Neutral Density, #8 Yellow, Baader 495, or Baader Fringe Killer for Chromatic Aberration

[TangoCharlie]

Battle of the Filters.  I was recently suggested to use a Neutral Density Minus Violet Filter for Chromatic Aberration versus Baader 495 or Baader Fringe Killer.  My scope again is an Explore Scientific AR152 Achromatic Refractor F6.5.  A number #8 Yellow was also suggested by another party.  So which one would be used to minimize the violet but not reduce the image brightness/contrast?  Thanks!  TC

[ronin]

Think you will find everyone has a slightly different opinion. Mine is that no filter really does what you want, you cannot turn an achro into an apo by a $20-$50 filter or else we would all be doing it.

One option is to start with the least expsnsive and see what it does for you.

A minus violet will remove some red and blue, leaving what should be a green biased image. A #8 yellow is simple in that it passes the yellow section so again removes Blue mainly and a proportion of the red. Not sure of the design of a  finge killer but I half guess a non-uniform ND where the central portion is reduced which means that it is effectively stopping down the aperture.

Oddly any sort of monochrome filter will remove CA as it only passes that wavelength and so CA is "removed", more relevantly made non-applicable, the drawback is you have a single colour image. Also it is relevant how wide the band that the filter passes.

In reality is it that great a problem ? Although I have ED's and apo's one of my often used is the Bresser 102 achro at f/6, and it doesn't really bother me, I just accept it and that's it.

[parallaxerr]

I agree with ronins sentiment re turning an achro into an ap, you will always have to accept the limitations of an achro.

Having said that, I too have a fast achro in which I wanted to remove some CA, so after some trial and error and reading lots of reviews I went with the Baader Semi-Apo filter. It does not have the yellow tinge of the fringe killer and does indeed remove significant amounts of CA. It's said the contrast booster is the most aggressive, but dims the image considerably and casts a yellow hue.

I still have to accept that the scope will never be any use on planetary work, on Jupiter recently, even at low mag and with the filter fitted, I found the CA intolerable.

Here's a review I found online when I was researching, makes an interesting read...baader_contrast_filters_US_ATT_review_0414.pdf

[Alan64]

When the white-light of an object, Jupiter for example, enters an achromatic-doublet, the light is essentially ripped apart into its various colours.  It is then the task of the figured doublet of crown and flint optical-quality glasses to reassemble the colours back into white-light, the image, by the time it reaches the eyepiece, and the eye and mind of the observer.

In this image, Issac Newton, the Einstein and Hawking of his age, is holding a glass prism in his right hand.  A beam of white-light enters from the right, and exits out the left as a rainbow of colours, but the stream of colours continues into infinity...

A refractor's doublet is figured, shaped, to recombine those colours at a finite point, however, and there at the eyepiece.

Said refractive process is akin to the breaking of a vase into numerous pieces, then reassembling it as though it had never been broken.  That's the goal, in any event; a complete reconstruction.  In the case of a short, fast achromat, quite a few "pieces" of the image are missing after the reassembly, hence the image arrives at the eyepiece damaged.  Said filters under discussion only mask, disguise, the damage, but the damage is still there, and the final image still resulting in a lack of sharpness... 

...and just as a damaged vase will leak. 

The longer the achromat, the more intact the image will be upon its arrival at the eyepiece, for crown and flint glasses can only do so much in reassembling the image in its entirety.

[Ben the Ignorant]

A pale yellow (#8) filter does the job for my 80mm f/7.5 achro. It dims quite a bit of violet, barely darkens the blue/indigo, and darkens the red just a shade. The effect on other colors closer to its own is: green, negligible difference; orange, barely perceptible change. It sharpens all the colors it filters, making the overall rendition cleaner, and after a while you might forget it's there because it almost matches the eye's color sensitivity curve.

Given it cost about 15€, it would be a shame not to give it a try. Even if it's not as perfectly tailored for reducing chromatic aberration as an interference filter, its price/performace ratio is so good you could still use it with no remorse for the expense. Very good Sun and Moon filter.

[Louis D]

I gave up on my ST80's chromatic aberrations and unsharp views even with a minus violet filter (light yellow) and put it away for years.  Recently I bought a used AT72ED.  Much happier with its sharp views up to 120x, near total lack of color, and awesome 2" focuser.  I paid $180 in 2000 for my new ST80 and $200 in 2012 for my used AT72ED.  I only wish the AT72ED had existed 17 years ago so I wouldn't have wasted those $180 back then.  A modern ED scope is the best fringe killer for me, but YMMV.