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Refactor field curvature and star bloat - its both focal length and ratio

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#1 hoes

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Posted 10 January 2020 - 11:56 AM

Hmmm - I'll delete until corrected


Edited by hoes, 10 January 2020 - 12:36 PM.


#2 sg6

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Posted 10 January 2020 - 01:15 PM

I see no consideration of the lens construction mentioned.

Are talking of an achro and apo triplet as being the same - as I seriously doubt that the image created will be identical at the focal plane.

 

I know which give the significantly better image quality between my 81mm triplet and my 70mm achro and although the view is acceptable, the bigger aperture, contrary to the information wipes the floor with the smaller. so the emboldened second conclusion:

" Two refractors with the same aperture but different focal lengths will have the same size star blur due to field curvature if operated at the same magnification and FOV.

 

A secondary conclusion – If you want to reduce star blur due to field curvature at a given magnification and FOV choose a smaller aperture refractor – going to a longer focal length refractor will not help.

"

I would doubt.

 

In some ways too many variables, doublet as achro or ED, is achro cemented or air spaced, then comes the triplets. Even to fair extent the lens format Fraunhoffer or other.

 

And in a way like one branch of maths it works for all instances or is invalid.

 

I would also suggest that one aspect is spherical aberration and is that ignored or just considered as part of the field curvature. I would kind od expect a rather similar experience to field curvature from SA or at least a contribution.



#3 Jon Isaacs

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Posted 12 January 2020 - 06:09 AM

I see no consideration of the lens construction mentioned.

Are talking of an achro and apo triplet as being the same - as I seriously doubt that the image created will be identical at the focal plane.

 

The curvature of the field of standard refractor, doublet, triplet, achromat, apo, is essentially the same, about 1/3rd the focal length.  What is seen at the focal plane in terms of curvature depends on both the focal length and the focal ratio, the focal length determining the radius of curvature, the focal ratio determining the relative size of the blur.

 

Modified Petzvals have an objective in front and a second set of lenses in back that function primarily as reducers, flatteners.  They essentially have flat fields.

 

Jon


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