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Chromatic Aberration Chart

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#26 Scott Beith

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Posted 23 December 2011 - 08:56 AM

The chart probably references planetary/lunar observing which is a tougher CA test.

#27 Vondragonnoggin

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Posted 23 December 2011 - 09:01 AM

The chart probably references planetary/lunar observing which is a tougher CA test.


:foreheadslap: that would make more sense.

Some of the filters are quite good at turning a scope into acceptable on Lunar and Planetary. I suppose it really is individual on CA sensitivity.

#28 jrbarnett

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Posted 23 December 2011 - 10:19 PM

Anecdotal, but interesting:

http://www.rfroyce.c...actor spots.htm

"To emphasize how extreme secondary spectrum can be and still have a telescope considered useful, the 36" Lick refractor has a longitudinal aberration of about 1.5"! I remember the late Sky and Telescope columnist Walter Scott Houston telling me about observing with the 40" Yerkes refractor, 'Everything was bathed in blue.'"

Here's an old write up (as in 1889, the year after the scope was completed) on the Great Lick's false color:

http://adsabs.harvar...PASP....2..160K

You can see from these pieces that the different colors actually come to focus on the Great Lick *inches* of focal length apart.

Regards,

Jim

#29 Mel M

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Posted 24 December 2011 - 12:28 AM

Anecdotal, but interesting:

http://www.rfroyce.c...actor spots.htm

"To emphasize how extreme secondary spectrum can be and still have a telescope considered useful, the 36" Lick refractor has a longitudinal aberration of about 1.5"! I remember the late Sky and Telescope columnist Walter Scott Houston telling me about observing with the 40" Yerkes refractor, 'Everything was bathed in blue.'"

Here's an old write up (as in 1889, the year after the scope was completed) on the Great Lick's false color:

http://adsabs.harvar...PASP....2..160K

You can see from these pieces that the different colors actually come to focus on the Great Lick *inches* of focal length apart.

Regards,

Jim






A few pages into the 1889 article it is said that the green seas of Mars seem to be drying up.

#30 jrbarnett

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Posted 24 December 2011 - 03:48 PM

Sure Mel. Laugh it up. Just wait until the *new* upcoming uber-rover runs into a masonry wall and teeters over into a dry brickwork canal. Then it'll be the Martians having a chuckle. :lol:

- Jim

#31 stevew

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Posted 25 December 2011 - 12:13 AM

So when did this chart originate, and who made it?
What is it based on?

Steve

#32 ValeryD

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Posted 25 December 2011 - 12:22 AM


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.


Dave



Dave,

This proves only one solid fact: you are relatively color blind.

I can EASLY see color aberration on stars 3,5m even in 80mm F/11 achromat. My wife see it easily too.

#33 buddyjesus

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Posted 25 December 2011 - 02:15 AM

Sorry, I just had to laugh at the previous post. haha

I have no problem noticing the CA in my 4" f/10. It didn't bother me when I first got the instrument, but now it does. It isn't the halo or rings around the planets that are bothering me, but the soft focus at high powers. I find the CA on stars pretty and I don't care to go without it totally, so my "next" scope is going to have a better polish and longer focal ratio. I am considering a 4" f/13 carton objective.

#34 plyscope

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Posted 25 December 2011 - 04:48 AM

This chart was made by Mardi (photonovore) for his own reference. He shared it on CN on 08/20/05, post #566983. There was a thread back then but I can't find it in search to provide a link.

Andy

#35 Rick Duering

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Posted 25 December 2011 - 06:11 AM

Neil English uses the chart as well in his book, Choosing and Using a
Refracting Telescope. If you google it, click on it under books. google.
com/books and you can preview it.

Rick

#36 Jon Isaacs

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Posted 25 December 2011 - 07:31 AM

So when did this chart originate, and who made it?
What is it based on?

