9 replies to this topic

[rmorein]

Somewhere, I caught a remark that the Zeiss Telementor reaches optimal performance with a prism type star diagonal, because the objective is designed to compensate for the spherical aberration of the prism.

 

Conversely, if an objective is not designed to compensate, it would perform better with a mirror-type diagonal. Which is apt to be the case for classic Japanese refractors ?

[Astrojensen]

The Telementor objective is not designed to compensate for the spherical aberration of a prism. The spherical aberration introduced by a small prism at f/13.3 is very small and can safely be completely ignored. The same is true of the classic Japanese refractors, which are often f/15, or even longer, and rarely shorter than f/10.

 

 

Clear skies!

Thomas, Denmark

[DAVIDG]

 A prism doesn't add spherical if made correctly but  it adds chromatic aberration because of the angle of the light entering it.  With slow f-ratio scope like  a F/15 refractor it adds little  chromatic aberration to the image but when the F-ratio gets faster it does add chromatic aberration. and no  it does not  compensate for the chromatic aberration of the objective . If it did it would have been done 100 years ago and commonly known and practiced  If you want the best color correction use a mirrored diagonal. 

  Here is  an example of  the ray tracing  of 10" f/8 Newtonian were the secondary is a mirror diagonal and one were it is a prism. The black circle is the size of Airy disk.  With the mirror diagonal the image is perfectly achromatic, and has no spherical aberration,  but  with the prism you can see the color blur with light now falling outside the Airy disk. 

 

                   - Dave 

 

[Astrojensen]

 

A prism doesn't add spherical if made correctly

Yes, it does. See https://www.cloudyni...the focal ratio.

 

Posts #5, 8, 10, specifically. 

 

See also https://www.telescop...s_optics.htm#be

 

 

Clear skies!

Thomas, Denmark

[jkmccarthy]

This thread recalls to my mind Wolfgang Rohr's optical performance measurements and report on the Vixen ED115S (D=115mm, F=890mm, f/7.74) refractor, in which he describes how including a prism diagonal (there being 50mm of optical path length in the diagonal he used) actually improves the optical performance of the OTA (most notably in the violet and red extremes) to a near APO level., which I found quite interesting.

 

See:  http://r2.astro-fore...t-ein-apo-phnoa

 

Specifically, compare the measured performance Mit Glasweg (with glass path through the prism diagonal) to the measured performance Ohne Glasweg (without glass path) to assess the improvement in this case.

 

Clear Skies,

 

        -- Jim

[photiost]

I have always believed that slow refractors will perform better with prism diagonals, anything faster than say f/10 will perform better with a mirror diagonal.

 

In any case some of our clubs "older members" insist on observing straight through with no diagonals.

[jragsdale]

In any case some of our clubs "older members" insist on observing straight through with no diagonals.

This is the way. 

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[photiost]

This is the way. 

Thats the way to do it !!  

[k5apl]

I used the inexpensive Celestron 1.25 inch prism diagonal wth my f13 4 inch achro refractor because it had good images and it was

collimatible.

Wes

[jkmccarthy]

This is the way. 

Agreed ... at least until the zenith angle decreases to less than 30-degrees or so !!