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Chromatic Aberration Reducing Filters?

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#1 Cometeer

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Posted 12 October 2014 - 12:47 PM

Are there any color filters that reduce the amount of chromatic aberration? I have an ED refractor, so the CA is little to begin with,  but I want to further eliminate it. I heard of Baader filters, but as of now that's out of my budget.

 

I saw this somewhere...Hoya 48mm 85B. Would it work?



#2 mogur

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Posted 12 October 2014 - 01:24 PM

When I had my 102 Acro I used a light yellow (#8) filter to reduce CA and enhance contrast. Good light throughput (doesn't dim the image too much). You could also try a minus violet filter. Those are fairly cheap. I use one in my 120 ST and it reduces CA by about 50% I'd say.



#3 stevew

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Posted 12 October 2014 - 02:53 PM

When I had my 102 Acro I used a light yellow (#8) filter to reduce CA and enhance contrast. Good light throughput (doesn't dim the image too much).

Agreed. #8 light yellow will help remove the violet, but will give a slight yellow cast to the entire image.

 

Steve



#4 russell23

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Posted 12 October 2014 - 04:52 PM

The #8 light yellow is a great option for a scope that already has excellent control of CA.  It costs less than the Baader Fringe Killer with virtually identical CA reduction and color shift - very mild.

 

Dave



#5 Carl N

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Posted 12 October 2014 - 08:19 PM

I prefer theBaader semi-APO.



#6 De Lorme

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Posted 15 October 2014 - 10:19 PM

I read a while back on the refractor forum that a prism diagonal would be helpful for a under
corrected ED refractor because a prism diagonal extends the light.

Edited by De Lorme, 15 October 2014 - 10:20 PM.


#7 Cometeer

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Posted 16 October 2014 - 06:58 PM

I read a while back on the refractor forum that a prism diagonal would be helpful for a under
corrected ED refractor because a prism diagonal extends the light.

 

When I tried out a 1.25" prism diagonal, the scope did not reach focus.



#8 AR6

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Posted 16 October 2014 - 09:15 PM

 

I read a while back on the refractor forum that a prism diagonal would be helpful for a under
corrected ED refractor because a prism diagonal extends the light.

 

When I tried out a 1.25" prism diagonal, the scope did not reach focus.

 

My AR-6 came with a 2" mirror diagonal and a 1.25" prism diagonal.  With the stock focuser both would focus with pretty much any eye piece due to the exceptionally long stock draw tube on the focuser.  However that long draw tube and cheap focuser left a lot to be desired in terms of alignment, and focus was critical anyway with the f/8 focal ratio.  

 

With a Moonlight focuser and a 4.5" draw tube, it was focus challenged with the shorter focal length 1.25" mirror diagonal, but a 1.5" extension tube fixed the problem.   



#9 Eddgie

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Posted 17 October 2014 - 08:13 AM

Are there any color filters that reduce the amount of chromatic aberration? I have an ED refractor, so the CA is little to begin with,  but I want to further eliminate it. I heard of Baader filters, but as of now that's out of my budget.

 

I saw this somewhere...Hoya 48mm 85B. Would it work?

 

Not sure that you realize it but these filters do not reduce charomatic aberration.   CA results in energy being removed from the Airy Disk of of a star, or from details adjacent to one another energy is smeared).

The filter does nothing to put this energy back to where it is supposed to be.

 

If fixes the cosmetic effect, but it does this by removing even more energy from the Airy Disk of a star, or more energy from the features on an extended target.

 

It is like washing a dirty car with a dent in the door.    The car looks cleaner, but the dent is still there.

 

Anyway, not sure of your goal, but filters don't reduce CA.  There is nothing that can reduce CA, but only mask the symptoms.

 

If the fringing bothers you though, the fitler can enhance the view cosmetically.   Always comes at the expense of removing more energy though.   It does not "Fix" CA.   A filter only alters the data further by removing even more energy from the target item. can enhance the appearance of some details, but almost always at the expense of reducing the contrast of other details.



#10 russell23

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Posted 17 October 2014 - 12:58 PM

To varying degrees refractors will produce CA - even ED APO's as you are seeing.  The CA is generally detected a purple halo around bright objects such as stars, planets, the Moon.  For many achromats there will also be a fringing produced at the red end of the spectrum. 

 

There several aspects here

 

1.  Objective producing CA

2.  Visual detectability of the CA

3.  Image degrading effects of the CA 

 

As Ed notes, the production of CA results in defocus of some light that is no longer in the image.  Lost light means some loss in image brightness.  Visually the CA has several negative effects with achromats especially.  First, the CA softens the image of the Moon and planets.   Second, the CA mades it difficult to get sharp star images at high magnifications.  Third, the  defocused light reduces contrast against the sky background. 

 

A filter cannot return the defocused light.  So it does not fix the cause.  However a good CA filter will treat the symptoms of CA.  The objective still produces CA but the filter reduces (most filters) or  completely removes purple CA (Baader 495 LP filter, #12 light yellow) the defocused light and thereby eliminates the negative effects of CA (except the lost light).  So the sky background is cleaner, Lunar and planetary features are sharper, and star images can be tightly focused at higher magnifications with the CA filters than without. 

