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APO vs. ACHRO

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#1 oneaudiopro

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Posted 06 July 2007 - 01:13 AM

I've read these forums for some time now and have read many posts on what scope should I buy and the discussion always seems to center around lens types, aperature and APO vs ACHRO. As a consumer myself, I have always wanted to compare apples with apples to make an informed decision on what equipment to buy BUT some manufactures seem to stretch the boundaries of technical terms that determine the differences between APO and ACHRO designed scopes.

Apocromatic means “without color”.

With this in mind, I have concluded the following:
1) "true" APO's are always triplets regardless of lens type
2) ACHROS always show false color
3) aperature only means something if the rest of the optical train is properly and correctly aligned

I'd like to keep this discussion free of manufacturer comparisons, but rather of the 2 designs and how they decide what is an APO and what is not. I look forward to hearing your comments and sharing your ideas.

#2 Benjamin B

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Posted 06 July 2007 - 01:47 AM

Apochromat

An apo can also be of two lenses. Some triplets shows color as well.

#3 naglertized

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Posted 06 July 2007 - 06:33 AM

oneaudiopro,

What type of observing do you enjoy most? Are you planning on doing any imaging?

Planetary and lunar observing are very "hard" on an achromat. Meaning those objects will reveal the chromatic abberation time and again. If you are imaging the CA will be a detraction to image quality.

On DSO's the CA is nearly undetectable. So some of the larger 6" achros represent a real value if you are primarily into DSO's and do planets and luna on the side. If this is your cup of tea, invest in a minus violet filter for those opportunities.

Collimation is a decreasing factor the longer the F/# of the scope. Shorther F ratio scopes it is more critical. The great thing about refractors is once they are collimated they stay that way. The only things that will alter this are upgrades done to the optical train of the OTA and mishandling of the OTA, ie. dropping it! I have removed the objective of my C6-R several times and have not had to re-collimate, I still get perfect diffraction patterns. I did mark the screw positions so I replaced the objective in the same orientation.

Good luck if you are planning to purchase.

#4 jrcrilly

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Posted 06 July 2007 - 06:48 AM

With this in mind, I have concluded the following:
1) "true" APO's are always triplets regardless of lens type
2) ACHROS always show false color


The initial problem with this is that APO is a performance parameter while achromatic is a specific telescope design. You can choose to decide what level of performance qualifies for your personal definition of APO but a telescope is either an achromat or not - and performance doesn't enter into that. It's possible to build an achromat with no false color by choosing the appropriate parameters. This is rarely done because of size and weight constraints.

APO designs are directed at achieving that performance in a smaller package. Any number of APO designs exist, incorporating from two to four elements.

Many other designs exist - lots of non-APO telescopes are not achromats.

#5 BillP

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Posted 06 July 2007 - 07:41 AM

Hello. Well, a "difficult" subject at best. I can easily see how you have drawn your 3 conclusions based on discussions, as popularly that is what seems to get expressed. But in reality I thing it is somewhat different.

A rather technical definition of achro vs apochro is as follows - that I pulled off a patent application. As you will see, it's not really about lens elements, but more about color crossings and how well varying wavelengths converge to a point within the airey disk.

"An achromatic lens system is, by definition, one which has paraxial chromatic correction at two discrete wavelengths. The residual color error at wavelengths other than these two is often called secondary color. Apochromatic lens systems have much smaller secondary color because they are corrected at three wavelengths. The residual color error of an apochromat is often referred to as tertiary color."

APOs come in many flavors to achieve the 3 color crossings, 2 elements, 3 elements, and 4 elements. Basically there are many ways to skin a cat as it were, so the number of elements are just a way to "implement" the definition. I suppose probably the best design for achiveing apochromatic performance in an optical system, it could be argued, would be the parabolic mirror! The patent I pulled the definition from is for yet another new design which used standard flint-crown glasses for the 1st 2 elements, then placed a liquid lens after them to achieve APO performance because it's just plain cheaper to get the specialty low dispersions needed for APO performance using liguids rather than glass - 17th century idea finally making it to high technology.