Steve


The chart is based on some analysis but basically they are two rules of thumb, the 3X rule and the 5X rule... The basic idea is that chromatic aberration in an achromat is well defined and the CA depends on two factors, aperture and the focal ratio of the objective, it is proportional to the aperture and inversely proportional to the focal ratio.

Thus a 3 inch F/5, has the same level of chromatic aberration as a 6 inch F/10.

The 3X rule: This is associated with Sidgwick. The way I interpret this rule is: "If the focal ratio is at least 3 time the aperture as measured in inches, the chromatic aberration will not be overly distracting. It will be visible on bright objects.

The 5X rule: This is associated with Conrady. The way I interpret this rule is: If the focal ratio of the objective is at least 5 times the aperture as measured in inches, then the chromatic aberration will be minimal and only detectable on the most difficult targets.

In my experience, these rules are quite accurate in their prediction of the level of false color. An 80mm F/11 has a CR of about 3.5, on Jupiter, one sees a purple haze but it is not so distracting. A 76mm F/16 has a CR of about 5.2, the views of Jupiter will be essentially free of CA.

The problem with building larger Achromats to these specifications is that it soon gets out of hand. A 6 inch designed to the 5X rule would be F/30 and so about 15 feet long.

Jon

#37 Cotts

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Posted 25 December 2011 - 07:50 AM

[quote name="ValeryD"][quote]

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.


Dave [/quote]


Dave,

This proves only one solid fact: you are relatively color blind.

I can EASLY see color aberration on stars 3,5m even in 80mm F/11 achromat. My wife see it easily

Each person's eyesight is different. The threshold for me is mag 3.5. For you and your wife this threshold exists too. Or are you siggesting that CA is visible on all stars in a short achromat?

I stand by my statement that for mag 5, 7 and 9 stars the 6-in f/5.9 showed no false color. It showed red stars as red, yellow as yellow and white as white. Your dismissive statement notwithstanding.

Dave

#38 Maverick199

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Posted 25 December 2011 - 12:20 PM

Remy, dumb question. Is brand of any consequence? Lots of brand talk about hardly noticeable CA. I have AR102 which is F/6.5 so am I safe? Thanks. :)

#39 Scott Beith

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Posted 25 December 2011 - 01:55 PM

brand doesn't matter. optical design matters for ca. some people are more sensitive to ca than others.

#40 Jon Isaacs

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Posted 25 December 2011 - 05:20 PM

Remy, dumb question. Is brand of any consequence? Lots of brand talk about hardly noticeable CA. I have AR102 which is F/6.5 so am I safe? Thanks. :)


Maverick:

This is a good read, once you are finished, hit the return button and look at some images and more on optics and refractors.

Roland Christen: More on Color Correction

Jon

#41 buddyjesus

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Posted 26 December 2011 - 04:27 AM

Remy, dumb question. Is brand of any consequence? Lots of brand talk about hardly noticeable CA. I have AR102 which is F/6.5 so am I safe? Thanks. :)


If it isn't broke, don't fix it. Your scope is made for low power deep sky views. CA is generally worse at high power and on brighter targets. Also if it doesn't bother you, enjoy. And if you want to enjoy your scope longer, try to avoid looking through an apochromat.

#42 Scott Beith

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Posted 26 December 2011 - 07:30 AM

Maverick,

On a side note your scope should be awesome on open clusters, M31, and a bunch of other targets in night sky. :waytogo:

#43 Vondragonnoggin

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Posted 26 December 2011 - 10:00 AM

Remy, dumb question. Is brand of any consequence? Lots of brand talk about hardly noticeable CA. I have AR102 which is F/6.5 so am I safe? Thanks. :)


Maverick, get a fringe killer or semi-apo and enjoy the scope on everything. My AR127 at F/6.5 puts up great views on everything with the right filters in. The Semi-Apo stays in most of the time. My ND13 makes the moon very nice to look at and that is about the most apparent on CA. The UHC or OIII while looking at nebula, takes out any CA. Deep sky low power is great unfiltered. I have been testing my new 150 mak and an absolutely color correct view and I still love the AR127 just as much. These achros from ES have very sharp optics and they picked a great focal length to really excel at color control with a filter and getting a very wide view.

