Re: [APML] Achromats?

[lkt <tan@cox.net>]

Since color lies in a continuous spectrum, doing tricolor with an
achromat will alleviate the misfocused colors but you will still see a
lot of it. For example, even the blue range of light thru the blue
filter will range from focused to out of focused. 

One good use for an achromat is for h-alpha imaging. The Custom
Scientific h-alpha filters are 3-4nm in width and so the color within
this range has pretty much the same focus. 3-4nm is much narrower than
say a red tricolor filter which might be a 100nm wide. 

Alternately you can try artificial tricolor with O-II, h-alpha, h-beta
filters. These filters are very narrow for extraction of emission lines.
Light within the passband of the filter will be focused but image scale
will be slightly different. The image scale problem can be fixed by
Registar as already suggested. For this to work though you must make
sure that the cheap achromat is not too cheap. Coma is not a color
abberation and the lens will show this very well even in a 4nm range.

Loke





> Tony Hallas wrote:
> 
> What about doing tricolor with an achromat? Would you be able to focus
> each color at a time and then combine them digitally? Or would the
> different focus points create different image scales? Even so,
> Registar could straighten that out. By using a R, G, & B filter maybe
> nice work could be done with a cheap lens?
> 
>      Tony

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