4 replies to this topic

[tom77]

I have read that prism diagonals may introduce chromatic aberrations.  Right now, I use a 1.25" Baader Zeiss spec prism diagonal with my FC-100DZ and I can't detect any false color.  My question is whether I will introduce false color if I upgrade to a larger 2" Baader prism diagonal.  Or, in other words, are larger diagonals more prone to chromatic aberrations?  I like the idea of using a 2" prism diagonal because I can use both my 2" and 1.25" eyepieces with an adapter, but I don't want to introduce false color.  Does anyone have experience with this?  Thank you.

Edited by tom77, 02 February 2023 - 10:02 PM.

[TOMDEY]

Longitudinal chromatic is directly proportional to the prism size. So that's 2/1.25 = 60% worse.    Tom

[PJBilotta]

Theoretically, yes. In practice, really difficult to say. I have both, and can detect no additional CA in the 2". So much of this depends on the particular scope and your eyes, but I really cannot see any difference.

A suggestion ... If the field stops of your 2-inchers are under 34mm (the clear aperture of the T2 Zeiss), why not just use a 2" barrel and eyepiece holder on it? Baader doesn't recommend this, but it works just fine as long as your eyepiece field stop doesn't exceed 32-33mm. Works like a charm up to my ES 28/68, though vignetting begins to appear if I go wider than this. All depends which eyepieces you are using and the size of their field stop.

[Chris K]

My understanding was that the CA was detectable in f6 and faster. I have the Baader 1.25 non-zeiss in an f6 60mm EDL refractor and I detect a sliver of yellow on bright moons. No other CA has been detectable for me.

[Eddgie]

My understanding was that the CA was detectable in f6 and faster. I have the Baader 1.25 non-zeiss in an f6 60mm EDL refractor and I detect a sliver of yellow on bright moons. No other CA has been detectable for me.

It is impossible to say this with any accuracy. 

 

If the telescope has overcorrection in red and blue, this will add to the sphrochromatism in the diagonal and if the diagonal is 2", it will add much more than a 1.25" diagonal.

If the scope is undercorrected in red and blue, this will subtract from the spherochroatism in the diagonal.

 

Said another way, if the diagonal overcorrects red and blue by 1/8th wave of SA in red and green, and the scope over-corrects 1/8th wave in red and blue, then you would add them and the total error would be 1/4 wave in red and blue.

 

If the scope was undercorrected by 1/16th wave in red and blue, and the diagonal overcorrected 1/8th wave, you would subtract the 1/16th wave from the 1/8th wave and the light coming out of the diagonal would only be overcorrected by 1/16th of a wave. 

 

Scopes used to be designed for use with prisms and when this was the case, the designer could try to intentionally undercorrect the scope in red and blue. 

 

The point here though is that even at f/8, if there is overcorrection in red or blue (or both) then the longer the light path in the prism, the worse the spherical aberration in those colors gets.

 

If you don't know the correction for red and blue though (and no designer really tells us what it is) you don't really know if the diagonal will make it better or worse.