Edge Of Field Distortion And Magnification

[cloudsweeper]

Blessed (or sometimes a tad cursed) with en enquiring mind, I was wondering - 

EoFD is, I gather, greater in wide angle EPs.  But what about in a given  EP at higher magnification  ?

You're still looking at the same image area through the EP, although it represents a smaller area of sky.  So is edge distortion the same as at lower mags?  I would guess so, but what do others think?

Doug.

[John]

I guess we need to be sure that we are talking about the same thing here. Eyepiece abberations which cause distortions can include:

Astigmatism

Field curvature

Distortion (also known as Pincusion I think)

The latter causes a round shape to be stretched out into an elipse as it nears the field edge. Is that what you are referring to ?

[Louis D]

Edge distortion is can be of either the pincushion or barrel type (negative or positive).  It is due to design trade-offs to maintain edge of field sharpness.  An eyepiece with constant angular magnification shows a double star with the same separation everywhere in the field of view.  However, this necessitates a lot of rectilinear distortion such that straight lines will be bent near the edge.  Both cannot be achieved at the same time in wide angle eyepieces.  Spotting scope eyepieces by comparison will tend to favor minimizing rectilinear distortion to keep straight lines straight.

Part way down on this page there's a good description of distortion and its effects on the view.

[cloudsweeper]

Thanks John and Louis.

I'm referring to all distortions - p/c, barrel, FC, coma, astigmatism....and was wondering if they are remain the same for a given EP when it is used at different mags.

I guessed those distortions would be the same (effect the same amount/region of what you see) when the mag is higher in a longer FL 'scope.

Do you reckon I'm right, or is there some effect I've failed to account for?

Doug.

[Pig]

I would think its safe to say if you see distortion at low magnification then yes distortion would appear intensified at higher magnification in the same scope.

The focal ratio / design of the scope and eyepiece design are important considerations when trying to understand the distortion observed.

[YKSE]

Using the correct terminology will minimize possibility of missunderstanding.

What you were thinking about is called aberration, even though the word distortion felt right.

Geometrical distortions include Rectiliear distorion(RD) and Angular magnification distortion(AMD), where positive RD is pincushion distortion, negative RD is barrel distiion. The visual difference is that RD bend straight lines to curves, while AMD keeps the line straight with varing space between the straight lines. Most of eyepieces have mixed distortion types, most of them with more RD than AMD.

There're more conplex mustasche distortion too

You can some pictures of distortions types here

http://www.cloudynights.com/topic/553874-afov-fs-and-different-distortions/

Geometrical distortion is in eyepiece design, it doesn't change what scope is used.

Coma in eyepieces is non-issue(all eyepieces are well-corrected for its own coma), coma we see is from the scope, it depends on focal ratio, not focal length, for 5"f5 or 20"f5, you'll see the same coma effect, despite the 4x more magnification.

Astigmatism is very much focal ratio dependant too, an 8"f10 and a 16"f5 has the same focal length, therefore same mag, but a not well-corrected EP will show much more astigmatism in 16"f5.

Field curvature is both scope and EP dependant, short focal refractors has quit some FC, makes it a bit more complecated, a flat field EP will likely to show the FC of the scope, while a EP with FC might be having the opposite FC than the scope, therefore showing no FC the combo.

[cloudsweeper]

Thanks!  I meant all aberrations/distortions, so that's everything covered!

Bottom line - these effects are not constant, but manifest differently in different EPs in different 'scopes.

Thanks again,

Doug.

 

 

[Louis D]

9 hours ago, cloudsweeper said:

Thanks!  I meant all aberrations/distortions, so that's everything covered!

Bottom line - these effects are not constant, but manifest differently in different EPs in different 'scopes.

Thanks again,

Doug.

 

 

Geometric distortion remains constant across all telescope types and focal ratios as YKSE stated.

Aberrations are highly dependent upon the scope design and focal ratio, again as YKSE stated.