You need to just enjoy your scope on whatever targets you feel like and forget about a chart. A chart will not dictate whether I am going to find my scope acceptable or not. Unless you are doing AP and looking for perfect color, enjoy your scope period. If you are doing AP, the filters and post-processing should fix a lot. This is one reason I don't give a hoot about a chart. They don't do a thing for me in usefulness.

Just my two cents.

#44 Maverick199

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Posted 26 December 2011 - 12:02 PM

Thanks Jon. I read the whole matter and then some. Clicked on 1.25" and 2" Dielectri diagonals to find out the dielectric designs have a turned down edge. That is something I didn't know and thought opposite.

Thanks Scott. :) Appreciate your advise and look forward to some clear views the coming days.

Thanks Buddy. I am waiting for the ES Diagonal to arrive so as soon as I get them, will test out. This would be my first view through a Refractor so hence my question.

Thanks Von. I need to look up this semi-Apo. ( I groan at the thought of spending more but looks like there may be little choice when it comes down to perfection on a limited budget ).

Finally, thanks Remy for this excellent topic. :D

#45 Remy Bosio

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Posted 26 December 2011 - 01:55 PM

Haseeb, I always knew CA was a problem in fast scopes. I had anticipated this when I got the scope with it's relativly fast focal ratio. I don't feel that CA is a huge compromise when you consider the Omni 120 is at the upper limits of what this antique amateur astronomer can effectively and safely handle. This instrument will be used mostly at my residence as my traveling days are waning.

Notwithstanding the above, every amateur should familiarize him/herself with the various optical pitfalls from scope to scope and attempt to make reasonable corrections to same. I have never been disappointed with the views from any telescoe. I am just thankful I have been preveliged and able to observe through many scopes over the years from 60mm. to 82" and believe me I wouldn't trade any of those memories for any sum.

I would also like to thank my friend and scope mentor, Barry Simion for sharing his wisdom and expertise with me over the years. Like my old grandmother used to tell me: "Every little bit helps!". :bow:

#46 Maverick199

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Posted 27 December 2011 - 12:44 PM

Yes it does Remy and thanks again for enlightening me. :bow:

#47 rwiederrich

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Posted 27 December 2011 - 01:26 PM

This chart was made by Mardi (photonovore) for his own reference. He shared it on CN on 08/20/05, post #566983. There was a thread back then but I can't find it in search to provide a link.

Andy


Mardi is a woman.

#48 galaxyman

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Posted 27 December 2011 - 02:07 PM

[quote name="Cotts"][quote][quote]

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.


Dave [/quote]


Dave,

This proves only one solid fact: you are relatively color blind.

I can EASLY see color aberration on stars 3,5m even in 80mm F/11 achromat. My wife see it easily

Each person's eyesight is different. The threshold for me is mag 3.5. For you and your wife this threshold exists too. Or are you siggesting that CA is visible on all stars in a short achromat?

I stand by my statement that for mag 5, 7 and 9 stars the 6-in f/5.9 showed no false color. It showed red stars as red, yellow as yellow and white as white. Your dismissive statement notwithstanding.

Dave [/quote]

Hi Dave

Your right on :waytogo:

Some times I would love to have these naysayers observe with us (CAS). That would be interesting :cool:


Karl
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#49 plyscope

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Posted 27 December 2011 - 05:01 PM

This chart was made by Mardi (photonovore) for his own reference. He shared it on CN on 08/20/05, post #566983. There was a thread back then but I can't find it in search to provide a link.

Andy


Mardi is a woman.


Oop's sorry about that, I didn't realise.

Andy

#50 Cotts

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Posted 27 December 2011 - 09:23 PM

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


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