 

A filtered achromat is superior in performance to an unfiltered achromat.  In the case of your ED refractor, the CA is so little you probably can get away with a #8 light yellow filter.   For large achromats I recommend the Baader 495 Longpass.

 

Dave



#11 Eddgie

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Posted 17 October 2014 - 04:20 PM

I have a low opinion of achromats, so won't go there.

But to the OP, consider this.   If you see a small fringe around a bright star, remeber that most of the blue enegy is still concentrated in the Airy Disk.

In an Achromat it is so defocused that it spreads the energy in a large area around the star.   Anyone using a big 6" f/8 achromat will tell you that on a bright star, there is a big purple glow.   Now of course that energy came from the star, but imaging how much of it there must be that even though it is so defocused, you can still see it!!!!

 

Enter the modern ED scope.  The level of correction is so much better that often, the only thing visible is a slight fringe.   The amount of blue energy still encircled inside the green light Airy Disk (by comparison to the achromat) is huge.

 

The filter not only removes the fringing around the star, but if it is effective enough to do this in any meaningful way, it also removes a great deal of that same blue energy from inside the green light Airy Disk.

 

And it does it to every star in the field.

This is the way filters work.  They can only remove energy.  In the Achromat, the purple in the field is of course bothersome, but it is so diluted that it does little to further damage the image.  The contrast loss is due to the energy not being where it is suppsed to be, not where it ends up.

 

If you use a filter that is strong enough to eliminate the fringe, you also take that same amount of encircled energy that is still contributing to the limiting magnitued performance and take it away.  All of the stars in the field suffer the same fate.

 

I tried to turn a 6" f/8 achromat into something better by using filters, but they were so inferrior to a 6" Mak Newt that I gave up completely.

And  the first time I used a 6" ED scope (Meade 152ED) the differnce in performance was pretty staggering.  No filter I used on the achromats really did much to improve the detail (though again, a filter can enhance some details, but always at the expensce of others.  Think about red and green filters on planets.. Some people love them because they feel that each allows better detection of some specific kinds of details.  I am sure a yellow fitler would do the same).



#12 mogur

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Posted 17 October 2014 - 04:44 PM

I'd rather have a clean car with a dent, than a dirty car! ;)



#13 Cometeer

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Posted 17 October 2014 - 05:02 PM

 

Are there any color filters that reduce the amount of chromatic aberration? I have an ED refractor, so the CA is little to begin with,  but I want to further eliminate it. I heard of Baader filters, but as of now that's out of my budget.

 

I saw this somewhere...Hoya 48mm 85B. Would it work?

 

Not sure that you realize it but these filters do not reduce charomatic aberration.   CA results in energy being removed from the Airy Disk of of a star, or from details adjacent to one another energy is smeared).

The filter does nothing to put this energy back to where it is supposed to be.

 

If fixes the cosmetic effect, but it does this by removing even more energy from the Airy Disk of a star, or more energy from the features on an extended target.

 

It is like washing a dirty car with a dent in the door.    The car looks cleaner, but the dent is still there.

 

Anyway, not sure of your goal, but filters don't reduce CA.  There is nothing that can reduce CA, but only mask the symptoms.

 

If the fringing bothers you though, the fitler can enhance the view cosmetically.   Always comes at the expense of removing more energy though.   It does not "Fix" CA.   A filter only alters the data further by removing even more energy from the target item. can enhance the appearance of some details, but almost always at the expense of reducing the contrast of other details.

 

 

I understand, this clears up a lot. I guess I should have phrased my question in a different manner, but I wanted to mask the symptoms.

 

Thanks for taking the time to type that up.


Edited by astronomylife, 17 October 2014 - 05:02 PM.


#14 russell23

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Posted 17 October 2014 - 07:40 PM

I'd rather have a clean car with a dent, than a dirty car! ;)

 

And you understand the point.  A large achromat with a CA reducing filter is a much better performing scope than an identical large achromat unfiltered.  It is not a hard concept but some people haven't tried the Baader 495 Longpass filter on a large achromat and do not realize just how effective that filter is at improving the performance of a large achromat.  The refractors forum has numerous reports of the effectiveness of the 495 LP filter.    Those of us that have seen the 495 LP in action do not make a claim that the filter makes the achromat the equivalent of an equal aperture APO.   But some people have an anti-achromat axe to grind.  In the process of grinding said axe they do their best to try and make people feel that achromats are junk that should never be purchased.   This is unfortunate because there are a lot of factors that go into what ultimately makes the best equipment choice for each individual.   A lot of people get many years of enjoyable use out of achromats.  And many of those people find that a variety of CA reducing filters (Baader Fringe Killer, Baader Semi-APO, #8 light yellow, Baader 495 LP, ... others) are effective at improving the visual performance of the achromat - because they are. 