IMO Achros do show false color...however on some it jumps out at you and on others you really have to search hard to find it. But as a general rule, your #2 I think is a good working assumption as long as one understands that the color may not always be obvious or it may only show on a very few brightest targets, making the color immaterial practically.

Finally, collimation is important regardless of aperture. Collimation is more sensitive in a faster optical system than a slow one. By that I mean how bad the resulting image presents itself when off. But above all, aperture always always rules IMO and IME. So more is always better, until such things as convenience and price get involved. In this realm I have found for me personally, that 6" is a minimum for a scope if it is to be used for all types of observing. Anything smaller compromises the image IMO (resolving power and light gathering). Now that's not to say a 3" scope is no good, it just means for me that it is a "supplementary" scope with limits in it's use. But then again, my opinion is based on what I got used to - a 10" scope. So I'm used to the 10" imagery and whenever I use an aperture that is less, I pretty much easily notice what I'm losing. With 6" scope I find I can still stay satisfied viewing the broad range of all targets. With a 3" I find I get satisfaction from Luna, doubles, and planets somewhat. All others I feel compromised. So aperture rules for me...until it gets too pricey or large and inconvenient.

The other thing you stated is quite true...manufacturers really stretch the definitions for their own convenience. I have seen them all stretch and bend the meanings of things for their convenience. This should not be taken in a bad light as they are operating businesses and this is what is done. Standard practice. It's just marketing and probably most of the time they don't even realize they do this as it has just become habit. So no faulting, just a reality. I mean, if all you sell is APOs for example, you will never hear many good words from those manufacturers about achros. To be expected as they have to make a living selling their wares.

Anyway, good topic.

#6 snart

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Posted 06 July 2007 - 08:03 AM

Even if the chromatic error in an achromat isn’t immediately obvious, it is always there. When all the colors do not focus at, or very near, the same point then there has to be some blurring. This robs you of contrast and of definition. An achromat will miss high resolution features that can be seen in an apochromat but unless you are comparing an achromat and an apochromat side-by-side you will never know what was missed. A good analogy to this problem is like looking through a screened window verses a glass-only window. The screen causes slight diffraction of the light and the waves are now coming to focus at slightly different points causing a hazy, soft image. Through a glass-only window the light is affected very little and you see a sharp, crystal clear image. There is indeed a good reason why many people choose to spend five times as much money on an apochromat than a similarly sized achromat.

#7 Jon Isaacs

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Posted 06 July 2007 - 08:39 AM

>>>Apocromatic means “without color”.

With this in mind, I have concluded the following:
1) "true" APO's are always triplets regardless of lens type
2) ACHROS always show false color
3) aperature only means something if the rest of the optical train is properly and correctly aligned
-------

As others have pointed out, there are three element achromats and two element APOs, that is just how it is. Since the color correction is a function of not only the glasses used and design but also the focal ratio and aperture, doublet designs are sufficient in smaller apertures. In essence one is trying to hide the color error behind the airy disk, that is easier to do with a smaller scope because the airy disk is larger.

Roland Christen on Color Correction in Refractors

There are also other interesting essays by the guy that started the affordable APO revolution.

As far as how to decide whether a scope is an APO or not, I look for false color. During the day, looking at a high contrast situation will show false color. My favorite is a telephone pole at maybe 100 meters against a bright blue sky. Sufficient magnification is necessary to make sure the your eye pupil is not masking the aperture. At low magnifications viewing off axis can accentuate the false color because your eye is only looking at the edge of the lens.

At night, I like Venus as a test.

jon

#8 Don W

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Posted 06 July 2007 - 09:39 AM

This subject has been discussed and rehashed a number of times here in this forum. You may want to try doing a search.


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