addressing ACHRO not APOs Telescopes. I have a couple of those APO OTAs laying around but color is not and has never been one of their strong points!! In fact I don't remember seeing any color in the 9" AP or my 6" F7 EDSF or even the TAKs pretty much color free,,,, Viewing. Its those pesky airspaced ACHRO doublets I was thinking of. You know the ones your so good at fixing up. :bow:


Clint
:shocked:

#57 GilATM

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Posted 29 December 2011 - 03:29 AM

On the psychology of color:

My wife and son recently declared that they preferred seeing Jupiter through my recently cobbled Jaeger 6" f5 over my 12.5" f6 reflector because they thought the violet fringe was so beautiful!

(and, to be honest, it is a lovely color).

Beauty, and telescope correctness, are in the eye of the beholder.

- Gil
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#58 RussL

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Posted 29 December 2011 - 03:52 PM

I have an Orion 120ST. BUT...I use it for its intended purpose---richfield observing. CA never bothers me at all when I'm doing that. However, when I get the moon, Jupiter, Venus, or any very star in view CA is there. But, I tame most of it by using some home-made masks of different sizes. i esprecially like my 80mm mask, although it doesn't get rid of all the CA, but ebnough so that I don't mind it. The 54mm mask supplied with the end cap gets rid of just about all CA, or at least all that I can see. Of course, masks can cost you some resolution at higher mags. But again, I didn't buy the scope for planetary and lunar use. If I need to get in good with them I can always get out my 8" SCT.


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