 

My feeling is you should buy whatever scope meets your interests and budget.  That may be an APO, achro, Cat, Newt ...  So if an achromat doesn't meet your personal needs there are a lot of other options.  But it seems absurd to me to go on a crusade against one class of instrument that is affordable and capable of providing quality views and a lifetime of enjoyment for observers of all experience levels.   It also seems absurd to me that with all the usage of filters in this hobby anyone would argue that in the case of CA reduction - filters should be seen as a dirty word.

 

Dave



#15 mogur

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Posted 18 October 2014 - 03:57 PM

Not to mention thousands of $$$ cheaper at the same aperture. Must be nice to have $$$ to burn!



#16 gunfighter48

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Posted 18 October 2014 - 09:58 PM

I use the Longpass 495 filter on my ES AR 152 scope with great results.  If I had $6,000.00 laying around unused I'd have bought a APO. But I live in the real world and I love the AR152!



#17 coopman

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Posted 18 October 2014 - 10:19 PM

Leslie Peltier sure liked his short focal length 6" achro.



#18 JustaBoy

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Posted 18 October 2014 - 10:28 PM

Yes he did. - But, that was back in the stone age, before the enlightenment.

 

Today we all know that he really couldn't have actually seen anything, because he didn't have an APO.

 

Think about it - What could he have really seen with such a primitive setup?

 

 

I have even heard that he wrote his book Starlight Nights on  something called a 'typewriter' - For those of you that don't know what that is, it's sort of a really Stupid Computer that uses faulty memory, but with a printer built right into the keyboard.

 

 

Thanks,

 

:-)


Edited by JustaBoy, 18 October 2014 - 10:28 PM.


#19 Cometeer

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Posted 19 October 2014 - 12:52 AM

So I'm a little confused now as to what I should get. Is it recommended that I buy a #8 or #12 yellow filter for my ED scope? Many people recommended a #8, but the baader 495 is a #12. Which would show less yellow tint, but still hide the CA visible?

Edited by astronomylife, 19 October 2014 - 12:55 AM.


#20 EuropaWill

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Posted 19 October 2014 - 08:37 AM

Back when I owned a number of achromats (most notably the excellent Meade AR5) I also tried various things to reduce the CA shown on the planets and lunar mainly. I never liked the way color filters attenuated the entire image with a wash of color over everything.

 

The only thing that truly reduced the CA to my satisfaction was the employment of an aperture mask. This effectively reduces the scope entrance aperture but also increases the scope's operating focal ratio (FR). There is an inverse relationship between CA and short FR achromat optics. Increasing the FR of your achromats by reducing the aperture is the cheapest and best way IME to get a cleaner color corrected view from an inexpensive achromat refractor. This will also generally improve the scope's optical correction and your eyepieces will all perform better as well at a longer FR. 



#21 Sarkikos

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Posted 19 October 2014 - 08:44 AM

The best CA reducing filter is GREEN ... more cash for an ED or APO!

 

:grin:

Mike



#22 Sarkikos

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Posted 19 October 2014 - 08:50 AM

I have a C80 ED from AWB. I see virtually no CA through this scope. Compared to my C102 and ST80 achros, the C80 ED is CA free.  

 

For planet/lunar/doubles, I keep a 1.25" Baader T2 Prism Diagonal in the focuser of the C80 ED.  Now, I've never tried planet/lunar through this scope with anything but this prism, so I'm not sure how the prism might be compensating for any residual CA.  I have used the C80 ED with a Meade dielectric 2" mirror diagonal, but only for DSO.  I do not notice any CA when viewing DSO or stars.

 

Mike


Edited by Sarkikos, 19 October 2014 - 08:53 AM.


#23 Sarkikos

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Posted 19 October 2014 - 09:03 AM

Maybe the best aperture mask for a 100mm f/9.8 achro is an 80mm f/7.5 ED? :thinking:

 

I can say that because I have both!

 

:grin:

Mike


Edited by Sarkikos, 19 October 2014 - 09:04 AM.


#24 russell23

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Posted 19 October 2014 - 10:19 AM

So I'm a little confused now as to what I should get. Is it recommended that I buy a #8 or #12 yellow filter for my ED scope? Many people recommended a #8, but the baader 495 is a #12. Which would show less yellow tint, but still hide the CA visible?

 

You will would have to try them and see.  A #8 cuts ~50-60% of the purple CA whereas the 495 LP cuts 100% of  the purple CA. With an ED  scope I would think the #8 LY would be sufficient. 

 

Also, having directly compared the 495 LP to the #12 yellow, the 495 LP does not significantly attenuate brightness of nebular light whereas the #12 does.  The Baader filters seem to have slightly different transmission curves than the typical curves for the wratten #'s.  The #12 visually does show a stronger yellow tint than the 495 LP.  I do not find the 495 LP color shift objectionable.  You will notice it with the Moon and bright stars, but it improves sharpness with an achromat significantly - especially above 100x.

 

Dave



#25 russell23

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Posted 19 October 2014 - 10:22 AM

The best CA reducing filter is GREEN ... more cash for an ED or APO!

 

:grin:

Mike

 

I agree.  It is just that the cost/aperture ratio is so different for the two classes of scopes that the amount of available green to meet the aperture goal for the refractor is often not enough. 

 

Dave




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