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Ken....
Pooh-Bah
Reged: 01/22/04
Posts: 1115
Loc: Nashua, NH, USA
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Is the comatic area as a percentage of the FOV constant regardless of the AFOV of the EP being used?
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Ken....
Discovery 12.5"
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deSitter
Carpal Tunnel
Reged: 12/09/04
Posts: 2928
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The comatic area as a percentage of a given apparent FOV is constant regardless of the focal length (magnification) used. This statement becomes moot at low powers because of edge astigmatism and field curvature.
-drl
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Ken....
Pooh-Bah
Reged: 01/22/04
Posts: 1115
Loc: Nashua, NH, USA
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The variable I'm discussing is the AFOV of the EP, not it's f/l.
So in other words, if coma becomes apparent at 75% out from the axial center of a 50 degree AFOV EP, it will also become apparent at 75% in an 80 degree AFOV EP.
This is what I trying to find out is true or not.
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Ken....
Discovery 12.5"
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Luigi
Post Laureate
Reged: 07/03/07
Posts: 4947
Loc: MA
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If you see coma at 75% from the axial center in a 50 degree EP, that's 75% of 25 deg or 18.75 degrees from the axis. It will appear the same at 18.75 degrees in the 80 degree EP, or 18.75/40 = 47%.
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17.5" f/5 Dob. IM-715 MCT. 120ED. Lunt 60mm Ha.
Zeiss, Leica, Fujinon, Nikon, Pentax, Bushnell bins
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Ken....
Pooh-Bah
Reged: 01/22/04
Posts: 1115
Loc: Nashua, NH, USA
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Quote:
If you see coma at 75% from the axial center in a 50 degree EP, that's 75% of 25 deg or 18.75 degrees from the axis. It will appear the same at 18.75 degrees in the 80 degree EP, or 18.75/40 = 47%.
So you're saying that the percentage of the comatic area increases as the AFOV increases. Right?
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Ken....
Discovery 12.5"
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Luigi
Post Laureate
Reged: 07/03/07
Posts: 4947
Loc: MA
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Let's put it this way. Basically, if you compare the coma visible in a 50 degree and an 80 degree EP, it will look the same in both EPs at every given angle from the optical axis. It'll look the same at 5 degrees in both, 10 degrees in both and 20 degrees in both. It'll look the same a 25 degrees in both but the 50 deg EP ends there while the 80 degree EP goes on another 15 degrees, with increasingly more coma. Out to 25 degrees off axis, the coma will be same in both.
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17.5" f/5 Dob. IM-715 MCT. 120ED. Lunt 60mm Ha.
Zeiss, Leica, Fujinon, Nikon, Pentax, Bushnell bins
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MitchAlsup
super member
Reged: 08/31/09
Posts: 186
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Coma is an aberation that increases linearly with off axis distance.
Given a elescope with only coma visible (Astig=0) and optically perfect EPs::
If you can see coma in a 50 dFoV Ep at 1/2 way out--you are seeing coma at 25 degrees. Thus, you will see coma in a 100 dFoV at 1/4 the way out--at that same 25 degrees.
Given that telescopes with coma and astigmatism, at some magnification, astig will start to blue the edges of the FoV; and note most EPs have significant astigmatism near the edge of the FoV.
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