Re: [APML] Avoidance of cromatic aberration

["Matt BenDaniel" <matt@starmatt.com>]

Paul,

Good question.

I've been meaning to write an article about this.  If the chromatic
aberration is primarily lateral, it can be corrected during post
processing.  I would not use any sort of filter on the exposure to attempt
to correct the problem.  A filter reduces the T-stop of the system and may
introduce reflections and/or skew the color balance.

If the image is rich in field stars, you can use RegiSar to realign the
channels.  I have used that on some of my images and it works well.  See
article here:
http://home.attbi.com/%7Ejeboud/xchrom.htm

If the image does not have field stars (e.g. star trails), or if you
cannot afford RegiStar, you can align the channels in Photoshop or
PictureWindow.  That is what I would write the article about.  I have done
this on a number of my images with good results.  For example, this image
originally had significant LCA, but it has been fixed:
http://starmatt.com/gallery/astro/azTrails.html

Matt

> Apologies for such a novice question - but you have to start somewhere
> :)
>
> In attempting to create a high resolution mosaic of the moon I notice
> slight blue and red bands either side of high contrast features in my
> images. Definitely some chromatic aberation going on here. I'm afocally
> mounting a 5MP Nikon CP5000 digital camera to an LX200/12 and the lens
> is obviously not as good as it could be....
>
> To try and remove the aberation should I use a "minus violet" filter (I
> hear that this can help remove color aberrations in refractors) or
> simply some type of green filter to remove the reds and blues from the
> image? What is a minus violet filter anyway?
>
> Alternatively is it possible to remove the aberration in a post
> processing step in photoshop simply by removing the red and blue
> channels?
>
> BTW, I'm not so worried about imaging true colors, its resolution I am
> after - I'm hoping that the results when combined can be scaled to cover
> several A3 sheets per phase - OK, it will not be true resolution, but
> when enlarging by this amount every bit of resolution I can squeeze out
> should help.
>
> Many Thanks, Paul.


-- 
Matt BenDaniel
http://starmatt.com


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